Acute isolated optic neuritis may be the first manifestation of both multiple sclerosis (MS) and neuromyelitis optica (NMO). The discovery of the aquaporin 4 autoantibody (AQP4-Ab) has provided serological markers to distinguish NMO from MS and led to the description of neuromyelitis optica spectrum disorder (NMOSD) (1,2) (Table 1).
Patients with NMO may experience a long temporal delay after acute optic neuritis before a relapse in the form of transverse myelitis occurs (2). In such cases, an episode of optic neuritis caused by NMO may be indistinguishable clinically from optic neuritis caused by MS. Comparison of brain magnetic resonance imaging (MRI) findings may be limited as Matsushita et al (3) have shown in patients who are seropositive for AQP4-Ab and those with typical MS. However, would MRI of the anterior visual pathways be more useful in distinguishing patients with NMO from MS?
Khanna et al (4) have reported a trend for more posterior lesions within the anterior visual pathways in patients with NMO and chiasmatic involvement occurring only in NMO. They found no significant difference in the length of the inflammatory lesion between the 2 groups.
In this pilot study, we compared the MRI appearance of the anterior visual pathways in acute optic neuritis in NMOSD to MS. We devised a simple scoring system to evaluate 2 aspects of the MR abnormalities: the linear location and thickness of the cross-sectional area (CSA).
This was a retrospective pilot study in which the MRI results of 27 patients were studied. Fifteen patients had confirmed MS and 12 patients had NMOSD. All patients presented over a 3-year period with acute isolated optic neuritis and were scanned using a 1.5-tesla or 3.0-tesla scanner during the acute phase (all within 6 weeks of symptom onset). Patients with coexisting neurological or systemic illness causing other visual pathway or brain lesions were excluded.
A diagnosis of NMOSD was given to patients who met established diagnostic criteria (2) (Table 1). Multiple sclerosis was diagnosed according to the revised McDonald criteria (5). All patients were tested for AQP4-Ab. Testing was carried out at the Wetherall Institute of Molecular Medicine, University of Oxford, by a method using the fluorescence immunoprecipitation assay technique described elsewhere (6). Multiple sclerosis patients were all seronegative for AQP4-Ab.
The majority of patients had MRI of the anterior visual pathways using standardized clinical protocols, performed on a General Electric Discovery MR450 1.5-tesla MRI unit (GE Healthcare, Waukesha, WI) or Siemens Trio 3-tesla MRI unit (Siemens AG, Erlangen, Germany). As data was collected over several years, some examinations were acquired with other scanners with minor variations in acquisition protocols. All MRIs included coronal T2 fat-suppressed and T1 images of the anterior visual pathways in addition to imaging of the brain and/or spine. Intravenous contrast was used in selected cases.
Imaging parameters for the coronal T2 fat-suppressed sequences were 1) General Electric Discovery MR450 1.5-tesla MRI: fat saturation, echo delay time (TE) 102.0, repetition time (TR) 4983.0, sample averaging (NEX) 3, base resolution 384, field of view (FoV) 18.0, slice thickness 3.0 mm and 2) Siemens Trio 3-tesla MRI: fat saturation, TE 84.0, TR 5020.0, averages 3, base resolution 384, FoV 18.0, slice thickness 2.0 mm.
Imaging parameters for the coronal T1 sequences were 1) General Electric Discovery MR450 1.5-tesla MRI: TE 8.0, TR 597.0, NEX 4, base resolution 256, FoV 18.0, slice thickness 3.0 mm and 2) Siemens Trio 3-tesla MRI: fat saturation, TE 8.2, TR 500.0, averages 2, base resolution 256, FoV 18.0, slice thickness 2.0 mm.
Magnetic resonance images were assessed independently by 2 neuroradiologists (I.D. and M.R.), who were blinded to the patients' history and diagnosis. A consensus decision was reached in case of disagreement.
Protocol for the Presence of Inflammation
Contrast-enhanced MRI has been reported as the gold standard for the detection of inflammation in the visual pathways (7). In accordance with local hospital protocol, gadolinium was not used in the majority of our cases. Increase in the thickness or CSA of the affected part of the anterior visual pathways occurring during acute optic neuritis was used as an absolute marker for inflammation (8). The presence of optic nerve T2 signal hyperintensity supported the presence of inflammation; it was not considered an absolute marker for inflammation as its persistence following the resolution of acute optic neuritis has been reported (8).
Anterior visual pathways were divided into 10 segments: orbital, canalicular, and intracranial segments of the left and right optic nerves, the left and right halves of the optic chiasm, and the left and right optic tracts (Fig. 1). T2 fat-suppressed and corresponding T1 sequences were used to assess CSA and T2 signal hyperintensity.
The number of anatomical segments affected by an increase in CSA at any point on the segment was noted in each case. A score of +1 was given for each affected segment, such that a patient with the involvement of all segments would be given a score of 10. A segment was not required to be thickened along its entire length in order for it to be given a score of +1. The entire anatomical extent of the lesion did not need to be continuous along the extent of positive scoring.
Lesion extent scores were compared between MS and NMOSD groups using the Mann–Whitney rank sum test. A P value of 5% was used to define statistical significance. The relative risk (RR) of higher scoring was calculated for the 2 groups.
The involvement of each segment was compared between the 2 groups using the 2-tailed Fisher exact test. A P value of 5% was used to define statistical significance. The RR of the involvement of each segment for the 2 groups was calculated.
Twelve patients were diagnosed with NMOSD and 15 patients with MS. The female to male ratio was 10:2 in the NMOSD group and 11:4 in the MS group. The mean age of patients was 39 years (range 27–51) and 34 years (range 26–42) in the NMOSD group and MS group, respectively. Caucasians comprised 69% of the MS patients and 17% of those with NMOSD. Figures 2 and 3 are schematic illustrations demonstrating the pattern of visual pathway involvement in the NMOSD and MS groups.
Figure 4 shows the lesion extent scores in optic neuritis patients with NMOSD and MS. Patients with MS demonstrated a mean score of 2.2 (range, 1–5) compared with a mean score of 4.0 (range, 2–7) in NMOSD patients. The difference between the means was statistically significant (P = 0.007). The RR of having a lesion extent score 4 in NMOSD vs MS was 7.5 (95% confidence interval: 0.33–17.3). A score of greater than 6 was seen only in patients with NMOSD.
Table 2 shows the frequency of involvement of each site and the RR for NMOSD over MS at each site across the patients within each group. A trend for anterior involvement was seen in MS patients. The RR of segment involvement within the NMOSD group increased with a more posterior location (RR for optic tract involvement = 3.13 vs RR for intracanalicular involvement = 1.25). The number of NMOSD patients with chiasmal involvement was significantly greater than the number of MS patients (P = 0.021). Both MS (n = 2) and NMOSD (n = 5) patients displayed bilateral optic chiasmal involvement.
Our study demonstrates that a novel MRI-based scoring system may help differentiate optic neuritis in patients with NMOSD vs MS. A lesion extent score ≥4 is highly suggestive of NMOSD. Anterior visual pathway inflammation in optic neuritis secondary to NMOSD may mirror the longitudinally extensive spinal cord lesions found in NMO.
While lesion distribution was not demonstrably different between NMO and MS patients, predilection was found for more posterior segments in NMOSD patients and for more anterior segments in MS. This is consistent with previous reports (4,9). Chiasmal inflammation was more frequent in patients with NMOSD than MS. This is in contrast to the findings of Khanna et al (4), where chiasmal involvement was found exclusively in NMO patients. In that study, the use of smaller sample sizes (NMO: n = 6; MS: n = 11) and differing imaging techniques (exclusive use of 1.5-tesla magnet) may explain these differences.
Our study has a number of limitations including small number of patients and the lack of use of intravenous contrast. The presence of increased CSA as the criterion for assessing the presence of inflammation along the visual pathway may have excluded patients with prior optic atrophy. As this was a pilot study, there was no standard protocol for the time interval between onset of optic neuritis and the time of scanning or examination of visual parameters. Although a trend for more extensive visual pathway inflammation was observed in NMOSD, the degree of inflammation may have been underestimated as corticosteroid therapy was sometimes initiated on patients with NMOSD prior to MRI.
In conclusion, the results of this study suggest that a scoring system based on the findings of MRI of the anterior visual pathways may help to identify the etiology of acute optic neuritis. This has important clinical implications given the differences in evaluation and treatment of patients with NMOSD vs MS.
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