Prism Cover Test — Complete Explanation

Fig. — Prism cover test in practice (Kanski's Clinical Ophthalmology, p. 743)

Overview

1. Cover tests (to detect and dissociate the deviation)
2. Prism neutralization (to measure its magnitude)

Prerequisite Tests — Understanding the Cover Test Sequence

A. Cover-Uncover Test (Detects Phoria vs. Tropia)

1. Patient fixates a distant accommodative target straight ahead.
2. The examiner covers one eye.
3. Watch the uncovered (fellow) eye for any movement:
   • No movement → that eye is fixing (may be orthophoric or a tropia of the covered eye)
   • Movement to pick up fixation → the uncovered eye was deviated = heterotropia of that eye

B. Uncover Test (Detects Heterophoria)

1. Cover one eye for 2–3 seconds.
2. Remove the cover and watch that eye:
   • No movement → orthophoria
   • Nasal recovery (adduction) → exophoria
   • Temporal recovery (abduction) → esophoria
   • Vertical recovery → vertical phoria

Most examiners perform the cover test and uncover test together — hence the term cover-uncover test.

C. Alternate Cover Test (Dissociates Fusion, Reveals Total Deviation)

1. Cover the right eye for several seconds.
2. Quickly shift the occluder to the left eye for 2 seconds, then alternate repeatedly.
3. Note movement of the eye being uncovered on each shift.

The Prism Cover Test (Quantification)

• Occluder
• Prism bar (a column of prisms of progressive strength, e.g., 1Δ to 45Δ or more)
• Fixation targets (distance: 6 m; near: 33 cm)

Step-by-Step Technique

• Place prisms in front of one eye (usually the deviating eye or the eye with less vision).
• The base of the prism is placed opposite to the direction of deviation (i.e., the apex points in the direction of the deviation):

• While alternately covering, introduce progressively stronger prisms from the prism bar.
• The refixation movement seen during alternate cover testing gradually decreases as the prism strength approaches the true angle of deviation.

• The endpoint is the prism strength at which no movement is observed during alternate covering.
• To confirm the maximum angle, increase the prism slightly further until a movement in the opposite direction is observed (the point of reversal).
• Reduce back to the neutral value.
• The angle of deviation = strength of prism at which no movement occurs.

Measurement Positions

Practical Tips

• Always use an accommodative target (letters/pictures), not a light source — using a light causes the patient to relax accommodation and may give false readings.
• The test must be done before the patient becomes fatigued from prolonged dissociation.
• In intermittent exotropia, measure at far distance (>6 m) as the deviation may only manifest at distance.
• In incomitant strabismus (e.g., cranial nerve palsies), PCT is also useful, performed in multiple gaze positions to map the field of deviation.
• When measuring a vertical and horizontal deviation simultaneously, use a compound prism or measure one component at a time.

What the PCT Measures vs. Does Not Measure

Clinical Significance

• Preoperative planning: The PCT angle directly determines the amount of surgical correction (muscle recession/resection).
• AC/A ratio: Comparing distance vs. near PCT measurements allows calculation of the accommodation-convergence to accommodation ratio.
• Monitoring: Serial PCT measurements track progression or response to treatment (glasses, patching, surgery).
• Incomitant squint: Measurements in all nine positions of gaze are plotted on a synoptophore chart or Hess chart for full assessment.

Summary

Cover-uncover test → detects tropia (manifest)
Uncover test       → detects phoria (latent)
Alternate cover    → reveals total deviation (tropia + phoria)
Prism cover test   → QUANTIFIES the total deviation in prism diopters

• Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., pp. 742–743
• The Wills Eye Manual, p. 1112

Krimsky Test

Fig. — Krimsky test. (A) Right exotropia; (B) base-in prism placed before the right eye aligning the corneal light reflexes. (Kanski's Clinical Ophthalmology, 10th ed.)

Definition

Background — Hirschberg Test (Foundation)

• A penlight is shone at both eyes simultaneously while the patient looks straight ahead.
• In the fixing eye, the corneal light reflex is approximately centred in the pupil.
• In the deviating eye, the reflex is displaced in the direction opposite to the deviation:
  • Reflex displaced nasally → exotropia
  • Reflex displaced temporally → esotropia
  • Reflex displaced inferiorly → hypertropia
• Each 1 mm of displacement ≈ 7° ≈ 14 prism diopters (Δ):
  • Reflex at pupil border (2 mm off center) ≈ 15°
  • Reflex at limbus ≈ 45°

Krimsky Test — Principle

"The Krimsky test involves placement of prisms in front of the fixating eye until the corneal light reflections are symmetrical." — Kanski's Clinical Ophthalmology, 10th ed.

Technique

1. Shine a penlight (or use a muscle light) equidistant from both eyes so the patient can fixate on it.
2. Observe both corneal reflexes — note which eye is deviating and in which direction.
3. Place a prism in front of the fixating eye with the base in the direction opposite to the deviation:

1. Increase prism strength until the corneal light reflex of the fixing eye is displaced to a position symmetrical with the reflex of the deviating eye.
2. The prism strength that achieves symmetry = angle of deviation in prism diopters.

Krimsky vs. Prism Reflection Test

Krimsky vs. Prism Cover Test

Indications (When to Use Krimsky Instead of PCT)

• Infants and very young children who cannot cooperate with cover testing
• Patients with very poor vision in one eye (cannot fix on a target)
• Patients with eccentric fixation — the deviating eye fixes off the fovea, making cover test results unreliable
• Nystagmus patients — where the cover test is difficult to interpret
• Quick bedside estimation when full orthoptic assessment is not possible

Limitations

• Less accurate than the prism cover test (no dissociation of fusion, no assessment of phoria)
• Does not reveal latent deviations (heterophoria)
• Angle kappa (the angle between the visual axis and the pupillary axis) can cause errors — a positive angle kappa can simulate exotropia and a negative angle kappa can simulate esotropia on corneal reflex tests

Summary

Prism Reflection Test

Definition

"The prism reflection test involves the placement of prisms in front of the deviating eye until the corneal light reflections are symmetrical." — Kanski's Clinical Ophthalmology, 10th ed.

Principle

• A penlight is shone equidistant from both eyes.
• In a normal (fixing) eye, the corneal light reflex sits near the centre of the pupil.
• In the deviating eye, the reflex is displaced opposite to the direction of the deviation.

Technique

1. Shine a penlight equidistant from both eyes.
2. Identify the deviating eye and note the direction of reflex displacement.
3. Place a prism in front of the deviating eye, with the base in the direction opposite to the deviation:

1. Increase prism strength until the corneal light reflex of the deviating eye aligns symmetrically with the reflex of the fixing eye.
2. The neutralizing prism strength = angle of deviation in prism diopters.

Prism Reflection Test vs. Krimsky Test

Indications

• Uncooperative or pre-verbal children
• Patients with very poor vision or eccentric fixation
• Patients with nystagmus
• Bedside or rapid estimation of deviation angle

Limitations

• Does not measure latent deviations (heterophoria) — only manifest deviations (heterotropia)
• Less accurate than the prism cover test
• Susceptible to angle kappa errors:
  • Positive angle kappa (visual axis nasal to pupillary axis) → simulates exotropia
  • Negative angle kappa (visual axis temporal to pupillary axis) → simulates esotropia
• Parallax is a greater issue here than with the Krimsky test

Summary

Angle Kappa

Definition

"Angle kappa is the angle, usually about 5°, subtended by the visual and anatomical axes." — Kanski's Clinical Ophthalmology, 10th ed.

Understanding the Two Axes

Normal Angle Kappa

• Normally ~5° and positive
• Because the fovea is temporal to the posterior pole, the visual axis is directed slightly nasally relative to the anatomical axis
• When a penlight is shone at the eye, the corneal light reflex falls just nasal to the centre of the pupil in both eyes — this is the normal, positive angle kappa appearance

Positive vs. Negative Angle Kappa

Large Positive Angle Kappa → Pseudoexotropia

Fig. — Pseudostrabismus from Kanski's Clinical Ophthalmology, 10th ed.

• Retinopathy of prematurity (temporal macular dragging)
• Coloboma displacing the macula
• High hypermetropia (relative temporal displacement)

Negative Angle Kappa → Pseudoesotropia

• High myopia — the elongated globe causes the fovea to be placed more nasally relative to the posterior pole

Clinical Significance

1. Pseudostrabismus

2. Errors in corneal reflex-based tests

• Hirschberg test — the reflex position may suggest deviation where none exists
• Krimsky test — an unjustifiably large prism may be required to centre the reflex
• Prism reflection test — same issue

3. Refractive surgery planning

How to Detect / Measure Angle Kappa

• Clinically: Hirschberg penlight test — observe where the corneal reflex falls relative to the pupil centre
• Corneal topography / Pentacam — measures the distance between the pupil centre and the visual axis (corneal vertex)
• Synoptophore — used to measure angle kappa more precisely in an orthoptic setting
• Confirmation of pseudostrabismus: Cover test shows no movement, confirming no true deviation despite reflex asymmetry

Summary

Angle kappa = angle between visual axis and anatomical axis
Normal      = ~5°, positive (fovea temporal, reflex slightly nasal)
Large +ve   = pseudoexotropia (e.g. ROP, temporal macular drag)
Negative    = pseudoesotropia (e.g. high myopia, nasal fovea)
Key rule    = Cover test normal → pseudostrabismus, not true squint