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It is almost never simple in glaucoma. We have all seen it. You ask a panel of four glaucoma specialists about what would be the single most important piece of information to know about a hypothetical glaucoma patient or what treatment to use, and you will likely get five answers. Although this can sometimes be difficult for those who want clear answers, one of the best things about glaucoma is the variability of opinions on how to care for patients. This occurs for many reasons all put together. Patients present not only with different features of disease like POAG or a secondary glaucoma, but also at different stages, from early to late. There are many different tests that can be done like OCT and HVFs. This is compounded as patients can do better on some tests but not on other tests, and even sometimes do better with one test and not as well the next time, such as with a visual field. To add to the complexity, there are many different types of treatment options, from drops to laser and surgery, with a variable amount of risks and benefits, which are sometimes unknown. Needless to say, there is no cookie-cutter approach to treating glaucoma patients.
One question I am asked frequently is which piece of information is most important for me to know. Of course, my answer is usually, “That’s complicated.” There is so much to pick from, with IOP, appearance of the nerve, visual field, OCT, pachymetry and family history to name a few. I usually will at least break it down to either visual field or OCT when choosing between the two. At that point my response is to know approximately what stage of disease the patient presents with before picking. This comes down to a discussion about structure vs. function.
By definition, glaucoma is optic nerve changes with associated visual field loss. What is really going on is the loss of retinal ganglion cells (RGC). The imaging modality to evaluate the structure of RGC is OCT and for function it is visual field (VF). The difficulty when using these two modalities is that they are very different in what they do, which leads to different results. We also know that given a certain amount of structural change, you don’t get a linear change in function. This relationship shouldn’t be surprising. OCT is measured in microns and visual field in decibels, which is log rhythmic. Also, there needs to be a fair amount of structure damage with loss of approximately 50% of RGC before function changes are noticed with standard threshold VF. This relationship leads to placing more weight on one more than the other, depending on the stage of the disease process.
On average, every eye begins with 1 million RGC. Glaucoma patients who are not controlled will see a faster reduction in the number of RGC compared with normal. Some will see a rapid decrease if their IOP is far from target while others will be more gradual if they are closer to the desired goal. OCT of the RNFL attempts to quantify this number through measuring thickness of the peripapillary region of the nerve in units of microns. With a loss of cells, the RNFL thickness decreases. At early stages, thinning of this structure provides clinicians information about the progression of the glaucoma. Structural changes occur before any changes in the VF are seen and classified as pre-parametric. These changes, however, are not proportional as there appears to be a floor effect around 50 µm where progression of glaucoma is occurring but changes in RNFL are not seen. This stage of glaucoma is advanced and better followed with VF by evaluation function. Although there will not be changes in VF early in disease, this modality is more reliable as the disease progresses.
Remember that both OCT and HVF are not perfect. For example, tilted myopic nerves interfere with the reliability of OCT RNFL just as a patient’s inability to accurately perform a VF influences it reliability. It is critical to understand the limitations of each test. This way, useful tests will be used at the right time and more importantly will be avoided when not applicable.
Taking care of glaucoma patients is like putting a puzzle together. We are fortunate to have the ability to obtain many different pieces of information in order to put each patient’s glaucoma puzzle together. It is just that some pieces are more important than others for particular patients or at different times in a single patient’s life.