Glaucoma can be found by checking the eye's pressure. Normal eye pressures range from 12 to 21. Glaucoma patients often have pressures higher than the normal range. However, it is possible to have glaucoma and a pressure that is in the normal range. It is also possible to have an elevated eye pressure and not have damage to the optic nerve. Thus, pressure alone is not sufficient to determine whether or not someone has glaucoma; it is only part of the equation. Quick screening exams that only check vision or eye pressure and lack a doctor evaluating the optic nerve can miss glaucoma. Additionally, it is very important to know the thickness of the patient’s cornea. We use a device called an ultrasound pachymeter to measure corneal thickness. Thicker corneas will give erroneously high pressure readings and thin corneas will be erroneously low. Thus corneal thickness is an important part of glaucoma diagnosis and treatment.
The examining eye doctor can look into the anterior part of the eye with a special mirror lens. This test is called gonioscopy. It should be performed initially on all glaucoma suspects to see if there is a physical blockage of the eye’s drainage system called the trebecular meshwork. If the drainage angle is narrow, eye dilation can be dangerous until a laser can be used to treat this condition. The eye pressure can be reduced in many narrow angle cases by laser treatment.
Visual field testing checks how sensitive our side vision is and compares it to a database of normal patients. This is the gold standard test and it should be done yearly on all glaucoma patients. Anyone suspicious for glaucoma should have a visual field test as well. If there is significant damage to the optic nerve it shows up as a loss of sensitivity in the side vision (which by the way is not noticed by the patient). It has been shown that a large portion of the nerve fibers in the optic nerve must be lost before the visual field test can detect glaucoma. Some studies have suggested as many as 30 to 50% of the nearly 1.2 million nerve fibers are lost before standard visual field tests can detect the loss. This has led eye doctors to search for more sensitive testing to try and find glaucoma earlier. Early detection and treatment would thus spare more of the optic nerve.
Two new testing devices are optic nerve analyzers and Blue-Yellow visual field tests. The optic nerve analyzers use scanning lasers to look inside of the optic nerve and surrounding nerve fibers. These remarkable devices give vary accurate objective information regarding the shape and thickness of the optic nerve. Thes data is compared to normal population data and thus glaucoma damage can be detected early. The optic nerve and retinal nerve fiber layer analysis can be repeated later (1 year or more) to see if the optic nerve has changed over time suggesting progressive damage to the optic nerve. This not only helps in making the diagnosis of glaucoma but it also helps determine if the current treatment is adequately preventing further damage for those already receiving treatment for glaucoma.
Early detection of glaucoma is also enhanced by the use of Blue-Yellow visual field testing (also called perimetry). This Short Wave Automated Perimetry (SWAP) differs from standard perimetry only in that a carefully chosen wavelength of blue light is used as the stimulus, and a specific color and brightness of yellow light is used for the background illumination. Other than these wavelength specifications, the test is very similar to standard visual field tests. Visual field defects with blue-yellow can be an early indicator of impending glaucomatous visual field loss not yet detected by standard white-on-white fields. Patients who have eye pressures higher than normal, for example, may show visual field defects earlier with the blue-yellow field than standard white-on-white fields. If a patient is very suspicious for glaucoma but has a normal white-on-white visual field then a Blue-yellow field may be indicated to look for early defects in the side vision.
In summary, glaucoma detection requires more that an eye pressure check. It also requires evaluation of the optic nerve’s appearance and evaluation of its function (visual field testing). Our patients benefit from the state-of-the-art technology we have available to diagnose and treat glaucoma. We have recently obtained the Blue-Yellow SWAP visual field analyzer as well as Zeiss Cirrus OCT scanners to evaluate the optic nerves and retinal nerve fiber layer. Both of these glaucoma-testing devices are available in our Parkersburg and Marietta offices. Please call us for more information or a glaucoma evaluation at 1-800-758-3937.