Glaucoma is defined as an optic neuropathy, primarily induced by an intraocular pressure (IOP) that is higher than the optic nerve can tolerate. But what determines the pressure the optic nerve can tolerate? Seemingly, the answer is that the pressure a given optic nerve can tolerate is related to the eye’s biomechanics, local optic nerve anatomy, optic nerve perfusion, and finally the cerebral spinal fluid (CSF) pressure. The CSF pressure serves as the back pressure on the optic disc, and a growing body of evidence shows that eyes with a lower CSF pressure are more susceptible to glaucoma. One could imagine that without CSF providing a counterpressure across the lamina cribrosa, even a normal IOP could stress the optic nerve and lamina, causing damage. Measuring the CSF pressure is not easy. Currently, the most practical way to measure CSF pressure is with an invasive medical procedure (lumbar puncture) that involves placing a needle into the spinal canal between the lumbar vertebrae. This is akin to performing manometry through a corneal paracentesis to measure IOP. It is safe to say that we have an unmet need in the management of glaucoma (and probably many other neurologic diseases) where we would like to be able to measure the CSF pressure in a noninvasive manner.
A recent publication in JAMA Ophthalmology may illuminate the first step toward noninvasively assessing CSF pressure noninvasively with optic nerve optical coherence tomography (OCT). The authors used OCT of the optic nerve to assess for elevated CSF levels in children. The study showed that average retinal nerve fiber thickness levels showed a moderate sensitivity (89%) and specificity (62%) for detecting pathologically elevated CSF values. In glaucoma, nerve damage may develop in a patient with a low-normal CSF pressure, and measuring the difference between CSF pressure and IOP may eventually become important. Progress with OCT in the measurement of CSF is a step in the right direction—but will we ever be able to accurately and noninvasively measure CSF pressure? The glaucoma-managing optimist in me says that we can and must be able to do so.