Medical Mondays 2

Dr. Pat likes to work as a collaborative physician. This week, during National Glaucoma Awareness Month, she calls upon the expertise of ophthalmologist Leila Rafla-Demetrious, M.D., who specializes in glaucoma. Here, Dr. Rafla-Demetrious provides a detailed overview of  “the sneak-thief of sight,” a symptomless disease that can cause devastating visual damage before a patient even knows she has it.

Dear Dr. Pat:

The articles on eye diseases that run on your site have been really interesting and informative. I never knew, however, that I would need to write to you about an eye condition I never knew I had.

I am 64 and still working as a proofreader full time, so my vision is my lifeline to employment as well as to daily living. I have had a long history of migraines with visual aura. I know I am going to have a migraine when part of my vision in my right eye goes dim in the lower half of what I am looking at. Generally, if I take my preventive medication right, I don’t get the migraine.

My primary care doctor told me that at my age, with this kind of aura, I should probably be seen once by an ophthalmology professor she knew at the medical school in my city. I have been seeing a perfectly capable optometrist for years, and have worn contacts for nearsightedness for decades. However, I made the appointment while I still have insurance that the doctor accepts, and had an evaluation.

It turns out that I have glaucoma, and the new specialist has recommended that I take two kinds of eye drops daily and see her again in three months. The good news is that I have only a small defect in my visual fields at this time. She said I might have to have surgery at some point, but she won’t know until she sees how I respond to this local treatment. I did some research on glaucoma, and I am still confused. Will it get worse if I don’t use these drops, which I don’t like to put into my eyes twice a day? Can glaucoma be cured?  Will the surgery she mentioned in passing cure the glaucoma?

Pamela

Dear Pamela:

We are so pleased to hear that you have found our medical articles helpful. As you may know, January is National Glaucoma Awareness Month, and we have already focused one article on “Glaucoma, the Sneak-Thief of Sight.” Two weeks ago, Dr. Leila Rafla-Demetrious, WVFC’s ophthalmology specialist, provided that informative response to a reader’s question concerning a worrisome delay in obtaining a referral to a glaucoma specialist.

Your primary care doctor has been very proactive in recognizing that you have some risk factors for developing glaucoma as you age and then sending you to a specialist before you had real damage. You have had migraines (and with a visual aura) for years, have myopia (nearsighted) requiring contacts for many years, and are growing older. Glaucoma cannot be cured, but in many cases there are ways to prevent it from worsening. That is why it is so important to have a baseline evaluation with sophisticated equipment used by someone well trained who can interpret it correctly. The age at which this should be done is based on your ethnic background (see below) and your your family and personal medical history.

Dr. Rafla-Demetrious, a member of Women’s Voices for Change’s Medical Advisory Board, will address your question here. Her extensive response covers what patients always need to know and often are not told—the anatomy of the eye (its many complicated parts, and how they work). She also discusses the different types of glaucoma and what treatments are available.

All primary care doctors need to arrange for proper age-appropriate screening for this very prevalent eye disease. Do thank your doctor for this great gift of thinking about your risk factors and arranging for screening before real damage was done. Glaucoma is a major cause of blindness, and yet many people know nothing about it.

Best,

Dr. Pat

Dr. Rafla-Demetrious Responds:

Dear Pamela:

Of the three main eye diseases that can occur with age—cataract, glaucoma, and macular degeneration—glaucoma is usually the subtlest. While cataracts cause a generalized and noticeable degradation in vision, and macular degeneration becomes obvious due to central visual distortion, glaucoma’s presence raises no such flags. It is called the “sneak-thief of sight” for good reason, as it can cause quite devastating vision loss even before it is discovered.

Risk Factors
Glaucoma is a disease in which the pressure of the eye is too high for the nerve in the eye, and tends to cause a gradual, painless loss of vision. This vision loss usually occurs first in one’s peripheral vision, and if unchecked can progress to involve central vision as well. It is estimated that up to 3 million Americans have glaucoma, though close to 50 percent may not know it. The prevalence in white Americans over the age of 40 is about 1.7 percent; in Hispanics over the age of 40, 2-4.76%; and in African Americans of any age, 3 times higher.  It is the leading cause of blindness in African Americans and Hispanics and among the top causes of blindness in whites and Asians.

Glaucoma’s prevalence only increases with age, and the risk of having it is 5-6 times greater in someone in her seventies than in her forties. General non-ocular risk factors other than older age and race include family history (parent, child, or sibling); prior eye trauma; diabetes; as well as lower-tiered risk factors, such as hypertension and ischemic vascular disease. Ocular risk factors include myopia, a thin cornea, and, of course, high intraocular pressure.

How Does Eye Pressure Do Its Damage?
So what is it about eye pressure that causes such a problem in our eyes, and where does this pressure come from? A brief lesson in ocular anatomy is necessary to make this clear.

(Fig 1). Simply speaking, the eye is a globe divided into two segments, posterior and anterior. The larger—posterior—segment occupies the back two-thirds, and includes the vitreous jelly, the retina, the choroid, and the optic nerve, which is like a cord leading from the brain to the back of the eye. It ends within the eye at the optic disc. The anterior segment includes the lens of the eye, which serves to focus light to the back of the eye, as well the iris (the colored circular part of the eye). An important fluid called the aqueous is made at a steady rate in the anterior segment, just behind the iris; it circulates up through the pupil and then drains out through an area between the peripheral iris and the cornea. This drainage area is known as the angle of the eye (fig 1a).

Essentially, glaucoma is when the fluid in the eye cannot exit properly through the angle, and pressure builds up within this closed system.  Even though most angles are anatomically open, malfunction is probably occurring at a molecular level. As pressure builds anteriorly, it is transmitted into the posterior segment, and damages the optic disc in a characteristic fashion, causing an enlargement and deepening of its central cup (fig 2). As the center cup enlarges, previously normal nerve fibers surrounding the cup are damaged, and this translates clinically and functionally into loss of peripheral vision.

fig 1. The globe of the eye

figure 1a. the angle of the eye

fig 2. Normal-appearing optic disc on the left. Note enlargement of the central cup on the right.

Why Does It Happen?
The theories behind how and why glaucoma occurs are many. They include actual mechanical damage, in which fragile nerve fibers are literally compressed by intraocular pressure; and vascular theories, in which there is a problem with blood flow to the optic nerve and its disc. Curiously, the damaging potential of intraocular pressure is relative, and even “normal” pressures can exert damage on the optic disc, depending on individual susceptibility.  While the national average of eye pressure is 16 +/- 2 mm Hg in the U.S., there is a very wide scale; anywhere between 12-22 mm Hg is traditionally considered normal. This is somewhat misleading, however, since many people with untreated glaucoma never even have documented high eye pressures.  In fact, there is a large subset of glaucoma known as low tension or normal tension glaucoma, which is responsible for 20-30 percent of all glaucoma in the U.S.  It tends to cause devastating field loss at even very low intraocular pressures, and is associated with vasospastic phenomena such as Raynaud’s syndrome and migraines. This finding further reinforces the fact that glaucoma is certainly not all about high eye pressure, but, more likely, the delicate balance between the pressure exerted on the optic disc and its easily compromised blood flow.

The diagnosis of glaucoma thus involves a gestalt of clinical measurements and examinations. Intraocular pressure, traditionally the gold standard and the parameter most easily tracked, is best measured at a slit lamp, though it can also by done by a portable tonopen if a person cannot be positioned correctly at the slit lamp or is otherwise unable to tolerate a slit lamp exam. The central corneal thickness is often documented at this time as well, as a thin cornea can lead to artificially low intraocular pressure readings, and needs to be taken into account when evaluating potential glaucoma.
Optic nerve exam is next, and the optic discs are studied critically; a clinician looks for signs of damage to the nerve, and may take color pictures and/or digital maps of the discs to document appearance and signs of damage. These pictures and maps can then be repeated periodically over years, to assess for evidence of progression of optic nerve damage, which is often too subtle to assess with certainty on intermittent clinical exams.

Lastly, the visual field test is the oft-dreaded, but oh-so-important, measure of optic nerve functionality. A patient is positioned in a specialized visual field machine (fig 3), facing a half-bowl apparatus.  One eye is tested at a time, and lights are flashed around the bowl in varying intensities.  These lights are adjusted according to patient responses, and serve to map out defects in peripheral or central vision.  Traditional visual field testing can be somewhat tedious if it drags on too long, and a good study inherently relies on a cooperative, awake, and reliable patient; newer modalities such as frequency-doubling testing are evolving to speed up the process, though are not yet in widespread use. Whatever the type, reliable automated visual field testing is essential to assessing the impact of optic nerve damage on one’s vision.  This is especially important since these defects tend to be subtle, at least early on in the course of the disease, and are almost never noticed by a patient until they are very advanced, or creep into the central vision (fig 4).

fig 3. Visual field machine

fig 4. Example of visual field test. Study on the right is a normal exam, with a normal physiologic blind spot. Study on the left shows a very dense superior and central defect.

You Have Glaucoma. What Now?
So, now you’ve been to your ophthalmologist, and she’s measured your eye pressures, scrutinized your optic nerves, and evaluated your visual field results.  After putting this information together, your doctor discusses the results with you, and says that you have glaucoma. What now?

While glaucoma is indeed a treatable disease, damage that has already occurred cannot be reversed. The goal of therapy is therefore to prevent further damage to the optic nerves, and to your visual fields. Generally, the initial treatment is with prescription eye drops, aimed at lowering the intraocular pressures to a level that should stop, or at least drastically slow down, glaucoma’s progression. A wide arsenal of eye drops exists, some of which decrease the production of aqueous fluid within the eye, and some of which speed up its outflow from the eye. (fig 5)  The common goal of each of these is to lower the intraocular pressure, and you may require more than one type working synergistically to achieve the goal pressure that your ophthalmologist is hoping to reach.

As with any type of medicine, each type of eye drop has its own benefit/side effects profile, and your ophthalmologist should discuss with you what type of drop would be best for you, based on anything from your concurrent medical issues to your eye color. For example, Timolol, a medicine that serves to decrease intraocular pressure by slowing aqueous production, can cause a flare in asthma symptoms in susceptible patients. Conversely, a prostaglandin analogue such as Xalatan, which increases fluid outflow from the eye, has very little systemic impact, but can cause red eyes, and can even darken one’s eye color.  As such, your ophthalmologist will make sure that your new medical regimen is right for you, and may need too change or adjust eye drops as time goes on.

There are also systemic medicines, such as Acetazolamide pills, that are sometimes prescribed to lower the intraocular pressure. While often effective, these tend to be poorly tolerated due to malaise and other side effects, and are usually used only as a temporizing measure before further steps can be taken.

fig 5  glaucoma eye drops

If Eye Drops Fail . . .
If eye drops fail to adequately control the intraocular pressures, or become difficult to take due to the need for multiple medicines and/or noxious side effects, your ophthalmologist may then discuss laser treatment.  Depending on the type of anatomic angle your eye has, you may be eligible for one of several types of lasers.  If your angles are open, your doctor may offer Argon laser trabeculoplasty (ALT), Selective laser trabeculoplasty (SLT) , or Micropulse laser trabeculoplasty (MLT). These lasers are in-office procedures, and are aimed at the angle of the eye. (fig 6) They are painless, and in promoting outflow of fluid from the eye, can be used in conjunction with glaucoma eye drops to further drop the intraocular pressure. If your angles are narrow, your doctor may need to do a laser iridotomy with a different type of laser. This procedure makes a very tiny hole in the iris, and can sometimes help open the angle; in doing so, it may drop the intraocular pressure, with or without the aid of additional glaucoma medicines. An iridotomy will also prevent a glaucoma attack, an uncommon but dramatic event, in which patients with pre-existing narrowing of the angles have an acute rise in intraocular pressure.

fig 6. Patient undergoing glaucoma laser treatment

Fig6!

Lastly, if eye drops, systemic medicines, and lasers do not work to stop the progression of glaucoma, your ophthalmologist will then discuss surgery.  Most glaucoma surgeries decrease the pressure in the eye by diverting the aqueous fluid into a surgically-created pocket, or into an implanted artificial chamber.  These surgeries, known as trabeculectomy and valve surgery, respectively, are usually effective, at least initially. They are not without risk, however, and do have a significant failure rate over time, which may eventually require restarting glaucoma drops to bring the eye pressure back down.  These surgeries also carry risks of infection, hypotony (too-low eye pressure) and loss of best-corrected vision, among other problems.

Newer surgical modalities are just starting or are in the pipeline. Your ophthalmic surgeon may be able to do an endolaser within the eye, aiming laser energy at the source of the aqueous production, in order to lower the intraocular pressure.  Alternatively, a procedure known as MIGS, or micro-incision glaucoma surgery, involves the implantation of a small stent, and can be done during concurrent cataract surgery to control the intraocular pressure in appropriate candidates. This technology is still evolving, though has promise in surgically controlling mild-moderate glaucoma, with a lower risk profile than those of traditional glaucoma surgeries.

In summary, glaucoma is a lifelong eye disease, which if unchecked, can lead to irreversible blindness. It is a quiet process, and a person must be proactive in seeking out proper eye care to look for it.  Once diagnosed with glaucoma, a patient will need to work in cooperation with her doctor, to come up with the best therapy tailored for her. The end goal of all glaucoma treatment is to prevent further visual loss and limit its impact on the all-important activities of one’s daily living.