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A prism is a wedge-shaped glass piece made up of transparent material. They have the property of a light ray's deflection towards the base. Prisms are routinely used in orthoptic practice to assess the degree of squint and manage deviations so that the eye becomes orthotropic. Prisms have been used in ophthalmology to treat binocular pathologies for over 100 years. However, only after the 1950s did instrumental orthoptics not provide the desired results; pediatric ophthalmologists started using prismotherapy as a prime treatment modality. As new prisms became available, the old conventional ones with high powers were aborted. After 1960, the Fresnel principle, which existed for more than 150 years, was used for the first time to manage strabismus. Fresnel prisms are made up of plastic sheet, which is thin and transparent, and it induces a prismatic effect. One side of the prism has angular grooves, and the other smooth side is attached to the lenses of the glass. This activity will describe the anatomy and physiology, indications, contraindications, equipment, preparation, technique, complications, and clinical significance of Fresnel prisms. Objectives: Describe the optics and advantages of the Fresnel prism. Explain the process and technique of Fresnel prisms application. Outline the clinical uses of Fresnel prisms. Summarise the contraindications and clinical problems associated with Fresnel prisms. Access free multiple choice questions on this topic.

After 1960, the Fresnel principle, which existed for more than 150 years, was used for the first time to manage strabismus. In 1970, the optical scientific group from San Rafael of California designed a combination of a series of Fresnel press on prisms.[1] These prisms are made up of plastic membranes with powers ranging from 0.5-30 prism dioptres. The Fresnel prism is derived from the concept of hand-ground lenses prepared by the physics and French specialist Augustine Fresnel in 1921.[2] Initially, they were intended for use in lighthouse beacons. Prisms have been used extensively in the field of ophthalmology, especially related to neuro-ophthalmology and strabismus.[3] Prisms are used extensively to increase the binocular field, relieve diplopia, or expand the field of vision. Fresnel prisms are thin transparent plastic membranes with multiple tiny prisms attached from base to base. Augustin Fresnel articulated the optics of Fresnel prisms.[4] The angle of refraction of a prism depends upon the angle of the surface and the refractive index of the material of which the prism is made.[5] The angle of refraction is independent of the thickness of the prism. These offer the advantage of being lightweight and easy to handle. The most common use is to temporarily relieve the diplopia in cases like nerve palsies secondary to trauma, blowout fractures, decompensated phoria, divergence insufficiency, or convergence insufficiency.[6] Fresnel prisms are most commonly used as stuck-on prisms. Traditionally, prisms have also been used in patients with stroke and homonymous hemianopia. The aim is to shift the peripheral image towards the central retinal meridian.[7]

Cosmetic blemish Harder to clean Difficult to reapply Discolours with age Chromatic aberrations[28] Deterioration of visual acuity due to reflections is common with prisms greater than ten prism dioptres. The vision deterioration with Fresnel prisms is more than with conventional prisms. A 30-prism dioptre conventional prism might reduce visual acuity from 20/20 to 20/30. This is lesser than the Fresnel prism, which reduces visual acuity from 20/20 to 20/100.[8]

Patients who complain of double vision need a detailed squint evaluation. Clear history regarding onset, duration, progression, diurnal variation, and prior episode of trauma/ fever/ association with underlying systemic disease needs to be explored. An interprofessional collaborative approach involving orthoptists, optometrists, ophthalmologists, and physicians is required. A strabismologist or neuro-ophthalmologist opinion should be sought whenever needed.[29] Some of these patients might have underlying uncontrolled diabetes or hypertension, which needs urgent intervention and be life-threatening to the patient. Interprofessional coordination between physicians, orthoptists, nurses, and ophthalmologists, including squint or neuro-ophthalmologists, ensures better management and helps the patient make better decisions. Once underlying causes have been ruled out, dispensing Fresnel prisms requires coordination between strabismologist and orthoptists.[21]