Glaucoma is a common cause of vision loss and blindness worldwide. Why it occurs is not fully understood by scientists. Increased pressure within the eye is a leading risk factor. Over time, permanent damage to the fibers making up the optic nerve occurs and vision is slowly lost.
When I completed my ophthalmology training at the University of Pittsburgh in 1980, there were few tests that helped doctors diagnose glaucoma. More importantly, the tests that were available did a poor job at recognizing early damage to the optic nerve. The standard test for analysis of the health of the optic nerve is called perimetry. In the early 1900s, Dr. Jannik Bjerrum found a characteristic pattern of blindness in the vision of patients who had advanced glaucoma. This so-called arcuate scotoma arches over or under the central point of vision. With progression of glaucoma damage, the arc becomes bigger and, in extremis, vision is limited to a central island before being completely lost. Patients with glaucoma typically lose their peripheral vision over time. This also affects the quality of vision as sight in darkly lit places can be difficult.
In 1945, Hans Goldmann introduced the kinetic perimeter. Many patients know this as the white bowl-shaped device in which small dots of light are moved to plot the map or field of vision. In the early 1980s, the Humphrey and Octopus computerized visual field analyzers were introduced. Refinements of this technology has progressed over the decades and they remain a key component to the evaluation of the patient suspected of having glaucoma. But, there is more to the story.
The comprehensive evaluation of a patient suspected of having glaucoma begins with the time-honored medical history. Risk factors are explored: age; family (genetic) medical history; current medical history focusing on things like high blood pressure and blood vessel (circulatory) disease; treatment with steroids; history of sleep apnea or an episode of very low blood pressure; and social habits such as the use of tobacco.
The pressure within the eye is then determined. It is important to note that the pressure within the eye (intraocular pressure or IOP) varies day to day, hour to hour. The highest pressure tends to be in the very early morning hours at a time when the optic nerve may be most vulnerable. “Normal” IOP is said to range between 10 and 21 mm Hg (millimeters of mercury as used to measure barometric pressure). The median pressure is 16. However, recent studies have shown that 30% of glaucoma patients in this country have normal eye pressures. Glaucoma is NOT a number.
The specialist will then examine the anatomy of the front and back of the eye looking for clues as to what might cause optic nerve damage. Are brown pigment granules found on the iris and cornea? Is the anatomy of the front of the eye restricting the outflow of the fluid, the source of eye pressure? Is the configuration of the colored iris such that it prevents fluid from escaping the eye? Are “dandruff-like flakes” called exfoliation seen on the surface of the god-given lens? A technique called gonioscopy uses a special mirrored lens to look at the actual filtering membrane that permits fluid to exit the eye.
Other special lenses are used to examine the optic nerve. In most patients, the nerve is shaped like a doughnut (see the image above). The hole in the center is called the “cup” and allows retinal blood vessels entrance to the eye. The “dough” of the doughnut is the actual nerve fibers that transmit the light impulses from the eye to the brain for conversion into the miracle of sight. Each retina has about 1.25 million nerve fibers, all of which fit into the nerve that measures 1.5 millimeters across (about 6/100th of an inch!) A large cup, smaller amounts of “dough” (the optic nerve’s nerve tissue), an unusual shape of the nerve especially when the two eyes are compared, a small bruise or hemorrhage at the edge of the optic nerve increases the experienced examiner’s suspicion of trouble.
Now, two aspects of the patient’s eyes must be considered: structure and function. Testing is performed. The computerized version of Goldmann perimetry tests the function of the nerve fibers. A damaged nerve will show specific patterns of decreased or lost vision. One such pattern is the so-called Bjerrum or arcuate scotoma described above. However – and this is important – research has shown that 40% or more of an optic nerve can be damaged and lost before the field of vision test shows abnormalities. This is NOT a good test for early glaucoma detection.
Fortunately, a newer technology called optical coherence tomography (OCT) has been introduced and provides a computerized image of the structure of the optic nerve and, more importantly, its nerve fibers. This image of the nerve’s structure can then be followed over a series of such testings to identify subtle changes alerting the eye specialist to progressive damage. The OCT is the new gold standard for the evaluation and treatment of early glaucoma and increased pressure. It and high resolution images of the optic nerve produced by a digital fundus camera are the standard of care relating to the structure of the optic nerve.
If you have been diagnosed with glaucoma or are a glaucoma “suspect”, all components of the prior discussion are part of the management of your disease. The goal of management is to prevent further damage to the nerves of sight and subsequent vision loss. If any part of this regimen is missing on a consistent basis, one expert is quoted that you may be “seeing” your doctor for glaucoma but you are not being “managed”.
Next? The treatment options for glaucoma. Check back soon.