Secondary Bruch's membrane defects and scleral staphyloma in toxoplasmosis.

Abstract

Providing the structural basis for the retinal pigment epithelium (RPE), Bruch′s membrane (BM) plays a major role in maintaining the blood–retina barrier. It is formed by the basal membrane of the RPE, an elastic layer, collageneous layer in its middle, a second elastic layer, and the basal layer of the choriocapillaris. The thickness of BM ranges between 3 μm and 7 μm (Jonas et al. 2014). The physiological opening of BM at the level of the optic nerve head is the inner one of three opening layers of the optic nerve head. The choroidal opening forms the second layer, and the scleral opening the third of the openings. In some eyes, the optic nerve head-related opening of BM is not exactly aligned to the choroidal opening and scleral opening. It leads to an overhanging of BM at the nasal disc side, while on the temporal disc side, BM does not reach up to the disc border in these eyes. This BM-free zone at the temporal border optic disc has recently been called parapapillary gamma zone (Jonas et al. 2012). Recent studies have shown that BM can develop secondary defects in the macular region in eyes with high axial myopia (Jonas et al. 2013; Spaide 2014). The prevalence of these macular BM defects was strongly associated with longer axial length, with the prevalence of increasing beyond an axial length of 26.5 mm (Jonas et al. 2013). Here, we report on three patients who underwent spectral-domain optical coherence tomography (OCT) (Spectralis®, Heidelberg Engineering Co., Heidelberg, Germany) with enhanced depth imaging modality and who showed a defect in BM not related to axial length or axial myopia. A 21-year-old female emmetropic patient showed a retino-choroidal scar at a distance of about 2.5 mm to the superior disc border (Fig. 1A). Central visual acuity was 1.0, and axial length was 24.3 mm. The retino-choroidal scar, presumably due to a previous toxoplasmotic inflammation, showed multiple BM defects on optical coherence tomograms. Due to the collateral lack of RPE, the areas of the BM defects were associated with a marked window effect allowing a deeper penetration of the scanning laser and visualization of the sclera. The BM defects additionally showed a lack of retinal pigment epithelium, deep retinal layers, choriocapillaris and of most of Sattler′s layer and Haller′s layer of the remaining choroid. A 64-year-old female patient showed a toxoplasmotic retinochoroidal scar nasal inferior and nasal to the optic nerve head with contact to the optic disc border (Fig. 1B). Refractive error was +2.50 = −0.25/78° and visual acuity was 1.0. On the OCTs, a defect in BM could be detected with similar characteristics as described for the first patient. A 50-year-old male patient had a toxoplasmotic retinochoroidal scar in the macula with an adjacent localized scleral staphyloma (Fig. 1C). Best-corrected visual acuity with a refractive error of +1.25 = −1.25/169° was 0.01. In addition to the characteristics of the BM defects described above, this patient showed a marked localized scleral staphyloma (Fig. 1C). As also described for another single patient with retinal toxoplasmosis by Lujuan (2014), the clinical examples demonstrated in our study demonstrate that toxoplasmotic-like retino-choroidal scars can show localized defects in the BM. Such a BM defect was accompanied by a localized deep scleral staphyloma, if the BM defect was located at the posterior pole. Our report extends the list of causes for secondary BM defects and includes toxoplasmotic-like retino-choroidal scars in addition to high axial myopia, Stargardt disease (Park et al. 2012) and pseudoxanthoma elasticum (Spaide & Jonas 2015) as causes for BM defects. Our report also showed that a secondary BM at the posterior pole defect can be associated with a pronounced scleral staphyloma. It may raise the question whether the intactness of BM is necessary for the globelike shape of the eye and whether BM, instead of the sclera, is the primarily important coat to give the eye form and shape.