The Optic Nerve
In eyes with pathologic myopia, the optic nerve head is undercut by a shifted Bruch membrane opening, the scleral canal may be stretched and tilted, the circle of Zinn–Haller is greatly enlarged, and the optic nerve may ordinarily appear stretched and pallorous. Glaucoma is much more common in highly myopic eyes and frequently goes undetected. Measuring the retinal nerve fiber layer with OCT is problematic because of the varying shape of the eye, the potential for schisis, and because normative databases were developed for eyes that are not pathologically myopic. Visual field tests may show defects because of the shape of the eye, some of which can be “fixed” by using a refractive correction for that portion of the eye. Dehiscences in the lamina cribrosa are common in eyes with high myopia (Fig 10-9).
Figure 10-9 Optic nerve changes in pathologic myopia. A, The optic nerve head seen in the color fundus photograph does not accurately show the size of the Bruch membrane opening. B, The enhanced depth imaging OCT shows the actual Bruch membrane opening (arrows). Note how far Bruch membrane extends into what appears to be the nerve (left arrow). C, A vertical section through (C) in the color photograph shows the extent of Bruch membrane. The arrow shows Bruch membrane extending into the nerve tissue. The nerve fibers have to arch nasally under Bruch membrane to reach the lamina cribrosa (yellow arrow). D, A vertical section through (D) in the color photograph shows 2 dehiscences (arrowheads) in the lamina cribrosa. Although this is a common finding in both glaucoma and pathologic myopia, it is not known whether every patient with a lamina defect in high myopia also has glaucoma. When an eye with glaucoma develops a Drance hemorrhage, there is typically an appearance of a lamina cribrosa dehiscence; however, in pathologic myopia dehiscences in the lamina are not typically seen to have any associated hemorrhage.
(Courtesy of Richard F. Spaide, MD.)
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.