Some critical issues to consider when diagnosing glaucoma are the recognition of an association between sleep dysfunction and glaucoma, the effect of corneal thickness on the accuracy of intraocular pressure (IOP) measurements, and the importance of first determining optic disk size when evaluating cup-to-disc ratio.
Many factors can lead optometrists to the diagnosis and management of glaucoma. Some critical issues to consider are the recognition of an association between sleep dysfunction and glaucoma, the effect of corneal thickness on the accuracy of intraocular pressure (IOP) measurements, and the importance of first determining optic disc size when evaluating cup-to-disc ratio, according to Jim Thimons, OD, and Peter Libre, MD.
An association between optic neuropathy and sleep apnea was initially reported in 1998 by Mojon, et al. The fact that there is a high prevalence of glaucoma among patients with obstructive sleep apnea was particularly brought to the forefront by a study from Bendel, et al.
"Collectively, research on this topic has shown that within any large population of patients with obstructive sleep apnea, there is an inordinately high number who convert to glaucoma. The rates vary between studies, but the increased association never disappears," said Jim Thimons, OD, medical director, Ophthalmic Consultants of Connecticut, and chairman of the National Glaucoma Society.
Dr. Thimons proposed that although sleep apnea is likely not a dominant causal factor in glaucoma, it is an entity that should be considered in a patient with an atypical presentation in whom glaucomatous optic neuropathy cannot be explained by elevated IOP, an abnormality in ocular perfusion pressure, or high myopia affecting the structural integrity of the nerve.
He noted that he currently orders sleep function studies a few times a month for glaucoma patients who do not fit into the "natural mold." He also noted that sleep apnea is associated with a number of other medical comorbidities, including an increased risk of cardiovascular disease, irregular menstrual cycles, psychologic dysfunction, and, in children, failure to thrive.
"Optometrists need to be taking on a larger role in the overall health of our patients and assess the impact of having this disease on an individual's general welfare," Dr. Thimons said.
Peter Libre, MD, added that an alternative explanation to consider for the normal tension glaucoma patient with an atypical presentation is a habit of sleeping with the hand pressed on the eye.
"This is not a common finding, but when present it is usually in middle-aged men, and if not uncovered and addressed, it can limit effective treatment of glaucoma," said Dr. Libre, assistant clinical professor of ophthalmology, Columbia University, New York.
Accurately measuring IOP
Results of the Ocular Hypertension Treatment Study and Early Manifest Glaucoma Study that established corneal thickness as a risk factor for conversion to progression of glaucoma have helped focus attention on the effect of corneal thickness on IOP readings with an applanation tonometer (Goldmann applanation tonometer [GAT], Haag-Streit).
New tonometry technology is available that measures IOP independent of corneal thickness. Dr. Libre said that the dynamic contour tonometer (Pascal Dynamic Contour Tonometer [PDCT], Swiss Microtechnology) is probably the most accurate instrument currently available.
Recognizing that most clinicians do not have access to this technology, he suggested that they should at least be performing pachymetry in all patients to get a sense of whether the patient has a normal, thick, or thin cornea.
There also has been significant research interest in evaluating the accuracy of IOP measurements in eyes where corneal thickness is altered by pathology or surgery. Results of a recently published paper by Price, et al. points to the importance of considering this issue in eyes that have undergone Descemet stripping endothelial keratoplasty (DSEK) where the cornea is thickened because of postoperative edema, noted Dr. Thimons.
"The number of DSEK cases being performed is growing, and the results of this study showed pneumatonometry and PDCT read IOP appropriately, whereas on average, GAT underestimated true IOP. As an exception, elevated IOP measured with GAT was probably real, and such a reading in a postDSEK eye should raise suspicion of increased IOP," Dr. Thimons said.
"IOP and corneal thickness were not significantly correlated in this population. These data speak to the point that an IOP measurement in an eye with a thick cornea due to edema is different than in an eye where the cornea is naturally thick," Dr. Libre said.
Recognizing the importance of diurnal IOP measurements, Dr. Thimons also expects that IOP evaluation will be improved by technology that provides an overnight ambulatory IOP measurement. This instrumentation is expected to be released later in 2009, and within a few years, an implantable IOP device may become available that can monitor and record IOP 24 hours a day.
Measuring optic disc size
Drs. Libre and Thimons also reminded optometrists about the importance of noting the size of the optic disc as they use the cup-to-disc ratio to evaluate optic neuropathy. The key information to note is the vertical disc diameter, which is indicative of the individual's physiological cup size-the greater the vertical height of the optic disc, the greater the normal cup size. It also is important to take into account that the measured disc diameter is affected by the lens used in the slit lamp.
Dr. Libre noted his preference is for a 78 D lens, and with that instrumentation, an average vertical disc diameter would be about 1.5 mm. The average size would be greater when using a lower power lens and less with a higher power lens. Both the vertical disc diameter and power of the slit lamp lens power should be recorded in the patient's chart.
"Once you have a sense of the height of the average nerve based on the lens you are using," Dr. Libre said, "if you see an eye where the vertical diameter is significantly larger or smaller, you need to take that feature into account when interpreting the cup-to-disc ratio. If the cup-to-disc ratio is 0.5 and the patient has a small nerve, probably one-third of the nerve fibers have already been lost to glaucoma, whereas that same cup-to-disc ratio in a patient with a large nerve probably represents normal physiology."
Dr. Thimons cited a study he participated in, where optic disc area was measured using scanning laser ophthalmoscopy (HRT 3, Heidelberg Engineering). Data of 244 patients were retrospectively reviewed. The mean optic disc area was 2.17 mm2. However, approximately 18% of patients had large nerves (> 2.82 mm2, more than two standard deviations above the mean). All of the patients with a large nerve had a very large cup-to-disc ratio, but only a minority (20%) of the latter subgroup had glaucoma defined by visual field loss.
"Looking at the cup-to-disc ratio only, 90% of eyes might be considered to have glaucoma, demonstrating why it is critical to consider the disc size," Dr. Thimons said.