In what may be a turning point in glaucoma research, scientists have determined that the disease, the leading cause of irreversible blindness, shows up first in the brain, not the eye.
The finding, published in the 1 March edition of Proceedings of the National Academy of Science, ties it to other neurodegenerative disorders such as Alzheimer's and Parkinson's diseases.
Headed by Dr David Calkins, director of research at Vanderbilt University's Eye Institute, the team made the discovery after injecting glaucoma-afflicted rodents with a special fluorescent dye that illuminated sections of the middle of the brain where the optic nerve forms its first connections.
They found that the first signs of the disease were not, as expected, in the retina. Instead, it turned that out the earliest damage was at the other end of the optic nerve, in the mid-brain, which lost its ability to receive information from optic nerve fibers.
The optic nerve is a cable that connects the retina - the light-sensitive tissue lining the inner surface of the eye - with the brain.
"It's a very interesting study," says Dr Darrell WuDunn, residency program director of the Department of Ophthalmology at Indiana University School of Medicine. "It does have potentially profound implications for treatment, and even diagnosis, of glaucoma, if it holds true for humans."
Conventional thinking is that glaucoma is a disease where the optic nerve gets damaged right where it enters the eye.
"This study shows that the deficits start in the brain, not the eye," says WuDunn.
"We feel the results ... are changing the ways people think about glaucoma," says Thomas Brunner, president and chief executive officer of the privately funded Glaucoma Research Foundation, which supports Calkins' work.
Current methods to detect glaucoma include testing for peripheral vision loss and looking for changes in the pressure of the eye.
"Our technology right now is limited in how early we can detect glaucoma. We can only detect some structural changes, but not very early," says WuDunn.
Not all cases of glaucoma begin with pressure changes in the eye, says Calkins. "It's an insidious disease."
Glaucoma is strongly associated with aging and is more prevalent among some ethnicities, including African-Americans and Hispanics. The disease is predicted to afflict about 80 million people worldwide by 2020, according to the US National Eye Institute.