Appendix
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Ophthalmology

Prisms

A prism is a triangular refracting surface with an apex and a base. The incident light ray passing through the prism is refracted so that it is bent towards the base. Thus the image is shifted towards the apex. The amount of light refracted through the prism depends on the power of the prism defined in prism diopters. Prisms play an essential role in orthoptics and are invaluable in an ophthalmologist's clinic as they have definitive optical, therapeutic, and diagnostic uses. Prisms are used in

v1 · Reviewed Nov 8, 2026 by Automated Clinical Reviewer (Strict Verbatim v1)
  • Deterioration of vision
  • Double vision
  • Dragged fovea syndrome – In patients with pathology at the fovea like an epiretinal membrane, the patient's fovea might get displaced.
  • If the underlying disease/mechanism causing original deviation is still progressive, the patient can adapt to the added prism and can redevelop the deviation.
  • Objective measurement of squint by prism cover test (Krimsky method), simultaneous prism cover test, modified Krimsky method
  • Subjective measurement of squint by Maddox rod
  • Measurement of fusional reserve amplitudes
  • Four prism diopter test for microtropia
  • To detect abnormal retinal correspondence
  • Prism Alternate Cover Test
  • Simultaneous Prism Cover Test
  • Vertical Prism Test
Prentice's rule
Prentice's rule measures the deviation produced by the prism.
Prentice's rule formula
D = cF
Prism Diopter Definition
One prism diopter represents the deviation of light by 1 centimeter and perpendicular to the initial direction of the light ray on a plane placed 1 meter away from the prism.
Prism Diopter Symbol
The power of the prism in prism diopters is represented by the symbol D.
  • Building up fusional reserve in patients with convergence insufficiency
  • Building up divergence capacity in patients with divergence insufficiency
  • To relieve diplopia in patients with small vertical squints, decompensated phorias, or paralytic squints to relieve diplopia in primary or reading positions.
  • It is advisable to prescribe the smallest amount of relieving prism that neutralizes the distinction.
  • The prism base should be oriented based on the deviation being corrected.
  • Split the amount of correction equally between two eyes
  • The base of the prism should be oriented opposite to the direction of the deviation of the eye
Prism
Not specified in source
Adult
0.5D to 10D
Renal
Not specified in source
Duration
Not specified in source
  • If prism adaptation occurs, there occurs an increase in underlying deviation and thus needs to be closely observed by the prescribing orthoptist.
  • The prisms might reduce the central diplopia temporarily, but the diplopia reoccurs as the peripheral fusion takes over the central fusion.
  • Patients prescribed with prisms need to be followed up closely for compliance.
  • Those prescribed prisms for exercise might experience an initial exacerbation of asthenopic symptoms and thus require the motivation to continue the exercise till the fusional reserves improve.
  • Patients might experience confusion and practical difficulties with the use of prisms.
  • A few patients might show an increased angle of deviation with the prescribed prisms, and thus close monitoring is again vital in these cases.
  • Headache
  • Eyestrain
  • Nausea/vomiting
  • Confusion
  • review
    Prisms. StatPearls [Internet] — Kirandeep Kaur, Bharat Gurnani — StatPearls Publishing, 2023

Decision support only. Verify against local protocols and current labeling. Source: www.ncbi.nlm.nih.gov