Generally speaking, myopic eyes are different. They are longer, yes, but also shaped differently. The optic nerve tends to insert into the sclera at more of an angle, resulting in optic disc tilting and parapapillary atrophy. The retinal nerve fiber layer is also distributed differently around the optic nerve.
It is no surprise, then, that one of the major challenges the ophthalmologist faces is discriminating a normal, myopic eye from the myopic eye with glaucoma. Optical coherence tomography (OCT), which can measure optic nerve topography as well as retinal nerve fiber layer thickness, would seem to be the ideal instrument to sort this out. The optic nerve scan of the myopic eye is compared with the normal eye in the OCT database, and if a difference exists, glaucoma is diagnosed.
The problem is that for all of the commercially available OCT diagnostics, myopic eyes have been excluded from the normative data base (typically above 6 diopters of myopia). We do know that compared with emmetropic eyes, eyes with myopia have thinner retinal nerve fiber layer values, in part from signal attenuation and in part from anatomical differences. It has been suggested that the macula might be a good place to evaluate myopic eyes to avoid optic nerve anatomical artifacts. However, myopic anatomical differences can be as much of a problem in the macula as they can be around the optic nerve.
It is therefore pleasing to see a recent publication in JAMA Ophthalmology on a myopic eye normative data base for optical coherence tomography. This database allowed the comparison of apples to apples (myopic patients with suspected glaucoma to myopic patients without glaucoma), rather than apples to oranges (myopic patients with suspected glaucoma to emmetropic individuals). The study found that a greater degree of specificity could be obtained for glaucoma detection using the normative database from myopic eyes. While the database in its current form provides values for retinal nerve fiber layer thickness measurements, perhaps over time one could incorporate optic nerve topography and macular findings. In any case, this study represents a much needed evolution in OCT technology, one that allows us to tailor our diagnostic tests specifically to the patient under evaluation.