The cover test is the most important clinical tool for detecting and quantifying ocular misalignment. As a COA candidate, you need to understand not only how to perform it correctly but also what each variation measures, why you do them in a specific order, and how to translate your findings into proper documentation. The COA exam tests this content heavily under the Assessments domain, particularly in ocular motility and binocular vision sections.
Before picking up the cover paddle, understand the three possible binocular states you are assessing: orthophoria (perfect alignment, no deviation), phoria (latent deviation held in check by fusion), and tropia (manifest deviation present even during binocular viewing). Your job with the cover test is to determine which of these applies to each patient, at both distance and near.
There are three distinct tests in the cover test battery, performed in a specific sequence. Starting with the cover/uncover test, then moving to the alternate cover test, and finally the prism and alternate cover test if quantification is needed. Skipping steps or performing them out of order gives you incomplete or misleading information.
The Cover/Uncover Test: Detecting Manifest Deviations (Tropias)
The cover/uncover test is performed first because it only reveals manifest deviations -- tropias that are present during normal binocular viewing. You must do this before the alternate cover test, because the alternate cover test will dissociate fusion and can temporarily unmask or worsen deviations, making tropia versus phoria discrimination impossible afterward.
Step-by-Step: Cover/Uncover Test
- 1. Have the patient fixate on a target appropriate for their age. For adults: a letter on the Snellen chart at distance (6 m) or a near card at 33 cm. For young children: use a small, interesting fixation target such as a penlight with an Harrington Flocks occluder or a small fixation sticker.
- 2. Cover the right eye with the occluder. Watch the left (uncovered) eye. If the left eye moves to pick up fixation -- movement toward the target -- it was misaligned before the cover was applied. This means the left eye has a tropia. The direction of movement tells you the type: movement inward (nasally) = exotropia of the left eye; movement outward (temporally) = esotropia.
- 3. Remove the cover from the right eye and watch it. If the right eye moves to resume fixation (it picks up the target again), it was deviated under the cover. If it stays still after removal, it either has no deviation or was already fixating. Record what you observe.
- 4. Repeat the same process covering the left eye and observing the right eye.
- 5. No movement of either eye during the cover/uncover test = no tropia. The patient may still have a phoria -- you will need the alternate cover test to check.
Key Clinical Principle
The cover/uncover test detects a tropia by observing the uncovered eye. Movement of the uncovered eye when the other eye is covered means the uncovered eye has a tropia. Do not confuse this with observing the covered eye -- that eye is irrelevant during the cover phase.
The Alternate Cover Test: Revealing Total Deviation
The alternate cover test dissociates binocular vision by rapidly alternating the occluder from one eye to the other without ever allowing simultaneous binocular viewing. This breaks down fusional vergence -- the mechanism that keeps phorias hidden -- and reveals the total deviation, which is the sum of the manifest tropia plus any latent phoria.
Because it destroys fusion, you cannot use the alternate cover test alone to distinguish a tropia from a phoria. You must perform the cover/uncover test first. The alternate cover test only tells you the total magnitude of deviation when fusion is eliminated.
Step-by-Step: Alternate Cover Test
- 1. Have the patient fixate on the same target as above. Begin with the right eye covered for about 2-3 seconds.
- 2. Move the occluder quickly to the left eye -- covering it -- before the right eye has time to resume binocular fusion. Watch the right eye as it is uncovered. If it moves inward (toward the nose), the patient was in an exodeviated position under the cover, suggesting exodeviation.
- 3. Continue alternating, watching the eye as it is uncovered each time. The direction it moves from its dissociated position back toward fixation indicates the deviation type. If the uncovered eye always moves inward from a temporal position, there is an exodeviation (the eye drifts out under cover).
- 4. Perform the alternate cover test at both distance (6 m) and near (33-40 cm). A deviation that is larger at near than distance may suggest an accommodative esotropia with a high AC/A ratio.
Prism and Alternate Cover Test: Quantifying the Deviation
Once you have established that a deviation exists and characterized its direction, you quantify it with the prism and alternate cover test (PACT). This is the gold standard for measuring strabismus magnitude and is performed under dissociated conditions using the alternate cover technique.
Prism Placement Rules
- Esotropia: Base-out prism (apex toward nose)
- Exotropia: Base-in prism (apex toward temple)
- Right hypertropia: Base-down before right eye
- Left hypertropia: Base-down before left eye
- Large deviations: Split prism between both eyes to reduce optical distortion
Neutralization Endpoint
- Increase prism power until refixation movement is eliminated
- Neutralization = no movement OR movement reverses direction
- Record the prism value where neutralization occurs
- Use loose prisms for small deviations, prism bar for larger ones
- For very large deviations (>40 PD), Krimsky test with corneal reflex is an alternative
Hirschberg Test: Corneal Light Reflex Estimation
The Hirschberg test uses a penlight held approximately 33 cm from the patient to estimate deviation magnitude by observing corneal light reflex position. It is fast, requires no patient cooperation, and is especially useful in young children or patients who cannot perform the cover test. However, it only estimates -- the prism and cover test is required for precise measurement.
| Reflex Position | Estimated Deviation | Clinical Significance |
|---|---|---|
| Slightly nasal (0.5 mm) | Normal (Kappa angle) | Pseudostrabismus appearance, no true deviation |
| At pupil margin (1-2 mm) | ~7-14 PD | Small deviation, confirm with cover test |
| Midway between pupil and limbus (3 mm) | ~21 PD | Moderate deviation, visible misalignment |
| At limbus (5 mm) | ~35 PD | Large deviation, obvious strabismus |
| Beyond limbus (>5 mm) | >45 PD | Very large deviation (e.g., exotropia with sensory loss) |
Documentation Standards
Accurate documentation allows any clinician to understand the patient's strabismus status at a glance and track changes over time. Use standardized abbreviations and always specify the testing conditions: with or without correction, at distance or near, and whether the deviation is constant or intermittent.
Standard Abbreviations
Deviation Type
- ET -- Esotropia (eye turns in)
- XT -- Exotropia (eye turns out)
- HT -- Hypertropia (eye turns up)
- HXT -- Hypotropia (eye turns down)
- E(T) -- Intermittent esotropia
- X(T) -- Intermittent exotropia
Qualifying Terms
- sc -- Without correction (sine correctione)
- cc -- With correction (cum correctione)
- D -- Distance (6 m)
- N -- Near (33 cm)
- PD -- Prism diopters
- Alt -- Alternating fixation preference
Documentation Examples
Intermittent exotropia:
Cover test: X(T) 20 PD at D, 14 PD at N, cc. Intermittent, R eye preferred. PACT neutralized at 20 PD BO.
Constant esotropia:
Cover test: ET 35 PD at D and N, sc. Constant. Hirschberg: reflex 3 mm temporal OD. PACT: 35 PD BO to neutralize.
Orthophoric / small phoria:
Cover test: No tropia OD or OS. Alternate cover test: Exophoria 4 PD at D, 6 PD at N, cc. No refixation movement on cover/uncover.
AC/A Ratio and Distance vs. Near Differences
The accommodative convergence/accommodation (AC/A) ratio describes how much convergence occurs for each diopter of accommodation exerted. A normal AC/A ratio is approximately 3:1 to 5:1 -- meaning 3 to 5 prism diopters of convergence per diopter of accommodation. Deviations in this ratio explain why some patients show very different measurements at distance versus near.
A high AC/A ratio (greater than 6:1) means the patient over-converges during accommodation. This is the mechanism behind accommodative esotropia -- the patient fuses adequately at distance but converges excessively when accommodating for near, producing esotropia at near. These patients often respond to bifocal spectacles or reading glasses that reduce the accommodative demand.
Deviation Larger at Near
Suggests high AC/A ratio. Common in accommodative esotropia. Consider bifocals or reading glasses to reduce near esotropia.
Deviation Larger at Distance
Suggests low AC/A ratio or divergence-type pattern. Seen in divergence insufficiency esotropia or exodeviation with convergence excess.
Special Considerations for the COA Exam
Intermittent vs. Constant Deviations
An intermittent deviation controls to orthophoria at some or all times. Document control level: excellent (controls spontaneously and rapidly), good (controls with blink or effort), fair (breaks frequently), or poor (nearly constant). Control level guides urgency of referral and treatment planning.
Variable Deviations
Some deviations change magnitude with gaze position (incomitant) versus remaining the same in all positions (comitant). Incomitant deviations suggest a muscle restriction or paresis. Document measurements in primary position plus all nine diagnostic positions of gaze when incomitance is suspected.
Testing in Young Children
Young children require visually interesting fixation targets -- a small toy, an illuminated spinner, or a detailed fixation stick. The Harrington Flocks occluder (a paddle with a small picture the child looks at through the non-occluded eye) helps maintain attention. For children under 2, the Hirschberg test combined with Bruckner reflex is often more practical than a formal cover test.
When to Refer
Any newly detected constant tropia, any tropia in a child under 8 (amblyopia risk), a deviation with poor or worsening control, or associated diplopia should be referred to the attending ophthalmologist promptly. Do not attempt to interpret findings beyond your scope -- your role is to accurately measure and document.
Practice COA Ocular Motility Questions
The COA exam includes cover test scenarios and strabismus measurement questions. Practice with real exam-style questions and AI explanations.