People wonder what is the importance of assessing ocular alignment. The assessment is essential to know if there are problems such as crossed eyes or misaligned eyes. Early detection is important so it can be corrected as early as possible. There are different tests to perform to check ocular alignment.
A cover test will be performed as an evaluation for ocular misalignment or strabismus. The cover test will be performed by asking the patient to seat at a fixed distance while wearing their best correction for refractive error. The patient needs to place their head in a straight position. Ask the patient to avoid tilting or turning their head around.
A single cover test is used to determine if there is a heterotopia or tropia. It will take approximately 1-2 seconds to cover the first eye. The uncovered eye will observe any shift in fixation. The occluder will be removed and any refixation movements will be noted under binocular conditions. There are two meanings if there is no shift in fixation:
- The patient has no misalignment which needs confirmation by doing the same thing on the opposite eye; or
- The uncovered eye is the preferred eye in the setting of a heterotropia.
Make sure to wait a few seconds from the prior eye to allow phoria to manifest and not to suspend fusion. Note any shift in fixation of the unoccluded eye. Exotropia is present if the unoccluded eye shifts in or medially. Esotropia is present if the unoccluded eye shifts out or laterally. Hypertropia is present if the unoccluded eye shifts down when the opposite eye is occluded. Hypotropia is present if the unoccluded eye shifts up when the opposite eye is occluded.
How to Determine if There Is Heterophoria or Phoria
A cover uncover test will be performed to know if there is a heterophoria or phoria. The test is performed in almost the same manner except that the attention is focused on the eye that has been occluded as the occluder is removed. Phoria is present if the covered eye shows a refixation movement in binocular conditions.
How Alternate Cover Test Is Performed
The alternate cover test measures the full deviation which brings out any phoria present in addition to the tropia that is determined in the single cover test by suspending binocular fusion. The test is usually performed after a single cover test. The test covers one eye and holding the occluder for several seconds and then shifts it to the other eye. Rapid alternation is done back and forth without making the patient binocular.
Eso deviation means there is an outward or lateral refixation. Exo deviation means there is an inward or medial refixation. Hyper or hypo deviation means there is a vertical refixation movement.