GLAUCOMA—WHAT SHOULD MY DOCTOR MEASURE AND HOW OFTEN
Patients with elevated intra—ocular pressures are at risk to develop glaucoma. There are many types of glaucoma and just as many subtleties when it comes to making absolute statements about glaucoma. As a starting point we will only cover the most common type of glaucoma, chronic open angle glaucoma (COAG) and its warning sign or forerunner, known as ocular hypertension (OHT).
OHT is your ophthalmologist’s shorthand way of saying that your eye pressure is elevated (commonly accepted as 22 millimeters of mercury and higher) yet there is NO EVIDENCE of PROGRESSIVE DAMAGE TO YOUR OPTIC NERVE and NO CORRESPONDING LOSS OF VISION (initially side vision).
COAG means that not only is your pressure elevated beyond what is considered normal, but also some form of testing has demonstrated that there is either progressive physical erosion of your optic nerve and or loss of vision in a characteristic pattern associated with glaucoma.
These definitions can be dense, so let’s talk about the specific testing that your ophthalmologist should be doing and reviewing with you, as well as why they should be doing it repeatedly and with regular frequency. No message is more important than this. There are 3 types of tests you should be experiencing if your eye doctor says you are at risk for or have glaucoma.
#1: Intra—ocular pressure (IOP)
There are at least 2 ways to measure IOP. One instrument is a Goldmann applanation tonometer. You might recall this as a blue illuminated plastic tip attached to a large table mounted device known as a slit lamp biomicroscope. With the surface of your eye anesthetized this blue illuminated tip gently pushes on the eye’s surface. Alternatively, a hand held device (about the size of a cell phone), can be employed to gently push its tip on the ocular surface. It emits a beep when a valid pressure is obtained by the examiner. Finally, there is a method not commonly used in ophthalmology offices that blows a puff of air at the eye’s surface, known as an air puff tonometer. In the final analysis, all these devices measure IOP. Which one is used will depend on the examiner’s preference and your comfort with one method compared to another.
#2: Progressive optic nerve damage
The optic nerve is the cable along which vision travels as it connects the eye to the brain. Often your eye will be dilated to get the best look at your optic nerve. We know that an optic nerve that is being damaged by elevated IOP will eventually show physical changes in a characteristic pattern. You may have heard your ophthalmologist talk about “optic nerve cupping” or a measurement known as “cup-to-disc ratio”. Simply put, you can think of the front surface of your optic nerve as a donut. In glaucoma, with progressive optic nerve damage, the donut hole begins to enlarge from the inside going outwards. These changes occur slowly and can be very subtle. Until about 20 years ago the only useful and reproducible method of measuring optic nerve erosion was by taking color photographs of the optic nerve and comparing the recent photos to older ones. That technique still has a role in glaucoma evaluation. There are now devices that actually measure the volume of the optic nerve (HRT) or the thickness of the many fibers that combine as they approach the optic nerve (OCT). Each method has its proponents. The important idea is that your ophthalmologist should periodically be employing one of these techniques to help him assess if there is ongoing damage and death of the cells and nerve fibers that make up your optic nerve.
#3 Visual Field Test-- Pressure numbers are of value, especially in assessing the likelihood that you may get optic nerve damage or whether or not the eye drops used to manage your pressure are effective in lowering your IOP. Images of your optic nerve convey the same kind of information to your doctor.
But what do you really care about? Not so much about numbers and images!
You care about loss of vision and neither of those other tests really measures your vision. Your ophthalmologist measures both your peripheral (side) vision and your central vision by a test known as a visual field test. This is a slightly tedious test in which each eye is separately tested with a series of targets of varying brightness and size that are shown to you, one at a time, usually on a white screen. Each time you indicate (usually by pressing a button) when you appreciate each target. The testing device records your answers of targets that you do and do not see. Its computer draws a “map” of your vision based on which target size and brightness you saw, and in what location you saw it. If you have damage to your vision from glaucoma (and not damage from any other variety of conditions and diseases) your map of vision will show very characteristic patterns known to occur in glaucoma. If your pressure and glaucoma are being well controlled, each time you take the test, year after year, the severity and size of any defects in your field of vision will not be changing—and that’s the goal of excellent treatment. In most eye care offices the visual field machine will be referred to as a Zeiss/Humphrey visual field machine. A similar device, no longer in such widespread use is called an Octopus visual field machine.
If your ophthalmologist believes you are at risk for developing glaucoma or if she is treating you for glaucoma she should be performing each of these 3 tests:
1. IOP—your pressure
2. HRT or OCT—a physical measurement of your optic nerve volume or its surrounding nerve layer thickness.
3. Visual Field Test—a map of peripheral and central vision of each eye
A minimum for most patients is a check of IOP at least every 6 months, a visual field test at least once every 12 months, and some assessment of your optic nerve health (possibly via HRT or OCT ) at least once every 12 months. These are not hard and fast rules but should help you appreciate the amount and type of data that your ophthalmologist requires if she is to take the best care in managing your OHT or COAG.
Martin R. Shapiro, MD