Optical coherence tomography (OCT) is a noninvasive method that has been used to assess macular and optic disc morphology as well as peripapillary retinal nerve fiber layer thickness (PRNFLT). In particular, some authors have recommended using PRNFLT to assess the potential for visual recovery in patients with optic neuritis (1
Because OCT demonstrates anatomical structures quite well, many authors have attempted to correlate the thickness of the PRNFL with visual function. The normal average PRNFLT is age dependent (2 3,4 5 6
However, because the PRNFL not only contains visual axons but also supports glial cells and blood vessels, even complete optic nerve atrophy would not be expected to result in complete loss of the PRNFL as measured by OCT. Indeed, glia constitute at least 18% of the primate RNFL (7 8 9 10
METHODS
This prospective study was conducted at the Neuro-ophthalmology Unit of the Wilmer Eye Institute, The Johns Hopkins Hospital, with institutional review board approval. Patients with NLP from an acquired nonglaucomatous lesion of one or both optic nerves were included in the study. Specifically, the duration of documented NLP had to be at least 1 year, and the patient had to have an absent pupillary reaction to direct light stimulation and no intraocular pathologic condition that could explain the visual acuity or affect the PRNFLT. Eight such patients were identified.
OCT was performed on both eyes of each patient using a Stratus optical coherence tomograph 3.0 (Carl Zeiss, Meditec, Dublin, CA) with software version 4.0.4. A fast RNFL thickness (3.4) scan acquisition protocol was used, and the results were analyzed with the RNFL thickness average analysis (both eyes) function. In scanning the blind eye, the external fixation target was used to allow the patients to fixate with their sighted fellow eye if necessary. A signal strength of at least 5 was achieved in all patients except Case 4 (see below) in whom the signal strength was compromised by an uneven cornea due to concurrent trigeminal and facial nerve palsy causing keratopathy, but the data acquisition was determined to be adequate for analysis.
The visual function of the fellow eyes was determined by Snellen visual acuity at distance, Jaeger visual acuity at near, color vision using Hand-Rand-Rittler pseudo-isochromatic plates, visual field examination using Humphrey perimetry, and ophthalmoscopy.
RESULTS
Baseline demographic data are summarized in Table 1 and the PRNFLT results are summarized in Table 2 . The average PRNFLT of the 8 eyes with NLP was 45.42 μm (range 37.65-51.46 μm). The average PRNFLT of the 4 normal eyes was 97.90 μm (range 94.82-100.89 μm). For Case 3, a woman with bilateral optic neuropathy thought to be inflammatory or ischemic in origin, the PRNFLT was actually thicker in the NLP eye than in the fellow eye (which had 20/200 best-corrected visual acuity and inferior visual field loss). Normal eyes should show a characteristic configuration for disc rim thickness of inferior ≥ superior ≥ nasal ≥ temporal, termed the ISNT rule (11 Fig. 1 ). In the 4 normal eyes in this study, the average PRNFLTs in the inferior, superior, nasal, and temporal quadrants were 126.0, 114.8, 86.8, and 64.0 μm, respectively, obeying the ISNT rule. This pattern was lost in the 8 NLP eyes, which averaged PRNFLTs of 44.8, 52.5, 44.1, and 40.1 μm in those four quadrants, respectively.
TABLE 1: Baseline demographics of study patients with long-standing nonglaucomatous primary optic atrophy resulting in no light perception vision in at least one eye
TABLE 2: Average PNFLT in the 8 study eyes (with NLP) and fellow eyes
FIG. 1: Optical coherence tomography images of the right eye (normal) of Case 4, demonstrating the ISNT rule (inferior ≥ superior ≥ nasal ≥ temporal).
DISCUSSION
This small case series demonstrates that the minimal average PRNFLT in eyes with NLP from long-standing, nonglaucomatous optic neuropathies is approximately 45 μm, a figure consistent with the average PRNFLT of 44.93 μm found in glaucomatous blind eyes by Sihota et al (10 9
None of these mechanisms would explain why the PRNFLT in the seeing eye of Case 3 was thinner than that of the fellow eye with NLP (Fig. 2 ). Because the underlying cause for her optic neuropathy has not yet been determined, it is possible that she had other intraocular pathologic conditions that could have contributed to her loss of vision and reduction of PRNFLT. One possibility is that over the 9 years that the patient's nonseeing eye had been blind, gliosis had occurred, thus thickening the PRNFL compared with the fellow sighted eye which had not yet developed substantial gliosis.
FIG. 2: Optical coherence tomography images of Case 3. Visual acuity in the right eye is no light perception (NLP) and in the left eye is 20/200. Note that the peripapillary nerve fiber layer thickness is greater in the right eye (NLP eye) than in the left eye (20/200 eye). For a possible explanation, see text.
Our findings should be taken into consideration in the interpretation of OCT findings in patients when one is attempting to determine the prognosis for visual recovery in patients with nonglaucomatous optic neuropathies.
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