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Evaluating pathologic changes in the brain during life has always been an indirect process at best, largely shielded from view by the barrier of the bony skull. Even MRI provides only a computer-analyzed picture of electromagnetic radiation from the nuclei of hydrogen atoms in the brain, excited by radiofrequency pulses in a magnetic field. In a sense, we are viewing the “ shadows thrown by firelight on the wall of the Cave” described by Plato,1 rather than the reality itself. Yet, part of the brain is directly visible: through the window of the eye, the optic nerve can be seen, and with modern techniques, the integrity of the nerve fibers and their area of origin from retinal ganglion cells can now be precisely visualized during life.
In 1850, Hermann von Helmholtz constructed the first “modern” ophthalmoscope capable of directly—if crudely—visualizing the retina and optic nerve. In 1972, William Hoyt et al.2 emphasized how permanent damage to the optic nerve could be identified in many patients using a simple, hand-held direct ophthalmoscope, allowing detection of early, asymptomatic damage from disorders such as glaucoma and permitting prevention of permanent visual loss. But ophthalmoscopy is a highly subjective technique, and more objective and precise ways to assess early retinal and optic nerve changes were needed. In recent years, several objective techniques for the evaluation of the optic disc and the peripapillary retinal nerve fiber layer (PRNFL) (the layer of axons from retinal ganglion cells …
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