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Neil Choplin considers the importance of retinal nerve fibre loss in early glaucoma.
"Of all the diagnostic instruments on the market, this is the only one that was designed specifically to look at the RNFL. It does that very well, and it doesn't do anything else. It doesn't look at the retina, and it doesn't look at the optic nerve head, just strictly the RNFL," said Dr Choplin, who has worked with this instrument since its inception more than a decade ago and has consulted with its developers on upgrades.
"The clinical significance of the instrument's exclusive emphasis on the RNFL is that this area is where the earliest glaucomatous damage occurs, beginning with axonal dysfunction and the eventual death of the axon," Dr Choplin claimed.
The device uses a physical property of the RNFL known as form birefringence to assess the thickness of the tissue, by determining the retardation of polarized light passing through the tissue. The laser scans 16,000 data points in a 15° grid centred on the optic nerve head, and the resultant data - the degree of polarized light retardation - is proportionate to the thickness of the RNFL.
The data collected produce an image printout for each eye. These printouts show an orientation image of the fundus represented in a colour-coded retardation, or thickness, map, in which warm colours represent areas of higher retardation and cooler colours represent lower values. The printouts also show areas of significant departure from an age-matched normative database, several summary parameters, and a plot graphing the values contained in a peripapillary circle, providing the physician with a very accurate picture of any damage present.
"This instrument has gone through several iterations over the years to address issues related to the signal that it acquires," noted Dr Choplin. The current model includes a built-in, fully automated variable corneal compensator (VCC) designed to remove a portion of the signal attributed to the anterior segment, which can confound the data.
"The addition of the VCC has improved the instrument's sensitivity and specificity," Dr Choplin added.
Why is it important to exclusively monitor the RNFL?
In his own practice, for all patients referred for consultation, except for those with advanced visual field loss or cloudy media, Dr Choplin uses the laser during the initial visit. Patients referred to him typically have glaucoma risk factors, such as elevated IOP or suspicious-looking optic discs, and so he uses the laser scan to reveal whether the eye presents with any damage to the RNFL that is compatible with glaucomatous damage.
The instrument can also be used for following patients with established glaucoma and assessing changes over time.
"The results can help the clinician determine whether the RNFL loss has occurred over time in a patient whose eye was normal at the initial evaluation, whether the loss has progressed in a patient with known loss, or whether the patient's status has remained stable over the follow-up period," said Dr Choplin, who noted that he repeats the scans once a year.
Approximately 20% of eyes demonstrate an atypical birefringence pattern (ABP), which shows an irregular distribution of retardation values with alternating areas of warm and cool colours and relatively high nasal and temporal values. According to Dr Choplin, these 'noisy', or atypical, scans can be difficult to interpret, making the device less useful in these eyes.
"A new software module - the Extended Corneal Compensation from Carl Zeiss Meditec - has been developed to address these atypical scans and, in testing, has been effective at reducing ABP to 2%," Dr Choplin explained. "A normative database has been collected, and the software package is currently awaiting FDA approval."
With this software offering the benefits of the device to an even wider range of patients, this tool for diagnosing and monitoring early glaucoma will soon be available even for eyes that are difficult to diagnose.
Special ContributorNeil T. Choplin, MD is in private practice in San Diego, California, and is an adjunct clinical professor of surgery at the Uniformed Services University of Health Sciences in Bethesda, Maryland, US. He may be reached by E-mail: firstname.lastname@example.org
or Tel: +1 619 296 8525. Dr Choplin has financial relationships with Alcon Laboratories Inc., Allergan Inc., Carl Zeiss Meditec, and Merck US Human Health.