Lecture 12, 13 - Prescribing Magnification Flashcards
Steps for prescribing LVA’s for distance tasks:
• Measure BVA
• Think about TA:
- TV ~ 6/18
- Street signs < 6/12
• Calculate magnification, Try predict M, Modify from there
• Decrease viewing distance
• Lva (Monoc/Binoc?)
- VA < expected: Focusing? Baseline VA/M calculation correct? Scotoma? Cosmesis?
- VA as expected: Trial LVA, Give instructions + check up
Factors affecting baseline VA:
• Illumination
- Rod monochromatic: better acuity in dim illumination
- Retinitis pigmentosa
- Media opacities
• 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 techniques
- 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
Contrast sensitivity and contrast reserve:
• Ability to read fluently or scan a visual scene quickly depends on contrast reserve
• CR is the ratio between Px contrast threshold (min cont) and targets contrast (e,g book or TV)
- Optimum contrast reserves are 20:1 while min req for reading is 10:1
• Testing a Px’s CS will help defide if light enhancement (>6%) or contrast enhancement (>4%) would be beneficial
Field loss and scotomas with ARMD:
• Scotoma, 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
Calculation for distance magnification:
• Snellen notation : M= TA/BVA
I.e BVA = 6/60, TA = 6/12
(6/12)/(6/60)= 5x
• Logmar : M = (1.25)^Number of steps
- Each step = 0.1log unit
Options available for improving distance vision:
- Spectacle lenses
- Contact lenses
- Coatings & tints
- Telescopes
- Multiple pinhole lenses
What steps are done after magnification need has been established?
• 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
Management strategies for near vision:
• Same for near as for distance.
- Calculate Magnification
- Try predict M/modify
- Chose LVA for task
- Trial
• “Divide by 3 Rule”
- Distance Va = 6/X near VA should be : (X/3) at 25cm (eg 18/3= N6)
Reasons for not using the “Divide by 3 rule”
There are good reasons for not using only distance VA:
While this may sometimes work there are good reasons for NOT using distance acuity to determine near magnification needs:
- 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
• It is always more accurate to use near acuity chart then calculate mag using this at 25cm
Examples of near vision notations:
• Point System - N point
Example:
N6 (‘near vision standard test 6 point) at
33cm
• Sloan M-notation
Example:
0.8M
• Keeler A chart (LogMAR chart)
Example:
A2 (5’ * 1.25 = 6.25 armin at 25 cm)
• logMAR near charts
Example:
0.5 logMAR
Near VA testing procedure:
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?
• Well designed near chart combines long + short words which are unrelated
- If this proves too difficult can use words that are related but have constant crowding
• Always record near vision with best N Rx, with LVA’s. Record chart type, viewing distance, eccentric viewing position and illumination
Determining near target acuity
• Estimated : Newspaper print is N8, While books N10
• Otherwise can measure letter sixe snd convert into N point or Logmar
• “Rule 144”: Count number of letters/spaces in 1 inch of text, Divide into 144 for N point
How is magnification calculated for Near VA: (N point notation)
• When TA/BVA have been determined:
- M = BVA/TA
Example:
BVA = N32 @ 25cm
Required for newsprint = N8
M = N32/N8=4x
The equivalent viewing distance:
• The viewing distance between Without LVA (i.e 25cm) and with LVA will decrease in proportion to magnification
- i.e 4x mag = 6.25cm Equivalent viewing distance
• This is the same as the focal length of the magnifier
Magnifier calculation for Log MAR:
• Magnification = 1.25^Number of steps
Example: BVA = logMAR 1.2; TA= logMaR 0.6 = 6lines
So Mag = 1.25^6=4x
