CN VI (abuscens nerve) controls the lateral rectus muscle of the eye. This muscle pulls the eye outward. If the muscle is weakened, the eye will turn inward. Therefore, the patient will have an esotropia. Older patients will complain of double vision (diplopia).
Orthophoria is the condition where a patient's two eyes are perfectly aligned. A phoria is an ocular deviation that is kept hidden by the fusional mechanism (i.e. when both eyes are open). A tropia is an ocular deviation that is not kept under control even when both eyes are open.
Terms used for muscle imbalance are: eso (inward), exo (outward), hyper (up), and hypo (down).
The monocular cover-uncover test is used to differentiate a phoria from a tropia. The examiner covers one eye (e.g. the right eye) and watches for movement of the opposite eye (e.g. the left eye). If the left eye moves when the right eye is covered, the patient has a tropia. If there is movement of the eye that is covered, the patient has a phoria. The alternate cover test involves the examiner rapidly moving the cover from one eye to the other. When used with prisms, this measures both the phoria and tropia of the patient's eyes.
cover test
alternate cover test
