Retinal nerve fiber structure versus visual field function in patients with ischemic optic neuropathy. A test of a linear model.

PURPOSE: To test a linear model relating the regional loss in retinal nerve fiber (RNFL) thickness to the corresponding regional loss in sensitivity with data from patients with previous anterior ischemic optic neuropathy (AION). DESIGN: Case-control study. PARTICIPANTS: Twenty-four individuals with AION and 20 with normal vision were tested. The time since the AION attack ranged from 5.2 months to more than 20.3 years (median, 2.95 years). METHODS: Eyes were tested with standard automated perimetry (SAP) and with optical coherence tomography (OCT), both RNFL thickness scans. The average RNFL thickness of the inferior and superior disc sectors was plotted against the average total deviations (linear units) of the corresponding superior and inferior arcuate field regions, and a linear model was fitted. According to the model, the RNFL thickness R=s(o)T+b, (1), where T is the relative SAP sensitivity loss (on a linear scale; e.g., for -3 dB, T = 0.5), s(o) is the RNFL thickness attributable to axons in the healthy or normal state (T = 1.0), and b is the residual RNFL measured when all sensitivity and axons are lost. MAIN OUTCOME MEASURES: Optical coherence tomography RNFL thickness and SAP sensitivity. RESULTS: The data from the AION patients resembled the data from glaucoma patients previously tested and were described by the linear model. For patients with SAP losses of more than -10 dB in the arcuate region, the RNFL thickness provided an estimate of residual RNFL thickness, b. The median value of b (45.5 microm) was similar to the value for patients with glaucoma. It varied among individuals (range, 30.4-63.3 microm), showing a very weak correlation with patient's age (r = 0.30) and the time since the AION episode (r = 0.26), but an excellent correlation (r(2) = 0.94; P<0.01) with the value of s(o), estimated from the unaffected eyes. CONCLUSIONS: The relationship between a structure (OCT RNFL thickness) and function (SAP sensitivity loss) is the same for patients with AION and glaucoma and can be approximated by a simple linear model. The model may provide a framework for identifying those patients with ganglion cell axons that are malfunctioning but are alive.

A comparison of multifocal and conventional visual evoked potential techniques in patients with optic neuritis/multiple sclerosis.

PURPOSE: To compare conventional visual evoked potential (cVEP) and multifocal visual evoked potential (mfVEP) methods in patients with optic neuritis/multiple sclerosis (ON/MS). METHODS: mfVEPs and cVEPs were obtained from eyes of the 19 patients with multiple sclerosis confirmed on MRI scans, and from eyes of 40 normal controls. For the mfVEP, the display was a pattern-reversal dartboard array, 48 degrees in diameter, which contained 60 sectors. Monocular cVEPs were obtained using a checkerboard stimulus with check sizes of 15' and 60'. For the cVEP, the latency of P100 for both check sizes were measured, while for the mfVEP, the mean latency, percent of locations with abnormal latency, and clusters of contiguous abnormal locations were obtained. RESULTS: For a specificity of 95%, the mfVEP(interocular cluster criterion) showed the highest sensitivity (89.5%) of the 5 monocular or interocular tests. Similarly, when a combined monocular/interocular criterion was employed, the mfVEP(cluster criterion) had the highest sensitivity (94.7%)/specificity (90%), missing only one patient. The combined monocular/interocular cVEP(60') test had a sensitivity (84.2%)/specificity (90%), missing 3 patients, 2 more than did the monocular/interocular mfVEP(cluster) test. CONCLUSION: As the cVEP is more readily available and currently a shorter test, it should be used to screen patients for ON/MS with mfVEP testing added when the cVEP test is negative and the damage is local.

An analysis of normal variations in retinal nerve fiber layer thickness profiles measured with optical coherence tomography.

PURPOSE: To assess the normal variations in retinal nerve fiber layer (RNFL) thickness measured with optical coherence tomography (OCT). SUBJECTS AND METHODS: Both eyes of 48 individuals (age 56.4+/-9.5 y) with normal vision and refractive errors between +/-6.0 D were tested with the fast RNFL scan protocol of the OCT3 (Zeiss Meditech). Their 256-point RNFL profiles were exported for analysis. The location and peak amplitude of the maxima of the RNFL profiles were measured. Intersubject and interocular variations were assessed with a coefficient of determination, R2. An R2 of 1.0 indicated that the average profile from all 48 individuals (or of the 2 eyes) accounted for 100% of the variation of an individual eye's profile. RESULTS: The R2 for the interocular comparison was good, with averages of 0.91+/-0.07 (right eye) and 0.92+/-0.05 (left eye). The R2 for the comparison of the individual's profile to the mean group profile was only 0.61+/-0.29 (right eye) and 0.65+/-0.24 (left eye), with 27% of the R2 values below 0.5. Even after normalizing each individual's profile by its mean, R2 was only 0.75+/-0.16 (0.75+/-0.16) for the right (left) eye. The location of the peaks for the right (left) eye ranged over 91 degrees (88 degrees) for the superior peak and over 64 degrees (66 degrees) for the inferior peak. The range of peak amplitudes for the right (left) eye spanned a factor of 1.7 (1.8) and 2.0 (1.7) for the superior and inferior peaks, respectively. CONCLUSIONS: There was a wide variation in the amplitude and shape of the individual RNFL profiles. However, the RNFL profiles of the 2 eyes of an individual were extremely similar. Adding an interocular comparison with OCT RNFL tests should help identify some false positives.