The deviation(s) are easy to deduce based on the refixation movements when one of the eyes is unoccluded. In this example, the left eye moves toward the patient's right. In other words, the left eye moves inward. This implies that the left eye is deviated outward (i.e. exodeviation exists) when it is occluded. The left eye also moves upward which implies that the left eye is deviated downward when it is occluded. This situation is described as either a left hypodeviation or a right hyperdeviation. Thus for this patient, both an exo- and right hyperdeviation exists (answer choice "E").
For this patient, we cannot specify whether this is a -tropia versus a -phoria since the alternate cover test measures the total deviation (i.e. latent + manifest) -- it cannot quantify the relative contributions of a heterophoria versus heterotropia.
To determine the actual heterotropia that exists, the simultaneous prism and cover test must be used. As its name implies, this involves covering the fixating eye at the same time that a prism is placed in front of the deviating eye. The test is repeated with increasing prism powers until the deviating eye no longer refixates. The prism power that accomplishes this is the total -tropia of the deviating eye.
