A key component of an ophthalmic examination is the assessment of binocular vision, also known as eye teaming. Good eye teaming consists of the two eyes pointing in the same direction while maintaining single, comfortable vision at distance and at near, while ocular misalignment places stress on a binocular system.
The cover test is a measurement of eye posture (or eye alignment). The test is performed in two steps. First, the clinician covers an eye with a hand or paddle and looks for movement of the fellow eye. The cover paddle is then removed, and the clinician looks for movement of the previously covered eye. This step is called the cover-uncover test. The second step of the test is called the alternating cover test. Here the clinician covers an eye and moves the paddle to the next eye. While moving the paddle back-and-forth, the clinician is looking for subtle movement of each eye.
The cover test is commonly performed at distance and at near (~40cm), though in some cases the cover test is performed in multiple positions of gaze (up-left, up, up-right, right, down-right, down, down-left, left, and central gaze).
The cover test is a critical component to understanding how a patient's eyes move and team. Abnormal eye positioning could be a cause for numerous symptoms, such as double vision (diplopia), eye strain (asthenopia), reading difficulties, headaches, and more.
The results of a cover test are reported in type and direction of misalignment, duration,
Ocular misalignment that is visible without dissociating the patient with a cover paddle or prisms is termed a heterotropia. If only manifest under dissociative conditions, the misalignment is termed a heterophoria.
Constant - the eye is deviated at all times and all distances.
Intermittent - the eye does not deviate constantly but may deviate under certain conditions or stressors (such as reading or near work).
Ortho - the eye(s) shows no misalignment.
Exo - the eye(s) tends to turn outwards.
Eso - the eye(s) tends to turn inwards.
Hyper - the eye(s) tends to turn upwards relative to the fellow eye.
Hypo - the eye(s) tends to turn downwards relative to the fellow eye.
Incyclo - the eye(s) tends to rotate inwards.
Excyclo - the eye(s) tends to rotate outwards.
Comittancy is essential how consistent the deviation is in all positions of gaze. Most forms of strabismus, for example, are the same magnitude in all positions of gaze. However, strabismus due to nerve damage or muscle tissue scarring is often different in magnitude in various positions of gaze (termed incomittant or non-comittant).
Measuring eye alignment is done both at near (40cm) and far (6m). The measurements are recorded in prism diopters (Δ), which is the displacement of 1cm visible at 1m.
Distance - 1Δ exophoria +/-2Δ
Near - 3Δ exophoria +/-3Δ