Prism and Cover Test

The prism and cover test is a fundamental examination technique for measuring eye misalignment (strabismus). Prisms of increasing strength are used to quantify the deviation, which guides diagnosis and treatment.

Why It's Done

• Quantify eye misalignment in conditions like sixth nerve palsy, fourth nerve palsy, and third nerve palsy
• Monitor cranial nerve palsies over time
• Prescribe prism glasses for double vision
• Plan strabismus surgery
• Track changes over time in conditions like myasthenia gravis and thyroid eye disease

How It Works

Basic Principle

• Cover one eye, observe movement of other
• If eyes are misaligned, uncovered eye moves to fixate
• Prisms bend light to eliminate movement
• Prism strength = amount of misalignment

What Prisms Do

• Bend light toward their base
• Eye doesn't need to move as much
• Measure in "prism diopters"

Types of Cover Tests

Cover-Uncover Test

• Cover one eye, watch other
• Detects manifest deviation (tropia)
• Movement indicates misalignment

Alternating Cover Test

• Alternate cover between eyes
• Detects total deviation
• Breaks fusion

Prism Cover Test

• Add prisms until no movement
• Quantifies deviation in prism diopters

Measuring at Distance and Near

• Distance: looking at far target (6 meters/20 feet)
• Near: looking at close target (33 cm)
• May be different-important for diagnosis
• Measurements may also be repeated in side gaze, upgaze, downgaze, and head tilt positions when a cranial nerve palsy is suspected

Results

Horizontal Deviations

• Esotropia/esophoria-eyes turn in
• Exotropia/exophoria-eyes turn out

Vertical Deviations

• Hypertropia-eye turns up
• Hypotropia-eye turns down

Use in Prism Prescription

The measurement guides:

• How much prism to prescribe
• Which direction prism should face
• Whether prisms will help symptoms

Related Tests

• Visual field testing-may be needed if neurological cause suspected
• MRI brain and orbits-to evaluate underlying cause of strabismus
• Ice test-if myasthenia gravis suspected
• OCT-to evaluate optic nerve if associated findings

Why Repeat Measurements Matter

One prism measurement is a snapshot. In neuro-ophthalmology, the trend is often more useful:

• A microvascular cranial nerve palsy should usually improve over weeks to months.
• Myasthenia gravis can vary during the day, so measurements may not match from visit to visit.
• Thyroid eye disease often needs stable measurements before permanent prism or strabismus surgery is planned.

Frequently Asked Questions

Does the prism and cover test treat double vision?

No. It measures the misalignment. The measurement can then guide prism glasses, temporary Fresnel prism, patching, botulinum toxin, or surgery.

Why are distance and near measurements different?

Some eye alignment problems show up more at near, others more at distance. That difference helps separate convergence problems, cranial nerve palsies, and decompensated childhood strabismus.

Can the test be wrong if my double vision comes and goes?

It can miss the worst pattern if symptoms are quiet that day. Bring notes about when the double vision is worse, and tell the examiner if fatigue, reading, driving, or certain gaze directions trigger it.