Low Vision Notes Flashcards

Question

4. Near magnification requirement

Answer 1

􏱃 Identify the visual task & estimate TVA as above 􏱃 Measure BVA (usually at 25cm with the appropriate near add of +4.0 DS) & estimate the magnification required to achieve TVA 􏱃 Assess central fields- if a defect is suspected (Amsler) before prescribing magnifiers; may need to consider eccentric fixation 􏱃 Try predicted magnification – check VA with magnifier; modify magnification as required to achieve target acuity 􏱃 start with simple magnifiers: high add, hand or stand magnifier 􏱃 Illumination 􏱃 Prescribe the LVA with clear instructions!!

Answer 2

􏱃 Establish binocularity (e.g. Cover test) and eye dominancy (many LVAs are monocular) 􏱃 Contrast sensitivity if indicated 􏱃 Fields (Amsler may be most useful in LVA assessment) 􏱅 Ophthalmoscopy/fundus photography, OCT, IOPs etc 􏱃 Spend your time wisely and do not do unnecessary tests

Answer 3

􏱃 Important- patients often forget what the aid was intended for or how to use it 􏱃 Ideally review patient in 2-4 weeks 􏱃 may need to change LVAs

Answer 4

􏱁 Speak slowly & clearly o May have hearing loss - speak to best ear 􏱁 Be encouraging & positive 􏱁 Don’t rush the patient, but stay in control

Answer 5

􏱁 Well known, commonly used standard clinical measure of VA & vision 􏱂 Very sensitive to blur and uncorrected refractive error 􏱁 Good size (portability) 􏱂 Easy to move closer to patient 􏱁 Has letters ‘O’ suitable for x-cyl

Answer 6

􏱁 Unequal numbers of letters on each line (crowding not constant) 􏱁 No relationship between line size & number of letters - from 1 to 8 􏱁 Poor control of ‘contour interaction’ 􏱁 Unequal letter size progression- 1.2X increase from 6/5 to 6/6, 1.67X increase from 6/36 to 6/60 􏱁 Designed to measure ‘normal’ acuity 􏱁 Scale intervals change at non-standard distances (crowding inconsistent)

Answer 7

Example: A 6 metre letter letter viewed at a distance of 6 m (6/6), has a minimum angle of resolution (MAR) of 1arcmin (gap size or limb width; MAR =1as 6/6=1’). A 24 metre letter is 4 times larger so MAR=4’ if viewed at 6m (24/6 = 4’) If a Px with VA 6/24 views the chart at 3m they should be able to see the 12m letter (written as VA of 3/12) because this subtends the same visual angle 􏰧􏱃􏰨 as the 24m letter viewed at 6 m (MAR 12/3=4’).

Answer 8

􏱁 Size, letter and line spacing equivalent throughout chart 􏱂 Equal numbers of letters per row (5) 􏱁 Sensible progression of letter size (0.1 log or 1.25X) 􏱁 All letters have equal legibility 􏱁 Constant crowding for all VA levels 􏱁 Final score takes into account all letters that have been read successfully

Answer 9

􏱁 LogMAR not used routinely as a clinical measure of VA 􏱁 Scoring and conversion not as easy as Snellen 􏱁 there’s no O letter for X-cyl 􏱁 A bit on the big side and may be hard to illuminate

Answer 10

Each letter = 0.02 log (5 letters per line) Rule of thumb: Every time the viewing distance is halved add 0.3 to the logMAR -score. Example (logMAR chart designed for 6 m) Px views chart at 6 m 􏱁 Reads the 0.8 line + 2 letters on the line below 􏱁 logMAR VA is 0.8 - 0.04 = 0.76 Px views chart at 3 m 􏱁 Px now reads 0.5 line + 2 letters on the line below 􏱁 logMAR VA is 0.5 - 0.04 = 0.46 + correction factor of 0.3 for moving from 6 to 3 metres 􏱄 0.46 + 0.3 = 0.76

Answer 11

The Faculty of Ophthalmologists Times New Roman near chart N-Point Notation 􏱁 Standard reading chart widely used 􏱁 Based on printing standards 􏱁 1 point is 1/72” 􏱁 Top of ascending to bottom of descending limb of letter 􏱁 Times Roman type face 􏱁 Easy to understand and convert e.g N24 = twice size of N12 􏱁 Blocks of text Bailey-Lovie Word-Reading chart LogMAR chart 􏱁 2to6wordsonaline 􏱁 Unrelated words, no guessing 􏱁 Limited number of words on line - good test of reading 􏱁 Difficult for children, those with poor English or poor cognition

Answer 12

E.g. telescopes BVA ... Threshold (Best corrected) VA TA ... target acuity M... required magnification M = BVA/TA Example: 􏱂 Px can see 6/60, needs to see 6/12 􏱂 M = BVA / TA = 60/12 = 5 􏱂 5 x magnification required

Answer 13

Use N – point notation and test patient’s acuity at a distance of 25cm (+4.00 D Near Add if required) M = BVA / TA BVA ... Threshold (Best corrected) VA TA ... target acuity M... required magnification Example= Px can see N24 at 25 cm with the appropriate add of + 4.0 D?…needs to see N12 M = BVA / TA = 24 / 12 = 2 2 x magnification required

Answer 14

􏱁 Magnifiers 􏱂 High add 􏱂 Spectacle 􏱂 Hand held 􏱂 Stand magnifier 􏱂 Electronic LVAs

Answer 15

􏱁 Keep it simple! 􏱁 Start with low magnification if possible 􏱁 Illumination! 􏱁 Consider 􏱂 Task & age of patient 􏱂 Physical infirmities e.g. can the patient hold the magnifier/task steady? 􏱂 Cost

Answer 16

Will be needed if the prescription does not create enough improvement and if VA is insufficient for identified tasks Need to make retinal image size greater hence easier to see

Answer 17

1. Increase size of object 2. Decrease viewing distance (move closer) 3. Real image or transverse magnification (E.g. zoom camera) 4. Telescopic magnification

Answer 18

M = new object size / old object size Examples: Large screen TV, Large print (books, newspapers, telephone dials, medicine labels, bills, computers and mag software, labelling machines)

Answer 19

(M) magnification = old viewing distance / new viewing distance - very simple and effective works at distance or near if sufficient accommodation – hence used frequently by children and young adults with good accommodation; myopes can remove their spectacles Using a short viewing distance without the necessity for accommodation requires the use of a plus lens (magnifier). Examples: - Distance: viewing TV from 6 feet rather than 12 feet gives 12/6 = 2x magnification - Near: bring print to 5cm rather than 30cm gives 30/5=6x magnification

Answer 20

(M) magnification = size of image/size of the original object Not an optical method – see electronic LVAs Advantages include the ability to adjust the magnification factor and manipulate contrast and colour

Answer 21

magnification = angle subtended by telescope image (0') / angle subtended by original object (0) - theta angle on the retina? - Optical method - Versatile (used at distance, intermediate and near)

Answer 22

Example: What is the magnification obtained by changing print size from 6mm to 15mm, and viewing distance from 30cm to 10cm? M1 = new object size / old object size = 15 / 6 = 2.5x M2 = old viewing distance / new viewing distance = 30/10 =3x The total magnification Mt=M1 x M2 =2.5 x 3 = 7.5x

Answer 23

Increase size = no change F.O.V, Working space = no change, Distance of Task = Nr (Int or Dis limited) Decrease distance = no change F.O.V, Working space = decreased , Distance of Task = Nr, Int or Dis Using a plus lens = Increase size = F.O.V decreased , Working space = decreased , Distance of Task = Nr real image (camera) = F.O.V decreased , Working space = no change, Distance of Task = Nr (Int or Dis limited) Telescopic = F.O.V decreased, Working space = no change, Distance of Task = Nr, Int or Dis

Answer 24

Magnification (M) is relative. Let’s compare two lenses F2 and F1 of different powers, placed at the eye’s nodal point (Figure 8). - Lens F1 “standard” lens is taken to be +4.00 D (f=0.25 m, “standard” viewing distance if the object is placed at the focal point of a plus lens) - Lens F2 (magnifier) whose magnification is unknown - Lens F2 has a magnification of M= F/4 relative to lens F1 -If a patient does not achieve the magnification stated on a magnifier their current reading specs may not have a +4.00 add (viewing distance 0.25 m) and the baseline acuity may therefore be incorrect. Æ measure vision at 25cm/with +4.00 add to determine a correct baseline acuity. Image at infinity – no near add or accommodation required as parallel light leaves the lens and enters the eye Eye to magnifier distance has no effect on magnification Called “Nominal magnification”

Answer 25

1. General case: M = qL / 1- dL' q … ‘nearpoint’ or ‘least distance of distinct vision’, usually assumed to be 0.25m d …distance lens to eye L… incident vergence L’…emergent vergence Note: M depends on the distance of the lens from the eye and the distance of the object from the lens 2. Image at near point M = ( F / 4 ) + 1 Object between F and Lens Image produced by magnifier at near point (e.g. q=25 cm) -> + 4 m-1 add or accommodation required Magnifier held close to Px’s eye (d=0) see Figure 12 called “Maximum magnification” or “Isoaccommodative magnification”

Answer 26

Reading speed is greatly affected by the extent of text seen through a magnifier and hence it is desirable for LVAs to provide as large a field of view (FoV) as possible. The usable FoV mainly depends on three factors: lens power, lens size and aberrations. Field of view = Diameter of magnifier / (d x Power of magnifier) Where d is the magnifier-to-eye-distance and D is the diameter of the magnifier. Example: How does the field of view change when a patient holds a round 90 mm +6 DS magnifier 25 cm from his eye and then brings it up to a distance of 6 cm from his eye? D= 90 mm; F=+6 m-1, d1=25 cm; d2=6 cm FoV1 = 0.09/(0.25*6) = 60 mm M=F/4 = 1.5x FoV2 = 0.09/(0.06*6) = 250 mm It can be seen that the magnifier must be held at a close distance to the eye to maximise the field of view.

Answer 27

D= 90 mm; F=+6 m-1, d1=25 cm; d2=6 cm FoV1 = 0.09/(0.25*6) = 60 mm M=F/4 = 1.5x FoV2 = 0.09/(0.06*6) = 250 mm

Answer 28

To maximise the field of view

Answer 29

The power of a thick lens or a lens system consisting of two thin lenses can be calculated as: Feq = F1 + F2 - ( d x F1 x F2) Feq = power of thick lens or system F1 = power of 1st surface or lens F2 = power of 2nd surface or lens d = distance between lenses (surfaces) e.g. the patient wears add +2.50 DS and holds a +10 DS magnifier 25 cm from their bifocal (or reading specs). (With nominal magnification: M = F/4 = 10/4=2.5X magnification) FMag= +10 m-1 FAdd = +2.5 m-1 z =25 cm (this is d) = 0.25m Hence: FEQ = 10 + 2.5 –(0.25m *10m-1 *2.5 m-1) FEQ = +7.5 m-1 So the magnification this patient achieves is: MEQ = FEQ / 4 = 7.5 / 4 = 1.9 X Conclusion: When a magnifier is used together with a near add and the magnifier to eye distance is: 1. less than the magnifier’s focal length the equivalent magnification will be higher than the magnifiers nominal magnification 2. greater than the magnifier’s focal length the equivalent magnification will be lower than the magnifiers nominal magnification 3. equal to the magnifier’s focal length the equivalent magnification will be equal to the magnifiers nominal magnification Key Point: Using a magnifier with reading Rx requires a short magnifier to eye distance. Long magnifier to eye distances can result in very low enlargement ratios.

Answer 30

FEQ = F1 + F2 –d F1 F2 FMag= +10 m-1 FAdd = +2.5 m-1 z =25 cm (this is d) = 0.25m Hence: FEQ = 10 + 2.5 –(0.25m *10m-1 *2.5 m-1) FEQ = +7.5 m-1 So the magnification this patient achieves is: MEQ = FEQ / 4 = 7.5 / 4 = 1.9 X

Answer 31

The magnification will be overestimated.

Answer 32

1. Handheld 2. Spectacle mounted 3. Stand mounted

Answer 33

-Familiar to patients, most common type of magnifier, inexpensive -easy to prescribe, can be used with Nr or Dis rx -Compact, lightweight and easy to carry around -Can have long eye-to-mag distance -Useful for px with reduced peripheral fields

Answer 34

Low powered (4x) Medium powered (4x-8x) High Powered (8x+) As increase power, field of view increases (40mm F.O.V. low powered - 25 mm F.O.V. for high powered)

Answer 35

-Can be ineffective is used incorrectly with reading add -Difficult to maintain correct position for extended periods -Poor field-of-view if long eye-to-magnifier distance -increased distortions as lens held further from eye

Answer 36

-Lay magnifier on page and slowly pull away from page until image clear -advise on spex to be worn -best to use distance rx -Nr rx only for shorter mag-to-eye distance -Obtain largest FOV by holding magnifier as close to eye as possible -most curved (steeper) part towards patients eye for least distortion -Hold magnifier parallel to reading material -Move head and magnifier as a unit together -

Answer 37

-On SV or Bifocals -Paired lenses with base-in prism in standard frame -Lenses glazed to special mounting or carrier -Clip-on lenses -Other LVAs intended for occupational use

Answer 38

Binocular gives: -Larger F.O.V -Greater depth of field -Better acuity -Depth perception -Monocular overlap in case of field loss -conventional + psychologically better appearance

Answer 39

-Px has no demonstratable binocularity at distance required for magnifier - greater than 2x diff in VA between two eyes -Central distortion in previously dominant worse eye -Bin VA worse than Mono VA -too great convergence demand, resulting in diplopia or discomfort (balancing/occlusion may be necessary)

Answer 40

NCD = (PD x WD) / (WD + a) -Where NCD is the near centration distance -WD is the working distance -a is the distance from the Centre of rotation to the spectacle plane 1. Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +3.00, requiring a working distance of 33 cm NCD = (PD x WD) / (WD + a) NCD = (2 x 30 x330) / (330 + 13 + (24/2)) NCD = 57mm 2.Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +20.00, requiring a working distance of 5 cm NCD = (PD x WD) / (WD + a) NCD = (2 x 30 x 50) / (50 + 13 + (24/2)) NCD = 40mm

Answer 41

1. Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +3.00, requiring a working distance of 33 cm NCD = (PD x WD) / (WD + a) NCD = (2 x 30 x330) / (330 + 13 + (24/2)) NCD = 57mm

Answer 42

NCD = (PD x WD) / (WD + a) NCD = (2 x 30 x 50) / (50 + 13 + (24/2)) NCD = 40mm

Answer 43

For “normal” working distances NCD approx. 2-5mm less than distance PD (e.g. inset of bifocal segment is usually 2-2.5mm for each eye). For the very short working distances required for magnifiers, lenses need to be decentred by a much larger amount. Remember to decentre the trial frame when testing patients with a high binocular add! Even if the eyes look through the optical centre, the convergence requirement is considerable (eg: approximately1' base out for each mm of PD to view at 10 cm), which is why the patient usually requires additional base in prism. There are various clinical “Rules of Thumb” to determine the NCD for example: 1. Fonda’s recommendation Total decentration for near = 2 x WD (D) For the example on the previous page this would give 2*5 =10 mm NCD = PD – Decentration = 60 -10mm = 50mm

Answer 44

Advantages of spec-mounted magnifiers: - hands free, useful for prolonged reading - widest field-of-view (because of short eye-to-magnifier distance) - cosmesis - similar appearance to “normal” spectacles - can incorporate astigmatic correction - maybe used for patients with hand tremors –e.g. with a reading stand Disadvantage's of spec-mounted magnifiers - close working distance - illumination may be difficult - fatigue , HA’s, dizziness - reduced reading speed generally occurs with very close WD - limited magnification binocularly

Answer 45

Because of the problems (convergence demand and viewing distance) associated with high near adds, binocular adds are: easy up to +6.0 tricky up to +8.0 risky up to +10.0 highly unlikely to be successful if +12.0 or greater

Answer 46

Change in l (triangle l) = ( - triangle L') / (4xM)^2 e.g. Calculate depth of field i.e. How much can the reading material be moved before the patient notices defocus if the patient’s depth of focus is 0.8 dioptres (+- 0.40 DS) and a simple magnifier of + 5 DS is used? Triangle l = ( - triangle L') / (4xM)^2 (M = F/4 = 5/4 = 1.25) Triangle l = 0.8 / (16 x 1.25^2) = 0.032 = 32mm Key point- Magnifiers have to be held very steadily to achieve fluent reading.

Answer 47

Triangle l = ( - triangle L') / (4xM)^2 (M = F/4 = 5/4 = 1.25) Triangle l = 0.8 / (16 x 1.25^2) = 0.032 = 32mm

Answer 48

1. Fixed Focus 2. Variable focus

Answer 49

Key Point: When prescribing or dispensing stand magnifiers ensure that the patient has adequate accommodation or suitable reading glasses. The higher the near add + the closer the eye-to-mag distance = the higher the mag achieved will be

Answer 50

1. View object through magnifier and find the maximum plus lens which when held up against the magnifier still gives a clear view 2. As in 1.) Use a telescope to eliminate the effect of accommodation 3. As in 1.) but reverse light path and focus a distant object on the object plane of the magnifier

Answer 51

– Good for patients with hand tremors – Most are small and portable – Relatively inexpensive

Answer 52

– Limited FoV – Accommodation or add required – Difficult to determine equivalent power – Posture & fatigue problems – Illumination can be problematic unless built-in light source

Answer 53

- An interesting feature of the design is that it is self-illuminating. - The thicker a flat-field magnifier is (in relation to r), the greater the magnification (usually < 3X) - Image is formed very close to original object so there is no “relative distance magnification” - Patient must wear reading correction or accommodate - Can have bar magnifier with magnification in only one direction (Plus cylinder lens)

Answer 54

- Normal reading posture possible - Very bright - light gathering - Useful for patients with hand tremor etc. - Clear image across the whole lens – aberrations have minimal effect - Combination of bar and spectacle mounted magnifier can increase the magnification while maintaining a long viewing distance

Answer 55

- Large lenses - heavy - Magnification typically low - Reading material has to be flat and on firm surface

Answer 56

Telescopic magnification is an optical method of magnification, which is more versatile than a plus lens because the magnification can be produced at any viewing distance: - Distance (street signs, bus numbers, blackboard, theatre) - Intermediate (playing cards, music, TV, computer) - Near (reading, writing) Telescopes can be hand-held (for spotting) or spectacle mounted, but it is rarely possible to wear them constantly or whilst mobile.

Answer 57

M = Fe / Fo t = f 'E + f ' o

Answer 58

Astronomical (Keplerian) telescope : - Fe and Fo positive - M negative, image inverted, - prism/mirror erecting system required - both f’e & f’o positive so telescope long - prism allows folding of light path to reduce length

Answer 59

- FE negative, FO positive - M positive so image erect – (no erecting system required) - one focal length negative - short system length

Answer 60

Galilean: Lighter, shorter length, exit pupil inside, image erect, mag low (up to 4 x), FOV smaller, image quality poorer + cost lower Keplerian - Heavier, longer, exit pupil outside, image inverted, mag upto 10x + ,

Answer 61

M = Fe / Fo Fe = - 30 m^-1 1/l' = 1/l + F =(1/-0.0667m ) - 30 m ^-1

Answer 62

The first number gives the magnification and the second number refers to the diameter of the objective lens in millimetres.

Answer 63

a) 7x50 50/7 = 7.14 mm b) 8x35 35/8 = 4.38 mm c) 10x20 20/10 = 2.00 mm Answer: a followed by b and c.

Answer 64

Answer: a first, followed by b, d then c a) 7x50 50/7 = 7.14 mm b) 8x35 35/8 = 4.38 mm c) 10x20 20/10 = 2.00 mm d) 10x40 40/10 = 4mm

Answer 65

- Magnification - use minimum power - vertex distance: 5mm = closest distance without specs + 20mm = likely with specs and rubber eyecup - object distance: not practical to change this -objective diameter: telescope becomes heavier and more difficult to handle To optimise the field we need to match size of exit pupil to the patient’s pupil, but if the alignment slips part of the field will go dark. It is therefore actually better if exit pupil > patient’s pupil, which allows some misalignment, but obviously means some loss of field. Another point to consider is that the calculations of the field of view do not accurately reflect the actual patient experience, because the patient’s pupil is the limiting aperture rather than the telescope’s objective lens. The effect of aberrations is also not accounted for by FoV calculations based on Gaussian optics.

Answer 66

required accommodation= ~ L’ L’ = M^2 x L = (3)^2 x 4 = 36 m-1 Conclusion: Even young patients viewing objects at intermediate distances do not have sufficient accommodation and need to modify the telescope

Answer 67

1. full correction for viewing distance over objective (Fo) 2. increased correction for viewing distance over eyepiece (Fe) 3. increasing the separation of Fe and Fo

Answer 68

Mtotal = Mtel x Mrc Mrc = Frc (power of plus lens) / 4 1.What magnification is achieved when using a telemicroscope, which consists of a 3x telescope with a +4.00 DS reading cap? What is the working space? MTOTAL = 3 x 4/4 = 3x working space: fRC = 1/4m^-1= 25cm

Answer 69

Mtotal = Mtel x Mrc Mrc = Frc (power of plus lens) / 4 1.What magnification is achieved when using a telemicroscope, which consists of a 3x telescope with a +4.00 DS reading cap? What is the working space? MTOTAL = 3 x 4/4 = 3x working space: fRC = 1/4m^-1= 25cm

Answer 70

Mtotal = Mtel x Mrc Mrc = Frc (power of plus lens) / 4 MTOTAL = 2 x 10/4 = 5x working space: fRC = 1/10m^-1 = 10cm

Answer 71

Working space is greater with telemicroscope (25cm versus 8 cm)

Answer 72

Practical considerations: even if working space is 2x larger, not useful unless at least 5cm increase increased working space makes binocular viewing more practical need to be able to angle telescope tubes appropriately (convergence) overall working distance increased by length of telescope (minor point)

Answer 73

- Not a practical method unless patient already has high-powered reading add for other purpose - Magnification difficult to calculate unless all system parameters known

Answer 74

- A practical method, but more difficult in spec-mounting = can become “front-heavy” as tube length increases + difficult to angle correctly if binocular - More common in astronomical telescope = usually hand-held anyway = image quality better maintained

Answer 75

Conclusion: For working distances (greater than 25cm) it is always better to refocus a telescope than to add a reading cap, as this will provide higher magnification. Practical limitations include the increase in telescope length. The closest possible focusing distance corresponds to the LONGEST position of telescope (t=max) so there are practical/mechanical limitations and sometimes the tubes cannot expand far enough.

Answer 76

The major reason for using telescopes for near tasks is the increase in working space compared to magnifiers. (2x telescope increases by factor 2; 3x telescope increases by factor 3 etc.). This is usually needed for manipulative tasks. but: The field of view is very small.

Answer 77

3 Methods: 1. full correction for refractive error behind eyepiece (Fe) 2. partial correction for refractive error over objective (Fo) 3. changing the separation of Fe and Fo 1. full correction for refractive error behind eyepiece (Fe) = Simplest method: telescope clipped on to, or held over, spectacle correction = Or correcting lens can be fitted into holder behind eyepiece if required = No effect on magnification because telescope still afocal (see Figure 30) Figure 30 Schematic diagram of a Galilean telescope with Rx behind eyepiece for a hyperopic eye (left); clip-on Galilean telescope (right) 2. partial correction for refractive error over objective (Fo) = Use partial correction to achieve some divergence or convergence of light entering telescope, which would then be amplified by telescope to the required amount = Complex calculation to work out effect on magnification = Impractical, and never attempted 3. changing the separation of Fe and Fo = shorten to correct myopia and lengthen for hypermetropia = length change needed obviously depends on Rx = effect on magnification

Answer 78

MTEL = -Fe/Fo = -60/20 = -3x (as for emmetropic user because telescope is afocal) t = fo’ + fe’ = 50 + 16.7 = 66.7mm

Answer 79

t = fo’ + fe’ = 1/Fo’ + 1/Fe’ = 1/20m-1 + 1/(60-(-10)m-1) = 64.3 mm MTEL = -Fe/Fo = -70/20 = -3.5x Mfocused > Mnormal

Answer 80

1. If hypermetrope is corrected -> MTEL = 3x as for corrected myope, or emmetrope, length as in normal adjustment 2. If hypermetrope is uncorrected and telescope focused to correct hyperopia: t = fo’ + fe’ = 1/Fo’ + 1/Fe’ = 1/20m-1 + 1/(60-(+10)m-1) = 70 mm MTEL = -Fe/Fo = -50/20 = -2.5x Mfocused < Mnormal

Answer 81

Astronomical = MTEL higher with focusing telescope Galilean = MTEL higher with correction behind eyepiece

Answer 82

Astronomical = MTEL higher with correction behind eyepiece Galilean = MTEL higher with focusing telescope

Answer 83

1. For a plus-lens magnifier (2.5x) of powerFEQ=+10m-1: if object at FLens then: l = f = -0.1m -> L = 1/-0.1= -10 m-1; L’ = 0 now let the emergent vergence (L’) be +0.5 m-1 L’=L+F , L1 = +0.5-10 = -9.5 m-1 now let the emergent vergence be -0.5 m-1 L’=L+F , L2 = -0.5-10 = -10.5 m-1 Change in l = (1/-9.5) - (1/-10.5) = 1 cm 2. For a 2.5x telescope with +4.00 reading cap Emergent vergence (L’TEL) = M^2 x incident vergence (LTEL) LTEL = L'TEL / M^2 working distance = 25cm, vergence at plus lens -> L= -4.00 m-1, L’ = 0 vergence entering telescope LTEL = 0 calculate LTEL if L’TEL= +/-0.5m-1 LTEL = +/-0.50m^-1 / 2.5^2 =0.08m^-1 Incident vergence at reading cap: L = 1/-0.25m = -4.00m^-1 Change in L = -4.00m-1 +/- 0,08m-1 change in l = (1/(-4+0.08m-1) - (1/(-4-0.08m-1) = 1cm Conclusion: It appears that both systems have the same depth of field. However, subjectively patients usually feel the DoF is smaller for telescopes. One possible explanation is that the DoF of a telescope corresponds to a smaller percentage of the working space.

Answer 84

Generally 3 types: Hand held, Clip on and Spectacle mounted. You could further distinguish monocular versus binocular telescopes and focusable v fixed focus devices. There are special variants: - Autofocus - Contact lens telescope (with CL or with IOL) - Intra-ocular lens telescopes (Peli 2002) - IOL VIP system – combination of negative and positive IOL (Orzalesi, Pierrottet et al. 2007) - BTLT - Behind the Lens Telescopes (Spitzberg, Jose et al. 1989) - Bioptic or BITA (Bi-level Telemicroscopic Apparatus) For patients who fulfil certain Visual Field and Acuity requirements driving with BiOptics is legitimate in some States in the US Driving with BiOptics is not legal in the UK hence there is limited interest in the technology

Answer 85

Galilean system - Eyepiece created by (high-powered) negative contact lens - Objective created by (high-powered) positve spectacle lens - Vertex distance of spectacles is equal to length of telescope (sum of focal lengths) - Field of view better than with conventional telescope because objective & exit pupil close to eye, but the FoV also depends on the diameter of the objective lens (use blended aspheric to avoid ring scotoma) - Rarely used because of the many practical and cosmetic disadvantages - Fitting of CL telescopes: - Check that conventional telescope improves acuity as expected - Check nystagmat for oscillopsia - Fit contact lens, maximise BVD of trial frame, perform over-refraction - Contact lens must be stable!! - Vergence amplification means separate reading specs are required - Uncorrected ametropia changes the effective eyepiece power = +10 hypermetrope with -30 contact lens, effective eyepiece is -40 = -10 myope with -30 contact lens, effective eyepiece is -20

Answer 86

Advantages : - In theory patients would be allowed to drive under UK law if VA and field requirements were met - Can get acuity much better than expected in congenital nystagmus - Intra-ocular lens could be used instead of contact lens giving a longer vertex distance so better magnification but IOL can’t be changed if acuity worsens Disadvantages: - Low magnification,so only works for moderate acuity loss - Patient needs to be adapted to contact lens wear first - CL difficult to insert–poor near acuity with highly negative lens power - Need to wear system regularly for prolonged periods - Adaptation to spatial distortion required - Poor cosmesis, specs with long BVD and high plus lenses

Answer 87

If the parameters of a telescope are unknown, its type and magnification can be estimated by direct observation or by locating and measuring the size of the entrance/exit pupil Determination of Magnifying Power: 1. Direct Comparison - The magnified view seen through the telescope with can be compared with the non-magnified view seen by fellow eye. - Use tile pattern etc (something you can count). as target (Figure 39) 2. Estimating based on Exit pupil measurements - Measure size of exit pupil e.g. using mm scale on band magnifier (Figure 40) - Measure size of objective lens (entrance pupil) -> M = entrance pupil/exit pupil Determination of telescope type by exit pupil method: - In general, Keplerian (astronomical) telescopes are longer than Galilean telescopes, but the use of erecting systems which incorporate mirrors or prisms may be misleading. The exit pupil movement can be used to distinguish telescope types: Looking into the eyepiece observe the apparent movement of the exit pupil while moving the eyepiece lens from side to side. 1. Astronomical gives Against (2 A's) 2. Galilean gives with

Answer 88

When viewing a distant object an uncorrected simple myope will shorten the length of an afocal telescope, while an uncorrected simple hypermetrope will increase the tube length, such that the image produced by the telescope will form at the far point hence his/her refraction will determine the tube length of the telescope. A calibrated telescope as a simple and inexpensive instrument to determine spherical refractive errors: - may be applicable to populations in under-served areas where electricity and modern equipment are not available. - alternative subjective refraction method in low vision population Æ patients can be refracted at a ‘normal’ 6m testing distance. - may be applicable to patients with extreme Rx e.g >25 DS and/or patients where retinoscopy or autorefraction is difficult/inaccurate due to media opacities or extremely small pupils and a starting point for subjective refraction is needed One could also refract through an afocal telescope using a distant test chart and alter the trial lenses behind the eyepiece e.g. in the back pocket of the trial frame if a patient’s best corrected acuity is low and they cannot be moved closer to the chart.

Answer 89

1. Measure BVA 2. Think about target acuity e.g. Tv about 6/18 or street signs <6/12 3. Calc magnification, try predicted Mag, modify 4. Decreases viewing distance (e.g. go closer to tv) OR LVAid (monoc / binoc) 5. VA expected = trial px with instrucyions and follow up appt OR VA Contrast, (e.g. projector, computer vs printed charts) ->Crowding (number of optotypes and lines etc) -> Testing distance

Answer 90

Examples include: Factors impacting on VA testing: - Illumination, for example: = Rod monochromats – may have better acuity in dim illumination = Retinitis Pigmentosa – may have better acuity in bright illumination -Visual field status = Peripheral field loss may result in inability to see larger targets but good response to see smaller ones = Central scotoma may require eccentric viewing, isolation of optotypes (crowding) - Measurement technique = Patient & practitioner motivation is important = Different acuity charts may give different VAs, depending on: ->Contrast, (e.g. projector, computer vs printed charts) ->Crowding (number of optotypes and lines etc) -> Testing distance

Answer 91

The ability to read fluently or scan a visual scene quickly depends on the contrast reserve (CR) CR is the ratio between the patient’s contrast threshold (minimum contrast required to see targets of a certain spatial frequency) and the target contrast (e.g. book or TV-screen). Optimum contrast reserves are in the region of 20:1, while the minimum requirement for reading or watching a film is about 10:1. Testing the patient’s contrast sensitivity (for example with a Pelli-Robson chart) will help deciding whether lighting enhancement (>6%) or contrast enhancement (>4%) would be beneficial. If the patient’s CS is very poor, they will not be able to see with an optical LVA.

Answer 92

Scotomas, which are extremely common in ARMD, reduce VA but also affect the accuracy and efficiency of eye movements. Poor eye movement control reduces the ability to scan a visual scene quickly and affects reading speed and reading comprehension.

Answer 93

Snellen notation: M = TA (target acuity) / BVA Example: Snellen BVA=6/60 TA= 6/12 M = (6/12) / (6/60) = 60/12 = 5x logMAR notation: M= (1.25)^number of steps each step = 0.1 log unit = 1.25x Example: BVA= 1.0 , TA = 0.3 M= (1.25)^7 = 4.77 M = 5x

Answer 94

- Spectacle lenses - Contact lenses - Coatings & tints - Telescopes - Multiple pinhole lenses

Answer 95

When the magnification need has been established and it has been the decided that the patient requires a telescope, a trial with an appropriate device is carried out: - Focus LVA yourself for appropriate distance - Ask patient to locate and read chart - Show them how to optimise focus - Note handling ability, reaction to FoV and cosmesis If the VA achieved with the device matches your prediction, the patient needs to try this in reallife circumstances. This requires good instructions: - explain exactly how it works - what it can and cannot do (e.g a focusable distance telescope for bus numbers can also be used for shop windows and a spec-mounted telescope is not for walking around in!) - need to practice with the device - how to care for it

Answer 96

Accommodation at near Pupillary constriction at near Lighting at near maybe different Myopia at near (retinal image size greater without Rx) Corneal or lens opacities: o Posterior subcapsular cataract – tend to have poorer near vision than distance vision o Nuclear & diabetic cataract - tend to have better near vision than distance vision It is more accurate to assess the patient’s near acuity using a near chart and calculate the magnification required based on this measurement. Generally, this is done at 25cm viewing distance. A near add needs to be provided if the patient has insufficient accommodation.

Answer 97

As with distance VA, assess near VA: - monocularly and binocularly fogging or occlusion required? different levels of illumination affect VA? difference between letter VA and text VA? A well designed near chart combines long and short words which are unrelated (Bailey-Lovie near). If this proves too difficult for the patient use a chart design with related words but constant crowding (IOO chart). Assess near vision with the best near Rx and with the patient’s own LVAs. Record chart type, viewing distance (e.g. N8 @ 25cm), eccentric viewing position and illumination level.

Answer 98

Target acuity can be estimated. As a general rule newspaper print is about N 8 while books and magazines are about N10, for other approximate target acuities see appendix on page 148. If the exact print size is not known ask the patient to bring a sample and measure the height of the lower case letters. This can be converted into N point or logMAR

Answer 99

When both TA and BVA have been determined the required magnification can be calculated. If the patient wants to carry out several different near task with different magnification, the patient will require a number of different near aids. Magnification is calculated as follows: M = BVA / TA Example: BVA = N32 @ 25cm (our 'standard’ near viewing distance) Required VA for newsprint about N8 (TA) M = N32 / N8 = 4x The patient requires 4x magnification in order to see newsprint.

Answer 100

The patient in the above example can see N32 print at 25cm. We have just calculated that he needs 4x magnification to see N8, in other words if we make N8 print 4x larger the patient will be able to read it. Figure 47 shows that N8 print which is 4 times smaller than N32 will have to be brought 4 times closer to subtend the same visual angle as N32. This patient’s equivalent viewing distance for N8 is therefore 6.25cm (25/4) to achieve 4x magnification. If the patient wanted to use a simple plus lens magnifier what power would this have to be and where should the reading material be placed? We know that M = F'/4 So the required lens power would be F =M x 4 = +16m-1 We also know that the reading material should always be placed at the magnifier’s focal point, so we need to find this magnifier’s focal length, which is f' = 1/16m-1 = 0.0625m You can see that the equivalent viewing distance is really the same as the focal length of the magnifier which is required to see a particular print size.

Answer 101

1. 60/3=20 2. 144/20=7.2 -> N8 3. 24 / 8 =3x; -> 3x closer than 27 cm = 9cm 4. 1/0.09m = +11.1 m-1 Answer: The patient should be able to read the novel with a + 11 DS lens

Answer 102

Magnification = 1.25 ^number of steps eg: BVA: logMAR 1.2 (N32); TA: logMAR 0.6 (N8) -> 6 lines difference Mag = 1.25^6 = 4x mag

Answer 103

A number of more recent studies have demonstrated that it is not possible to read fluently if the print size is close to threshold level (Cheong, Lovie-Kitchin et al. 2002). This has been found for both, normally sighted and visually impaired patients. It implies that the print size of the task must be larger than threshold to achieve fluent and comfortable reading. Acuity reserve is the ratio of print size that the patient can read fluently, compared to the threshold print size that the patient can just resolve (text VA): Px can just see N6 but reads N12 print fluently. AR = 12/6 = 2:1

Answer 104

Reading rates (RR) vary greatly with age, education, difficulty of reading material and viewing conditions like print size, lighting & contrast etc. To read paragraphs of text satisfactorily RRs between 80 and 160 wpm are required. Patients with low vision will often read much more slowly. For spotting tasks like reading price labels or phone numbers, RRs as low as 40 wpm are usually sufficient.

Answer 105

1. Magnification requirement based on an assumed ‘fixed’ acuity reserve: In most cases it will be sufficient to use a fixed AR between 1.5:1 & 2:1 when calculating magnification needs. Example: A patient can just see N24 @ 25 cm with his bifocal lenses +2.00 DS / Add 4.00. The patient requires a portable hand held magnifier to read magazines on the train and in waiting rooms. What dioptric power should the magnifier have? BVA = N24; TA=N8; AR= 1.5 N8/1.5 = TA new = N5 M= BVA/TA = N24 / N5 = 4.8 (approx 5x magnification needed) M=F/4 , F = M x 4 , 5 x 4 = 20 m-1 -> Patient should have a +20DS magnifier. 2. Magnification requirement and acuity reserve are calculated individually. In some cases, where RR is insufficient using the fixed AR approach, reading can be improved by calculating AR for the patient individually. This is done as follows: 1a. VA assessment: establish threshold acuity using a near chart at a known distance. 2a. RR assessment -With the patient’s habitual near Rx in place, a text reading chart e.g. MNREAD or digital app is used. -The patient reads sentences out aloud and the time taken is recorded - RR is calculated for a range of print sizes taking into account any errors made RR(wpm) = ((n. of characters read - errors made) x 60) / (reading time in seconds x 6) - CPS can then be established 3a. Magnification can be calculated as: Required EVD = (TA/CPS) x current viewing distance Example: A patient can just see N24 @ 25 cm with a near add of +4.00 DS. How much magnification does the patient need, to be able to read novels containing N12 print fluently if their CPS is N64? What power magnifier should be given? BVA = N36; TA=N12 Required EVD = (N12/N64) x 25cm = 4.7cm ~ 5cm -> Mag 25/5 = 5x Px requires 5 x mag (+20 m-1 magnifier) to read N12 fluently 25cm 4.7 5cm N64 N12 Required EVD ¸u | ¹ · ¨ © § Î magnification 25/5 = 5x The patient would require 5x magnification ( + 20 m-1 magnifier) to read N12 fluently.

Answer 106

* Very close working distance -> too high a convergence demand * Limited aperture of high-powered lenses * Limited convergence of binocular telescopes

Answer 107

(e.g. reading, writing, sewing etc.) they may need a different device for each task.

Answer 108

x Hand held magnifiers x Stand magnifiers x Spec-mounted magnifiers x Telescopes/ Telemicroscopes x EVES

Answer 109

focused correctly positioned correctly baseline VA measurement is correct lighting is adequate

Answer 110

e a central scotoma or poor contrast sensitivity (CS). CS can be tested with a Pelli-Robson chart at a reduced viewing distance (e.g. 1m) or other suitable CS tests that do not require good resolution (for example the MET test)

Answer 111

The Low Vision Reading Comprehension Test Pepper Visual Skills for Reading Test (PVSRT)

Answer 112

It is important to instruct the patient which spectacles (Table 12) and which eye to use and make sure they understand that it is important to have the correct working distance. Patients often don’t know how to clean a magnifier or change bulbs and batteries. As discussed before, if possible, a follow up should be conducted 2-6 weeks after the initial appointment to find out if the LVA is suitable. Possible questions discussed during the follow up: x Have you been able to use the aid? x How long/often? x What for? - Is it suitable for the intended use? - Have requirements/priorities changed or been added? x What spectacles did you use? x “Show me what you have been doing?” x Are there any problems using it? - May need a different aid? - Extra training/instruction? - Non-optical aids? Lighting?

Answer 113

Telescope = Distance Rx used Hand held magnifier = Distance, sometimes Near rx used Stand magnifier = Near rx used, separate add may be necessary Spec mounted plus lens magnifier= Distance if clip on; otherwise take into account dist rx e.g. -4.00 myope with 3x mag (F/4) -> nr add = +12.00 -> final lens +8.00

Answer 114

1. peripheral loss, where the patient may have good central visual acuity 2. central loss, which will always result in poor visual acuity and is very disabling for the patient.

Answer 115

* Glaucoma (very gradual onset) * Retinitis pigmentosa (very gradual over many years, overall constriction (tunnel vision) * Cerebrovascular accident (hemianopia) - homonymous hemianopia or quadrant defects - usually sudden onset due to stroke, seldom gradual onset due to tumour - other associated problems hemiparesis, dysarthria (impaired speech) Any patient who has lost peripheral vision has lost rod photoreceptors and will therefore have poor night vision.

Answer 116

In early stages of field loss patients usually compensate by making eye movements and scanning the visual scene, but when the visible field has been reduced to 10 degrees or less the loss significantly impairs a patient’s mobility. Symptoms produced by field loss will affect the patients vision at any viewing distance, examples include: Distance : - bumping into obstacles = poor mobility - not being aware of layout of environment Intermediate : difficult to get whole task into view simultaneously (TV, computer) can’t find objects put down on cluttered table/desk Near: Page navigation - doesn’t realise reached end of line; difficulty in finding start of line without missing one

Answer 117

Hemiplegia: This will restrict the patient’s ability to position and grip magnifier and task. Loss of “higher visual functions”: e.g agnosia (inability to recognise objects, faces etc.) alexia (inability to read) etc. Oculomotor problems e.g. squint, decompensated phoria or impaired convergence Personality/behaviour/attention changes can also be drug-induced

Answer 118

Depending on the cause of the VF loss, management strategies include: 1. Illumination & glare control 2. Optical methods – 2.1. Visual field (VF) enhancement for sighting where a minified image is produced, so that more information can be squeezed into the patient’s remaining field of vision 2.2. VF enhancement for mobility where objects from the missing part of the field are projected into unaffected parts of the field. Unfortunately, optical VF enhancement can be quite confusing for the patient, which is why its success rate in practice is relatively low.

Answer 119

Reverse telescope –‘field expanders’ Amorphic lens Concave lens Convex mirror

Answer 120

Mirror system (hemianopia mirrors) Prism system (Fresnel, Inwave lens) Scanning Electronic systems

Answer 121

for tunnel vision NOT SUITABLE for hemianopia x magnification <1.0 (minification) increased FoV equal to power VA is reduced in proportion x Therefore, unlikely to be able to wear permanently hand-held for intermittent spotting tasks bioptic mounting, clip on x Door peep-hole viewer (substantial field expansion (>100 deg)) Magnification of reverse telescopes: Example: Galilean reversed Fo= -50 m-1 , FE= +25 m-1 M = -Fe / Fo = -25/-50 = 1/2 x -> 2 x minification

Answer 122

minifies only in the horizontal plane-expands the field horizontally (where it will be most useful), preserves VA – no size change vertically image distortion Every Day Uses: 1. Scanning. 3. Bike riding. 13. Watching tv/ sports. 15. To see a large group of people all at once . Reasons for a lack of success included dizziness, distortion, confusion caused by changes in the depth of field, excessive weight, and cosmetic factors. driving with extensive field defects is illegal in the UK!

Answer 123

Holding a negative lens 30-50cm from the eye produces a minified image and expands the visual field. Since for all telescopes = Fe / Fo ; increasing the power of the negative lens (FO) increases the minification effect, which leads to a bigger field of view, but poorer acuity. In practice, minification values between 3x and 8x seem to be most effective.

Answer 124

FE (Add) = +2 m-1; M =0.5 to double the available FoV; VA =6/6 M= -Acc / Fo ; Fo = -2m-1 / (1/2) = 4m-1 d=f'E+f'o = 0.5m-0.25m = 0.25m D = d x a / 60 = (250 x 10) / 60 = 42mm VA = 6/6 x 0.5 = 6/12 (0.5) Answer: The patient needs to hold a -4.00 DS lens of 42 mm diameter 25 cm from his eye. The VA with the lens would be 6/12.

Answer 125

Convex mirrors give a wider field of view than plane mirrors and can be used for field enhancement. Placed in kitchens, hallways, public places etc. they can help detect people, open doors and other obstacles. Note the minification effect

Answer 126

The goal is to stop the patient from “bumping into things”. As a general guide use as much prism as possible (15-30') with the base towards the field defect (non-seeing area). The system can be customised for general field constriction or smaller defects. The disadvantages of prisms systems mainly relate to the “jump” at the prism apex, which is proportional to the power of the prism (1o for each 1.75') and the corresponding prism scotoma at the apex. If Fresnel stick-on lenses are used this will lead to a reduction in VA. Where should the edge of the prism be placed? In practice, the edge of the visible VF can be found by moving a card across the across the FoV from the temporal side across the FoV until the patient just sees it. The spectacle lens can be marked about 1mm temporal to the edge of the field and the patient is asked to walk around to ensure that they don’t notice the mark before the prism is put in place. Monocular or binocular? The conventional approach is binocular which has the disadvantages mentioned above (jump, scotoma) Monocular prism placement has been suggested (Peli 1998, Peli 2000).Two separate prism segments are placed monocularly at the level of the limbus, and cover the width of the spectacle lens. Full-aperture binocular prisms of powers 6' to 20' with base towards field defect are put in place. This does not achieve field expansion and merely produces a small field shift, which would easily be achievable by making a small eye or head movement. It may be useful as a last resort in desperate situations.

Answer 127

Possible strategies involve minification by increasing the viewing distance. M = old object distance / new object distance e.g from 35cm to 60cm =35/60 -->M=0.6

Answer 128

Due to the relatively low success rate of field expansion devices, the complex nature of field loss and its associated complications many patients will benefit from appropriate referrals to additional professional services like: * Counselling * Rehabilitation teaching * Occupational therapy * Physical therapy * Auditometry * Other support groups

Answer 129

x ARMD- most common, mostly elderly but may affect younger patients x Macular holes x Central serous retinopathy x Cystiod macular oedema x Toxic maculopathies etc.

Answer 130

x Acuity loss: Mainly impaired reading & face recognition x Distortion x Photostress Impaired Dark & Light adaptation x Photophobia better vision at night x Impaired depth perception x Charles Bonnet Syndrome Visual Hallucinations after sudden loss of vision -13% AMD px with CBS

Answer 131

If the VF loss is unilateral, the patient will usually just ignore the affected eye and may sometimes not realise that the condition is present. Occasionally, the affected eye needs to be occluded, for example if it was previously the dominant eye. Therefore, central field loss is only a problem if the condition is bilateral. Because of the foveal involvement the patient will always suffer severe acuity loss in affected eye. The fovea is also the oculomotor reference for eye movements. In the presence of an absolute macular scotoma the patient will notice a blank patch covering the object of regard and may try to re-fixate using a parafoveal point, just off edge of the scotoma. This is called the patient’s Preferred Retinal Locus (PRL). Images formed away from the fovea are therefore a stimulus to “re-foveate”. Due to the acuity falloff away from the fovea one would expect the patient to use a PRL as close as possible to the edge of the scotoma, and if the size of the scotoma is known, the patient’s VA should be predictable. Older patients are less likely to change their oculomotor reference. In practice, an Amsler grid or field screener (e.g. macular VF on HFAII) can be used to locate the scotoma and identify the EV direction which gives the best vision. Sometimes, an area which is not the patient’s PRL may be identified and give better VA or higher reading speed than the PRL. In this case the patient could be taught to use this trained retinal locus (TRL). For reading the TRL should preferably be above or below the macula. Eccentric Viewing Training in Glasgow: The New View Project is run by Visibility. Before entering the training programme patients attend for an LVA assessment at Gartnavel General Hospital, Royal Alexandria Hospital or the Low Vision Clinic at GCU. This mainly involves mapping out the scotoma and assessing the patient’s magnification needs. Suitable patients enter the programme and receive 1 hour of instructed training per week, usually between 5 and 8 weeks. This is combined with 5-10 min of daily practice. Patients are also given advice regarding suitable lighting and the use of the steady eye strategy.

Answer 132

The idea behind this technique is that a prism could be used to move the retinal image out of the scotoma and place it on the optimum PRL without the need for the patient to re-fixate.

Answer 133

Macular translocation surgery has been carried out on patients with wet macular degeneration, where impaired retinal pigment epithelium (RPE) function leads to a loss of macular photoreceptors. The procedure involves detaching and rotating the sensory retina and reattaching it so that the macular receptors can be supplied by intact RPE, which will preserve macular function. technique is still in the research phase.

Answer 134

-The most common example of Real image magnification An advantage of electronic magnifiers is that the magnification produced is aberration-free, but image quality depends on the screen resolution. While the maximum magnification achievable with optical magnifiers is about 20x (80 D), this can be up to 70x or higher with electronic aids, but a limit of about 30x is common. Zoom control allows variable magnification (important for page navigation: probably only 4 characters required for reading but up to 15 required for optimum page navigation). Variable magnification may also be useful if the patient’s eye condition changes. Other advantages include the possibility of binocular viewing and variable camera-to-task and eye-to-screen distances. Disadvantages of EVES: x Cost - expensive to buy and service/repair x Size depending on type/model x Image quality – depending on screen The persistence of the image on the screen’s phosphor causes blurring/fading if the reading material is moved too quickly. Not an issue with LED/LCD. Patients with distance VA of better than 6/90 may gain little benefit from using EVES

Answer 135

CCTV – probably still one of the most commonly used designs x each component fixed and mounted vertically “in-line” x working space fixed or limited x for distance need second camera to be added x !! reading material needs to be flat – depth of focus !! x hand held/portable (P-EVES) greater versatility than CCTV x television/USB readers low cost x head-mounted video magnification devices: -reading, dealing with correspondence/filling in forms, school tasks, viewing photographs) ?ability to wear permanently? Distance and near magnification Independent of external illumination/reduced glare

Answer 136

It has been suggested that more practice is required to achieve optimum reading speed. – 15 to 20 sessions of daily practice) than with optical aids, but small print (approx.

Answer 137

Optical aids are provided free of charge through the Hospital Eye Service (HES). The aids are given to the patient on a “permanent loan” and there is no formal limit to the number of devices. Aids can be changed as required. There is no equivalent in GOS, although local schemes exist where patients can have an LVA assessment in the community (usually by a participating optometrist) and the Health Authority pays for aids and assessment. If bought privately, patients who are registered VIPs are exempt from VAT. EVES are available to use in local libraries and resource centres and patients may be able to borrow them from Social Services. They are also available through charities (e.g. “Electronic Aids for the Blind”) and there are special schemes for education and employment.

Answer 138

To improve the visibility of objects for visually impaired patients adequate lighting has to be provided in appropriate places. General lighting should be distributed evenly to avoid glare and extra lighting should be provided for hazardous areas. Light coloured walls & ceilings improve the light distribution in buildings and non-reflective surfaces (matt paint) reduce glare. Particularly patients with reduced macular function require substantially increased light levels for reading (Figure 76). While some patients and of course retailers tell you that “daylight” bulbs (these contain a blue filter to simulate the appearance of natural light) improve reading performance, there is no scientific evidence for this. Visual performance improves with increasing light level, but does not depend on colour temperature (Figure 75). Reading lights frequently recommended to VIPs are fluorescent anglepoise or LED lamps. Cold light sources can be brought very close to the patient’s head and reading material which maximises luminance.

Answer 139

Discomfort Glare: Subjective visual discomfort The patient feels visually uncomfortable or fatigued by this type of glare. Symptoms can range from mild to severe, but VA is not affected by and symptoms are usually relieved by tints. Examples include uveitis, ocular albinism, cone-rod dystrophy, retinitis pigmentosa. Disability Glare: Loss of retinal image contrast as a result of intraocular light scatter, or straylight. Reduces visual performance (VA) Severity depends on the angle between task and glare source & relative luminance Media opacities lead to intra-ocular light scatter which creates a veiling luminance across all parts of the retinal image. This reduces the retinal image contrast. Conditions which induce disability glare include ageing, cataract, posterior chamber IOL , posterior capsule opacification, keratoconus , corneal oedema, Radial Keratotomy (RK), vitreous opacities.

Answer 140

Simple clinical method for assessing disability glare: Measure VA or low contrast VA without glare source Re-assess with glare source (e.g. penlight) Standardise the angle and distance of light to eye typically 10cm and 30 degrees. drop in VA of two lines or more - significant degree of glare

Answer 141

Glare reduction includes environmental measures discussed before e.g.: the avoidance of shiny surfaces, use of blinds, appropriate task lighting (angle!), visors, hats tints Due to Rayleigh scattering, short wavelengths produce more glare than long wavelengths in patients with media opacities. This would suggest that glare can be reduced by wearing (yellow) filters. When prescribing tints, objective measures of visual performance like VA with high and low contrast charts or under different light levels can be taken into account. However, research has shown that filter colour does not have a significant effect on visual performance (Figure 78) and most tints are prescribed by ‘trial and error’ based on the patient’s subjective preference.

Answer 142

Daily living aids: -Reading/writing -kitchen/around house -medical/personal care Travelling/Mobility: -canes -dogs -electronic aids Non optical aids also include lighting & contrast enhancement, tints and environmental design features

Answer 143

Self threading needles x Liquid level indicators x Talking microwaves/kettles x Talking watches etc x Large number telephones, microwaves etc x ‘Bump ons’ or ‘Hi marks’ x Coloured stickers and buttons

Answer 144

large print books, bank statements & bills, clocks/watches/telephones x Talking; books, watches, clocks, newspapers x Typoscopes- for cheques, reading or writing x yellow paper/lined paper

Answer 145

3x2 pattern of 6 dots 26 letters + contractions st, ch etc and punctuation Can be generated by computer- keyboard or screen x Disadvantages: Thick paper with raised dots - thick & bulky books Not good for elderly due to loss of sensitivity in finger tips

Answer 146

These mainly include: x Canes & sticks x Electronic aids (e.g. ultrasound, sonar) x GPS systems x Talking signs x Built environment x Dogs & sighted guides (people)

Answer 147

Symbol cane ( lightweight, folding cane + indicates that the user is visually impaired. ) White walking stick (as above but aids support) Long cane (made of light-weight aluminium) Guide cane ( shorter and stronger than the long cane + back up for those with residual vision) A range of electronic mobility aids is available. Most of these use laser beams or ultrasonic sensors to detect obstacles and vibrate or emit a sound to warn the patient. Canes are available from Social services, but can be bought from various charities like the RNIB. If a white cane has red stripes this indicates that the patient also has impaired hearing.

Answer 148

Guide Dog Assessment (3 stages) Before pairing a dog with a potential owner, a guide dog assessment is carried out. This usually involves 3 stages: 1. General Information Visit The owner receives information on general services and mobility training. 2. Mobility Assessment Medical status and cause & degree of vision impairment are assessed and the possibility of additional non-guide dog training or mobility aids like a long cane explored. This is followed by an initial general suitability assessment. Contra indicators to guide dog ownership are sufficient sight, dislike of dogs and age <16 years. There is no upper age limit. 3. Guide Dog Assessment This involves an assessment by a guide dog mobility instructor and if the applicant is suitable they will be trained to work and walk with the dog, recognise and follow its movements and develop effective & consistent dog control and geographical awareness. The training starts in their home environment but usually extends into their neighbourhood (shops and other frequently visited places).

Answer 149

Simple things x Move TV away from windows to avoid glare x sit close to TV/ near blackboard x Location of lights for reading (contrast, glare) x Extra task lighting where required (kitchen, bathroom) x May need to have room lights on during the day x Sit on one side of teacher/TV etc. for hemianopic patient Architectural design: x Suitable colour schemes Matt light coloured paint on walls Dark paint round doors, contrast surrounds to switches, plugs Lighter door with contrasting handle Dark flooring Hard flooring- sound Contrasting skirting boards x Features to aid VIP’s Wide doors (guide dog access) Sliding doors Auditory indicators (e.g. lifts) Handrails Enclosed staircases Tactile elements (e.g. floors) No projections (radiators, fire extinguishers) Strip lighting Tactile and auditory signs, maps and floor plans in public buildings x Features that hinder VIP’s These include things like glass doors, open staircases and poor signage

Answer 150

The measurement of success of LVA provision is important to establish if healthcare is effective and to assess if LVA’s benefit the patients. The question is though, what constitutes a successful LVA assessment what does the patient perceive as success? x Increase in VA? x Increase in patient confidence? x Improved performance doing required tasks? x Improved reading speed/accuracy x Frequency of use of LVA It is important to remember that perceived success varies and depends on the individual’s goals and expectations. Patient expectations must be realistic – if you know that they can’t be met, be honest with the patient, but try to let them down gently.

Answer 151

Most importantly, patients prefer simple LVA’s!! x Spectacle prescriptions (high adds) x Hand magnifiers x Stand magnifiers x Illuminated magnifiers

Low Vision Notes Flashcards

(176 cards)