Glaucoma - Round 2: Myths/Facts & Testing
With the new year here our goal is not to simply get our patients seeing 20/20, but to provide them with the knowledge and means to maintain that vision beyond 2020. With January being the month of glaucoma awareness, I’m going to take this time to discuss some facts about the sight-threatening disease, as well as some of the common tests we can use to help us better diagnose and monitor it. (If you want to learn more about glaucoma, including different forms of it and risk factors, see Dr. Thirion’s post from January 28th, 2019).
The optic nerve plays a vital role in making vision possible. The nerve is made up of about one million microscopic nerve fibers. As these fibers enter the back of the eye, they form what is called the retinal nerve fiber layer (RNFL). In short, the RNFL allows communicating cells in the retina, called ganglion cells, to send visual information to the brain. Glaucoma is a disease that progressively destroys the RNFL and ganglion cells; consequently, the optic nerve is damaged. Since the optic nerve is the only bridge of communication the eye has towards the brain, damage to this pathway can lead to a gradual loss of vision.
Glaucoma Myths vs Facts
➤Myth: Glaucoma is a disease that only affects the elderly.
➤Fact: While the incidence of it is higher after the age of 40, glaucoma can occur at any age. This is one reason it is recommended that all infants have their first eye exam before the age of 1. Early signs of infantile glaucoma may include: excessive blinking, tearing and sensitivity to light. Enlargement of the eye(s) and corneal clouding can also occur in more advanced cases.
➤Myth: Being able to see 20/20 is a good indicator that I don’t have glaucoma.
➤Fact: Although glaucoma can ultimately lead to complete blindness, many people with mild or moderate glaucoma can still see 20/20. This is due to the fact that earlier stages of the disease generally affect the peripheral vision more significantly than the central vision. This is why early loss of vision is nearly impossible to detect without proper testing.
➤Myth: I cannot develop glaucoma if my eye pressures have always been normal.
➤Fact: While intraocular pressure (IOP) is an important measurement, it does not tell us the full story. Any eye can become damaged when the IOP is too high. A healthy IOP is generally considered to be between 10-21 mmHg. In other words, if an eye’s pressure is within this range, it is statistically less likely to develop glaucoma. However, glaucoma is a complex disease, the reality is that the IOP at which nerve damage first occurs is unique to every person and is largely determined by genetics, health status of the individual, age and other risk factors. While one eye may be able to tolerate an IOP of 23 mmHg without a problem, another eye may suffer damage with an otherwise “normal” pressure of 17 mmHg.
When glaucoma is suspected, it is important to establish several baseline measurements. Depending on the individual case, the eye doctor may choose to run a number of tests to ensure proper diagnosis and/or treatment. Below is a short description of some of the more commonly used tests.
Tonometry - An estimate of the intraocular pressure. There are numerous methods to obtain this measurement. Being that IOP is the main risk factor we can significantly reduce, this is a mainstay test for any glaucoma evaluation.
Pachymetry - Measures the thickness of the cornea. All forms of tonometry interact with the cornea in some way. The accuracy of the IOP measurement is based on the assumption that the cornea being tested is of average thickness. A thinner or thicker cornea will affect the accuracy of the estimated IOP.
Gonioscopy - Allows us to visualize whether or not the drainage system of the eye is open. This “drain” is often referred to as the trabecular meshwork and plays an important role in maintaining a balanced IOP.
Perimetry - A detailed test of peripheral vision sensitivity. This test can be very valuable in detailing the degree of vision loss.
Ophthalmoscopy - A lens is used to view the fundus (retina and optic nerve) of the eye. While it is an important test to screen for classic signs of optic nerve damage, it is not the most sensitive way to view subtle changes to the nerve or RNFL. Fundus photography if often used in addition to ophthalmoscopy, to further improve monitoring capabilities. Ganglion cells cannot be directly viewed using these methods.
Optical Coherence Tomography (OCT) - Non-invasive scan, which allows us to visualize and measure several ocular structures, including the thickness of the RNFL and ganglion cells. Since the OCT is capable of capturing microscopic changes to these structures, it is generally seen as a more reliable method of monitoring early glaucomatous thinning than more traditional methods, such as ophthalmoscopy. The OCT can also be used to measure pachymetry, and though it’s arguably not as accurate as gonioscopy, it can be used to estimate whether the trabecular meshwork is open.
Electrophysiology Testing - Similar to how an EKG can check heart function by testing it’s electrical signal, tests such as a visual evoked potential (VEP) or pattern electroretinogram (PERG) can be used to test the integrity of the RNFL or ganglion cells, respectively. Unlike other forms of glaucoma testing, which attempt to decipher how many cells have already died, electrophysiological tests can detect unhealthy cells before they’re lost. This theoretically allows for glaucoma changes to be caught at a much earlier point in the course of the disease. Since some still consider these types of tests somewhat experimental, the use of them in the clinical setting has not yet become as widespread as tests like the OCT, but it is likely they’ll become more commonplace in the future.
While there is no current cure for glaucoma, if caught soon enough, much can be done to impede disease progression. Thanks to technology and the knowledge we’ve accumulated over many years of research and practice, today’s optometrists and ophthalmologists are better equipped than ever to help tackle glaucoma and other diseases of the eye. Still, early detection is paramount, and the first step is a comprehensive eye exam.
As always, our Eye Doctors are available to help with any vision questions you have.