1st October 2014
Glaucoma—Better Detection of Early Signs
Glaucoma is the most prevalent neurodegenerative disease in the world. In fact, it outnumbers all others combined. Although the eye care community has made great gains over the last 50 years in detecting and treating this devastating disorder, we can do more. I invite you to join me as we review recent research on early diagnosis of glaucoma and clinical pearls from world leaders in the field.
A large study titled “Long-Term Trends in Glaucoma-Related Blindness in Olmstead County, Minnesota” had encouraging results. When blindness rates for patients diagnosed between 1965 and 1980 were compared with those diagnosed between 1981 and 2000, the findings showed:
- Diagnosis and treatment of glaucoma has improved
- The incidence of glaucoma associated blindness has dropped by nearly 50%
However, the exciting conclusions are tempered by the fact that 15% of patients in the contemporary group still progressed to blindness.
The rate of lost vision due to open-angle glaucoma remains unacceptably high. The study’s authors wrote,
“This is likely due to late diagnosis and our incomplete understanding of glaucoma, so it is critical that research into this devastating disease continues, and all eye care providers be vigilant in looking for early signs of glaucoma during routine exams.”
As optometric physicians provide the bulk of routine eye exams in this country, the call to vigilance for early signs is our charge.
Using Structural Tests
The optic nerve is where it’s at! When diagnosing and monitoring glaucoma, examination of the optic nerve compared with serial stereo disc photos remains the gold standard. Careful evaluation of the 5 R’s—the scleral Ring, neuro-retinal Rim, RNFL, Region of PPA and for Retinal or disc hemorrhages—is essential.
Spectral domain OCT (SDOCT) has become a huge, helpful addition when a nerve is deemed suspicious and it is necessary for any optometrist who wants to practice state-of-the-art care. In a study of eyes with suspicious discs but normal VFs, SDOCT was able to help separate preperimetric glaucoma from glaucoma suspects with good success when looking at global RNFL thickness. The diagnosis was based on the presence of disc changes documented by serial disc photographs. However, there was no repeatable VF loss.
Clinical Pearls for Structural Testing
These structural evaluation tips will help in the early detection of glaucoma:
- Use the 5 R’s. It may seem regimented but it is effective. Brush up by reading articles or recent texts. This refresher and refocus will make you a better “optic nervologist”.
- Do not look at IOP until after examining the ONH. It may be a hard habit to break, but it prevents bias. Look at every nerve for evidence of early glaucoma.
- Get a SDOCT on any patient with suspicious discs—no matter the IOP. Do not let IOP bias your decision. Serial OCT and serial photographs together help catch early disease. If you do not have an OCT, call PCLI. We can help.
- In regard to OCT, normal does not mean healthy. And abnormal does not mean disease. The entry criterion for an OCT database is a clean VF. Highly myopic and irregular nerves make OCT data suspect.
In the illustration below, we see 11 years of progressive, marked glaucomatous ONH change complete with a Drance hemorrhage—but normal glaucoma hemifield tests. This should make us pause and get refocused on the ONH.
Throughout follow-up, standard automated perimetry indices, including the glaucoma hemifield test (GHT) and mean deviation, remained within normal limits. At the most recent visit, spectral domain optical coherence tomography showed evidence of inferior retinal nerve fiber-layer loss. The estimated retinal ganglion cell (RGC) count at the most recent follow-up was 705,082 cells, and the combined structure–function index (CSFI) was 29%, indicating the eye had lost 29% of RGCs compared to that expected in a healthy age-matched eye.
Strategies for improving early detection of glaucoma: the combined structure-function index
Clin Ophthalmol. 2014;8:611-621.
Using Functional Tests
In addition to structural testing, functional tests play a critical role in detecting glaucoma and are an important method of monitoring progression. Ongoing research by Dr. Ron Harwerth at the University of Houston College of Optometry, with his VF-taking monkeys, and others like Medeiros and Weinreb at UCSD, has given us a model to evaluate visual fields more accurately and to understand the limitations of standard automated perimetry (SAP) in early disease.
The human eye has about 1.2 million retinal ganglion cells (RGCs). It takes a loss of 500,000 for SAP to pick up a detectable glaucoma hemifield test (GHT) type defect at the 3dB level. That is consistent with the old adage “You have to lose 30-50% of nerve fibers before we see a VF defect.” What is alarming is that the loss of 500,000 RGCs only results in a 3dB loss!
The RGC loss is marked, but VF defect is small due to system redundancy. Unfortunately, this small VF defect means moderate disease. With redundancy exhausted, patients only need to lose 300,000 more RGCs to go from the earliest of VF defects to a 10 dB loss—which research now shows causes functional life altering changes.
- 3dB loss = 500,000 RGCs gone
- 10 dB loss = 400,000 RGCs left
These are estimates but they may change the way you look at VFs.
Clinical Pearls for Functional Testing
The following tips for functional testing will help in the early detection of glaucoma:
- The Ocular Hypertension Treatment Trial gave us this pearl: 86% of patients who had clean VFs and then developed a defect were found to be normal on repeat testing and 66% of patients who had two consecutive abnormal VFs were normal on the third test. A bad test is just as likely as a bad nerve so be sure to repeat VFs.
- VFs may actually be better in detecting progression in moderate or advanced disease because we tend to see large structural change in early disease and less structural change in later disease.
- Consider a 10-2 VF on early glaucoma patients and suspects. 10-2s used to be reserved for end stage disease because central VF loss was thought to be end stage. However, VF andOCT data now tell us otherwise. Due to spacing of points tested, the 10-2 tests 55 central points compared with 12 in the 24-2.
The future of early glaucoma detection is presented in an article titled “Strategies for Improving Early Detection of Glaucoma: The Combined Structure-Function Index” Clinical Ophthalmology March 2014. The abstract summarizes,
“Although advances in ocular imaging offer the potential for earlier diagnosis, the best method is likely to involve a combination of information from structural and functional tests. Recent studies have shown it is possible to estimate the number of retinal ganglion cells from optical coherence tomography and standard automated perimetry, and then pool the results to produce a combined structure-function index (CSFI)...”
Until these new technologies become available, we need to stay attentive to the optic nerve head and avoid being lulled into a false sense of security by clean or stable VFs.
Call with Questions
I hope this discussion will increase our effectiveness in detecting early signs of glaucoma. If you have questions or wish to discuss a specific case, always feel free to call any of our optometric physicians.