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Glaucoma causes damage to the optic nerve, which can result in an eventual loss of vision.
The optic nerve is a cable of nerve fibers that carry electrical impulses from the retina to the brain. A layer of cells on the retina, called retinal ganglion cells, is one end of this "cable." Photoreceptors in the retina convert light into electrical impulses, which are then transmitted to the retinal ganglion cells. The ganglion cells in turn transmit visual information along their axons to the visual centers of the brain. It is there that the electrical impulses are interpreted into sight. Damage to the optic nerve interrupts this flow of visual information.
In glaucoma, researchers believe that the ganglion cells of the retina, as well as the support cells within the optic nerve, can die for various reasons. Some possible reasons include excessive fluid pressure within the eye, lack of blood flow or oxygen, or naturally occurring toxins within the eye. With sufficient damage, sight is lost.
Damage to the optic nerve is irreversible because the cable of nerve fibers doesn’t have the capacity to regenerate, or heal itself, when damage occurs. This is why glaucoma is an incurable disease at this point, and why early detection is so important.
At the present time, no. Research is still in the initial stages of investigating the mechanisms of regeneration and how they can be stimulated. However, there is a lot of exciting work going on in this area.
Optic nerve regeneration is possible in some lower vertebrates. For example, in animals such as fish and frogs, an injured optic nerve regenerates fully, allowing for a complete restoration of vision. In mammals, it has been shown that retinal ganglion cells, when put under conditions found in the peripheral nervous system, can successfully regenerate their axons.
Researchers are studying the visual systems of these animals to find out what factors stimulate retinal ganglion cells to regrow their axons and restore the transmission of visual information to the brain.
This is only one piece of the puzzle, however. An important consideration in glaucoma is the health of the retinal ganglion cells during the course of the disease. Unlike cells in most other tissues, we have a fixed number of retinal ganglion cells that must last a lifetime. Once they die, they cannot be replaced. In glaucoma, it appears that a precisely controlled process of retinal ganglion cell death, called apoptosis, is activated during the course of the disease.
A key concern, then, is determining how and when retinal ganglion cell death is activated in glaucoma, and what steps can be taken to stop or prevent this from happening. A Catalyst Meeting sponsored by the Glaucoma Research Foundation discussed new hypotheses and research priorities that will lead to a better understanding of retinal ganglion cell death in glaucoma.
Article by Larry Benowitz, PhD, Director, Laboratories for Neuroscience Research in Neurosurgery and Professor of Neurobiology and Neurosurgery at Harvard Medical School, Children’s Hospital Department of Neurosurgery, Boston, MA.
Last reviewed on February 27, 2011