Glasses & Ametropia

Question 1

Caculer l’équivalent sphérique à partir d’un spherocylinder format

Answer 1

Average of 2 powers in the lens
ADD 1/2 cylinder power to the sphère

Question 2

How much does each -0,25D of myopia reduce a person’s VA on the Snellen Eye Chart?

Answer 2

Chaque -0,25D of myopia diminue la ligne de 1 sur Snellen

Question 3

Qu’est-ce qu’occasionne la correction d’un astigmatisme monoculaire versus binoculaire?

Answer 3

Monocular : distorsion in meridional magnification
Binocular : differentiel tilting (+distorsion)
CAT : minimiser la vertex distance (VDC)

Question 4

Qu’est-ce que l’anisometropia?

Answer 4

Différence réfractaire entre les 2 yeux
Most adult cannot tolerate a difference greater than env. 3D, and loss even less.
Ex. monocular aphakia is an exemple of a large anisometropia which cannot be corrected by glasses. It requires lens contact.
CAT : minimize vertex distance

Question 5

Qu’est-ce que l’aniseikonia?

Answer 5

Refers to a difference in size or shape of the retinal images formed in the two eyes.
Spectacle correction of anisometropia causes differential magnification and is the most common cause of anieseikia.
It can be caused by retinal disorders and occasionally by unilateral cataract removal even with an IOL implant.
CAT : minimize vertex distance

Question 6

Anisometropia

Answer 6

Eyes looking at Near, ESOTROPIC for DISTANCE
Uncrossed diplopia

Eyes looking at Distance, EXOTROPIC for NEAR
Uncrossed diplopia
Crossed diplopia

Question 7

Components of hyperopia

Answer 7

Absolute : the minimum PLUS required for clear (20/20) vision
Manifest : the maximum PLUS accepted before vision blur
Total (cycloplegic) : The TOTAL amount of hyperopia revealed once the patient is dilated
Facultative : manifest-absolute, the range in which the patient could adjust to different amounts of correction
Latent : total-manifest, the amount only revealed by dilating the patient

Question 8

How does an under corrected spectacle wearing myope/hyperope compensate to see clearly in the distance?

Answer 8

Hyperope : slides glasses FURTHER away = increases effective plus power

Myope : pushes glasses CLOSER = increases effective minus power OR tilts lens forward = increases astigmatism of oblique incidence, thereby increasing minus sphere (and minus cylinder)

Question 9

General features of contact lenses (VF, image size, accommodative need, prismatic effect)

Answer 9

Increased field of view

Image size
- Myopes : increase
- Hyperopes : decrease

Accommodative needs
- Myopes : increase
- Hyperopes : decrease

Decrease prismatic effect of glasses (« image qui shift quand l’oeil n’est plus centré dans la lunette »)

Question 10

Nomenclature des soft contact lenses

Answer 10

Base curve/diameter/power
Base curve : mm or D (D= 337,5/r)

Question 11

Power calculations of soft lenses

Answer 11

1. Over-refraction method
2. Calculation method
   - Obtain refraction
   - Spherical equivalent
   - Convert to zero vertex distance

Question 12

How to change the fit of rigid or soft contact lenses?

Answer 12

To change fit = alter base curve (BC) or diameter

If fit is steep/tight : loosen by flattening BC (increasing r) or decreasing diameter

If loose : tighten by steepening BC (decreasing r) or increasing diameter

Question 13

How to change the fit of rigid or soft contact lenses?

Answer 13

To change fit = alter base curve (BC) or diameter

If fit is steep/tight : loosen by flattening BC (increasing r) or decreasing diameter

If loose : tighten by steepening BC (decreasing r) or increasing diameter

Question 14

How to change the fit of rigid or rigid contact lenses?

Answer 14

1. Over-refraction method
2. Calculation method
   - Choose BC 0,05 D steeper than flat K
   - Convert to minus cylinder spectacle power
   - Drop the cylinder (the rigid lens will mask the cylinder)
   - Vertex the spherical power
   - Add -0,5 D to compensate for minus tear lens

SAM-FAP
Steeper add minus
Flatter add plus

Question 15

Définition de l’accommodation et de la presbytie

Answer 15

Accommodation : increase in total dioptric per of the eye achieved by increased convexity of lens (via contraction of ciliary muscle)

Presbyopia : loss of accommodative amplitude with age

Question 16

Définitions near point of accommodation, amplitude of accommodation and range of accommodation.

Answer 16

Near point of accommodation (m) : closet the eye can see clearly with accommodation maximally active

Amplitude of accommodation (D) : maximum diopters an eye can accommodate

Range of accommodation (m) : linear distance over which a patient can accommodate and maintain clear vision

Question 17

Determination of Reading Add

Answer 17

1. Measure amplitude of accommodation
2. Use a table to predict amplitude of accommodation
3. Empiric/clinical method

Determine proffered working distance
Substrat 1/2 of AA taken from table/Substract half of amplitude

Question 18

Dimensions optimales du pinhole et de la pupille pour Pinhole Acuity

Answer 18

Pinhole acuity :
- Reduces blur circle
- Corrects up to 3-4 D of refractive error
- Optimal pinhole size : 1,2 mm
- Optimal pupil size 2,4 mm
- Stenopeic slit = elongated pinhole

Question 19

Dimensions of the reduced schematic eye

Answer 19

Primary focal point à cornée/principale plan = 17 mm
Cornée/principale plan à nodal point = 5,5 mm
Cornée/principale plan à secondary focal point = 22,5 mm
Nodal point à secondary focal point = 17 mm
N oeil = 1,33

Question 20

Types d’acuité

Answer 20

Minimum visible : measure of brightness discrimination, detection of black dot on a white background

Minimum discriminable : aka vernier acuity, hyperacuity, detection of a break in a line

Minimum separable/resolvable/legible : smallest visual angle at which 2 objects can be distinguished

LogMAR visual acuity : pour la recherche surtout, objectif