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HRT offers ‘objective measurement’ of structural injury
When used to determine glaucomatous damage, the HRT gives clinicians reliable and reproducible measurements, physicians say.
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PHILADELPHIA — Reliable, reproducible and objective measurement of structural injury in eyes with glaucoma may be achieved with computerized imaging. The Heidelberg Retinal Tomograph provides a calculated assessment of damage based on contour measurements, according to several physicians familiar with the technology.
They spoke during the program “3-D Scanning Topography: Diagnostics for Glaucoma Detection and Management,” held here during the American Society of Cataract and Refractive Surgery meeting. Speakers addressed the use of HRT in following glaucoma patients and diabetic patients. They also addressed special considerations in using retinal imaging technologies in refractive surgery patients.
Heidelberg Retinal Tomograph (HRT, Heidelberg Engineering) images correlate well with visual field information gleaned from clinical visits, according to Raymond LeBlanc, MD, FRCSC. The technology is best used to detect glaucomatous progression in high-risk patients and in patients with early and moderate glaucoma.
HRT is most useful in assessing glaucomatous progression when used at baseline scans and then during routine intervals, he said. If the patient presents with advanced disc damage or the patient’s initial scans are poor, the HRT will be of little help in clinical assessment, he added.
“Glaucomatous disc changes determined with HRT occur more frequently than field changes,” Dr. LeBlanc said. While patients with visual field changes tend to have disc changes as well, fewer than half the patients with disc changes had visual field changes, he said. Dr. LeBlanc and colleagues studied 77 patients with early glaucomatous visual field damage for a median of 5.5 years to draw their conclusions.
HRT post-LASIK
Jeffrey Liebmann, MD, said he found that retinal nerve fiber layer (RNFL) thickness as measured by laser polarimetry (GDx, Laser Diagnostic Technologies) is altered after LASIK. Believing these changes were a direct result of altered corneal birefringence rather than a real change in thickness, Dr. Liebmann sought to confirm this theory using other imaging modalities. He performed a comparison of optical coherence tomography, scanning laser polarimetry and scanning laser tomography in measuring RNFL thickness,
In a prospective study on 20 eyes of 20 patients with myopia, Dr. Liebmann and colleagues performed GDx, HRT and OCT (Zeiss Humphrey Systems) 1 week before patients underwent LASIK and then 1 week and 1 month after the surgery.
All patients underwent at least three HRT scans per session to evaluate RNFL thickness, cross-sectional area, rim area and rim volume. With GDx, four to six scans per session were performed, obtaining mean RNFL thickness along the entire ellipse and four sectors. With OCT, three to six scans per session were taken, obtaining mean RNFL thickness for the entire circumference and each quadrant.
RNFL thickness, when measured by either OCT or HRT, remained unchanged after LASIK, while the GDx analysis showed some alterations. Dr. Liebmann said surgeons using GDx should acquire new baseline images post-LASIK, but that new images are not necessary when using either OCT or HRT.
HRT for macular edema
John G. Flanagan, PhD, MCOptom, FAAO, told attendees that an objective measure of retinal thickness is necessary for the clinical management of diabetic macular edema (DME). DME is the most common cause of visual impairment and blindness in patients with diabetes, he said. At 15 years after diagnosis with diabetes, 42% of type 1 and 80% of type 2 diabetics will develop DME, Dr. Flanagan said.
Stereo fundus biomicroscopy is the gold standard for detecting and assessing patients with DME, Dr. Flanagan said.
But “substantial differences” exist between experienced medical retinal specialists when defining the extent and location of retinal thickening for a particular patient using stereo images, he said.
In a study of 16 eyes with clinically significant DME, 15 showed a significant increase in HRT edema map values either at the time of or before conversion to clinically significant DME.
Dr. Flanagan explained that in the normal retina, the shape of the confocal intensity profile is slightly asymmetric, with a slightly longer tail towards the deeper layers. Light scattered from the deeper retinal tissue adds to the high signal. If edema is present, the amount of scatter inside the swollen retina increases. The tail of the confocal intensity profile extends in the deeper layers, the peak reflectance reduces and the profile becomes more asymmetric and its width increases, he said.
In a normal eye, the signal width is smallest at the fovea. Signal width increases with increasing distance from the fovea. If macular edema is present, the thickness of the retina is largely increased and the signal width map shows wider signals (lighter colors) at the site of the edema.
Some limitations to the system include signal width measurements that are disrupted in the presence of hemorrhage, exudate or lesions of the retinal pigment epithelial layer, according to Dr. Flanagan.
For Your Information:
- Jeffrey Liebmann, MD, is a professor of clinical ophthalmology at New York Medical Center and the associate director, glaucoma service, at The New York Eye and Ear Infirmary. He can be reached at East 14th St., New York, NY 10003. (212) 477-7540; fax: (212) 420-8743. Dr. Liebmann has no direct financial interest in any of the products mentioned in this article nor is he a paid consultant for any company mentioned.
- Raymond LeBlanc, MD, FRCSC, is professor and head of the Department of Ophthalmology, Dalhousie University and can be reached at Eye Care Centre 1278 Tower Rd., Room 2035, Halifax, NS B3H 2Y9 Canada. (902) 473-4343; fax: (902) 473-2839. Dr. LeBlanc has no direct financial interest in any of the products mentioned in this article nor is he a paid consultant for any company mentioned.
- John G. Flanagan, PhD, MCOptom, FAAO can be reached at (519) 888-4567; fax: (519) 725-0784; e-mail: jglanag@quark.uwarterloo.ca. Ocular Surgery News could not confirm whether Dr. Flanagan has a direct financial interest in any of the products mentioned in this article or if he is a paid consultant for any company mentioned.
Reference:
- Heidelberg Engineering, makers of the Heidelberg Retinal Tomograph, may be reached at 1499 Poinsettia Ave., Ste. 160, Vista, CA 92037; (800) 931-2230; fax: (619) 930-3575; e-mail: contact@heidelbergengineering.com; Web site: www.heidelbergengineering.com.
- Chauham BC, McCormick TA, Nicdela MT, LeBlanc RP. Optic disc and visual field changes in a prospective longitudinal study of patients with glaucoma: Comparison of scanning laser tomography with conventional perimetry and optic disc photography. Arch Ophthalmol. 2001; 119:1492-1499.