What is anisometropia and its causes? Optometry fans

 When refractive status of two eyes is equal, it is known as isometropia. When refractive status of two eyes is not equal, it is known as anisometropia. 

A difference of 1D in refractive status of two eyes creates a difference of 2% in size of retinal image of two eyes. 

Up to 5% difference in retinal image size of two eyes is well tolerated. Thus an anisometropia of up to 2.5D is acceptable. 

Anisometropia of up to 4D may be acceptable in some individuals and more than 4D creates diplopia.

Causes of Anisometropia

It may be congenital or developmental or acquired as in unilateral aphakia.

Clinical Significance

Small degree of anisometropia is of no concern and is quite common. 

Patient develops normal binocular single vision. However, if one eye is emmetropic and other is highly hypermetropic, the later will develop suppression and amblyopia (lazy eye). Patient will use only emmetropic eye for vision.

 If one eye is myopic and other is hypermetropic, then patient will develop alternate vision. It means that he will use myopic eye for near vision and hypermetropic eye for distant vision.

Types of Anisometropia

1. Simple anisometropia: Here one eye is normal (vision 6/6 without  glasses) and other eye is myopic e.g. 6/6 vision with–2.00DS. 

It is  known as simple myopic anisometropia. If other eye is hypermetropic  e.g. 6/6 vision with +2.00DS, it is known as simple hypermetropic  anisometropia. 

2. Compound anisometropia: Here both eyes are either myopic (compound  myopic anisometropia; one eye requires–2.00DS and other–5.00DS)  or hypermetropic (compound hypermetropic anisometropia; one eye  requires +2.00DS and other +5.00DS) but one eye has higher refractive  error than the other.