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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]
Surgical correction of the hypertropia is desired to achieve binocularity, manage diplopia and/or correct the cosmetic defect. Steps to achieve the same depend on mechanism of the hypertropia and identification of the offending muscles causing the misalignment.
The Maddox rod test can be used to subjectively detect and measure a latent, manifest, horizontal or vertical strabismus for near and distance. The test is based on the principle of diplopic projection.
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.
A video covering the Bagolini Striated Test. The BSGT is used for patients with strabismus to test for suppression, normal retinal correspondence or abnormal retinal correspondence, particularly in cases of manifest strabismus.
When this muscle's function is diminished due to a fourth cranial nerve palsy, the affected eye will extort, deviate upward (hypertropia), and, to a smaller extent, drift inward. Treatment
Esotropias measuring more than 15 prism diopters (PD) and exotropias more than 20 PD that have not responded to refractive correction can be considered candidates for surgery. Techniques Disinserting the medial rectus muscle. The goal of strabismus surgery is to correct misalignment of the eyes.
Depending on the symptoms, the surgical correction of cyclotropia may involve a correction of an associated vertical deviation (hyper- or hypotropia), or a Harada–Ito procedure or another procedure to rotate the eye inwards, or yet another procedure to rotate it outwards.
Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base". The base is the thickest part of the lens and is opposite from the apex. Light will be bent towards the base and the image will be shifted towards the apex.
Based on prism-FD curves (Fig. 3b), one can find the aligning prism sP 0 that nullifies the naturally prevailing fixation disparity sFD 0. This test procedure is typically made in near vision of 40 cm, e.g. with the Mallett-unit, the Disparometer, or the Wesson card (see above).