Orthophoria is the condition where a patient's two eyes are perfectly aligned. A phoria is an ocular deviation that is kept hidden by one's 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 detect the presence of 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). For example, if the left eye moves when the right eye is covered, the patient has a tropia. If a phoria is present, the uncovered eye does not move, however the eye that is behind the occluder will deviate and return to a straight position when the occluder is removed.
The alternate cover test involves the examiner rapidly moving the cover from one eye to the other. When used with prisms, this quantifies both the phoria and tropia of the patient's eyes.
In the example above, the left eye has an exotropia which means that it is constantly turned away from the nose. This patient most likely fixates with the right eye and ignores the left eye. However, if you cover the right eye, you force the patient to use the left eye. This causes the left eye to turn inward to look straight ahead.
