The study was conducted in the Department of Ophthalmology at the University of Illinois at Chicago. It was approved by the institutional review board and was performed in accordance with tenets of the Declaration of Helsinki.
A representative sample of patients, older than 18 years and with a known diagnosis of RP and a spectrum of disease severity, was asked to participate in the study. Patients were randomly selected and enrolled based on their willingness to undergo OCT testing. The diagnosis of RP was based on history of night blindness, impairment of peripheral visual fields, reduction of electroretinogram rod and cone amplitudes, and presence of characteristic fundus pigmentary changes. Exclusion criteria used for screening included visual acuity less than 20/80 on a Snellen acuity chart (with one exception), refractive error of more than ± 6.0 diopters spherical or ± 2.0 diopters cylindrical correction, inability to maintain steady fixation, media opacity sufficient to hinder an OCT examination, previous history of glaucoma or intraocular pressure higher than 21 mm Hg, excessively large or small size of the optic nerve on clinical examination, and diabetic retinopathy. Twenty-six patients were included in the study, all of whom provided informed consent. One patient was subsequently excluded because of nerve fiber layer drusen that resulted in inaccurate measurement of the nerve fiber layer thickness value on OCT testing. Of the 25 patients included, 1 had visual acuity of less than 20/80 (20/400) but could hold steady fixation using the external fixation target on the OCT and, hence, was asked to participate in the study.
All patients underwent complete eye examination, including best-corrected visual acuity, slit lamp biomicroscopy, 2 intraocular pressure readings at 5-minute intervals, fundus stereophotography, and corneal pachymetry. In the participating patients, the best-corrected visual acuity was determined by the Early Treatment of Diabetic Retinopathy Study chart (The Lighthouse, Long Island City, NY). The better eye was then dilated in each patient using 2.5% phenylephrine and 1% tropicamide, and detailed fundus examination was performed.
RNFL thickness was measured with a third-generation OCT machine (StratusOCT, software version 4.0.1; Carl Zeiss Meditec, Inc., Dublin, CA). The fast RNFL thickness protocol, consisting of 256 individual A-scans along a circular scan path, was used. Internal fixation was used in all patients except one, who required external fixation, as already discussed. The optic disc was then centered within the scan circumference. The software automatically used a scan diameter of 3.46 mm and performed 3 successive scans, with a total acquisition time of 1.92 seconds. Scans were only accepted if they had a signal intensity of 5 or more and were free of artifacts. The average of the 3 scans in each acquisition was automatically calculated by the OCT software. To increase the reliability of the test, each patient underwent testing 3 times, with the third acquisition the repeat sequence of the second one. In 14 patients, all 3 acquisitions were found to be reproducible in the number of abnormal quadrants and, hence, were used for analysis. In the remaining 11 patients, 2 of the 3 acquisitions were reproducible and were used for the purpose of analysis.
For each eye, we studied RNFL thickness in the temporal (316°–45°), superior (46°–135°), nasal (136°–225°), and inferior (226°–315°) quadrants. We also measured 3 smaller segments within each quadrant, all automatically calculated with the use of existing OCT software. The data thus obtained were compared to the normative database already incorporated in the StratusOCT unit. The RNFL was considered abnormally thin if its value was below the 5th percentile of the age-related normal value and thicker if it was above the 95th percentile. Abnormal thinning of the RNFL in at least 1 quadrant, or abnormal thinning of the RNFL in at least 2 smaller segments, each present consistently in at least 2 acquisitions, was considered significant and therefore abnormal.
Goldmann perimetry (Haag-Streit AG, Koeniz, Switzerland) using target sizes V4e, III4e, and II4e was performed to measure the kinetic visual field. The area of visual field for each target size was then measured by a planimetric calculation in square inches and was converted to square millimeters.
One of the authors (DPE), who was masked to the findings of the nerve fiber layer measurements, reviewed the stereo photographs of the optic disc of each patient for any evidence of glaucomatous damage. Factors that were considered in designating an optic disc as glaucomatous included a large cup-to-disc ratio greater than 0.7 that did not follow the ISNT rule,
24 25 presence of polar notching, increase in sloping of the cup margins, and baring of blood vessels on the disc surface.
An unpaired t-test was used to calculate the 2-tailed P value to test for significance between 2 means. P < 0.5 was considered statistically significant.