The Relationship of the Clinical Disc Margin and Bruch's Membrane Opening in Normal and Glaucoma Subjects.

Glaucoma is a chronic optic neuropathy characterized by slow loss of the retinal ganglion cells (RGC) and their axons. Early damage to RGC axons is considered to occur at the level of the lamina cribrosa (LC), a multilayered connective tissue structure through which all the RGC axons pass when leaving the eye to form the optic nerve.1 Hence, studying the anatomy of the optic nerve head (ONH) and the peripapillary area is important for studying the mechanisms by which glaucoma damages these structures. Historically, the inner edge of the sclera, called the border tissue of Elschnig, has been presumed to form the clinical optic disc margin or outer boundary of the anterior neural canal. The complex relationships of the tissues bordering the neural canal now is better understood thanks to availability of in vivo imaging of the peripapillary region with spectral domain optical coherence tomography (SD-OCT). Strouthidis et al.2 recently proposed that the appearance of the clinical disc margin depended mainly on the colocation of the inner edge of the Bruch's membrane (BM) and the border tissue of Elschnig (BTE). The direction of the BTE has been shown to vary around the ONH and internally oblique, externally oblique, and nonoblique configurations have been well defined.3,4 The termination of the BM around the optic nerve forms the BM opening (BMO), which defines the anterior most boundary of the neural canal and commonly is the narrowest part of the neural canal. In monkeys, the clinical disc margin consistently localizes to BMO,3,5 while based on findings by Reis et al.,6 this is not the case in humans. In human eyes, BM overhang (i.e., extension of BM beyond the BTE) is common. The BMO area may better predict the number of ganglion cell axons in individual eyes or indicate the best location for centering SD-OCT's disc/retinal nerve fiber layer (RNFL) measurement cube,7 although it is likely that the narrowest region of the neural canal determines the axonal complement of a given eye. In addition, it has been demonstrated that neuroretinal rim area measurements based on the minimum distance between the inner edge of BMO and the internal limiting membrane (minimum rim width [MRW]) are superior to those based on clinical disc margin (horizontal rim width).8 Also, MRW measurements have been found to perform better or at least as well as peripapillary RNFL thickness measurements for discriminating glaucoma from normal eyes and demonstrate similar structure–function relationships compared to RNFL.8,9 To date, most clinical disc area measurements are based on identification of the clinical disc margin by experienced observers either on optic disc photographs or on images obtained with scanning laser ophthalmoscopy (such as the Heidelberg Retina Tomograph [HRT]). Some of the current SD-OCT machines have an automated algorithm for locating the inner edge of the BMO. Whether the relationship between the BMO and the clinical disc margin (CDM) is a function of the size of the optic disc is unknown; likewise, it is unclear whether BMO area can change over time in glaucoma. This study was performed to test the hypothesis that the anatomical relationship of the CDM and BMO is a function of the BMO area (BMOA). We also tested the hypothesis that the relationship between the clinical disc margin and BMO was different between glaucoma and normal eyes with less mismatch in glaucoma eyes.