The cover test is a simple eye examination used to determine the presence of a tropia (a fixed eye muscle imbalance). The exam is a measurement of ocular deviation, the misalignment of a patient’s eye. It helps the doctor to assess binocular vision or eye teaming. It is the efficiency with which the patient’s eyes coordinate and work together. Correct eye teaming is essential for single, sustained and comfortable vision. It forms the basis for depth perception. The cover test is a vital part of a comprehensive eye examination.
It is the most common and simplest technique for finding out if a person’s eyes are working together or coordinating to focus. It is typically done on children and the elderly. The patient looks at a small target across the room, with each of their eyes covered alternately. The procedure is then repeated with the patient looking at a near target. If one eye drifts inwards or outwards, the patient has what is commonly called a lazy eye.
Also Known As
- Cover-uncover test
There are three types of cover tests:
- Single cover test/ Cover-uncover test
- The alternate cover test
- The simultaneous prism and cover test
All the tests can be conducted with fixation at near or distance.
The cover test can be conducted at the nurse’s or doctor’s office, eye clinic or hospital by:
- A nurse
- A technician
- An optometrist
- An ophthalmologist
It is the gold standard objective technique for establishing the existence, type, and degree of strabismus (ocular misalignment). It also helps to classify concomitant squints further. Those that are present throughout the day are manifest squints called tropia. Those that only occur when binocular fusion is broken are latent squints called phorias.
The doctor will be interested in tropias because their presence indicates that the patient’s eyes are always misaligned. While one eye focuses on a visual object, the tropic eye moves (deviates) either temporally (exotropia), inferiorly (hypotropia), superiorly (hypertropia), toward the nose (esotropia), or a combination of horizontal and vertical deviation. An alternating tropia is a situation where the patient has a deviating eye, but may and does focus with either eye. For instance, the patient might be focusing with the left eye while the right one may be deviating inward. A moment later, the patient can change the fixation to the right eye while the left one moves in an inward deviation.
These disorders usually arise in early childhood and the brain can compensate for the bewilderment by subduing the signals from the misaligned eye. It causes underdevelopment of vision in the suppressed eye, normally referred to as amblyopia (lazy eye). Phorias are temporary misalignments where the eye can drift outwards or inwards in a weak binocular fusion. It is common when a person is tired, like at the end of a long day or during a major illness. Binocular fusion weakens with age, and elderly patients have more episodes of phoria.
Preparation & Expectation
The cover test is a simple and straightforward eye exam that requires no preparation.
The patient will sit or stand, looking at a target. The test is commonly done at 6 meters (distance) and 40 centimetres (near), though in some cases it can be conducted in multiple positions, such as central, up, up-right, up-left, right, down-right, down, left, down-left gaze.
Cover-uncover test/single cover test
It is conducted first in most cases. The test helps to determine whether misalignment is a tropia or phoria. It is performed using a fogged covering. The doctor will use the covering to block one of the patient’s eyes for several seconds and remove it while checking both eyes for movement. He/she will conduct the test on both eyes. The misaligned eye deviates outwards or inwards. In tropia, the eyes are misaligned when the patient is gazing with all eyes uncovered. A latent deviation or phoria only occurs where the binocular sight is broken; each eye is looking at a different object.
The alternate cover test
It is done after the cover-uncover test. The doctor will cover one of the patient’s eyes and then the other, moving back and forth without giving the patient time to fuse between the obstructions. The faster the patient’s eyes can pick up when the cover is removed the more control the patient has over the variation.
The simultaneous prism and cover test
It is like the alternate test in which a prism is added. The doctor holds the cover over the fixating eye and a lens over the deviating one to measure the misalignment. He/she repeats the test using lenses with higher power until the deviating eye stops moving. The misalignment is assessed by the power of the prism needed to neutralize the variation. The test is particularly useful in monofixation syndrome, a condition where patients with the disorder can reduce the level of variation identified in the alternate cover test by controlling a coexisting deviation by peripheral fusion when all their eyes are open. The eye exam might be useful in evaluating the divergence in the real-life situations when both of the patient’s eyes are viewing.
The outcome of a cover test is presented in type, duration and direction of misalignment.
Type of misalignment
Heterotropia is an eye misalignment that is visible without a cover paddle or prisms. Heterophoria is an eye misalignment that is only manifest under dissociative conditions, i.e., by dissociating the patient with a cover paddle or lenses.
Duration of misalignment
Where the patient’s eye always deviates at all distances, the patient has constant misalignment. Intermittent misalignment is where the patient’s eye may only deviate under specific stressors or conditions, such as being near work or reading.
Direction of misalignment
The eye(s) tends to:
- turn inwards - Eso
- turn outwards - Exo
- rotate inwards - Incyclo
- rotate outwards Excyclo
- shows no misalignment - Ortho
- turn upwards relative to the fellow eye -Hyper
- turn downwards relative to the fellow eye -Hypo
The consistency of the divergence in all positions of gaze is an essential factor. For example, most types of strabismus have the same measure in all locations of gaze. However, ocular misalignment due to muscle tissue scarring or nerve damage is often different in magnitude in various positions of gaze. It is called non-comitant or incomitant.
Abnormal eye positioning might be a cause for many symptoms such as eye strain, reading difficulties, double vision, headaches, etc.
Risks & Complications
The cover test has no risks or complications associated with it.