What are the cover uncover tests for strabismus and what does a positive test indicate?

Cover-Uncover Test for Strabismus

The cover-uncover test detects manifest strabismus (tropia) by covering one eye and observing whether the uncovered fellow eye makes a refixation movement; any refixation movement indicates a positive test and confirms the presence of a tropia. 1

How to Perform the Cover-Uncover Test

• Use an accommodative target at both distance and near fixation to control the patient's accommodation during testing 1
• Cover one eye completely and observe the uncovered fellow eye for any movement 1
• If the uncovered eye moves to take up fixation, a manifest deviation (tropia) is present 1
• The test requires sufficient visual acuity and patient cooperation to fixate on the target 1
• Perform alignment testing before cycloplegia, as alignment may change after cycloplegic drops 1

What a Positive Test Looks Like

A positive cover-uncover test demonstrates a refixation movement of the fellow eye when the opposite eye is covered 1. The direction of this movement indicates the type of strabismus:

• Esotropia (inward deviation): The uncovered eye moves outward (temporally) to take up fixation 1
• Exotropia (outward deviation): The uncovered eye moves inward (nasally) to take up fixation 1
• Hypertropia (upward deviation): The uncovered eye moves downward to fixate 1
• Hypotropia (downward deviation): The uncovered eye moves upward to fixate 1

Critical Testing Sequence

Perform sensory tests (stereopsis) BEFORE the cover-uncover test when evaluating intermittent strabismus, as monocular occlusion during cover testing can dissociate the eyes and unmask a latent deviation or reduce stereoacuity measurements 2. However, when manifest strabismus is already suspected based on observation (such as a "wandering eye"), the cover-uncover test becomes the direct confirmatory diagnostic step 3.

Alternate Cover Test vs. Cover-Uncover Test

• The cover-uncover test detects only manifest deviations (tropias) 1
• The alternate cover test (prism and alternate cover test) measures the total deviation including both manifest and latent components (phorias), making it more sensitive than simple observation 1, 2
• The alternate cover test is used to quantify the amount of surgery required 1

Common Clinical Pitfalls

• Do not rely on a single examination to characterize strabismus fully, as fusional control can vary substantially between visits, particularly in intermittent forms 2, 3
• The most common error is performing monocular occlusion tests (like visual acuity) before assessing binocular alignment in intermittent cases, which can dissociate the eyes and interfere with accurate assessment 2
• If the patient cannot cooperate with cover testing, use the corneal light-reflection test (Hirschberg or Krimsky test) to estimate alignment, though cover testing remains superior when feasible 1, 3
• Cover tests require sufficient visual acuity and cooperation to fixate on the target; alternative methods may be needed for infants or uncooperative children 1

Documentation

Document the following when recording cover test results 1:

• Whether refractive correction was worn during testing 1
• The method used to measure the angle of deviation 1
• Distance and near measurements 1
• Whether the deviation is constant or intermittent (recorded as X(T) for intermittent exotropia or XT for constant exotropia) 2