Introduction to muscles and muscle movements

There are six muscles of the eye.

Four of them are rectus muscles, meaning that they yank the eye straight back toward the origin of the muscle. These four muscles are either medial, near the nose, lateral, near the temple, superior, above the eye, inferior, below the eye.

The other two muscles are oblique muscles which means that they pull the eye at an angle causing a slight toric motion. These two muscles are the inferior oblique and the superior oblique muscles.

Thus the six eye muscles and motions are:

1. Inferior rectus (IR) – down
2. Superior rectus (SR) – up
3. Medial rectus (MR) – in toward nose
4. Lateral rectus (LR) – out towards temple
5. Superior oblique (SO) – depresses and rotates the eye
6. Inferior oblique (IO) – turns inward and rotates the eye

Versions and Ductions

Versions are a tropia or phoria of both eyes in the same direction. Ductions are a tropia or phoria of one eye alone.

Tropias and Phorias

A tropia is deviation which is always present. Tropias can be seen when a patient is trying to fixate on an object. Both eyes do not work together. This results in a loss of fusion. Which is the ability of the eyes to work together. When eyes are not focusing on the same target then they will have double vision

A phoria is a deviation which has to be teased out a little. A patient may look like they are fixating on an object, but when the cover test is preformed, when fusion is disturbed, the eye will deviate.

Cover Tests

Cover tests are used to disrupt fusion. There are two kinds of cover tests the cover/uncover test and the alternate cover test.

The cover/uncover tests for phoria. It is preformed by covering one eye at a time and seeing if the uncovered eye deviates.

The alternate cover test will test for total deviations (phobia + tropia).

Stereoacuity

Stereoacuity is the ability to detect distances in depth. The eyes use environmental clues to determine what objects are close and which are far away.  Depth perception is not the same as stereoacuity. A monocular patient can have depth perception, but not stereo vision. Stereoacuity is measured in seconds. The less amount of time the better the stereo vision.

Nystagmus

Nystagmus is the random jerking of the eyes. When a patient has nystamus one eye should not be covered while testing visual acuity. Instead try to fog out the eye or use a lens with will effectively blur the eye that would normally be occluded.

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