THE EFFECT OF OPTIC DISC SIZE ON DIAGNOSTIC PRECISION WITH THE HEIDELBERG RETINA TOMOGRAPH

Purpose: The authors evaluated the ability of a confocal scanning lase r ophthalmoscope to detect glaucomatous visual field loss by using the ir previously described discriminant formula on a prospectively obtain ed cohort. The relationship of optic disc size to diagnostic classific ation was also evaluated. Methods: One eye was chosen randomly from ea ch of 153 subjects. Sixty control eyes had intraocular pressure less t han 21 mmHg and normal visual fields; 93 glaucomatous eyes had intraoc ular pressure greater than 21 mmHg and abnormal visual fields. The opt ic disc status purposely was not used for classification purposes. Ail subjects were examined with the Heidelberg Retina Tomograph (HRT; Hei delberg Engineering GMBH, Heidelberg, Germany) and Humphrey Perimeter, program 30-2 (Humphrey Instruments, Inc., San Leandro, CA). Visual fi elds were considered abnormal by the authors' previously published cri teria. The HRT classification used age, adjusted cup shape measure, ri m volume, and height variation contour to classify the optic disc as n ormal or glaucomatous. Then the authors assessed the sensitivity, spec ificity, and diagnostic precision for the entire group, and for three subsets classified by disc area: disc area less than 2 mm(2), between 2 and 3 mm(2), and more than 3 mm(2). Results: The entire group had a sensitivity, specificity, and diagnostic precision of 74%, 88%, and 80 %, respectively. The specificity was 83% when disc area was less than 2 mm(2) and improved to 89% when disc area was more than 2 mm(2). The sensitivity tended to improve from 65% to 79%, and to 83% if the disc area increased, but the difference was not statistically significant. Conclusions: In a prospective cohort of patients, the HRT discriminant analysis formula was capable of detecting glaucomatous visual field l oss with good precision. Unusually small optic discs continue to prese nt diagnostic difficulties.