Glaucoma The 'Sneak Thief' of Sight
Regular checkups necessary to stay ahead of this silent disease
Glaucoma refers to a group of eye diseases characterized by progressive damage to the optic nerve over time, leading to a very typical, predictable, and irreversible pattern of peripheral and central vision loss. This is usually but not always associated with increased internal eye pressure. Glaucoma results from impaired circulation of clear fluid (aqueous humor) within the eyes and may occur in isolation (primary glaucoma) or as the result of other eye diseases (secondary glaucoma).
Several large epidemiological studies have suggested that people with diabetes are at increased risk of developing ‘primary open angle glaucoma,' the most common form of the disease, while other studies have found no such relationship. Patients with severe diabetic retinopathy are definitely at increased risk for a rarer type of glaucoma caused by growth of abnormal blood vessels within the eye. Regardless, glaucoma is one of the leading causes of permanent vision loss and blindness in the U.S., and rarely causes symptoms until damage is quite advanced, so it makes sense for all of us to be regularly checked for development of this serious eye disease.
Aqueous humor is ultra-filtered blood, produced by a specialized gland within the eye (just behind the colored part of the eye – the iris), that serves to nourish transparent parts of the eye that aren't supplied by blood vessels (e.g. the cornea and lens). Aqueous humor normally flows through the pupil and then drains out of the eye and back into the blood stream through a circular drainage system (trabecular meshwork & Canal of Schlemm) located just in front of the iris (see figure 1). We are not aware of this flow (e.g. it does not lead to external ‘watering of the eyes'); rather it is, like circulation of blood throughout our bodies, continual and imperceptible.
Figure 1- normal flow of aqueous humor
Glaucoma occurs when this normal flow of aqueous humor is interrupted, either by a clogged drainage system (analogous to hair clogging a drain in your bathroom sink), what is called ‘open angle glaucoma,' or by an iris that bulges forward to mechanically block the drainage system (analogous to closing the drain cover of your sink), termed ‘narrow angle glaucoma.' When the eye's internal drainage system becomes blocked, fluid pressure inside the eye rises and this may crush some of the 1.5 million delicate fibers that make up the optic nerve at the back of the eye.
Because the optic nerve sends electrical signals to the brain that allow us to see, damage to a significant number of nerve fibers results in the progressive development of abnormal blind spots (visual field defects) and, if left untreated, blindness (see figure 2). Interestingly, some patients with relatively normal eye pressures develop glaucoma, whereas some patients with pressures above the ‘normal' range do not, so there is a range of individual tolerance to eye pressure that we have yet to fully understand.
Figure 2 – typical visual field of patient without glaucoma and with glaucoma
Though glaucoma affects between 1% and 2% of the population, certain groups are at higher risk including: African Americans; those with a family history of glaucoma, especially in siblings; people above age 50; people with higher internal eye pressures; people with thin corneas; people with a history of other eye diseases, especially uveitis (inflammation of the blood vessels within the eyes); and people who have diabetes. The fact that both glaucoma and diabetes are more common in African-Americans, as well as the fact that patients with diabetes are at higher risk for vision loss from diabetic retinopathy and are thus followed more regularly and closely by their eye doctors in general (what is termed ‘detection bias'), may help at least partly explain some association between glaucoma and diabetes.
Amazing new technologies now allow us to scan optic nerve fibers and detect damage much sooner than ever before, and research continues to give us more insight into diagnosing, treating and, hopefully, preventing glaucoma. Treatment of glaucoma is directed at lowering internal eye pressure with the use of (primarily eye drop) medicines, laser treatment of the eye's internal drainage system, and surgery. Newer treatments on the horizon include medicines to fortify the optic nerve and possibly regenerate damaged optic nerve fibers. In addition, genetic tests may one day allow us to predict which patients will develop glaucoma and, possibly, give preventive treatment years before this ‘sneak thief of sight' begins to rob patients of vision. For the time being, regular eye examinations are the best way to diagnose and treat glaucoma at its earliest stages – hopefully, long before patients have any symptoms whatsoever.
For more information on diabetic eye disease, consult Dr. Chous' book Diabetic Eye Disease: Lessons From a Diabetic Eye Doctor, Fairwood Press, Seattle, 2003.
Read more about Dr. Chous here.
Visit Dr. Chous' website here.
NOTE: The information is not intended to be a replacement or substitute for consultation with a qualified medical professional or for professional medical advice related to diabetes or another medical condition. Please contact your physician or medical professional with any questions and concerns about your medical condition.
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