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Home arrow Environment arrow Inflammatory Disorders of the Nervous System: Pathogenesis, Immunology, and Clinical Management
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Investigations

An MRI of the spinal cord and brain is mandatory in any patient presenting with suspected NMO. The location and length of the spinal cord lesion on MRI can provide vital clues about the diagnosis. Based on clinical and radiologic data, transverse myelitis can be categorized as longitudinally limited or longitudinally extensive [32]. Longitudinally limited partial TM with purely or predominantly sensory manifestations is more typical of MS [32]. In MS, the lesions often affect the cervicothoracic spinal cord and are typically located in the posterolateral or lateral portions of the spinal cord on axial sections [32]. On the other hand, LETM (which refers to a contiguous lesion that extends over three or more vertebral segments and involves more than two-thirds of the spinal cord thickness on axial sections) is distinctly rare in MS and strongly indicates NMO [32]. Furthermore, T1 hypointensity of the central gray of the spinal cord is more suggestive of NMO [48, 49]. Lumbosacral myeloradiculitis has also been described in NMO [50].

A brain MRI is critical to differentiate MS from NMO and is part of the supportive criteria for the diagnosis of NMO (discussed later). The presence of lesions typical of MS would argue against, but not completely exclude, the diagnosis of NMO. Brain lesions occur in 60% of patients of NMO and may affect areas typical for the disease (discussed above), appear nonspecific, or, in rare cases, mimic MS lesions [8]. However, the presence of any lesion adjacent to the lateral ventricle and inferior temporal lobe, a subcortical U-fiber lesion, or a lesion reminiscent of a Dawson’s finger could distinguish MS from NMO with 92% sensitivity and 96% specificity [51]. Diencephalic lesions are distinctly rare in MS and, if present, are more indicative of the diagnosis of NMO [8]. MRI changes suggestive of PRES have also been observed in NMO patients, although it is unclear if PRES was the result of NMO or a complication of therapy [47]. Kim et al. [52] described five categories of brain MRI lesions in NMO patients: (1) corticospinal tract lesions that were often related to LETM and likely represent Wallerian degeneration; (2) extensive, tumefactive, hemispheric white matter lesions with vasogenic edema; (3) periependymal lesions surrounding the cerebral aqueduct, third ventricle, or fourth ventricle; (4) periependymal lesions surrounding the lateral ventricles; and (5) medullary lesions, which were often contiguous with cervical cord lesions.

In patients with suspected AON, it is important to obtain an MRI of the orbits with and without gadolinium. Compared to MS, NMO-related AON often results in abnormal signal and gadolinium enhancement extending to the posterior portions of the optic nerves and even involves the optic chiasm (which some have termed this more extensive distribution along the anterior visual axis “longitudinally extensive optic neuritis”) [53, 54].

NMO-IgG remains the most specific serologic marker of NMO and is one of the supportive criteria in the revised 2006 NMO diagnostic criteria [55]. Improved laboratory techniques have enhanced the sensitivity of NMO-IgG detection. NMO-IgG can be detected in almost three-quarters of recurrent NMO and predicts higher relapse rates [2, 56]. However, NMO-IgG titers are not reliable indicators of disease activity or prognosis [57].

Approximately 20-30% of patients who meet the criteria for NMO are NMO- IgG seronegative [57]. Antibodies against myelin oligodendrocyte glycoprotein (MOG-IgG) have been identified in a subset of NMO-IgG seronegative patients with NMO [58, 59]. MOG-IgG seropositivity has been strongly associated with bilateral relapsing AON, as well as simultaneous and sequential AON and TM [60, 61]. Interestingly, MOG-IgG seropositive AON is typically associated with corticosteroid-responsive papillitis on fundoscopic examination [60], a clinical finding that is distinctly atypical in MS-related AON.

NMO is often associated with organ-specific and nonspecific autoantibodies (e.g., antinuclear antibodies, SS-A, SS-B, ribonucleoprotein antibodies), often in the absence of corresponding disease [34, 62, 63]. The presence of autoantibodies does not militate against, but rather strengthens the evidence for a diagnosis of NMO.

Cerebrospinal fluid (CSF) should be obtained when evaluating a patient for possible NMO. CSF pleocytosis (>50 WBC/mm3) may occur in NMO, particularly during an acute attack [34]; when present, a neutrophilic (>5 neutrophils/mm3) and eosinophilic preponderance is usually detected [34]. CSF oligoclonal bands are infrequent in NMO (20-30%) [17, 24, 34, 55] and may only be detected during attacks [63] but are present in 85-90% of patients with MS [32]. In fact, in patients with TM and normal brain MRIs, the presence of CSF oligoclonal bands and an increased IgG index portend a higher risk of developing MS [32]. Other promising potential CSF biomarkers of NMO include glial fibrillary acidic protein (a marker of astrocytic destruction) [64] and IL-6 levels [13, 65, 66].

 
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