Diagnosis of glaucoma and its progression

The optic nerve is the cable that transmits the information from the eye to the brain which allows us to see. Glaucoma is damage to the optic nerve that typically occurs when the pressure in the eye is too high for one‘s optic nerve. The pressure level at which an optic nerve may develop damage may vary considerably from person to person. Eye pressure is a risk factor for glaucoma, and if the pressure is extremely high there is a high likelihood of having or developing glaucoma. However, high eye pressure alone does not usually determine if someone has glaucoma. Glaucoma is typically diagnosed by noting changes in the structure and function of the optic nerve.

Glaucoma is most often a silent disease until it is very advanced, meaning that the patient doesn‘t notice a problem. Damage usually occurs over a period of years. The damage that develops from glaucoma is permanent and irreversible. The optic nerve has a certain reserve of tissue and may function well until a good portion of the nerve tissue has been damaged. Once functional damage sets in, it typically begins as peripheral vision defects which are not often noticeable to the patient until severe loss of peripheral vision or central vision loss has occurred. One eye can often make up for a deficit in the other eye, making it less likely the patient will notice damage. Glaucoma may be harder to treat in later stages, and there is less reserve left to work with. Thus, it is important to diagnose glaucoma early on, well before the damage is noticed by the patient.

Structural damage often occurs first and can be detected before functional damage. There are two ways to diagnose structural damage: change can be detected when compared to previous photographs or measurements, or damage may be evident immediately when comparing to the expected normal structure of the nerve. A baseline photograph may later allow definitive diagnosis of glaucoma if the optic nerve develops visible damage compared with the initial photo. Comparing a repeat photo to an initial photo may be preferred to minimize the effect of photo quality on the nerve’s appearance. Other possible indicators of damage like hemorrhages and progressive atrophy around the nerve may be better picked up with a photo. Many patients have suspicious features or risk factors for glaucoma, and good baseline studies are performed to help pick up change at an early stage, if it occurs.

Newer technologies may also provide precise measurements of nerve structure. As with any measurement, with these tests, careful attention to the quality and reliability of the test is important. HRT, or Heidelberg retinal tomography, provides a topographic map, like an elevation map, of the optic nerve. It may be used to compare the dimensions of the optic nerve tissue to age-matched normals. HRT can diagnose change or progression in an automated fashion.

OCT, or optical coherence tomography, provides almost an “optical biopsy” of the optic nerve and nerve tissue, as the level of detail is incredibly high. The layer of optic nerve tissue can be isolated automatically and precisely measured in a brief second, and then compared to age matched normal eyes. Progression analysis is relatively new but holds promise with the newer technology machines. The information from OCT and HRT are somewhat different and may complement each other. GDx is a similar but less commonly used technology to OCT in measuring nerve tissue (OCT is more versatile).

Study of the function of the optic nerve is also extremely important. A definitive diagnosis of glaucoma typically includes some functional deficit, usually demonstrated on an automated visual field test. Early glaucoma may be diagnosed more effectively, and sometimes exclusively, by structural changes. As glaucoma damage becomes more advanced, change in the visual field becomes more important in diagnosing progression or worsening of the glaucoma. In late stages of the diseases, visual fields may be the only reliable way to tell if glaucoma is stable or not. Visual fields show actual changes in vision due to glaucoma, which is ultimately what the doctor and patient are concerned about. In severe cases the size of the targets on the visual field test may be increased, and the area tested may be reduced, in order to focus on the remaining vision and better detect change.

Special visual field tests may pick up earlier damage to the function of the optic nerve. Certain features of vision such as color and motion are only served by a minority of nerve fibers, and so damage may be detectable earlier by testing these systems, which have less redundancy. A blue/yellow visual field specifically tests nerve fibers that respond to these colors. Frequency doubling technology, or FDT, tests ability to detect motion by using a patterned target that rapidly reverses itself.

Electrophysiology holds promise in diagnosing glaucoma as well. This is the study of electrical activity of the nerve pathway that supplies the vision. One advantage of this type of test is that it does not depend on the patient’s responses; however, concentration is still important. Pattern electroretinography (PERG) has been shown to indicate optic nerve dysfunction in several studies. It is possible that dysfunction may be discovered earlier with PERG than with other measures of function.

None of the tests for glaucoma are definitive by themselves. There is uncertainty in any measurement, and tests are not always completely reliable. Different tests compliment each other and help us to diagnose and treat glaucoma and its progression. Having more data from various different tests can increase the validity of the diagnosis. A complete evaluation is always important to rule out other problems.

Will I go Blind from Glaucoma?

Glaucoma is a disease where the eye pressure is typically too high, leading to damage to the optic nerve. The optic nerve supplies all of the vision for the eye, and any damage that occurs is typically irreversible. The pressure level at which damage occurs is variable from person to person. There are many different types of glaucoma. The differences between types of glaucoma usually relate to the cause of increased pressure in the eye. Primary open angle glaucoma is the most common type of glaucoma in Western countries, and this basically means that there is no particular reason that can be seen to explain elevation in eye pressure. Most ophthalmologists consider glaucoma that develops with eye pressures in the statistically normal range “normal tension” or “low tension” glaucoma, but this could be considered a subtype of “primary open angle glaucoma”.

For most people, glaucoma is a slowly progressive process that takes years to cause serious damage to the vision. There is no “cure” for glaucoma, but the majority of people with glaucoma can be controlled with medications. Laser may be helpful in some cases as a relatively easy and low-risk office procedure. A significant minority of patients will require surgery. These patients are at risk for losing vision, but many of them will maintain their vision as well with successful treatment.

There are certain risk factors for losing vision from glaucoma. Being diagnosed early in the course of the disease can be helpful. Glaucoma is often a silent disease, with no symptoms until late in the course of the disease. Fifty percent of people with glaucoma even in developed countries may be undiagnosed. Patients who already have more advanced disease may tend to be more difficult to control. With less reserve left in the optic nerve, there is less of a chance of maintaining the vision long-term. Patients who don’t follow their doctor’s recommendations for treatment and follow up put themselves at risk for losing vision. Patients who do research about their condition on their own seem to understand their disease better and comply better with treatment.

Certain types of glaucoma are more likely to lead to blindness. Glaucomas related to blood vessel problems (diabetes, a stroke in the retina) can be quite severe. Glaucoma related to inflammation in the eye, or glaucoma occurring after certain operations can be tougher to control. Angle closure glaucoma tends to be more severe and harder to control, and surgery carries a higher risk of complications. Glaucoma associated with higher pressure and greater pressure fluctuation tends to be more aggressive. Pseudoexfoliation glaucoma is characterized by dandruff-like flecks in the eye, and is an aggressive type of glaucoma, often with high pressures and wide pressure fluctuation. “Low tension” or “normal tension” glaucoma, on the other hand, may be slowly progressive, and in some cases may not even get worse over time.

The only well-proven treatment for glaucoma currently involves lowering the eye pressure. Studies show that about a one-third decrease in the eye pressure, and pressure in the low or mid teens with treatment, can stop progression in the majority of cases. However, pressure fluctuates, both at and between office visits. Different people may get worse at different pressures. Lowering eye pressure to an ideal level may not always be risk free. Pressure control and optic nerve damage may worsen over time. It is important that glaucoma patients follow up with careful regular monitoring of the eye pressure and the structure and function of the optic nerve to see if further treatment is necessary, and to minimize any damage from uncontrolled glaucoma. With modern diagnostic techniques, medications, laser and surgery, the risk of going blind from glaucoma has decreased significantly.

Narrow angle and glaucoma

The “angle” in the eye is the area where fluid drains out of the eye; it is also called the “drainage angle”. Fluid is constantly produced inside the eye. This fluid must be constantly drained or the eye pressure will rise. This may occur if the drainage angle closes. High eye pressure can damage the optic nerve, causing permanent loss of vision. If the eye pressure goes up rapidly and reaches a very high level, pain, blurred vision, halos, redness, and even nausea and vomiting can result. (Aside: the drainage of tears occurs outside of the eye and is not directly related to this internal drainage system of the eye.)

To understand angle closure, it’s essential to picture the angle structures. The back wall of the angle is made up of the iris. The iris is the blue, green, or brown-colored ring inside the eye that outlines the pupil. The front wall of the angle is made up of the clear cap on the front of the eye, called the cornea. These two structures meet at the drainage angle, which goes all the way around the eye. Looking directly at someone, the angle occupies a circle surrounding the iris.

In most eyes, the drainage angle is wide open. The more common forms of glaucoma in Western countries do not involve angle closure. Instead, there is microscopic blockage or damage inside the tissues of the drainage angle which cannot be seen by examining the eye. These are called “open angle glaucomas” to contrast them with angle closure.

The most common form of angle closure is called primary angle closure, and is discussed below. However, there are many other conditions that lead to angle closure. Conditions that cause scarring in the drainage angle, due to formation of membranes or abnormal blood vessels, may lead to angle closure, and often require surgery. As we get older, the natural lens inside the eye grows and can push the iris more forward, making the angle more narrow. The tissue behind the iris, called the ciliary body, can push the iris forward, making the angle narrow. Swelling or bleeding in the back of the eye and abnormal flow of fluid through the eye can also push the iris forward. After eye surgeries, more rare, complex forms of angle closure may also develop; these situations often require a careful evaluation and treatment by a specialist. These and other factors may all contribute to angle closure to a varying extent, and are often lumped together as “secondary angle closure”.

The principal problem in most primary angle closure is called pupillary block. This occurs when fluid is trapped behind the iris, pushing it forward. In severe cases this can close off the drainage angle. It’s hard to tell if pupillary block is present, and it probably contributes to at least 2 out of 3 cases of angle closure. Often times the first step in treating angle closure is to treat any pupillary block. If an eye is even felt to be at high risk for angle closure, pupillary block may also be treated to reduce the risk of this problem occurring.

Pupillary block is treated by making a small hole in the iris with a laser, called an iridotomy. This allows any fluid trapped behind the iris to flow freely into the front of the eye, relieving any pent-up pressure behind the iris. The iris, which was billowing forward like a sail in the wind, may then fall back and open the drainage angle, usually immediately.

The laser vaporizes the iris and causes inflammation, which can cause (usually temporary) discomfort and/or rise in the eye pressure. Occasionally a line of light may be noticeable by the patient from the light entering the opening, but it’s less common with current techniques. Retina problems are rare.

There is a vocal minority of patients who have posted serious complaints about problems they have had after the laser on the internet. In reality, serious or lasting complications are rare, and laser iridotomy is a very common procedure. Since the procedure is not risk-free, however, not everyone at risk for angle closure may get the laser. It is controversial who should have a laser iridotomy, since only a minority of patients with narrow angles develop angle closure, and other than patients who already have signs of angle closure, we can’t predict who will have a problem.

Even when pupil block is relieved, other factors, mentioned above, may keep the angle narrow. Unless significant scarring has set in, another type of laser treatment called iridoplasty can be used to open the angle. Iridoplasty involves heat-shrinking the iris where it meets the angle in order to widen the angle. Removal of a cataract may also be very effective in opening the drainage angle. Once the angle is sufficiently scarred, a new drain may have to be surgically created to lower the eye pressure. Thus, angle closure is best prevented or treated early.

Trabeculectomy, ExPRESS shunt, and Tube shunt

The initial treatment for glaucoma typically includes eye drops and laser, which are generally associated with minimal risk. Although most glaucoma can be controlled in this way, a significant minority will require surgery for glaucoma. There are newer, “minimally invasive” procedures for treating early or mild glaucoma, involving enhancing the natural drainage systems. Once glaucoma is severe or advanced and uncontrolled, typically a drainage or filtration surgery is required to construct a new drainage system for the eye.

After surgery, the fluid drains to a pocket under the surface of the eye, called a “bleb”, then returns to the blood stream. This often allows for the possibility of lower pressures, but there is some risk of the pressure becoming too low. Low pressure and other problems sometimes require further surgery. These types of surgery involve sutures on the eye surface and frequent eye drops to control inflammation, all of which can cause irritation and blurry vision for several weeks. Frequent visits may be necessary to monitor and alter the healing process to maximize results and prevent sight-threatening complications. Scarring around the drainage site must be just enough to ensure the pressure ends up in a safe range once the eye heals. Excess scarring can result in pressure too high, and not enough scarring can lead to low pressure problems, which can affect vision. Most of the healing seems to occur around the first few months after surgery. Different eyes tend to heal differently, but darkly pigmented races, younger age, inflammation and prior surgery are risk factors for scar formation.

The traditional, and still most common, drainage operation is known as a trabeculectomy. This involves creating a flap under which the fluid drains out of the eye. Potent anti-scarring medication is usually applied at the time of surgery to prevent excess scarring. The pressure may be adjusted during surgery using sutures that may be cut later using a laser in the office. The ExPRESS shunt is a modification of trabeculectomy using a small implant which sits under the flap and drains the fluid, perhaps helping to customize the surgery somewhat. Some report easier recovery and less postoperative care with the ExPRESS. Advantages of trabeculectomy with or without ExPRESS shunt may include more control over the amount of fluid drainage, and more options for further surgery if needed later. Drawbacks include long term infection risk through the bleb, which sometimes develops a thin and leaky wall that can allow entry of bacteria. Also, postoperative care is often critical to achieving a good long term result. Sometimes the trabeculectomy can be revised later to improve drainage, called a “needling”.

Tube shunt surgery has steadily increased in popularity in recent years. The surgery involves placement of a long plastic tube connecting to a plastic plate implanted way back under the eyelids. The bleb forms over this plastic plate, and tends to be thicker and not prone to infection. The tube is covered by a patch of biologic material for protection. The surgery is inherently more standardized than trabeculectomy, but results may be no more predictable, since healing varies significantly between different eyes and patients. The Ahmed tube comes equipped with a valve to prevent excessive drainage, allowing for immediate drainage through the tube. Nonvalved implants such as the Baerveldt or Molteno require the tube to be tied off for several weeks, often with a dissolvable suture, to allow for a capsule to form around the plate to prevent low pressure.

These larger implants can offer better long term pressure control. Different implants could be better for individual patients. Tubes may have specific advantages for certain types of glaucoma, and with issues of scarring, and may be a good choice once trabeculectomy has failed. Disadvantages include a small risk of double vision, cornea problems, and lack of ability to titrate the flow through the tube.

All of these types of surgeries have a similarly good track record of success, with long term success rates in the 70-80% range. A recent large well-conducted study showed higher 5-year success rates in patients with the Baerveldt tube shunt compared with trabeculectomy in a large group of patients with history of prior eye surgeries, many who had significant scarring or failed trabeculectomy. An ongoing study compares trabeculectomy with the Baerveldt as a first-time glaucoma operation.

Many factors determine which procedure is best for a given patient, including other eye diseases, ability to follow up, age and general health. Different surgeons may get better results with one surgery than another under specific circumstances. No one surgery is clearly superior to another for all patients.