Tomograph best used as one of several sources

Key Points

"The tomograph is useful for diagnosing and monitoring ocular hypertension and suspected glaucoma, as well as for monitoring early glaucoma. The tomograph output, however, should not be used in isolation but as one of several sources of information," said Dr Garway-Heath.

"Diagnostic reports, such as the Moorfields Regression Analysis and the Glaucoma Probability Scoring system, should not be used to assess eyes for change. A better tool," he explained, "is a software application for the tomograph (Topographic Change Analysis; Heidelberg Engineering). The rim trend chart can provide additional information."

During glaucoma subspecialty day at the 2008 meeting of the American Academy of Ophthalmology, Dr Garway-Heath described the tomograph and his use of this instrument in his practice.

The device provides monochromatic images and a surface height map or topography of the optic nerve head and peripapillary retina, scaled for the magnification characteristics of the eye, which enables quantitative measurements of the nerve structure to be made. Follow-up images can be compared with those taken at baseline to identify progressive glaucomatous damage.

"To obtain the high-quality images that are required for correct data interpretation, technician training and quality control are essential," stated Dr Garway-Heath, who noted that technician training is available online at the manufacturer's website. The training should incorporate techniques to optimize image quality; these include performing imaging prior to tonometry, using artificial tears in dry eyes, and dilating the pupil in eyes with cataract. In non-cataractous eyes, it is preferable to obtain the image through an undilated pupil.

Quality control measures include taking multiple images and selecting the highest quality for analysis, determining whether any features in the image would confound the software analysis, and checking that the disc margin contour or edge has been correctly placed.

The next step in data interpretation is to determine whether a valid test has been performed. The technician and clinician should look for any confounding factors that might affect the test quality and determine the appropriateness of the subject to the normative database. "Some subjects may be poor matches because of an exceptionally high refractive error, large disc size, or if their ethnic origin is not reflected in the normative data," Dr Garway-Heath explained. "In such instances, caution is warranted in interpreting the test."

Clinicians should keep the data in perspective.

The results demonstrate probabilities, not definitive diagnoses.

"I want to emphasize that the machines do not produce a diagnosis: they produce a statistical classification. Clinicians using the machines should try to think in terms of probabilities rather than a binary diagnosis," Dr Garway-Heath clarified. "They should use the report from the instrument either to increase or to reduce the probability that glaucoma is present."

An adaptation of Bayes' theorem may also be used in making the diagnosis. In what is known as Fagan's nomogram, the probability that glaucoma is present is considered before doing a diagnostic test, and the likelihood ratio is used to calculate the post-test probability that a patient has glaucoma. The tomograph is also very helpful in measuring optic disc progression, and may be more accurate for this task than for diagnosis, Dr Garway-Heath continued.

"To ensure best results, a high-quality baseline must be acquired. Once a series of scans has been obtained, the series should be reviewed, starting with the baseline. If it is of poor quality, choose another one," he explained. "Similarly, poor-quality images within the series may need to be excluded occasionally. The disc contour margin should be checked, as should the alignment of images to baseline."

The software supports identification of progression by identifying groups of pixels in the image, in which the surface height at follow-up is significantly lower when compared with the baseline image. The software automatically identifies a cluster of these pixels when they reach a significant size, then plots the size and volume of change over time.

"The visual display of these data is helpful in assessing whether the optic nerve head is stable or progressing," Dr Garway-Heath noted.

When should the tomograph be used?

No hard data are available on how frequently the tomograph should be used for monitoring. Dr Garway-Heath, however, suggested that imaging should be performed at a frequency similar to visual field testing, with the timing guided by risk factors.

"Eyes at high risk should be tested every four months for the first two years, patients at moderate risk should be tested every six months, and those at low risk should be tested once yearly," he proposed. "I also recommend that both visual field testing and imaging be performed in eyes with ocular hypertension or early glaucoma, to pick up all instances of progression."

Technology is good but no substitute for you

In a conversation with Ophthalmology Times during the 2008 meeting of the American Academy of Ophthalmology (held November 8–11 in Atlanta, Georgia), Harvard Medical School's Dr Rhee discussed his spotlight session presentation, "You don't need high tech to make the real decisions". Here, he explains why he agrees with some of Dr Garway-Heath's comments:

"New imaging technologies can be very helpful when you have a patient who cannot reliably take a visual field test, for whatever reason. They also really help me out in trying to decide patients who have very early glaucoma, and they have helped us to establish a sub-category of open-angle glaucomas: pre-perimetric glaucoma; where we can see structural evidence of damage and glaucomatous disease either in the optic nerve or the nerve fibre layer, but it hasn't yet shown up in the standard achromatic white-on-white visual field test. But to screen every single patient that comes into your office with these new technologies is just not practical, nor is it cost-effective, and it certainly is inconvenient for both the practitioner and the patient.

"In any case, these technologies do not yet have absolute sensitivity and specificity for diagnosing glaucoma; there is no form of glaucoma that exists that can only be diagnosed by one of these technologies. It still is dependent on you, the clinical practitioner, to make the diagnosis, and if you do not make the diagnosis with your clinical exam, simply owning technology will not help you; it's only an aid to you. The technologies that are available right now have not yet reduced their variability to a point where we would make an interventional decision based on anything other than optic nerve, intraocular pressure and visual field.

"In the future, maybe the situation will be different, but right now, technology is no substitute for the clinician. These technologies can only be there to help assist you, as the practitioner, to make the determination. You are still the most important thing to the patient; to determining whether or not they have damage, and whether or not therapy needs to be escalated."

---------

Special ContributorsDavid Garway-Heath, MD, FRCOphth is Glaucoma Theme Lead at the Biomedical Research Centre for Ophthalmology, Moorfields Eye Hospital and University College London Institute of Ophthalmology, London, UK. He may be reached by E-mail: David.garway-heath@moorfields.nhs.uk
or Tel: +44 20 7566 2973. Dr Garway-Heath has received grant support from Heidelberg Engineering.

Douglas Rhee, MD, assistant professor at Harvard Medical School, is affiliated with the Massachusetts Eye & Ear Infirmary. He may be reached by E-mail: Douglas_Rhee@meei.harvard.edu