View Full Version : The choroid thins with increasing age and myopia

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09-02-2012, 04:35 PM

The choroid in highly myopic eyes is very thin and undergoes further thinning with increasing age and degree of myopia, according to a new study.

High myopia is associated with excessive and progressive elongation of the globe, resulting in a variety of fundus changes that lead to visual impairment, including lacquer cracks in the Bruch membrane, choroidal neovascularization (CNV), and chorioretinal atrophy. There is evidence that axial elongation of the globe leads to stretching of retinal tissue and reduced retinal function. Based on these findings, retinal dysfunction would be expected to correlate with the degree of myopia regardless of the patientĺs age. However, visual function tends to be normal in highly myopic children regardless of the degree of myopia, and worsens with age.

Studies have demonstrated choroidal thinning and decreased choroidal circulation in myopic eyes. Because the choroid supplies nutrition to retinal pigment epithelial (RPE) cells and the outer retina, compromised choroidal circulation may account, in part, for the retinal dysfunction and vision loss that is seen in high myopia.

A method called enhanced depth imaging spectral-domain optical coherence tomography (EDI OCT) has been developed that enables in vivo cross-sectional imaging of the choroid. Using this method, choroidal thickness (CT) was studied in normal eyes and was shown to be thickest underneath the fovea and to be inversely proportional to age.

Methods and Results

Enhanced depth imaging OCT images were obtained in highly myopic eyes (> or =6 diopters [D]). Images of CT were obtained by positioning a spectral-domain OCT device close enough to the eye to acquire an inverted image. CT was measured from the outer border of the retinal pigment epithelium to the inner scleral border at 1000 Ám intervals of a horizontal section from 3 mm temporal to the fovea to 3 mm nasal to the fovea. Statistical analysis was performed to evaluate CT at each location and to correlate CT with age and refractive error.

The mean age of the 31 patients (55 eyes) was 59.7 years (range, 24 to 90 years), and the mean refractive error was -11.9 D. The mean subfoveal CT was 93.2 Ám and was correlated negatively with age, refractive error, and history of choroidal neovascularization. Regression analysis suggested that subfoveal CT decreased by 12.7 Ám for each decade of life and by 8.7 Ám for each D of myopia.

Discussion and Conclusions

In this study, EDI OCT measurements of CT demonstrated that the choroid of highly myopic eyes is significantly thinner than the choroid of normal eyes. CT was greatest 3 mm temporal to the fovea and decreased nasally, reaching a minimum 3 mm nasal to the fovea. Regression analysis suggested that subfoveal CT decreased by 1.27 Ám per year of age and by 8.7 Ám per D of myopia.

The age-related rate of choroidal thinning in myopic eyes is similar to the rate of choroidal thinning reported in normal eyes (1.56 Ám/year). Choroidal blood flow is the highest of any tissue in the body to satisfy the normal metabolic demands of the outer retina. However, as choroidal vasculature is lost with older age, choroidal perfusion may decrease as well, resulting in a level of oxygen and nutrients delivery to the outer retina that falls below a minimum that is required for normal visual function.

The age-dependent decrease in choroidal thickness described in this report suggests a role for choroidal atrophy in the pathogenesis of visual dysfunction in high myopia. The findings in this study suggest that in addition to undergoing choroidal thinning resulting from progressive stretching from increasing axial elongation, highly myopic eyes also experience the same age-related choroidal attenuation that affect normal eyes. The relatively thin choroids seen in younger myopes may be physiologically sufficient, even though if the same CT was found in an eye without myopia, such a condition may be considered pathologic. However, as the choroid undergoes age-related attenuation, the available supply may not be sufficient to support the outer retina, the RPE, and even the choroid itself. At some point, the thickness of the choroid seems to make a transition from myopic choroidal thinning to frank myopic choroidal atrophy. The transition point has not been defined at present.

This study demonstrated that the choroid is thin in highly myopic eyes and undergoes further attenuation with increasing age and increased myopia. These findings suggest that choroidal thinning may play a role in the pathophysiologic features of vision loss in high myopia, just as a relative ischemia of the outer retina, RPE, and the choroid itself may lead to pathologic decompensation. It seems likely that after the supply offered by the choroid is reduced to very lowlevels, ophthalmoscopically evident changes, such as broad areas of depigmentation and what is termed myopic atrophy or myopic degeneration, may become visible. Studies of choroidal circulation are needed to correlate CT with perfusion, which may guide future therapies aimed at increasing oxygen and nutrient delivery to the choroid and outer retina.