Overview
This lecture explains the theory, technique, and interpretation of the prism cover test for measuring ocular deviation (squint), including clinical application and precautions.
Optics and Principle of the Prism Cover Test
- A prism bends light toward its base; the image appears shifted toward the apex.
- In squint (e.g. esotropia), the deviant eye’s image falls on the peripheral retina, not the fovea.
- When forced to fixate, the deviating eye moves to align the image with the fovea (redressal movement).
- The magnitude of redressal movement is proportional to the deviation.
Performing the Prism Cover Test
- Place prisms of increasing strength in front of the deviated eye and perform the alternate cover test.
- Increase the prism power until no redressal movement is observed—this neutralization point equals the angle of deviation.
- If increased further, the redressal movement reverses, indicating over-correction; return to the last neutral point.
Types and Placement of Prisms
- Esotropia: use base out prism (apex towards nose).
- Exotropia: use base in prism.
- Hypertropia: use base down prism.
- Hypotropia: use base up prism.
- Always place prism in front of the deviated eye; fixing eye is uncovered.
Clinical Technique and Settings
- Perform the test at both distance and near, and in all nine gaze positions.
- Use appropriate visual targets—6/9 Snellen line for distance, suitable objects for near.
- Use loose prisms or prism bars for horizontal/vertical deviations.
Interpretation and Special Cases
- Comitant squint: deviation equal in all gazes.
- Incomitant squint: deviation varies across gazes (may indicate muscle paralysis or restriction).
- Test with and without glasses for static (basic) and dynamic deviations.
- For large deviations exceeding available prism power, use combination prisms and reference conversion tables (not simple addition).
Primary vs. Secondary Deviation
- Primary deviation: measuring the squinting eye while the normal eye fixates.
- Secondary deviation: measuring the normal eye while the squinting eye fixates; secondary > primary in incomitant squint due to Hering's Law.
Precautions and Troubleshooting
- Prevent fusion by covering alternate eyes properly and slowly.
- Allow time for the patient to fixate; use detailed targets for accuracy.
- High-power prisms may reduce visual clarity; use on better-seeing eye when possible.
- For mixed (horizontal + vertical) deviations, neutralize the larger component first.
- In uncooperative patients (e.g. children), start with an estimated prism value to save time.
Key Terms & Definitions
- Prism Cover Test — Test using prisms and alternate cover to measure eye deviation.
- Redressal Movement — The eye’s corrective movement to align the image with the fovea.
- Comitant Squint — Deviation remains the same in all gaze directions.
- Incomitant Squint — Deviation varies in different gaze directions.
- Primary Deviation — Deviation measured when normal eye is fixating.
- Secondary Deviation — Deviation measured when the squinting eye fixates.
- Static (Basic) Deviation — Deviation measured with refractive correction; accommodation relaxed.
- Dynamic Deviation — Deviation measured without correction; accommodation active.
Action Items / Next Steps
- Practice prism cover test at both distance and near, in all gaze positions.
- Review procedures for primary and secondary deviation measurement.
- Use conversion tables for combined prism measurements.
- Answer: What is the relation between primary and secondary deviation in comitant strabismus?