Therapeutic Use of Prisms
Small horizontal and vertical deviations can be corrected conveniently in spectacle lenses by the addition of prisms.
Asthenopic symptoms may develop in patients (usually adults) if fusion is disrupted by inadequate vergence amplitudes; if fusion cannot be maintained, diplopia results. Thus, in patients with an exophoria at near, symptoms develop when the convergence reserve is inadequate for the task. Some patients can compensate for this fusional inadequacy through the improvement of fusional amplitudes. Younger patients may be able to do so through orthoptic exercises, which are sometimes used in conjunction with prisms that further stimulate their fusional capability (base-out prisms to enhance convergence reserve).
Symptoms may arise in some patients because of abnormally high accommodative convergence. Thus, an esophoria at near may be improved by full hyperopic correction for distance and/or by the use of bifocal lenses to decrease accommodative demand. In adult patients, orthoptic training and maximum refractive correction may be inadequate, and prisms or surgery may be necessary to restore binocularity.
Prisms are especially useful if a patient experiences an abrupt onset of symptoms secondary to a basic heterophoria or heterotropia. The prisms may be needed only temporarily, and the minimum amount of prism correction necessary to reestablish and maintain binocularity should be used.
Vertical fusional amplitudes are small (<2.00∆D). Thus, if a vertical muscle imbalance is sufficient to cause asthenopic symptoms or diplopia, it should be compensated for by the incorporation of prisms into the refractive correction. Once again, the minimum amount of prism needed to eliminate symptoms should be prescribed. In a noncomitant vertical heterophoria, the prism should be sufficient to correct the imbalance in primary gaze. With combined vertical and horizontal muscle imbalance, correcting only the vertical deviation may help improve control of the horizontal deviation as well. If the horizontal deviation is not adequately corrected, an oblique Fresnel prism may be helpful. A brief period of clinical heterophoria testing may be insufficient to unmask a latent muscle imbalance. Often, after prisms have been worn for a time, the phoria appears to increase, and the prism correction must be correspondingly increased.
Methods of prism correction
The potential effect of prisms should be evaluated by having the patient test the indicated prism in trial frames or trial lens clips over the current refractive correction. Temporary prisms in the form of clip-on lenses or Fresnel press-on prisms can be used to evaluate and alter the final prism requirement. The Fresnel prisms have several advantages: (1) they are lighter in weight (1 mm thick) and more acceptable cosmetically because they are affixed to the concave surface of the spectacle lens, and (2) they allow much larger prism corrections (up to 40.0∆). With higher prism powers, however, it is not uncommon to observe a decrease in the visual acuity of the corrected eye. Patients may also observe chromatic fringes.
Prisms can be incorporated into spectacle lenses within the limits of cost, appearance, weight, and the technical skill of the optician. Prisms should be incorporated into the spectacle lens prescription only after an adequate trial of temporary prisms has established that the correction is appropriate and the deviation is stable.
Prism correction may also be achieved by decentering the optical center of the lens relative to the visual axis, although a substantial prism effect by means of this method is possible only with higher-power lenses. Aspheric lens designs are not suitable for decentration. (See earlier discussion of lens decentration and the Prentice rule.) Bifocal segments may be decentered in more than the customary amount to give a modest additional base-in effect to help patients with convergence insufficiency.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.