The cover test is a cornerstone of orthoptic practice, serving as a vital and objective diagnostic tool for assessing binocular vision anomalies. Its primary purpose is to identify and differentiate between heterophoria (latent deviation) and heterotropia (manifest deviation). The test hinges on careful observation of how a patient’s eyes behave while they maintain fixation, with each eye being covered and uncovered in turn.
Types of Cover Tests
The cover test includes several variations, each designed to provide specific insights into a patient’s ocular alignment:
1. Unilateral Cover Test (Cover/Uncover Test):This is the most commonly used method for detecting manifest strabismus and distinguishing it from heterophoria. The examiner briefly covers one eye and watches the uncovered eye for any movement, then uncovers the first eye to observe its recovery movement.
2. Alternate Cover Test (Duane Cover Test or Cross Cover Test): This variation fully dissociates the eyes, making it highly sensitive for revealing total deviation, including both manifest and latent components. The examiner alternates the cover between the two eyes quickly, preventing the re- establishment of binocularity.
3. Prism Cover Test (PCT): This test combines the alternate cover test with prisms to objectively measure the angle of deviation in prism diopters. The examiner adjusts the prism power until there is no movement of the eye behind the prism when the other eye is covered.
4. Simultaneous Prism Cover Test (SPCT): This method measures manifest deviation under normal binocular conditions, particularly useful for small-angle strabismus like mono-fixation syndrome. The prism is placed in front of the deviating eye while the cover is placed over the fixating eye.
Procedure and Important Considerations
The cover test involves several critical procedural elements:
Fixation Target: It’s essential to use an accommodative target with discernible detail, such as a letter or small picture, rather than just a spotlight. For infants, a brightly colored squeaky toy can be effective. The patient should be encouraged to identify the target to ensure they are focused on it.
Testing Distance: The test should be conducted at both distance (e.g., 6m or 20ft) and near (e.g., 33cm, 40cm, or the patient’s habitual working distance).
Refractive Correction: Ideally, the test should be performed both with and without the patient’s refractive correction, especially in hypermetropic individuals, as spectacles can affect fixation.
Occluder: An opaque or translucent occluder can be used. Translucent occluders allow the examiner to observe the covered eye while minimizing accommodation.
Duration of Occlusion: For the unilateral cover test, the eye is typically covered for 1-2 seconds to observe the response. In the alternate cover test, the occluder is held over each eye for several seconds to ensure full dissociation of fusion. Prolonged occlusion (10-15
seconds) can reveal intermittent strabismus.
Speed of Movement: The cover should be moved slowly enough to allow accurate fixation, avoiding overshoot movements. However, in the alternate cover test, the shift should be quick to prevent fusion from re-establishing.
Patient Cooperation: The test relies heavily on patient cooperation. For young children, examiners may use their hand as an occluder and engage the child with questions about the target to maintain attention. Infants as young as 3-6 months can exhibit adequate refixation reflexes for cover testing.
Information Gained from Cover Test
The cover test yields a wealth of information regarding ocular alignment and related conditions:
Presence of Deviation: It confirms whether a true manifest or latent squint is present, distinguishing it from pseudo-strabismus.
Type of Deviation: It identifies various types of deviations, including eso-, exo-, hyper-, hypo-, and cyclodeviations.
Size of Deviation: The test allows for estimation of the magnitude of deviation, often categorized as slight, small, moderate, or marked, based on the observed movement.
Laterality: It determines which eye is deviating (right, left, or alternating).
Frequency: The test assesses whether the deviation is constant or intermittent, and if intermittent, the proportion of time the eye is deviated.
Compensation and Recovery: It evaluates the speed and smoothness of recovery movement after the cover is removed, indicating the strength of motor fusion and degree of compensation.
Fixation Behavior: The test reveals the preferred eye for fixation, visual dominance, or amblyopia.
Effect of Accommodation: It can determine how accommodation influences the deviation by comparing results with a light versus a detailed accommodative target.
Incomitance: The test identifies if the angle of deviation varies across different gaze positions, which can indicate muscle overaction or underaction.
Nystagmus: It can reveal latent nystagmus, which may only be elicited when one eye is covered.
Limitations
Despite its utility, the cover test has limitations:
Eccentric Fixation: The validity of the test can be compromised by eccentric fixation, where the measured deviation may not reflect the true angle of strabismus.
Microtropia: Very small deviations may not be detected, especially if associated with eccentric fixation.
Repeated Testing: Unnecessarily repeating the test can interfere with fusion and increase deviation.
Uncooperative Patients/Poor Vision: The test requires sufficient central fixation and visual acuity. If vision is very low or there are severe motility defects, the test may be ineffective.
