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

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