Exploring synergies in glaucoma diagnosis
Our definition of glaucoma has changed, and our approach to diagnosing it must change as well. Here’s how.
Recently, clinicians have begun to recognize that glaucoma isn’t a single disease but a range of conditions. Perhaps, instead of talking about glaucoma, we should be talking about a spectrum of glaucoma disorders.
Traditionally, to have a diagnosis of glaucoma, you had to have a patient with high IOPs, cupping and visual field defects. Now, we know that normal-tension glaucoma exists. We also see glaucoma without cupping – and cupping without glaucoma – and glaucoma without field defects. We also know that corneal thickness impacts the disorder itself. Patients with thin corneas typically have a poor prognosis, whereas patients with thicker corneas, even if they have glaucoma, progress more slowly.
Is there anything that’s truly unique to the diagnosis? Yes, every patient with glaucoma exhibits retinal nerve fiber layer (RNFL) deficits. So, although we’ll continue to look at pressures, cupping and visual fields, our earliest diagnosis will be made with RNFL technology.
The newest system for examining the RNFL is the GDx VCC (Variable Corneal Compensator). Studies show that this device is excellent for early diagnosis, even without high IOPs, field loss and cupping. In fact, the GDx VCC can correctly identify more than 90% of glaucoma patients and do it sooner than any other technology.
Just as importantly, this technology can show change over time, even if visual fields aren’t changing. In glaucoma, if you wait too long to treat, neurotoxicity ensues and, even if you lower the pressure, those neurotoxins are still present in the nerve fiber layer, and will continue to cause progressive loss of nerve fiber layer.
The following cases illustrate these points.
Uncovering the whys
This 67-year-old African-American woman had no family history of glaucoma. She presented in 1995 with an enlarged left cup; however, the left disc was larger also. Her pressures were normal. Her visual field showed an odd defect with a right angle in the inferonasal quadrant of the left eye. By 1997, her pressures had risen to about 21 mm Hg, but she had no changes in her visual fields or cup-to-disc (C/D) ratios.
Her first RNFL scan, done in 1999 on an early model GDx, showed a very subtle, but very real, superior-inferior symmetry abnormality in the left eye. Most clinicians would not have characterized this defect as glaucoma. Her 30- to 60-degree fields showed a defect farther out that matched the deficit. In fact, the entire inferonasal quadrant was grossly abnormal.
Despite the visual field and nerve fiber defect, we observed no visible cupping. Based on the location of the reproducible field defect, we expected a corresponding RNFL defect, and we should see some type of cupping -that is, if you believe cupping occurs in every case of glaucoma. However, I can show you 200 glaucoma cases where cupping does not occur.
A scan of the right eye using a Heidelberg Retina Tomograph (HRT) shows all six zones as normal, and a Moorfield’s regression analysis is also normal. In the left eye, the two bottom zones are borderline, while the two superior zones are normal. With an inferonasal visual field defect, superotemporal cupping should be present, but that area is normal on the HRT.
At this point, the patient hadn’t been treated. Eventually, her pressures rose to 32 mm Hg. There’s no question that she has glaucoma, despite the lack of cupping in the area that corresponds to the field defect found within the central 30 degrees. So I treated her.
Her GDx VCC scan shows a massive superotemporal wedge defect, consistent with an inferonasal field defect, but you can’t see it with an ophthalmoscope or an HRT!
This case shows that you need to find a reason for a persistent visual field defect. Not all field defects can be explained on the basis of a neuroretinal rim assessment. We know that RNFL loss can occur far in advance of visual field loss in glaucoma. We also know you can have RNFL loss and visual field loss in the absence of obvious rim loss.
Don’t be hampered by what you learned about glaucoma diagnosis in optometry school. The answer is in the RNFL, not necessarily pressure, not necessarily even field defects and not necessarily cupping. Our definition of glaucoma has changed, and our approach to diagnosing it must change as well.
We began seeing this 59-year-old African-American woman in 1988. Her C/D ratio was 0.3 OU and her IOP readings occasionally were in the 30s. I started to treat her in the 1990s – despite normal fields – because of an increase in cupping to about 0.7 OD and 0.6 OS, consistently high IOPs to 30 mm Hg and above and an abnormal GDx. The diagnosis was pre-perimetric glaucoma.
This patient continues on medication and seems to be doing well. At a recent evaluation, her visual acuity was 20/20, her threshold fields and short wavelength automated perimetry (SWAP) were essentially normal. I generally do 30- to 60-degree fields now, and you’ll see that there’s some decrease when we do peripheral fields. We also did laser scanning tomography, optical coherence tomography, nerve fiber layer analysis and retinal thickness analysis.
Recent central fields show one abnormal point in the right eye inferiorly, but it’s transient on repeat fields. The HRT demonstrates a superior notch. The GDx shows a dramatically abnormal nerve fiber indicator of 71, even though the patient has an essentially normal visual field. It also shows dramatic RNFL loss on the deviation plot in the right eye that corresponds to the peripheral field defect. Now she’s 0.75 and 0.65. Remember, when I first started seeing her, her C/D ratio was 0.3, and I didn’t start to treat her until 10 years later when her cups increased and her pressures rose.
A 24-2 visual field done more recently demonstrates normal visual fields in both eyes, with only one abnormal point inferiorIy that corresponds to a superior notch. But the glaucoma hemifield test (GHT) is within normal limits. A SWAP looked normal also. We did a visual field out to 60 degrees, and detected an inferior loss in the right eye, which had that little superior notch. We also detected a nasal step not present in the central 30-degree field. The left central and peripheral field look fairly normal.
An HRT flags the zone that we’re very concerned about as borderline. A GDx scan shows an RNFL defect that’s absolutely abnormal, even though there’s only one point on the standard visual field that corresponds to it.
You may not believe the field defect. You may not believe the RNFL defect. But when you put the structural and functional tests together, and you find concordance, you know they have to be correct. What’s the probability of these two matching by coincidence? Very low. I call this diagnostic synergy, with two or more results that, on their own, may be inconclusive, but when put together, give a clear picture.
Glaucoma and more
The GDx VCC is a viable technology to detect and monitor the progression of RNFL disorders not just in glaucoma, but in a host of optic neuropathies for which we, as optometrists, are responsible. I believe the GDx VCC is capable of early diagnoses on the first visit and serial analysis for progression.
Jerome Sherman, O.D., F.A.A.O.
Dr. Shcrman practices at the Eye Institute and Laser Center in Manhattan and is a distinguished clinical professor at the State University of New York College of Optometry.
Copyright Boucher Communications, Inc. Nov 2003
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