The Choroid in Pathologic Myopia
More than 70% of the blood flow to the eye goes to the choroid, a structure that provides oxygen to the choroid, RPE, and the outer retina in addition to acting as a heat sink, absorbing stray light, participating in immune response and host defense, and acting as an integral part in the process of emmetropization. Optical coherence tomography (OCT), particularly either swept-source OCT (SS-OCT) or enhanced depth imaging OCT (EDI-OCT) using spectral-domain OCT (SD-OCT), can image the full thickness of the choroid in myopic eyes.
One study found that the mean subfoveal choroid thickness in children aged 11–12 years was 369 ±81 μm in girls and 348 ± 72 μm in boys. With increasing age, the choroid becomes thinner; for example, the typical subfoveal choroidal thickness in an emmetropic 60-year-old might be approximately 220–260 μm. Eyes with myopia, particularly those progressing into the range of pathologic myopia, undergo ocular expansion starting in late childhood, and this expansion is also associated with thinning of the choroid (Fig 10-5). A group of patients with myopia with a mean age of 59.7 years had a mean refractive error of –11.9 D and a mean subfoveal choroidal thickness of 93 μm, with a relatively large standard deviation of 63 μm. The thinning of the choroid per decade of life is approximately the same in myopic eyes as in nonmyopic eyes. Therefore, older individuals, or individuals with higher amounts of myopia, may have remarkably thin choroids. The most significant predictor of visual acuity in highly myopic eyes with no macular pathology is subfoveal choroidal thickness. Eyes with pathologic myopia have many reasons to lose visual acuity, and some of these are quite dramatic, such as retinal detachment. However, later in life many more highly myopic eyes suffer smaller amounts of visual acuity loss associated with decreasing choroidal thickness.
Figure 10-4 Expansion of CNV after treatment with photodynamic therapy followed by bevacizumab. A, This patient was treated with photodynamic therapy for myopic choroidal neovascularization with leakage. Note the rings of pigment centrally, indicating successive expansions of the lesion. B, After treatment, the lesion expanded even more. Note the increased pigment (arrow). C, After several photodynamic therapy treatments, the lesion expanded further (arrow), and a hemorrhage developed (arrowhead). Visual acuity was 20/80. D, Fluorescein angiography image reveals the extent of the neovascularization. The patient was given an injection of intravitreal bevacizumab 1.25 mg. E, The patient received 2 additional injections over time. Six years after first being treated with bevacizumab, the patient has some residual hyperpigmentation, but also a wide area of pigmentary loss. Visual acuity was 20/60. F, Nearly 10 years after injection, the atrophy continued to expand.
(Courtesy of Richard F. Spaide, MD.)
Figure 10-5 Graph of subfoveal choroidal thickness versus myopic refractive error in a group of 145 highly myopic eyes with no macular pathology. The trend line demonstrates the decrease in choroidal thickness with increasing refractive error, and the thinner bordering lines show the 95% confidence interval of the trend line.
(Used with permission from Nishida Y, Fujiwara T, Imamura Y, Lima LH, Kurosaka D, Spaide RF. Choroidal thickness and visual acuity in highly myopic eyes. Retina. 2012;32(7):1229–1236.)
When the choroid becomes very thin, the pigmentation of the RPE often becomes granular. The larger choroidal vessels, or what is left of them, are easily visible. The choroid may show a repeating pattern of pigmentation, blood vessel, pigmentation, blood vessel and so on in a pattern known as a tessellation. Eventually the choroid may become so thin that the choroidal tissue and the overlying RPE are no longer supported (Fig 10-6). This produces ovoid areas of white, called patchy atrophy, in which the underlying sclera is shown. If the central macula is involved, the patient’s visual acuity will be poor. In eyes with larger areas of atrophy, even Bruch membrane can rupture, leaving a truly bare sclera.
Figure 10-6 End-stage chorioretinal atrophy in pathologic myopia. Note the patches of full-thickness tissue loss; these appear white because of the direct visualization of the sclera. The emissary openings in the sclera become enlarged. Inset, an OCT image taken at the origin of the arrow, demonstrates remarkable thinning of the sclera and a near absence of scleral tissue in the emissary opening itself.
(Courtesy of Richard F. Spaide, MD.)
Around the optic nerve, between 5% and 10% of highly myopic eyes will have a yellow-orange pocket, which was at one time thought to be a localized retinal detachment, but more refined OCT imaging revealed it to be an acquired cavitation in the choroid (Fig 10-7). Therefore, these lesions are called peripapillary intrachoroidal cavitations. EDI-OCT demonstrated that these cavitations are associated with a posterior bowing of the sclera around the nerve.
Figure 10-7 Peripapillary intrachoroidal cavitation. A, Color fundus photograph shows the yellow-orange region of the intrachoroidal cavitation (white arrows). The green arrows show the locations of subsequent OCT sections. B, Fluorescein angiography image shows a modest late collection of dye within the cavity (white arrows). Note the upper edge of the cavity is sharply demarcated (yellow arrow). The edge of the retinal defect is more clearly evident than in the color photograph. C–F, Successive serial sections taken using SS-OCT show the inner retinal defect and the extension of the cavitation into the choroid. A veil of tissue extends through the thickness of the choroid at the border of the cavitation. In F, the hyperreflective band that corresponds to the retinal pigment epithelium is nearly straight, as illustrated by the blue dashed line. The red line follows a posterior bowing at the center-point thickness in the sclera.
(Used with permission from Spaide RF, Akiba M, Ohno-Matsui K. Evaluation of peripapillary intrachoroidal cavitation with swept source and enhanced depth imaging optical coherence tomography. Retina. 2012;32(6):1037–1044.)
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Ohno-Matsui K, Jonas JB, Spaide RF. Macular Bruch membrane holes in highly myopic patchy chorioretinal atrophy. Am J Ophthalmol. 2016;166:22–28.
Spaide RF, Akiba M, Ohno-Matsui K. Evaluation of peripapillary intrachoroidal cavitation with swept source and enhanced depth imaging optical coherence tomography. Retina. 2012; 32(6):1037–1044.
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.