Anatomy & Refractive States of the Eye
Principal Ocular Structures

Below is a diagram of the extra-ocular muscles. These six muscles are inserted into the sclera and enable the eye to rotate in all possible directions. They include the internal, external, superior, and inferior rectus, and the superior and inferior oblique. When these muscles are working properly the two eyes can work together in perfect unison. And in doing so the two images can be kept relatively close together, enabling the brain to fuse the two images it receives into one. But if a muscle imbalance is present, it may be difficult or even impossible for the brain to fuse the two images. To help alleviate the effects of a muscle imbalance, an element called prism is sometimes incorporated into the prescription.

There are two general conditions which can result from an imbalance of the extraocular muscles and these are called phorias and tropias. Referring to the diagram below, it is mentioned that a phoria is a constant tendency for the eyes to turn from the normal position for binocular vision while a tropia is a definite and obvious turning of the eyes.

Certain prefixes are also used to define phorias and tropias more specifically. As indicated in the table below, exo means out; eso in, hyper up, and hypo down. So, looking at the examples, a right exophoria would be a tendency of the right eye to turn outward, left esotropia, a definite turning inward of the left eye; right hyerphoria, a tendency of the right eye to turn upward; left hypotropia, a definite turning downward of the left eye, and so on.

 

Extra-ocular Muscles

 

Examples of Phorias & Tropias

Exo	out	right exophoria	a tendency of the right eye to turn outward
Eso	in	left esotropia	a definate turning inward of the left eye
Hyper	up	right hyperphoria	a tendency of the right eye to turn upward
Hypo	down	left hypotropia	a definate turning downward of the left eye