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The prism cover test (PCT) is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. [1]
Ophthalmology. Exotropia is a form of strabismus where the eyes are deviated outward. It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition.
Prism dioptres. Prism correction is commonly specified in prism dioptres, a unit of angular measurement that is loosely related to the dioptre. Prism dioptres are represented by the Greek symbol delta (Δ) in superscript. A prism of power 1 Δ would produce 1 unit of displacement for an object held 100 units from the prism. [2]
Types include esotropia, where the eyes are crossed ("cross eyed"); exotropia, where the eyes diverge ("lazy eyed" or "wall eyed"); and hypertropia or hypotropia where they are vertically misaligned. They can also be classified by whether the problem is present in all directions a person looks (comitant) or varies by direction (incomitant). [3]
With heterotropia, a correcting movement of the eye can be detected already by the simple cover test; with heterophoria, such correcting movement only takes place in the cross-cover test. People with heterophoria are able to create and maintain binocular fusion through vergence , and the cross-cover test purposely breaks this fusion, making the ...
However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia [14] and poor binocular function may be ...
Other options for strabismus management are vision therapy and occlusion therapy, corrective glasses (or contact lenses) and prism glasses, and strabismus surgery. The effects that are due only to the toxin itself (including the side effects) generally wear off within 3 to 4 months.
Prism spectacles with a single prism perform a relative displacement of the two eyes, thereby correcting eso-, exo, hyper- or hypotropia. In contrast, spectacles with prisms of equal power for both eyes, called yoked prisms (also: conjugate prisms, ambient lenses or performance glasses) shift the visual field of both eyes to the same extent.
In the cover part of the test, the examiner looks at the first eye as he or she covers the second. If the eye moves from in to out, the person has exotropia. If it moved from out to in, the person has esotropia. People with exotropia or esotropia are wall-eyed or cross-eyed respectively.
The management of strabismus may include the use of drugs or surgery to correct the strabismus. Agents used include paralytic agents such as botox used on extraocular muscles, topical autonomic nervous system agents to alter the refractive index in the eyes, and agents that act in the central nervous system to correct amblyopia.