Glaucoma and Choroidal Thickness

Since the landmark study by Spaide and colleagues on the development of enhanced depth imaging by optical coherence tomography, an increasing number of studies have addressed choroidal thickness in normal eyes, factors associated with choroidal thickness in normal eyes, and associations of choroidal thickness with various retinal and retinochoroidal disorders.1-4 These studies have revealed that mean subfoveal choroidal thickness (SFCT) in normal eyes is approximately 250μm in a population with mean age of 65 years,5 that it shows huge variability ranging from values as thin as 8μm to as large as 854μm, that it decreases with age by 4µm per year of older age and by 15 µm per diopter of increasing myopia, and that it is associated with male gender, a deeper anterior chamber and a thicker lens.5 Clinical studies also showed that patients with central serous chorioretinopathy have a thickened SFCT in the affected eye as well as in the contralateral unaffected eye, and that patients with polypoidal vascular choroidopathy have increased SFCT in association with dilatation of large choroidal vessels.2,3 A recent study suggested that choroidal thickness is additionally influenced by brain pressure.6

With respect to choroidal thickness in angle-closure glaucoma, Arora and colleagues recently reported that choroidal thickness was significantly greater in the angle closure glaucoma group than in the open angle glaucoma group and normal subjects, with no significant difference between eyes with open angle glaucoma and normal subjects.7 Also after adjusting for age, axial length, intraocular pressure and central corneal thickness, choroidal thickness was significantly greater in the angle closure glaucoma group than either normal eyes or those with open angle glaucoma. Interestingly, the severity of glaucomatous optic nerve damage as measured by cup/disc ratio or visual field mean deviation was not significantly associated with choroidal thickness. In another investigation, Arora and coworkers reported a significant increase in choroidal thickness and a decrease in anterior chamber depth when water drinking test was performed in eyes with anterior chamber angle closure as compared to eyes with open anterior chamber angles.8 In another recent study on fellow eyes of 44 patients with unilateral acute primary angle closure, Zhou and colleagues observed that the unaffected fellow eyes had thicker choroid than a group of control eyes after adjusting for age, axial length and gender.9

In this issue of JOVR, Hosseini and colleagues presented a study on macular and peripapillary choroidal thickness in patients with perimetric glaucoma.10 They confirm previous investigations which also did not find any significant difference in SFCT between patients with open angle glaucoma and non-glaucomatous individuals11-13 (Jonas JB, Steinmetz P, Forster T, Schlichtenbrede FC, Harder B. Choroidal thickness in open-angle glaucoma. J Glaucoma 2014; in Press). Hosseini and colleagues extended the examinations into the peripapillary region in which, except for the temporal region, choroidal thickness did not differ between glaucomatous and control eyes. As also discussed by Hosseini et al, these findings may imply that choroidal blood circulation may not be markedly involved or affected in open angle glaucoma.

As in many situations, there are caveats. The most important layer for nourishment of the retinal pigment epithelium and the outer retina is the choriocapillaris, which makes up less than 20 µm (or less than 10%) of the whole choroid. The choriocapillaris is too thin and the contrast between it and neighboring structures is too weak to make it visible on OCT images. The results of the study by Hosseini et al therefore do not allow to conclude whether or not blood perfusion in the choriocapillaris is affected or primarily involved in the process of open angle glaucoma. This issue is also of importance for discussion on the pathogenesis of beta zone parapapillary atrophy. According to recent histomorphological studies and clinical investigations, one may differentiate between an alpha zone, characterized by the presence of Bruch´s membrane (which can be visualized on OCT images) and the presence of an irregularly structured retinal pigment epithelium; a beta zone is characterized by the presence of Bruch´s membrane but absence of retinal pigment epithelium, while a gamma zone is characterized by the absence of Bruch´s membrane (and therefore by the absence of retinal pigment epithelium).14-17 This newly defined beta zone is associated mostly with glaucoma but not with axial myopia, while gamma zone is associated mostly with axial myopia, but not with glaucoma. Since the retinal pigment epithelium depends for nourishment on the choriocapillaris and since glaucoma associated beta zone is characterized by loss of retinal pigment epithelium, it may be seductive to think of a primary insufficiency of the choriocapillaris in glaucomatous eyes, leading to loss of the retinal pigment epithelium. The study by Hosseini did not support this notion, however, due to limitation in the spatial resolution of the OCT technology, the results do neither contradict it.

In conclusion, the carefully planned and conducted study by Hosseini and colleagues highlights the possibility of imaging the choroid by OCT technology and shows the potential impact choroidal imaging may have on discussions on pathomechanisms of major eye diseases and on decision making in daily ophthalmic practice.