The thickness of the sclera in a nonmyopic eye varies considerably with location; the thickest area, which is around the optic nerve, can be slightly more than 1 mm, whereas the area immediately under the rectus muscle insertions may be as thin as 0.3 mm. When ocular expansion related to myopia begins, the eye elongates, but the amount of material that makes up the sclera does not increase (see Fig 10-1). The collagen fibers become thinner, the typical gradient in fiber thickness in the sclera is lost, and the amount of extracellular matrix decreases. Over time, the sclera in a myopic eye shows more elasticity and greater viscoelastic creep. These factors appear to be necessary to allow the myopic eye to expand, but why it expands is unknown.
Form-deprivation or lens-induced errors in the eye are followed by axial length changes in animal models. Optic nerve sectioning or the destruction of the ciliary nerve does not prevent the development of experimental myopia. Form deprivation of a hemifield results in expansion of the eye that is conjugate with that hemifield, even if the optic nerve is sectioned. These findings support the hypothesis that remodeling of the eye results from local effects within the eye, beginning with signaling that originates in the retina and choroid and eventually affects the sclera. Connection to the brain does not appear to be necessary. Eyes that develop axial myopia lengthen, but in comparison to the posterior pole, the periphery becomes relatively hyperopic. Peripheral hyperopia can induce myopia in animal models, and curiously myopia can develop in eyes with peripheral hyperopia even if the posterior portion of the retina has been destroyed.
Ocular expansion can vary regionally, inducing formation of areas of the sclera that have differing radii of curvature. A regional expansion of the eye that produces a protrusion is called a staphyloma. These protrusions follow several patterns, but typically involve 3 general areas of the eye: (1) the area around the nerve, (2) the macular region, which leads to exaggerated thinning of the choroid and possibly myopic traction maculopathy, and (3) the inferior or inferotemporal portion of the eye (Fig 10-8). The superior portion of the eye has one radius of curvature while the inferior portion has another, and there is a visible boundary between these two curves. If the boundary occurs above the optic nerve, the optic nerve head will appear grossly tilted and rotated. If the boundary bisects the fovea, several alterations may be seen. In later life, there may be atrophy along the boundary line that affects the RPE under the fovea, and either subretinal fluid without CNV or frank CNV may also develop in these eyes. Because this last staphyloma may be accompanied by a set of possible ocular manifestations, it has been referred to as inferior staphyloma syndrome or tilted disc syndrome. The origin of the accumulation of subretinal fluid without CNV is not known. In some patients, the choroid may show slight regional thickening around the change in curvature, but nearly all patients show a localized thickening of the sclera, which may limit outflow through the sclera (see Fig 10-8).
Figure 10-8 Inferior staphyloma syndrome, also known as tilted disc syndrome. A, Color fundus photograph shows that the color of the superior fundus is darker than the staphylomatous inferior staphyloma. At the border between the two regions (arrowheads) there is a pigmentary change in the macula (yellow arrow). Because this border runs through the superior border of the optic nerve head, the patient has a tilted disc (arrow). B, A vertical OCT taken through the fovea shows the two curves. At the ridge between them, there is choroidal neovascularization (arrow) associated with a small amount of submacular fluid. The sclera is typically thicker at the border zone (asterisk) than anywhere in the neighboring areas.
(Courtesy of Richard F. Spaide, MD.)
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Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.