Electrophysiological Findings in Neuromyelitis Optica Spectrum Disorder

Neuromyelitis Optica Spectrum Disorder (NMOSD) has been differentiated from Relapsing Remitting Multiple Sclerosis (RRMS) by clinical, laboratory, and pathological findings, including the presence of the anti-aquaporin 4 antibody. However, it is recognized that patients with NMOSD are hard to differentiate from those with RRMS. Measurement of Evoked Potentials (EPs) is often used to diagnose RRMS and to find subclinical lesions in patients with RRMS, but EPs have not been investigated in detail in patients with NMOSD. Here, we have compared EPs in patients with NMOSD to those with RRMS. Characteristic findings in patients with NMOSD were the absence of both visual evoked potentials and of motor evoked potentials in the lower extremities, while in patients with RRMS there were delays in these potentials. Most patients with NMOSD did not present abnormal subclinical EPs, whereas many patients with RRMS did. None of the patients with NMOSD showed abnormalities in auditory brainstem responses. These data lead us to conclude that NMOSD can be differentiated from RRMS by EP data obtained in the early stages of these diseases. Multiple Sclerosis; SPMS: Secondarily Progressive Multiple Sclerosis; PPMS: Primary Progressive Multiple Sclerosis; EP: Evoked Potential; AQP4-Ab: Anti-Aquaporin 4-Antibody; EDSS: Expanded Disability Status Scale; VEP: Visual Evoked Potential; ABR: Auditory Brainstem Response; SEP: Somatosensory Evoked Potential; MEP: Motor Evoked Potential; UE: Upper Extremity; LE: Lower Extremity; CSCT: Central Sensory Conduction Time; TMS: Transcranial Magnetic Stimulation; CMCT: Central Motor Conduction Time; MRI: Magnetic Resonance Imaging antibody-positive patients reported by have been classified as NMOSD. MEP: Evoked Potentials, NMOSD: Neuromyelitis Sclerosis,


Introduction
Neuromyelitis Optica Spectrum Disorder (NMOSD) is an autoimmune, inflammatory, necrotic disease with lesions in the optic nerve and spinal cord. It has been challenging to distinguish NMOSD from Relapsing Remitting Multiple Sclerosis (RRMS), an autoimmune, inflammatory, demyelinating disease of the central nervous system. At present, NMOSD is recognized as a different disease from RRMS due to its specific association with the Anti Aquaporin 4 antibody (AQP4-Ab) [1,2]. On the other hand, patients positive for AQP4-Ab that presented with not only optic neuritis and transverse myelitis but also with symptoms of lesions in the brain stem and/or cerebrum have been classified as having NMOSD [3]. Moreover, not all NMOSD patients have serum AQP4-Ab, and some patients with NMOSD are similar to RRMS in their clinical symptoms and MRI findings [4,5]. Distinguishing NMOSD from RRMS is important because disease-modifying therapies are different for the two diseases. Evoked Potentials (EPs) have been used in patients with RRMS as an important examination for diagnosis. In this review, we summarize the utility of EPs in the differential diagnosis of RRMS and NMOSD.

Evoked Potentials
An EP is an electrical potential with a specific pattern recorded from a specific part of the central nervous system following a particular stimulus. Visual EPs (VEP) can be used to confirm damage to the visual pathway, including the retina, optic nerve, optic chiasm, optic radiations, and occipital cortex [6]. VEPs are induced by a visual stimulation consisting of looking at a checkerboard pattern on a computer screen. When using the standard 10-20 EEG pattern of electrodes VEPs are recorded at the Oz electrode with reference to the Fz electrode.
The P100 component of the VEP response, which is a positive peak with a delay of about 100ms from the stimulus presentation, is of major clinical importance. The P100 originates from the occipital cortex, the first visual field. An abnormal latency or no P100 suggests the presence of a lesion somewhere between the optic nerve and the occipital cortex. Visual pathway dysfunctions anterior to the optic chiasm may be the place where VEPs are most useful for diagnoses. For example, patients with acute severe optic neuritis often lose the P100 response [7].
The Auditory Brainstem Response (ABR) is used to evaluate the integrity of the auditory pathway up to the midbrain. An ABR is elicited in the auditory nerve within the auditory pathway in the brain stem by a sound stimulation. The response is generated in the cochlea, passes through the cochlear nerve, the cochlear nucleus, the superior olivary complex, the lateral lemniscus, to the inferior colliculus in the midbrain; it then passes on to the medial geniculate body, and finally to the cortex [8]. ABRs to clicks are recorded in the Cz electrode with reference to the ipsilateral and contralateral ears.
This stimulus elicits between five and seven positive waveforms designated by roman numbers. The I, III, and V waveforms are recorded as main peaks and originate from the auditory nerve, nucleus olivaris superior, and the inferior colliculus, respectively.
The latencies of these main peaks (I, III, V). and their inter-peak latencies (I-III, III-V, I-V) are commonly studied.
Somatosensory Evoked Potentials (SEP) are recorded from the brain or spinal cord while repeatedly stimulating a peripheral nerve [9]. SEPs are used to evaluate somatosensory pathways at the peripheral, spinal, cortical, and subcortical levels, and have proven their benefit in assessing disorders of the central nervous system more than of peripheral nerve lesions [10]. We obtained Upper Extremity (UE) and Lower Extremity (LE) SEPs by electrically stimulating the median nerve at the wrist and the tibial nerve at the ankle, respectively. Electrodes for recording the SEPs of the UEs were

EPs in RRMS Patients
Many clinical utilities of EPs in patients with RRMS have been reported [12][13][14]. Subclinical lesions in the visual, auditory, deep sensation, and pyramidal tracts can be revealed by EPs [15].
Abnormal VEP findings are included in the McDonald Criteria for RRMS [16]. The summed score of the SEP and MEP exhibit high sensitivity and specificity for RRMS and are correlated with the RRMS -related disability-ambulation scale including the Expanded Disability Status Scale (EDSS) [17]. The correlation between abnormal EPs and EDSS was reported to be higher than between conventional MRI and EDSS [18]. In this way, several studies have suggested a prognostic value for EPs in RRMS and an association between EPs and the severity of clinical disability.

EPs in NMOSD Patients
There have been several reports on EPs in patients with NMOSD [19,20]. More patients with AQP4-Ab were reported to show a lack of the P100 on VEP than those without the antibody [20]. In addition, AQP4-Ab positivity and a visually unevoked response were significantly related to the development of severe visual impairment in patients with NMOSD [20]. VEPs in patients with NMOSD were characterized by the absence of a response or a decreased P100 amplitude with normal latency [19]. It was also reported that abnormal MEPs and SEPs of the LEs in NMOSD were related to higher EDSS, making them a good indicator of disability status and relapsing activity in NMOSD patients [21].

Comparisons between NMOSD and RRMS
We studied the EP findings of NMOSD and RRMS patients [22].
All of the patients with NMOSD were AQP4-Ab-positive and fulfilled the diagnostic criteria for NMOSD [3]. Using the McDonald criteria [23], only patients with confirmed RRMS were evaluated, while those with secondarily progressive MS and primary progressive MS were excluded from the study. All patients with RRMS were AQP4-Ab-negative. Patients with NMOSD and RRMS in whom more than 5 years had elapsed since their time of onset were also excluded from the study. Thus, we analyzed 23 NMOSD patients and 28 with

RRMS.
The clinical characteristics of these two groups of patients are summarized in (Table 1). The NMOSD patients were older (P < 0.001) and the ratio of females was higher than those with RRMS (P = 0.044). The latencies from onset to diagnosis were similar in the two groups. The main initial symptoms of both patient groups were visual and sensory disturbances. The prevalence of visual disturbances and bladder dysfunction were significantly higher in the NMOSD group; and hiccups and nausea and bladder dysfunction were found only in patients with NMOSD. Other symptoms did not differ significantly between the two patient groups. However, the mean EDSS score at the first examination was higher in the NMOSD group than in the RRMS group (P > 0.001).
At the EP examination, patients with NMOSD showed variable clinical symptoms with EDSS scores ranging from 2 to 8, whereas those with RRMS had EDSS scores ranging from 1 to 3. While only 3 patients with NMOSD demonstrated the presence of the oligoclonal band in their CSF, 65.2% of patients with RRMS were positive for it (P = 0.002). Patients with NMOSD who had severe muscle weakness presented longitudinally extensive signal abnormalities on MRI, extending from the cervical cord to the thoracic spinal cord. An intramedullary lesion was also observed, involving predominantly the central gray matter. We also compared the prevalence of abnormal EPs in symptomatic regions of patients with NMOSD and those with RRMS (Table 3), but there were no differences in the frequencies of abnormal EP findings between the two groups.     aThe numbers after the slashes (/) represent total number of extremities examined. *p < 0.05 (NMOSD vs. RRMS; Fisher exact test).  (Tables 4-6). Similar to our findings [20] found that a significantly higher number of patients with NMOSD had no VEP P100 than did RRMS patients, whereas more patients with RRMS showed a delayed P100. These results agree with another previous report showing that NMOSD patients presented a loss of P100 [19] ( Tsao et al. [21] reported that MEPs in the LEs were a good indicator for disability status in NMOSD patients whose lesions were in the thoracic spinal cord [21]. In addition, recent studies using diffusion tensor imaging also showed significant white matter involvement combined with gray matter involvement in the spinal cords of NMOSD patients, which was not visible on conventional MRI, and such involvement was associated with the clinical disabilities of patients with NMOSD [27,28]. These studies suggest the presence in NMOSD patients of greater diffuse damage in the spinal cord than can be detected by conventional MRI. Therefore, measurement of EPs may reflect the diffuse damage and pathological severity more accurately than conventional MRI. Both Watanabe et al. [20] and Tsao et al. [21] reported that NMOSD patients more frequently presented abnormal MEPs than we observed in our study [22] (  (Table 5). But SEPs and MEPs in the LE were more affected than in the UE in both patients with NMOSD and those with MS.
While few patients with NMOSD presented abnormal ABRs, delayed ABRs were observed in 21.7% of patients with RRMS [22].
Abnormal ABRs were frequently seen in most patients with RRMS who had normal hearing or mild hearing loss [29]. In contrast, abnormal ABRs were not detected in NMOSD patients without auditory symptoms, even in the presence of medullary symptoms such as Intractable Hiccups and Nausea (IHN) in this study. IHN was also found in 17% of patients with NMOSD but in none of the patients with RRMS [30]. MRI showed that in those patients with NMOSD, the medullary lesions involved the area postrema and the nucleus tractus solitaris. In NMOSD patients with anti-AQP4 antibodies and brainstem lesions, the anatomical pathway involved in ABRs might be preserved due to their unique pathophysiological background, although there is one report on ABRs in patients with NMOSD, in which the ABRs were severely attenuated due to hearing loss without brainstem lesions as determined by MRI [31]. Therefore, it may be a critical point for the differentiation of NMOSD from RRMS that patients with RRMS likely present abnormal ABRs even in the early stage of the disease, whereas patients with NMOSD do not.

Conclusion
While nearly all of our NMOSD patients exhibited normal subclinical EP findings, more than half of the RRMS patients exhibited abnormal subclinical EP findings. However, significantly more NMOSD patients were lacking P100 VEPs and LE MEPs compared with the RRMS patients, suggesting that patients with NMOSD have severe clinical symptoms compared to those with RRMS. Because these EP findings appear to be characteristic in patients with NMOSD and RRMS, EPs may enable us to differentiate patients with NMOSD from those with RRMS in the early stage of the disease.