What is the difference between the cover/uncover test and the alternate cover test?

The cover/uncover test detects a tropia (manifest deviation) by covering one eye and watching whether the uncovered eye moves to fixate. The alternate cover test measures the total deviation (tropia plus phoria) by rapidly alternating the cover between eyes without allowing binocular viewing. The cover/uncover test should always be performed first to determine if a tropia exists.

What does a receded near point of convergence indicate?

A near point of convergence greater than 10 cm from the nose is considered receded and may indicate convergence insufficiency. This condition causes difficulty maintaining convergence for near tasks, leading to symptoms like eyestrain, headaches, blurred vision at near, and intermittent diplopia when reading. It is one of the most common binocular vision disorders.

Motility Testing: Cover Tests, Hirschberg, and NPC

Q: How does the Hirschberg test estimate the amount of strabismus?

The Hirschberg test estimates deviation based on the position of the corneal light reflex relative to the pupil center. Each millimeter of reflex displacement corresponds to approximately 15 prism diopters of deviation. A reflex at the pupil margin (approximately 2 mm displacement) suggests about 30 prism diopters, and a reflex at the limbus (approximately 4-5 mm) suggests about 60-75 prism diopters.

Overview of Motility Testing

Motility testing evaluates how well the eyes move individually and together. It detects strabismus (eye misalignment), identifies which muscles or nerves may be affected, and assesses how well the two eyes coordinate. Ophthalmic assistants perform these tests daily, and understanding each component is essential for the COA exam.

Hirschberg Test (Corneal Light Reflex)

The Hirschberg test is the quickest way to screen for strabismus. You hold a penlight at arm's length and direct it at both eyes simultaneously while the patient fixates on the light.

You observe the corneal light reflex (the tiny bright reflection on each cornea):

Centered in both pupils: Eyes are aligned (orthophoric)
Reflex displaced nasally in one eye: That eye is turned outward (exotropia)
Reflex displaced temporally in one eye: That eye is turned inward (esotropia)
Reflex displaced superiorly or inferiorly: Vertical deviation (hypotropia or hypertropia)

Each millimeter of displacement corresponds to approximately 15 prism diopters of deviation. For example, a reflex displaced 2 mm nasally suggests roughly 30 prism diopters of exotropia.

The Hirschberg test is especially useful for young children or uncooperative patients who cannot perform a formal cover test. It gives a quick estimate of deviation magnitude and direction without requiring patient cooperation beyond looking at the light.

Cover/Uncover Test

The cover/uncover test detects a tropia (manifest deviation). It is the single most important test in strabismus evaluation.

Have the patient fixate on a target (accommodative target for near, distance chart letter for distance)
Cover one eye with an occluder
Watch the uncovered eye: Does it move to take up fixation?
If the uncovered eye moves, a tropia is present. The direction of the refixation movement tells you the type of deviation:

Uncovered eye moves outward to fixate: The eye was turned inward = esotropia
Uncovered eye moves inward to fixate: The eye was turned outward = exotropia
Uncovered eye moves downward to fixate: The eye was turned upward = hypertropia

If the uncovered eye does not move, there is no tropia of that eye. Repeat by covering the opposite eye.

Alternate Cover Test

The alternate cover test detects the total deviation (phoria + tropia). It breaks fusion completely by alternating the occluder from one eye to the other without allowing binocular viewing.

Cover one eye for 2-3 seconds
Rapidly move the occluder to the other eye
Watch the eye being uncovered: Does it move to take up fixation?
Continue alternating, never allowing both eyes to see simultaneously

The direction and magnitude of the refixation movement upon uncovering reveals the total deviation. Prisms can be placed in front of one eye to measure the deviation quantitatively (prism and alternate cover test, or PACT).

The cover/uncover test distinguishes tropia from phoria. The alternate cover test measures the total deviation. Always perform the cover/uncover test first to determine if a tropia exists, then the alternate cover test to measure the full deviation.

Near Point of Convergence (NPC)

The NPC measures the closest point at which both eyes can maintain single binocular vision. You bring a small target (pen tip, RAF rule target) from approximately 40 cm toward the patient's nose while watching both eyes converge.

The break point is where one eye deviates outward and the patient reports seeing double
The recovery point is where the eyes regain single vision as the target is moved back out
Normal NPC is approximately 6-10 cm from the nose
A receded NPC (greater than 10 cm) may indicate convergence insufficiency

Versions and Ductions

Versions (Binocular Movements)

Versions test both eyes moving together through the nine positions of gaze (primary, eight peripheral). Hold a fixation target at the patient's eye level and move it into each position while both eyes follow. Observe for:

Smooth, full range of motion in all directions
Any restriction or overaction of individual muscles
Nystagmus in extreme gaze positions

Ductions (Monocular Movements)

Ductions test each eye individually by occluding the other eye. This isolates the function of specific muscles without the influence of binocular coordination. The four primary ductions are:

Adduction: Movement toward the nose (medial rectus)
Abduction: Movement away from the nose (lateral rectus)
Elevation: Upward movement (superior rectus, inferior oblique)
Depression: Downward movement (inferior rectus, superior oblique)

Performing the alternate cover test first instead of the cover/uncover test. The alternate cover test disrupts fusion completely and measures the total deviation, which includes any phoria. If you start with the alternate cover test, you cannot determine whether the deviation was a tropia or just a phoria. Always start with the cover/uncover test.

Key Takeaways

The Hirschberg test provides a quick estimate of strabismus by assessing corneal light reflex position
The cover/uncover test is the definitive test for detecting a tropia by watching the uncovered eye for refixation movement
The alternate cover test measures total deviation (phoria + tropia) by breaking fusion completely
NPC measures convergence ability; a receded NPC suggests convergence insufficiency
Versions test binocular movements through nine gaze positions; ductions test monocular movements individually