When To Use Prisms--------- Diagnostic and Treatment Decisions

1- Prisms are used for treatment and for diagnostic purposes.  Rarely more than 10 diopters per eye are tolerated when incorporated in spectacle lenses.  Weight and optical disadvantages preclude prescription of higher powers.  Fresnel prisms also cause distortion and decrease of visual acuity in higher powers but are well tolerated in lower powers.  They are ideal for short-term use because they are simply added to the present spectacles.

2- Prisms are indicated to treat diplopia in small-angle strabismus and are well tolerated by most patients.  The minimal prismatic correction necessary to maintain comfortable single binocular vision should be prescribed.  

Incomitant strabismus responds less favorably to prismatic therapy because of different prismatic requirements in different gaze positions.  The base of the prism should be placed in the opposite direction as the deviation.  For instance, esotropia is corrected with base-out; hypertropia is corrected with base-down.

3- A vertical and horizontal prismatic correction may be combined in one lens by placing the axis at an oblique angle.

4- Prisms of sufficient power to shift the neutral point of nystagmus to correct a compensatory head posture are rarely tolerated.  However, they may be useful preoperatively for a diagnostic trial.

5- Base-out prisms incorporated in the distance correction trigger convergence, which may dampen the nystagmus and improve visual acuity at distance.

6- In adult patients with long-standing strabismus, it is useful to be able to predict whether surgical alignment will cause postoperative diplopia.  To accomplish this, the preoperative angle is neutralized with prisms and the patient's response is studied at near and distance fixation.  If when the angle is neutralized a diplopia response is pardoxic and caused by anomalous retinal correspondence, postoperative diplopia is a good possibility but usually is transient.  Prism adaptation has been advocated by some to predict the outcome of surgery in acquired esotropia and to modify the amount of surgery accordingly.

7- Fresnel prisms are used to neutralize a previous prismatic spectacle correction so that the spectacles may be worn during the early postoperative phase and until the patient is ready for a new permanent prescription.

8- Absence of diplopia indicates suppression which may protect the patient from diplopia postoperatively.

9- Fusion and stereopsis after prismatic correction of strabismus is evidence of an excellent functional potential.