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Retinal mapping accurate despite low astigmatism, study shows
Working distance for image acquisition does not require strict control.
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CARDIFF, Wales — Scanning laser ophthalmoscopy can be performed on glaucoma patients with uncorrected low astigmatism in a clinic setting without jeopardizing the accuracy of the results, concluded researchers here. In a busy clinic where time is at a premium, not correcting for low astigmatism in every patient could save time in diagnosing optic nerve head changes.
Researchers wanted to learn whether uncorrected astigmatism would significantly affect the nature of the topographic retinal maps produced by the Heidelberg Retina Tomograph (HRT) scanning laser ophthalmoscope. James Morgan, DPhil, FRCOphth, a consultant ophthalmologist at the University Hospital of Wales, told Ocular Surgery News that theoretical and practical reasons motivated a study into ways to save time in a busy glaucoma clinic.
Dr. Morgan said that, theoretically, astigmatism may affect the topographic map.
“In clinical practice we noticed that significant numbers of clinicians and investigators were not correcting for astigmatism,” he said.
Operating instructions for the HRT recommend using the patient’s prescription during an exam if the person has more than 1 D of astigmatism. However, the researchers wrote in the American Journal of Ophthalmology, about 13% of the population over age 49 has at least 1.5 D astigmatism. Correcting large amounts of astigmatism is difficult, and it hinders capturing images at the correct working distance of 15 mm from the cornea to the scanning laser ophthalmoscope. Finally, correcting astigmatism for every patient is also time-consuming.
Recruiting subjects
With these problems in mind, researchers wanted to assess the effects of astigmatism and working distance on images of the optic nerve head. They recruited 51 normal subjects aged 19 to 44 from Cardiff University and imaged them using the HRT (Heidelberg Engineering GmbH).
The researchers tested the role of astigmatism in two ways. First they assessed natural astigmats (mean 1.6 D, range 1.25 to 2.25 D) with and without their spectacle prescriptions worn and then induced astigmatism optically by adding a +3 D lens to assess its effect on topography. Researchers next assessed 15 astigmatic patients (range 1.25 D to 2.25 D, mean 1.6 D) without spectacle correction and then again with their prescriptions in place. Finally, researchers assessed the effect of varying the working distance between 15 mm and 25 mm in 16 nonastigmatic patients.
Although good images were obtained in all exams, quality varied in the images with an induced astigmatic error. The researchers wrote that introducing astigmatism increased significantly the mean standard deviation of the topographic images obtained by the HRT, from 16.14 µm to 20.5 µm. However, this change did not increase the global and segmental parameters, they wrote.
Similar results occurred when correcting astigmatic subjects. When patients wore their prescription lenses the mean standard deviation increased slightly, but the results were not statistically significant.
Finally, no change in magnification occurred when an image was captured at a working distance of 25 mm instead of 15 mm, researchers wrote. The algorithm used by the HRT adjusts for the skew of the image caused by astigmatism, regardless of the amount the patient has. However, increasing the working distance can diminish the amount of laser light illumination as well as narrow the view enough to prevent obtaining an image.
“Our results are of practical significance in that they suggest that the HRT, at least up to 2.5 D of astigmatism, does not require optical correction,” Dr. Morgan said. “No significant difference for HRT parameters using paired Student’s t-test for normally distributed and Wilcoxon test for data not normally distributed. This is important not just for natural astigmats but also for those patients with temporarily induced astigmatism as a result of cataract or glaucoma surgery.”
Astigmatism of 2.5 D or less will not affect the HRT’s detection of glaucomatous optic disc, the researchers concluded. And increasing the working distance from the front of the eye to the front of the HRT’s objective tube did not affect scan parameters or image magnification.
“The algorithms used in the HRT to determine the location of the retinal surface are robust. We would not recommend that correction of 2.5 D of astigmatism is required. This result is important since the HRT, particularly the HRT II, is increasingly being used to screen glaucoma referrals and is therefore being used on large numbers of patients. Not having to correct for astigmatism should increase patient throughput in these busy clinics without compromising diagnostic accuracy,” Dr. Morgan said.
For Your Information:
- James E. Morgan, D Phil, FRCOphth, can be reached at the University of Wales College of Medicine, University Hospital of Wales, Cardiff CF4 4XW, Wales, United Kingdom; fax: (44) 1222 743222; e-mail: morganje3@cardiff.ac.uk. Ocular Surgery News could not confirm whether Dr. Morgan has a financial interest in any products mentioned, or if he is a paid consultant.
Reference:
- Heidelberg Engineering, makers of the HRT, can be reached at 5661 Palmer Way, Suite G, Carlsbad, CA 92008; (800) 931-2230; fax: (619) 930-3575.
- Sheen NJL, Aldrigde C, et al. The effects of astigmatism and working distance on optic nerve head images using a Heidelberg Retina Tomograph Scanning Laser Ophthalmoscope. Am J Ophthalmol. 2001;131:716-721.