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.