Is It Possible that Glaucoma Is a Brain Disease?
by Maureen Duffy
A group of American and Australian researchers is proposing that glaucoma may actually be a brain – not an eye – disease. Their research theorizes that it is the brain, not the eye, controlling the cellular process that results in glaucoma.
The study, entitled Refined Data Analysis Provides Clinical Evidence for Central Nervous System Control of Chronic Glaucomatous Neurodegeneration, was published in the May 2014 issue of Translational Vision Science & Technology. Published by the Association for Research in Vision and Ophthalmology (ARVO), Translational Vision Science & Technology is an online-only, peer-reviewed journal emphasizing multidisciplinary research bridging the gap between basic research and clinical care.
ARVO is an international organization that encourages and assists research, training, publication, and dissemination of knowledge in vision and ophthalmology, including low vision. Translational research helps to make findings from basic science useful for practical applications that enhance human health and well-being.
The authors are William E. Sponsel; Sylvia L. Groth; Nancy Satsangi; Ted Maddess; and Matthew A. Reilly, who represent the following institutions: the University of Texas at San Antonio; University of the Incarnate Word, San Antonio; Baptist Medical Center, San Antonio; the University of Texas Health Science Center-San Antonio; the Australian Research Council Centre of Excellence in Vision Science, Canberra; and the University of Minnesota Medical School, Minneapolis.
What Is Glaucoma?
The term "glaucoma" describes a group of eye diseases that can lead to blindness by damaging the optic nerve. It is one of the leading causes of vision loss and blindness. The human eye continuously produces a fluid, called the aqueous, that must drain from the eye to maintain healthy eye pressure.
Types of Glaucoma
In primary open-angle glaucoma, the most common type of glaucoma, the eye's drainage canals become blocked, and the fluid accumulation causes pressure to build within the eye. This pressure can cause damage to the optic nerve, which transmits information from the eye to the brain.
Normal-tension glaucoma, also called low-tension glaucoma, is a type of glaucoma in which individuals with the disease experience optic nerve damage and subsequent vision loss, despite having normal intraocular [i.e., within the eye] pressure (IOP).
Most eye care professionals define the range of normal IOP as between 10 and 21 mm Hg [i.e., millimeters of mercury, which is a pressure measurement]. Most persons with glaucoma have an IOP measurement of greater than 21 mm Hg; persons with normal-tension glaucoma, however, have an IOP measurement within the normal range.
Visual Field Loss
Glaucoma results in peripheral (or side) vision loss initially, and the effect as this field loss progresses can be like looking through a tube or into a narrow tunnel. This "tunnel vision" effect makes it difficult to walk without bumping into objects that are off to the side, near the head, or at foot level.
Glaucoma is an especially dangerous eye condition because most people do not experience any symptoms or early warning signs at the onset. Glaucoma can be treated, but it is not curable. The damage to the optic nerve from glaucoma cannot be reversed.
Glaucoma and the Brain: Might Glaucoma be a Brain Disease?
From Is glaucoma a brain disease? Scientists study the "jigsaw effect", via Science Codex:
Findings from the study … show the brain, not the eye, controls the cellular process that leads to glaucoma. The results may help develop treatments for one of the world's leading causes of irreversible blindness, as well as contribute to the development of future therapies for preserving brain function in other age-related disorders like Alzheimer's disease.
"Haphazard" Vision Loss?
In glaucoma, the loss of vision in each eye appears to be haphazard. Conversely, neural damage within the brain caused by strokes or tumors produces visual field loss that is almost identical for each eye, supporting the idea that the entire degenerative process in glaucoma must occur at random in the individual eye – without brain involvement.
The "Jigsaw" Effect
However, the team of investigators discovered … that when previously disabled optic nerve axons [i.e., nerve cells that conduct impulses] recover, the remaining areas of permanent visual loss in one eye coincide with the areas that can still see in the other eye. The team found that the visual field of the two eyes fit together like a jigsaw puzzle, resulting in much better vision with both eyes open than could possibly arise by chance.
"As age and other insults to ocular health take their toll on each eye, discrete bundles of the small axons within the larger optic nerve are sacrificed so the rest of the axons can continue to carry sight information to the brain," explains author William Eric Sponsel, M.D. "This quiet intentional sacrifice of some wires to save the rest, when there are decreasing resources to support them all, is analogous to pruning some of the limbs on a stressed fruit tree so the other branches can continue to bear healthy fruit."
According to the researchers, the cellular process used for pruning small optic nerve axons in glaucoma is "remarkably similar to the … mechanism that operates in the brains of people afflicted with Alzheimer's disease."
"The extent and statistical strength of the jigsaw effect in conserving the binocular [i.e., both eyes] visual field among the clinical population turned out to be remarkably strong," said Sponsel. "The entire phenomenon appears to be under the meticulous control of the brain."
"If the brain is actively trying to maintain the best binocular field, and not just producing the jigsaw effect accidentally, that would imply some neuro-protective substance is at work preventing unwanted pruning," said co-author Ted Maddess, Ph.D.
More about the Study from Translational Vision Science & Technology
From the article abstract:
Purpose: Refined data analysis was performed to assess binocular visual field conservation in patients with bilateral glaucomatous damage to determine whether unilateral visual field loss is random, anatomically symmetric, or nonrandom in relation to the fellow eye.
Methods: This was a case-control study of 47 consecutive patients with bilaterally severe glaucoma; each right eye visual field [location] was paired with randomly selected co-isopteric left eye [locations], with 760,000 [repetitions] performed per subject. [Note: An isopter is a line or curve representing areas of equal visual acuity/retinal sensitivity in the visual field.]
Results: A remarkable natural tendency for conservation of the binocular visual field was confirmed, far stronger than [explainable] by random chance or anatomic symmetry, and reaffirmed by subsequent … binocular visual field retesting of a [random] subset of the study population.
Conclusions: Refined data analysis of paired visual fields confirms the existence of a natural optimization of binocular visual function in severe bilateral glaucoma via interlocking fields that could be created only by central nervous system (CNS) involvement.
VisionAware will provide updates of this glaucoma research as they become available.
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