Prism Use in Adult Diplopia

Kammi B. Gunton; A'sha Brown

Disclosures

Curr Opin Ophthalmol. 2012;23(5):400-404. 

In This Article

Fresnel Prisms

The Fresnel principle states that prismatic or refractive power can be achieved by employing a concentric set of prismatic rings with the face of each ring having the prismatic power or curvature of the lens element it replaces, respectively. Fresnel prisms are 1.0 mm in thickness regardless of the power of their prismatic correction. The thinner prism allows a wider range of prismatic corrections to be used in spectacles. Once the Fresnel prism was modified with optical-grade polyvinyl chloride, allowing the application of the prism directly to spectacle lenses, their use became more widespread.[1] In most cases, the cost of Fresnel prisms is much lower than ground-in prisms. Fresnel prisms are particularly effective in temporary situations, such as sixth and fourth nerve palsies from microvascular insults when resolution of the diplopia is expected. They are also useful when deviations vary between near and distance. Fresnel prisms can be applied to only the top or bottom of a spectacle lens, allowing for positional variability in the deviation to be addressed. When the exact prismatic correction needed is unknown, the cost advantage also favors Fresnel prisms. Fresnel prism can be used in cases of larger deviations, as an initial trial prior to permanent prism spectacles, large lateral incomitance, and when uncertainty exists in a patient's subjective response to prism.[2]

Fresnel prisms have some disadvantages. The degradation of visual acuity in the distance is more with Fresnel lenses of greater than 12 prism diopters compared with traditional ground-in prisms.[3] There are also increased optical aberrations, loss of contrast and light scatter in larger Fresnel prisms. In addition, the grooves of the Fresnel are cosmetically visible. Further optical degradation occurs when dust and other small particles accumulate within the grooves of the Fresnel prism. Inadequate application of the Fresnel prism allows trapped air pockets between the Fresnel membrane and the spectacle lens, further degrading visual acuity. Twenty percentage of patients in one study discontinued Fresnel prism usage because of these side effects.[1] In another study, only 8% continued in Fresnel prism once satisfactory treatment of the diplopia was achieved.[2] The balance of these disadvantages against the advantages has led to the frequent use of Fresnel prisms to correct diplopia.

Because of the limitations, the Fresnel should only be applied to one eye. Typically, the nondominant eye is chosen for the application of the Fresnel because of the effect on visual acuity. Horizontal and vertical prism can be achieved through the oblique application of an appropriate prism onto the spectacles.[4] In cases of reduced visual acuity in one eye, for example, from macular disease or optic neuropathy, the Fresnel can be effectively used to eliminate diplopia.[5] The resulting further reduction in visual acuity is well tolerated.

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