Thickness of retinal nerve fiber layer correlates with disease duration in parallel with corticospinal tract dysfunction in untreated multiple sclerosis JRRDJRRD Volume 46, Number 5, 2009Pages 633–642 Journal of Rehabilitation Research & Development Thickness of retinal nerve fiber layer correlates with disease duration in parallel with corticospinal tract dysfunction in untreated multiple sclerosis Rebecca I. Spain, MD;1–3* Mitchell Maltenfort, PhD;4 Robert C. Sergott, MD;5 Thomas P. Leist, MD, PhD6 1Department of Neurology, Thomas Jefferson University, Philadelphia, PA; 2Department of Neurology, Oregon Health & Science University School of Medicine, Portland, OR; 3Neurology Service, Portland Department of Veterans Affairs Medical Center, Portland, OR; 4Department of Neurological Surgery, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA; 5Neuro-Ophthalmology Service, Wills Eye Institute, Thomas Jefferson University, Philadelphia, PA; 6Comprehensive Multiple Sclerosis Center, Department of Neurology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA Abstract—Optical coherence tomography (OCT) is an emerg- ing clinical and research measure of retinal nerve fiber layer (RNFL) loss in multiple sclerosis (MS) and may reflect neuro- degeneration. Few studies capture the effect of disease duration on the RNFL in subjects without exposure to disease-modulating therapies. We assessed the relationship of RNFL loss with dis- ease duration in subjects with untreated MS and determined if such loss paralleled corticospinal tract dysfunction in MS. Sub- jects underwent OCT (n = 52) and visual testing (n = 60). Either they were either examined or they participated in a vali- dated telephone interview so we could determine their Expanded Disability Status Scale (EDSS) scores. Both RNFL thickness (Spearman rs = –0.47, p < 0.001) and EDSS scores (rs = 0.51, p < 0.001) correlated with disease duration. RNFL thick- ness correlated with EDSS scores (rs = –0.43, p < 0.001). In conclusion, RNFL loss correlates with disease duration and EDSS scores in subjects with untreated MS, indicating that OCT may capture neurodegeneration. Key words: disease duration, disease-modifying therapy, Expanded Disability Status Scale, multiple sclerosis, neurode- generation, optic nerve diseases, optical coherence tomogra- phy, rehabilitation, retinal nerve fiber layer, visual acuity. INTRODUCTON In multiple sclerosis (MS), both inflammatory demye- lination and axonal loss in gray and white matter are observed early, even presymptomatically [1]. These losses lead to central nervous system (CNS) atrophy [2] and Abbreviations: CI = confidence interval, CNS = central ner- vous system, DMT = disease-modifying therapy, EDSS = Expanded Disability Status Scale, HCVA = high-contrast visual acuity, LCVA = low-contrast visual acuity, logMAR = logarithm of minimum angle of resolution, MRI = magnetic resonance imaging, MS = multiple sclerosis, OCT = optical coherence tomography, ON = optic neuritis, OU = oculus uter- que (both eyes), RNFL = retinal nerve fiber layer, RNFLt = RNFL thickness, RRMS = relapsing-remitting MS, VA = visual acuity. *Address all correspondence to Rebecca I. Spain, MD; Department of Neurology, Oregon Health & Science Uni- versity, 3181 SW Sam Jackson Park Road, Portland, OR 97239; 503-494-5759; fax: 503-494-7289. Email: spainr@ohsu.edu DOI:10.1682/JRRD.2008.11.0156 633 mailto:spainr@ohsu.edu 634 JRRD, Volume 46, Number 5, 2009 functional disability [3]. Standard outcome measures of relapse rate and magnetic resonance imaging (MRI) lesion load neither correlate well with functional decline nor reflect CNS neurodegeneration [4–5]. Optical coher- ence tomography (OCT) is under evaluation as an out- come measure of axonal degeneration for use in clinical trials and practice [6]. OCT is a noninvasive technique that uses infrared laser light to create cross-sectional images of the retina; this technique provides quantitative analysis of the retinal nerve fiber layer (RNFL) thickness (RNFLt), macular thickness, and macular volume. The mechanism and reli- ability of OCT are well described [7]. OCT detects changes in the RNFL, which contains unmyelinated axons subject to damage in glaucomatous and ischemic optic neuropathies [8]; more recently, OCT has been applied to optic neuritis (ON) [9]. Short-term follow-up studies after acute ON demonstrate an acute decrement of the peripapillary RNFLt that plateaus by 3 to 6 months [10]. OCT also detects changes in the macula after ON [11]. The calculated macular volume includes the inner macular thickness, which contains the retinal ganglion cells presumably affected after ON by a dying-back axonopathy, as seen in animal models [12]. Alternatively or additionally, primary neuronal degeneration may affect the retinal ganglion cells. OCT has been shown to correlate with functional optic nerve measures such as multifocal visual-evoked potentials [13] and low-contrast visual acuity (LCVA) [14]. As one of the longer white matter tracts in the CNS, the optic nerve is likely to show the effects of both acute inflammation and neurodegeneration early in MS. ON is the presenting symptom of MS in 25 to 50 percent of cases and eventually affects up to 80 percent of patients during the illness, causing significant morbidity [15]. RNFLt in subjects with MS without a history of ON is lower than in control subjects, indicating subclinical axonal loss [9,16]. Thus, OCT is an attractive candidate measure for capturing early changes in MS. The corticospinal tract is another long CNS white matter tract frequently affected in MS. Like the optic nerve, it is subject both to acute inflammation in the form of transverse myelitis and to neurodegeneration as seen by increasing gait dysfunction, MRI atrophy [2], and recently, gray matter demyelination on high-field MRI [17]. The Expanded Disability Status Scale (EDSS) is a standard measure of gait dysfunction and may correlate with other long tract signs, including RNFL loss and cog- nitive dysfunction [18], although it poorly predicts long- term disability [19] and its validity in relapsing-remitting MS (RRMS) has been recently challenged [20]. OCT may provide a more sensitive research and clinically applicable measure to capture neurodegeneration in MS. Most studies using OCT have not accounted for the effects of disease-modifying therapies (DMTs) on the RNFL, which may be significant if similar to DMT effects in early MS [21]. We studied OCT in subjects with MS of varying disease durations and with no expo- sure to DMTs to test the hypothesis that in untreated MS, RNFLt loss occurs both over the course of disease and in parallel with corticospinal tract dysfunction reflecting CNS neurodegeneration. METHODS Subjects The institutional review board approved all study procedures, and subjects gave their written informed con- sent. We recruited subjects over 18 years of age from the Thomas Jefferson University Comprehensive MS Center (Philadelphia, Pennsylvania) and the Neuro-Ophthalmology Service at Wills Eye Institute (Philadelphia, Pennsylva- nia) with no exposure to DMT. Subjects met revised McDonald criteria for MS [22]. We included subjects with clinically isolated syndrome if they fulfilled two or more modified Barkhof MRI criteria [23]. We deter- mined disease-duration subject history. We considered neurological symptoms occurring before the diagnosis date to be caused by MS if they were of a typical nature (brainstem, spinal cord, visual, or multisystem), lasted for at least 24 hours, and were unexplained by other neu- rological or systemic conditions. Exclusion criteria included any DMT use, including interferon beta preparations, glatiramer acetate, natali- zumab, mitoxantrone, chemotherapeutics, or immuno- suppressants. We did not consider corticosteroid treatment a DMT for the purpose of this study, although the effects of corticosteroids on RNFLt after ON are largely unknown. Of the subjects, 21 had been treated with corti- costeroids before enrollment; only 4 had received corti- costeroid treatment for acute ON in the year before enrollment. We delayed enrollment in subjects with acute ON for 3 to 6 months to allow the RNFL to stabilize [10] and for at least 1 month after intravenous corticosteroid treatment. We excluded subjects with comorbid systemic 635 SPAIN et al. Retinal nerve thickness in untreated MS diseases that could mimic MS, ocular disease that might affect the OCT results, or other abnormal fundoscopic findings unrelated to MS. We also excluded subjects with OCTs performed at nonparticipating facilities or who were unable to cooperate with testing. The primary end point was to establish a correlation between RNFLt and disease duration. Secondary end points included establishing correlations between EDSS scores and disease duration, RNFLt and EDSS scores, and RNFLt and both high-contrast visual acuity (HCVA) and LCVA and determining the effects of ON history on these relationships. The study was powered on the pri- mary end point only. We calculated a sample size of 45 subjects to achieve 80 percent power to detect a moderate Spearman rank correlation of 0.4 with a 5 percent two-sided test. After study initiation, data regarding the normal age-related RNFL decrement of 2 µm/decade over the age of 18 years became available [24]. We increased RNFLt by a factor of 2 µm for every decade over age 18 for each subject, and we recruited additional subjects to allow for a lower correlation coefficient. We did not age adjust macular data because limited data on normative age-related decrements exists [11]. Of the subjects, we screened 134, enrolled 83, and then excluded an additional 31 after enrollment, leaving 52 subjects. Screen failures were from lack of interest (n = 32), DMT use (n = 6), uncertainty of MS diagnosis (n = 4), ON within 3 months of OCT (n = 4), comorbid sys- temic or ocular disease (n = 3), clinically isolated syn- drome failing MRI criteria (n = 1), and OCT from a nonparticipating facility (n = 1). Reasons for exclusion after enrollment were DMT use (n = 16), clinically iso- lated syndrome failing MRI criteria (n = 6), ON within 3 months of OCT (n = 5), lack of informed consent (n = 2), uncertainty of MS diagnosis (n = 1), and comorbid ocular disease (n = 1). History of Optic Neuritis Prior studies have shown an absolute difference in RNFLt based on individual eye ON history [10,14]. We obtained ON history in our study by subject report. Because ON history is subject to recall and other biases, we performed multivariate analyses considering the effect of individual eye ON history and thinner side RNFLt in the manner proposed by Sepulcre et al., presuming the thinner side is most affected by inflammation [25]. Retinal Imaging OCT images were acquired with a Stratus OCT with (software version 4.0, Carl Zeiss Meditec, Inc; Dublin, Cali- fornia) by trained technicians at the Wills Eye Institute and Thomas Jefferson University, and Dr. Sergott reviewed them for quality. We dilated the subjects’ pupils with 2.5 percent phenylephrine hydrochloride and 0.5 percent tropicamide if a small pupil sized impaired image quality [26]. We captured three 3.4-mm diameter Fast RNFL Pro- tocol scans (scan length = 10.87 mm) centered on the optic disk for each eye. The Stratus OCT software calcu- lated mean RNFLt values from each 360° scan, and we averaged the three means for each eye using Microsoft Excel (version 2003, Microsoft Corp; Redmond, Wash- ington) to reduce the influence of technical error during scan acquisition. The Fast Macular Protocol (scan length = 6.0 mm, reliability within ±10 µm) used the average of six radial lines to compose a macular thickness/volume map. We reviewed additional fast optic disk and fast crosshair images to exclude other ocular pathologies. All images met the signal strength requirements of at least 7 (maximum 10) as per manufacturer recommendation. Visual Testing We measured the HCVA of each eye with appropriate refraction using a retro-illuminated Early Treatment Dia- betic Retinopathy Study chart (Precision Vision; La Salle, Illinois) at 4 m, and we recorded 4 m as the loga- rithm of minimum angle of resolution (logMAR) acuity. We tested LCVA at 2 m and evaluated given data sug- gesting improved sensitivity for optic nerve dysfunction [27] using a 1.25 percent low contrast Sloan letter chart (Precision Vision). We determined the eye with greater visual impairment by logMAR value; we considered eyes with equal logMAR values equally impaired. Corticospinal Tract Dysfunction Dr. Spain examined Thomas Jefferson University subjects (n = 43) for their EDSS scores [28]. Because they were unavailable to be evaluated in the clinic, Wills Eye Institute subjects (n = 9) participated in a stan- dardized and validated telephone interview conducted by Dr. Spain that established their EDSS scores [29]. Statistical Analysis We used Spearman rank correlation to assess correla- tions between OCT parameters, EDSS scores, visual 636 JRRD, Volume 46, Number 5, 2009 acuity (VA), and disease duration. OCT parameters from each eye included the average RNFLt; the thickness of the temporal, inferior, nasal, and superior quadrants of the RNFL; the total macular volume; and the minimum and average thicknesses of the fovea. We age-adjusted RNFLt as described previously. We performed multivari- ate analyses to determine the effects of ON history as described previously. We used the Tukey test method to limit type 1 error from performing more than one statistical test; individual p-values of 0.025 and 0.018 limited the overall type 1 rate at 0.05 as indicated. We used analysis of variance to compare the mean RNFLt for both eyes (ocu- lus uterque [OU]) between cohorts with and without ON history. We performed statistical analyses using JMP, ver- sion 8.0 (SAS Institute, Inc; Cary, North Carolina). RESULTS Table 1 shows subject demographics. Wills Eye Institute subjects (n = 9) were more likely to have RRMS (78% vs 53%), ON (100% vs 37%), a higher mean EDSS score (3.9 vs 3.0), and a lower mean RNFLt (75.57 µm vs 88.51 µm) than Thomas Jefferson University subjects (n = 43). We found no other systematic differences between recruitment sites. The distribution of subjects by disease duration demonstrated a wide spread, with 18 subjects in the 0- to 3-year duration group, 12 in the 4- to 6-year group, 8 in the 7- to 12-year group, and 14 in the >12-year group. The RNFLt OU correlated significantly with MS dis- ease duration (Spearman rs = –0.47, p < 0.001) (Figure 1(a) and Table 2). Other OCT parameters, including macular volumes and foveal thicknesses, did not demon- strate superior correlations with disease duration com- pared with the RNFLt (Table 2). The inferior quadrant RNFLt had the strongest correlation with disease dura- tion than any other sector, although all were significant (data not shown). While some studies suggest a greater absolute RNFLt loss in the temporal sector [30], others do not [31]; as yet, little data exist to suggest differing regional rates of RNFL decline. Corticospinal tract dysfunction as captured by EDSS also correlated with disease duration (rs = 0.51, p < 0.001) (Figure 1(b) and Table 2). Finally, RNFLt OU correlated with EDSS scores (rs = –0.43, p < 0.001) (Fig- ure 1(c) and Table 3). We performed multivariate analyses to determine the effects of individual eye ON history and the thinner RNFLt on the correlations between RNFLt and EDSS scores with disease duration. While RNFLt differed sig- nificantly between eyes with a history of ON (75.81 µm, 95% confidence interval [CI] 69.96–81.65 µm) compared with no history (90.93 µm, 95% CI 87.98–93.87 µm; p < 0.001) and between thinner (80.30 µm, 95% CI 76.38– 84.21 µm) and thicker side (92.25 µm, 95% CI 88.34– 96.16 µm; p < 0.001), neither had a significant affect on the relationship between RNFLt or EDSS scores and dis- ease duration (Table 4). Of note, 6 of the 24 subjects with known ON had been affected bilaterally, thereby decreas- ing a possible ON effect. Interestingly, the ON eye did not always demonstrate a more impaired OCT; of 18 sub- jects with monocular ON, 4 had a thinner RNFLt and 5 had a lesser macular volume in the opposite eye. As a proof of concept, VA worsened at a lower RNFLt. HCVA was shown to correlate with RNFLt (rs = –0.53, p < 0.001, n = 51) (Table 3), as did LCVA (rs = –0.34, p = 0.002), although we tested fewer subjects using LCVA (n = 39). Both ON history and thinner-side RNFLt influenced the VA results, as shown by multivariate analyses (Table 4), although none reached significance after we applied the Tukey test for multiple analyses. A categorical difference existed between subject cohort-based ON history. The mean RNFLt OU was sig- nificantly lower in subjects with a history of ON (79.06 µm, 95% CI 75.31–82.81 µm) compared with in those without (93.49 µm, 95% CI 89.74–97.23 µm; p < 0.001) (Figure 2). The distinction in baseline RNFLt based on subject ON Table 1. Subject demographics. Variable Number (%) Total 52 (100) Thomas Jefferson University 43 (83) Wills Eye Institute 9 (17) Female 43 (83) Age (yr, mean ± SD) 43.2 ± 11.1 (range 23–72) Type of MS Clinically Isolated Syndrome 11 (21) Relapsing-Remitting 30 (58) Secondary Progressive 3 (6) Primary Progressive 8 (15) History of Optic Neuritis 24 (46) Disease Duration (yr, mean ± SD) 9.3 ± 10.3 Disease Duration (yr, median) 5.5 (range 0.2–45.0) EDSS Score (mean ± SD) 3.2 ± 1.6 EDSS Score (median) 3.0 (range 1.0–6.5) EDSS = Expanded Disability Status Scale, MS = multiple sclerosis, SD = standard deviation. 637 SPAIN et al. Retinal nerve thickness in untreated MS history may have implications for using OCT as an out- come measure in populations with MS heterogeneous in ON status. DISCUSSION Few studies have assessed RNFL loss in subjects with MS who had not received DMTs. We found that in MS, a greater correlation existed between RNFL loss and desease duration than could be accounted for by age alone, corroborating other recent studies [25,32]. In addi- tion, the functional measure EDSS also correlated with disease duration, supporting the hypothesis that long white matter tracts (optic nerve and corticospinal tracts) are frequently affected by symptomatic inflammation and insidious neurodegeneration. While RNFL and EDSS scores correlated with each other, as has been shown in other studies [25,32], com- parison of their relative sensitivities to change over the Figure 1. (a) Retinal nerve fiber layer thickness (RNFLt) of both eyes correlates with disease duration. (b) Expanded Disability Status Scale (EDSS) scores correlates with disease duration. (c) RNFLt of both eyes correlates with EDSS scores. “+” = subject history of optic neuritis (ON), “×” = no subject history of ON. 638 JRRD, Volume 46, Number 5, 2009 short term cannot be evaluated in this cross-sectional design. Furthermore, OCT is a continuous physiological measure, while EDSS is an ordinal functional measure and thus not directly comparable. This study does, how- ever, provide baseline data for a prospective study of RNFL loss in subjects with untreated MS to assess the true rates of loss regarding ON history and sensitivity to short-term changes. Rates of RNFL loss need to be assessed in conjunction with whole brain or gray matter atrophy measures [3] and more sensitive measures of cor- ticospinal tract dysfunction than the EDSS [19–20] so we can gain a comprehensive measure of the neurodegenera- tive component of MS. Only then can neuroprotective strategies for preserving vision, gait, cognition, and the other debilitating effects of MS be more confidently tested. We found that peripapillary RNFL measures pro- duced stronger correlations with disease duration than did macular measures of thickness [11] or volume [33], which have been found in other studies to be more sensi- tive to the effects of ON. Further elucidation is required. Correlations between RNFLt OU and disease dura- tion were relatively modest. Exclusion of subjects with clinically isolated syndrome based on MRI criteria may have biased the subjects with shorter disease duration toward those with more advanced disease and potentially more RNFL involvement than a true early MS cohort. Additionally, some of the subjects with longer disease duration, having chosen not to initiate DMT, may have been biased toward less severe disease and potentially less RNFL loss. The cross-sectional design of the study and heterogeneous subject population regarding MS type and stage of disease may also have influenced the strength of correlations. Multivariate analysis using individual eye ON his- tory and thinner RNFLt side did not affect the relation- ships of RNFLt and EDSS scores to disease duration, although this study was not powered to address these sec- ondary outcomes. This finding may indicate that subclin- ical ON and/or ongoing neurodegeneration affect both optic nerves sufficiently to negate the influence of symp- tomatic ON. Interestingly, OCT data contradicted the recalled history of ON in one-quarter of subjects with lesser RNFLt in the reportedly unaffected eye. A larger prospective study of RNFL loss is necessary to determine the character and tempo of neurodegeneration given ON history. At a given time point, using the averaged RNFLt OU may prove a more useful global measure of disease burden. The smaller baseline RNFLt among subjects with a history of ON as found in other trials may require sub- ject stratification by subject history of ON rather than individual eye history of ON in clinical trials [25]. Table 2. Spearman correlation coefficients (rs) and p-values between optical coherence tomography (OCT) parameters and disease duration (in years) and between Expanded Disability Status Scale (EDSS) scores and disease duration. Significance set at p < 0.025 for multiple comparisons (Tukey test). Outcome Measure Disease Duration rs p-Value OCT Parameter RNFLt OU (µm) –0.47 <0.001 RNFLt Inferier Quadrant OU (µm) –0.55 0.001 Total Macular Volume OU (mm3) –0.30 0.002 Fovea Minimum Thickness OU (µm) 0.22 0.026 Fovea Thickness OU (µm) — — EDSS 0.51 <0.001 NS = not significant, OU = oculus uterque (both eyes), RNFLt = retinal nerve fiber layer thickness. Table 3. Spearman correlation coefficients (rs) and p-values between retinal nerve fiber layer thickness (RNFLt) of both eyes (oculus uterque [OU]), Expanded Disability Status Scale (EDSS) scores, and both high- and low-contrast visual acuities (HCVA and LCVA, respectively). Significance set at p < 0.05. Outcome Measure RNFLt OU (µm) EDSS EDSS rs = –0.43 — p < 0.001 — HCVA rs = –0.53 rs = 0.48 p < 0.001 p < 0.001 LCVA rs = –0.34 rs = 0.15 p = 0.002 p = 0.18 639 SPAIN et al. Retinal nerve thickness in untreated MS As proof of concept, we found worse VA to correlate with a lower RNFLt and a more impaired EDSS score, with stronger correlations for HCVA than LCVA likely influenced by the unequal numbers tested (Table 3). Overall, correlations between RNFLt and VA measures were sensitive to the effects of ON and thinner RNFLt side, although the study was underpowered to these sec- ondary outcomes. The poorer visual outcomes at lower RNFLt underscore the need to measure and preserve reti- nal axons. Larger prospective studies that include more sensi- tive measures of corticospinal tract dysfunction than the EDSS, coupled with advanced neuroimaging techniques and, ideally, histopathological investigations, will help detect differences in the pattern and rate of RNFL loss by disease subtype and ON history, pointing to underlying pathological mechanisms of neurodegeneration [3,34]. Studies comparing DMT-exposed (including corticoster- oids) and non-DMT-exposed subjects may help detect treatment effect on the RNFL. Finally, improvements to the current 8 to 10 µm resolution of the Stratus OCT will have greater power to differentiate neurodegeneration from normal age-related losses to the RNFL, as well as any potential DMT effects on preserving crucial visual function in MS. CONCLUSIONS In summary, we found RNFLt to correlate with dis- ease duration in parallel with corticospinal tract dysfunc- tion, as measured by the EDSS in a population with MS untreated by DMTs. Subjects with ON history had a lower baseline RNFLt. This study suggests that OCT may prove to be a useful instrument for capturing the CNS effects of inflammation and neurodegeneration for use in both clinical trials and practice. Table 4. Multivariate analyses to determine if factors of individual eye optic neuritis (ON) history or thinner retinal nerve fiber layer (RNFL) interacted with correlations between RNFL thickness (RNFLt) and disease duration, between Expanded Disability Status Scale (EDSS) scores and disease duration, and between RNFLt and both high- and low-contrast visual acuities (HCVA and LCVA, respectively). Significance set at p < 0.018 for multiple comparisons (Tukey test). Parameter Mean Parameter Estimate ± Standard Error p-Value RNFLt and Disease Duration by ON History 0.038 ± 0.248 0.09 RNFLt and Disease Duration by Thinner Side –0.021 ± 0.149b 0.87 EDSS and Disease Duration by ON History 1.009 ± 1.784 0.57 EDSS and Disease Duration by Thinner Side –0.671 ± 0.471 0.16 RNFLt and HCVA by ON History 9.251 ± 4.202 0.03 RNFLt and HCVA by Thinner Side –6.198 ± 2.731 0.03 RNFLt and LCVA by ON History 0.752 ± 5.857 0.90 RNFLt and LCVA by Thinner Side –5.938 ± 3.258 0.08 Figure 2. Subjects with history of optic neuritis (ON) (1) had significantly lower mean retinal nerve fiber layer thickness (RNFLt) in both eyes (79.06 µm, 95% confidence interval [CI] 75.31–82.81 µm) than subjects without history of ON (93.49 µm, 95% CI 89.74–97.23 µm; p < 0.001). 640 JRRD, Volume 46, Number 5, 2009 ACKNOWLEDGMENTS Author Contributions: Study concept and design: R. C. Sergott, T. P. Leist. Subject recruitment: R. C. Sergott, T. P. Leist, R. I. Spain. Collection of data: R. 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Spain, MD;1-3* Mitchell Maltenfort, PhD;4 Robert C. Sergott, MD;5 Thomas P. Leist, MD, PhD6 1Department of Neurology, Thomas Jefferson University, Philadelphia, PA; 2Department of Neurology, Oregon Health & Science Unive... INTRODUCTON METHODS Subjects History of Optic Neuritis Retinal Imaging Visual Testing Corticospinal Tract Dysfunction Statistical Analysis RESULTS Table 1. Figure 1. DISCUSSION Table 2. Table 3. Table 4. Figure 2. CONCLUSIONS ACKNOWLEDGMENTS REFERENCES