alternate cover test
The alternate cover test, also called the prism and cover test, measures the direction and size of a deviation. The examiner moves an occluder from one eye to the other while the patient fixates a target.1,2 One eye stays covered throughout, so the patient is never binocular and the test is fully dissociative. The alternate cover test therefore reveals the total deviation, but the test cannot tell a heterophoria from a heterotropia.2,3
Each eye makes a movement of redress as it is uncovered, in the direction opposite the deviation. An eye that moves out on uncovering has an eso deviation, and an eye that moves in has an exo deviation.1,2 To measure the deviation, the examiner holds prisms before one eye, base-out for eso and base-in for exo, and increases the power until the movement stops.1,2 The movement stops because the prism places the image of the target on the fovea of the deviating eye. The prism strength that offsets all movement is the measure of the deviation.1
More detail
Full dissociation matters most in heterophoria, because a strong compensatory innervation keeps the eyes aligned and does not relax as soon as one eye is covered.1 Each eye should be covered three or four times in turn for about 2 seconds, and the test is done at distance and at 40 cm.2,3 Accommodative fixation targets, in place of a penlight, keep accommodation controlled during the measurement.1 When a deviation has horizontal and vertical components, the horizontal deviation is neutralized first and vertical prism is then added. Cyclodeviations cannot be measured with the alternate cover test.1
Sources (9)
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 12
- Carlson and Kurtz, Clinical Procedures for Ocular Examination, ch. 2
- Eperjesi and Rundström, Practical Binocular Vision Assessment, ch. 3
- American Academy of Ophthalmology (2024) Cover Tests. Available at: https://eyewiki.aao.org/Cover_Tests (Accessed: 4 May 2024).
- Antona, B., Barrio, A., Barra, F., Gonzalez, E. and Sanchez, I. (2008) 'Repeatability and agreement in the measurement of horizontal fusional vergences', Ophthalmic and Physiological Optics, 28(5), pp. 475–491.
- Carlson, N.B., Kurtz, D., Heath, D.A. and Hines, C. (2004) Clinical Procedures for Ocular Examination. 3rd edn. New York: McGraw-Hill.
- Grosvenor, T. (2007) Primary Care Optometry. 5th edn. St. Louis: Butterworth-Heinemann Elsevier.
- Martínez-Thompson, J.M., Diehl, N.N., Holmes, J.M. and Mohney, B.G. (2014) 'Incidence, types, and lifetime risk of adult-onset strabismus', Ophthalmology, 121(4), pp. 877–882.
- Rutstein, R., Daum, K. (1997) Anomalies of Binocular Vision: Diagnosis & Management, Mosby
Reviewed October 2026