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OCT helpful in managing glaucoma, but more advancements needed
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Every ophthalmologist encounters glaucoma patients in their practice, but it is the comprehensive ophthalmologist who follows the majority of patients who are glaucoma suspects or have mild-to-moderate glaucoma.
The first challenge is to recognize undiagnosed glaucoma when performing a routine eye examination. Every patient is screened with an IOP measurement. For me, elevated IOP, a significant difference in IOP from one eye to the other on a single examination, or variable eye pressure from one examination to the other moves the patient into the glaucoma suspect category. Other risk factors, such as a positive family history especially on the maternal side, older age, high myopia, black race, pseudoexfoliation, pigment dispersion, diabetes mellitus, hypertension and arteriosclerosis, or an elevated IOP during topical steroid use raise the index of suspicion.
During the examination at the slit lamp, the angle can be screened for depth, and gonioscopy is indicated but underutilized when glaucoma is suspected. When examining the optic nerve, enlarged cups, but especially asymmetry in cup-to-disc ratio from one eye to the other, is an important finding. Ophthalmoscopy in expert hands can reveal nerve fiber defects, and I always look for a nerve fiber layer hemorrhage. In patients who are glaucoma suspects, corneal pachymetry is important, with the patient who has a thin cornea that is less than 500 µm at much higher risk for glaucoma damage than the patient with a corneal thickness that is more than 600 µm.
Combining all these factors, one can categorize the risk a patient faces of developing glaucoma damage in 5 years and 10 years using one of several predictive formulas. But the final arbiter of whether glaucoma damage is present has traditionally been a visual field examination. Demonstration of any of the classic patterns of glaucomatous visual field loss, such as a vertically enlarged blind spot, arcuate scotoma or nasal step along with an elevated pattern standard deviation (PSD), is confirmatory of glaucoma damage and indicates initiation of therapy.
Unfortunately, the classic studies by Quigley and colleagues confirm that by the time a visual field defect is present, significant damage to the optic nerve has already occurred. The use of disc photography, from stereo photos to scanning laser ophthalmoscopy and now optical coherence tomography, allows some cases of early glaucoma to be diagnosed in the absence of a visual field defect. For me, an enlarged vertical cup and disparity in vertical cup from one eye to the other is an important finding.
In addition, nerve fiber layer thinning not easily detected on ophthalmoscopy is important but fraught with more false positives and false negatives. Because glaucoma is a disease of the ganglion cell layer, with progressive loss of ganglion cells and nerve fibers when untreated, it makes sense to me that eventually we will want to be able to count ganglion cells in each sector of the retina as our imaging capabilities continue to advance.
In my practice, the typical patient with mild-to-moderate glaucoma who has achieved target pressures with medical therapy is seen twice per year. At one examination, IOP, slit lamp, fundus and a visual field are performed, and at the other, IOP, slit lamp, fundus and an OCT are performed. I look at the IOP to determine that I am in my target pressure range and discuss compliance by asking patients how frequently they miss their drops. I then look at the PSD on visual field and vertical cup and nerve fiber layer on OCT over the past several examinations.
For me, if IOP is good, compliance is satisfactory, and visual field PSD and OCT vertical cup and nerve fiber layer are stable, I am comfortable that my therapy is appropriate. Elevated IOP, significant asymmetry in IOP, or increasing PSD and OCT vertical cup or nerve fiber layer thinning means a much more intense evaluation of a patient’s therapy. I always remember that a single elevated IOP, abnormal visual field or OCT is not diagnostic, and repeat testing that confirms any abnormalities is required to confirm glaucoma progression.
The addition of OCT has been very helpful to me in managing glaucoma, but I remain disappointed in the variability of test results and am hoping for continued advancement in this diagnostic tool’s capabilities. It makes sense that one day we will be able to accurately and reproducibly have a direct quantitative measure of ganglion cells and/or nerve fiber layer count in each sector of the retina, and this will be a most welcome advancement. For now, significant skill and art in the interpretation of a series of tests, along with individual patient management, are required to properly diagnose and treat the often very frustrating and challenging glaucoma patient.