4.1.1 Identifies anomalies in a prescription and implements the appropriate course of action.

What is anisometropia?

A significant difference in refractive error between the meridians between the two eyes of more than 1.00D. This can induce a prismatic effect which is calculated by the following equation:

Problems that can arise:

P = cF (where c is in centimetres)

Anisometropia

Problems that can arise:

• Aniseikonia

• Prismatic effect/anisophoria

• BV problems

• Cosmetic & comfort issues

• Adaptation problems

What is Aniseikonia?

Unequal lenses cause different image sizes, making binocular fusion difficult.

Symptoms: headaches, eye strain, diplopia, suppression of one eye, reduced stereopsis

What is Prismatic effect/anisophoria ?

When the patient looks away from the optical centre of the lenses, unequal prismatic effects occur

This is particularly problematic in vertical gaze (since vertical prism is hard to fuse).

Symptoms: eye strain, discomfort when reading, diplopia in down gaze

What binocular vision problems can arise from anisometropia?

Brain has difficulty fusing images due to size or prismatic differences

Possible outcomes: suppression of one eye (risk of amblyopia), intermittent or constant diplopia, decreased depth perception

What cosmetic and comfort issues can arise from anisometropia?

One lens thicker/heavier than the other
Noticeable asymmetry (cosmetically unacceptable for some patients)

What adaptation problems can arise from anisometropia?

• Patients may experience distortion or imbalance when moving (objects appearing tilted, “swimming” sensation)

• Difficulty with stairs/curbs due to altered depth cues

What are the management strategies for anisometropia?

• Spectacle-based strategies

• Contact lenses

• Refractive surgery

What spectacle-based strategies can be used to manage anisometropia?

Partial correction
Best in high anisometropia in adult who cannot adapt to full correction
Reduces symptoms but may leave some blur in one eye

Slap-off or compensated prism techniques
Used if prismatic imbalance in downgaze is a problem

What are the advantages of contact lenses for managing anisometropia?

Often the preferred option for moderate to high anisometropia
Minimal image size difference
Avoids prismatic imbalance
Cosmetically better than asymmetric glasses