Researchers at the University of Toronto and St. Michael's Hospital (SMH) believe there's more to glaucoma than meets the eye -- they have discovered that the disease associated with blindness affects not only the eyes but the entire visual system, including the brain.
Current treatments for glaucoma -- a degenerative disease believed to be caused by the death of nerve cells at the back of the eye -- rely on eye drops or surgery to lower ocular pressure either by suppressing the formation of fluid or improving drainage from the eye.
"Our study shows for the first time that in glaucoma there is also a loss of specific nerve cells in the brain which control our ability to see colour and motion," says Dr. Yeni Yucel, a neuropathologist in U of T's Faculty of Medicine and lead author of the study which appears in the current issue of Archives of Ophthalmology. "The concept that glaucoma is a neurodegenerative disease affecting also the major vision centres in the brain is a major breakthrough in the understanding of this disease."
The nerve cells in the brain, or target neurons, receive signals from the nerve cells in the eye via the optic nerve and in return the nerve cells in the brain supply essential growth factors to the eye. "Good vision depends on a healthy eye with healthy connections to the brain," says study co-author Dr. Neeru Gupta, director of the glaucoma unit at SMH and assistant professor of ophthalmology at U of T. "This discovery really gives patients new hope because we can now focus on additional innovative treatment strategies similar to those aimed at Alzheimer's and Parkinson's patients."
Using three-dimensional computer reconstruction, the team of researchers counted the total number of nerve cells in brain tissue samples of experimental glaucoma and healthy controls. They found that forty per cent of the nerve cells -- both in the eye and in the brain in the experimental group were destroyed by the glaucoma, with greater losses occurring in the moderate to advanced stages of the disease.
According to the researchers, future studies will be directed at understanding why these nerve cells die. "It's clear that if we want to develop strategies to prevent the death of nerve cells in the eye, we need to also take into account what's happening to the target nerve cells in the brain," adds Yucel, director of the ophthalmic pathology laboratory in U of T's department of ophthalmology and research scientist at SMH.
Glaucoma is a leading cause of blindness and affects 67 million people worldwide. Although the disease can affect anybody, the elderly, blacks and persons with a family member with glaucoma are at greatest risk. Other risk factors that may be associated with glaucoma include diabetes, high blood pressure and near-sightedness.
This study was an international collaboration led by U of T and SMH. Other co-authors include Dr. Qiang Zhang (U of T and SMH), Dr. Paul Kaufman (University of Wisconsin, Madison) and Dr. Robert Weinreb (University of California, San Diego). This research was supported by the Smith Barney Inc. Research Fund of the Glaucoma Foundation (New York, NY), the Glaucoma Research Society of Canada (Toronto, ON), the Foundation for Eye Research (Rancho Santa Fe, CA), the National Eye Institute (Bethesda, MD), Research to Prevent Blindness Inc. (New York, NY) and the Joseph Drown Foundation (Los Angeles, CA).
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Journal
Archives of Ophthalmology