Glaucoma: How can screening tests help?

Doctors screen for glaucoma using a combination of tests that are also used to diagnose eye problems:

• In a test known as ophthalmoscopy, the eye doctor uses a special instrument called an ophthalmoscope to look into your eye. This makes it possible to detect optic nerve damage.
• Intraocular pressure (the pressure in your eye) is measured using a technique called tonometry. Here the eye is first numbed with a drop of liquid. The doctor then presses a small probe against the cornea to measure the pressure inside the eye.
• A slit lamp is used to examine the front part of the eye. This is done to see whether the fluid in the eye (called aqueous humor) can drain out of the eye freely.

These tests don't take very long, and they carry little risk and aren't painful. If your doctor believes you have glaucoma, he or she can also measure your field of vision. This is done to see whether there are any gaps in your field of vision and whether blind spots have already developed.

In Germany, statutory health insurers only pay for these tests if:

• you have typical glaucoma symptoms, such as gaps in your field of vision or trouble seeing,
• you regularly use steroids (such as corticosteroids) to treat another medical condition,
• diabetes has caused eye damage,
• a problem in the front part of your eye is preventing aqueous humor from draining out,
• you have eye injuries that make it necessary to rule out glaucoma,
• you have planned eye surgery, and need to rule out the possibility of glaucoma before or after the procedure,
• the back of your eye has changed and/or your intraocular pressure is too high.

If none of these apply to you, glaucoma screening will usually be classified as an individual healthcare service in Germany (individuelle Gesundheitsleistung, or IGeL for short) and you will have to pay for it yourself. Eye doctors usually offer this screening test for about 30 to 50 euros. Glaucoma has now become the most commonly individual healthcare service that doctors offer.

Over the last few years, different groups of researchers have looked into the question of whether offering glaucoma screening to everyone over a certain age has any benefits. They did a search for studies on whether screening prevents loss of vision. They also wanted to find out how reliably the eye tests can tell whether someone has glaucoma or not.

But the searches for studies didn't turn up much: There's hardly any good-quality research on the benefits of screening for glaucoma in the general population. This means that it's not possible to say whether screening for glaucoma will have any advantages or disadvantages for people who don't have any symptoms or aren't at greater risk of developing glaucoma.

Screening will generally always have advantages and disadvantages, regardless of what is being screened for. A test may deliver the wrong results – for example, if someone is diagnosed with glaucoma after the screening test, although their eyes are actually healthy. This is called a "false positive" result. If people are wrongly diagnosed with a disease, they may take medication they don't need and the diagnosis might cause unnecessary distress. But it's difficult to draw any conclusions about the disadvantages of screening for glaucoma because there are no good studies in this area.

Some eye doctors only measure the pressure inside your eye when screening for glaucoma, but it's not advisable to have that done. Testing intraocular pressure isn't enough on its own to diagnose glaucoma or determine whether you have a higher risk of developing glaucoma. Some people have glaucoma even though they don't have high intraocular pressure. And others may have high intraocular pressure, but don't develop glaucoma.

In order to get an idea of whether you have glaucoma or a higher risk of developing it, other tests would have to be done too. These tests involve examining the optic nerve and the nerve fibers of the retina, as well as testing your field of vision. You may need more tests if your doctor thinks you might have glaucoma. A thorough examination would also include your doctor asking about your medical history, your life circumstances and symptoms.

Why is screening not covered by statutory health insurers in Germany if glaucoma can be treated when caught in time? The answer is: Just because a disease can be treated, it doesn't automatically mean that it's a good idea to have everyone tested. To decide whether screening makes sense, you need to know whether it actually has the advantages it is thought to have. And you have to know about the possible disadvantages too.

Experiences with other screening programs have shown that misdiagnosis is inevitable. This can make patients out of people who don't actually have the disease. They are then given further examinations that may be unnecessary or unpleasant. Or they may be given treatment even though they won't benefit from it and it could have side effects. So there's a risk of overdiagnosis and overtreatment.

These fundamental concerns might not apply to glaucoma screening. But that's precisely the problem: The actual advantages and disadvantages of glaucoma screening can't yet be assessed with any certainty, simply because no good-quality research is available.