Optic Nerve Degeneration and Glaucoma

What Causes Optic Nerve Degeneration?

This is a difficult question to answer completely, as there are many different factors that contribute to the process. One major risk factor is eye pressure. However, it is possible to have elevated eye pressure or “ocular hypertension” and show no changes of optic nerve degeneration (although one would need to be monitored over time for signs of degeneration), and conversely it is possible to have normal eye pressure and significant optic nerve degeneration. What we do know is that there is likely mechanical damage at the site where the optic nerve inserts into the back of the eye. This is because the axons of the optic nerve leave the eye by passing through a structure called the “lamina cribrosa,” which is a mesh-like structure composed of pores or holes through which the axon bundles must cross. It is thought that the site of initial damage may be to the axons as they pass through the lamina cribrosa. Another contributing factor may be blood flow to the optic nerve. Some believe that in cases of normal-tension glaucoma, impaired blood flow to the optic nerve may be playing a more important role in the degenerative process.

Protecting the Optic Nerve (Neuroprotection)

The current treatments for the degenerative process of glaucoma, whether medical or surgical, are to lower eye pressure. This is achieved using eye drops, laser surgery, or incisional eye surgery. While lowering eye pressure has been proven to slow visual field loss, the unfortunate reality is that some patients whose pressures have been lowered still continue to show signs of worsening. Therefore, there is much attention paid to developing novel treatments that slow or halt optic nerve degeneration, also called neuroprotection. One of the currently available treatments that may have neuroprotective effects is the eye drop Alphagan (brimonidine), which is used to lower eye pressure in glaucoma patients. A recent randomized control trial compared patients with normal pressure glaucoma who received either timolol (another eye drop commonly used to treat glaucoma) or brimonidine.

Despite the fact that both groups had similar eye pressure lowering, patients receiving brimonidine had visual fields that did not deteriorate as much as the visual fields of patients receiving timolol. However, there are a few caveats in interpreting this study, including the fact that more patients dropped out of the study in the brimonidine group as compared to the timolol group. Certainly more studies are needed on neuroprotective treatments that demonstrate that vision is truly preserved, retinal ganglion cells survive, and optic nerve degeneration is halted.