Neuromyelitis optica spectrum disorder

Neuromyelitis optica (NMO), also known as Devic diseaseand, more recently, NMO spectrum disorder, is a severe demyelinating disease caused by an autoantibody to the aquaporin-4 water channel. Previously NMO had been considered a variant of multiple sclerosis (MS), but it has become evident that they are distinct entities showing many overlapping features. 

Classic presentation is with the triad of optic neuritis, longitudinally extensive myelitis, and positive NMO-IgG ⁸. 

On imaging, NMO usually manifests as bilateral swollen optic nerves with a T2 hyperintensity and contrast enhancement. Spinal cord involvement is commonly present and characterised by extensive (more than three vertebral levels), centrally located T2 hyperintense lesions. Brain demyelinating lesions are usually not present, but when they do occur, have specific features different from MS.

Terminology

The term neuromyelitis optica spectrum disorder (NMOSD) is also used to describe this condition ⁹. 

Epidemiology

Neuromyelitis optica is typically found in patients somewhat older than those with MS, with an average of presentation of 41 years, and there is an even stronger female predilection (F:M  6.5:1) ^6,8. It is found more frequently in patients of Asian, Indian, and African descent ⁸. 

Clinical presentation

NMO is characterized by bilateral optic neuritis and myelitis resulting in blindness and paraplegia. Although the two usually present concurrently, it is not uncommon for one to precede the other by up to several weeks ³. Additionally, it is now recognized that some patients present with unilateral optic nerve involvement. 

Although NMO was initially thought of as a monophasic illness, it is now evident that, as with multiple sclerosis, it is usually a relapsing-remitting disease with symptomatic events separated by many years ⁵. 

Pathology

In approximately 70% (sensitivity of 70-90%; specificity of 90%) of patients with established neuromyelitis optica, a specific immunoglobulin can be isolated (NMO-Immunoglobulin G) which targets a transmembrane water channel (aquaporin 4) present on astrocyte foot processes abutting the limiting membrane ^5,8. This accounts for some of the predilection for certain parts of the brain (e.g. periaqueductal grey matter) which are particularly rich in aquaporin 4 ⁸. 

Early in the disease, demyelinating regions will demonstrate similar findings to MS, such as macrophage/microglia activation and axonal damage. Additionally, however, and relatively specific for NMO, these regions will also demonstrate extensive eosinophilic infiltration, perivascular deposition of immunoglobulins (especially IgM) and local activation of the complement cascade ³. Another differentiating feature is that axonal damage precedes demyelination in NMO ⁵. 

Generally, the condition is sporadic, although some overlap in immunogenic features between certain viruses and aquaporin-4 water channel have been identified ⁸. 

Radiographic features

MRI

MRI is the modality of choice. In addition to the orbits, the brain and the spinal cord should be imaged. 

Orbits

Targeted imaging of the orbits (including fat saturated T1 C+ and T2 weighted sequences - see MRI protocol: orbit) may demonstrate typical features of optic neuritis: 

• optic nerves appearing hyperintense and swollen on T2 weighted sequences and enhancing on T1 C+
• bilateral optic nerve involvement and extension of the abnormal signal posteriorly as far as the chiasm is particularly suggestive of NMO ⁵
• atrophy of the optic nerves with associated hyperintensities on T2 weighted sequences may be seen in chronic stages of the disease ¹¹

Spinal cord

Spinal cord involvement is extensive, with high T2 signal spanning at least three vertebral segments, often many more (known as a longitudinally extensive spinal cord lesion) ^4,5,8,11. Also helpful in distinguishing NMO from MS demyelination is the involvement of the central part of the cord (MS lesions tend to involve individual peripheral white matter tracts) ⁸. 

Imaging features include ^5,8: 

• T1
  • hypointense
  • follow-up scans may demonstrate cord atrophy and low T1 signal ⁵
• T2
  • hyperintense (>3 vertebral body lengths) 
  • central grey matter involvement
  • "bright spotty lesions" describing inhomogeneity of signal change ⁸
• T1 C+ (Gd)
  • patchy "cloud-like" enhancement of the aforementioned T2 bright lesions may be present
  • thin ependymal enhancement similar to ependymitis
  • enhancement in a lens-shaped appearance on the sagittal images can be distinctive for NMO ¹¹
  • NB: open ring enhancement is not a feature of NMO

Brain

Although traditionally NMO was thought to have normal intracranial appearance it is increasingly evident that asymptomatic abnormalities are present in the majority of patients. These can be divided into four categories ^5,8:

1. lesions which mirror the distribution of aquaporin 4 in the brain, which is particularly found in the periependymal regions abutting the ventricles:
   • periventricular (hemispheric) confluent white matter involvement (unlike MS, there are usually no Dawson's fingers)
   • periaqueductal grey matter
   • hypothalamus/medial thalamus
   • dorsal pons/medulla 
   • corpus callosum
     • multiple callosal lesions with heterogeneous signal leading to a marbled pattern ⁷ 
     • the splenium may be diffusely involved and expanded
2. deep (or less frequently subcortical) punctate white matter lesions (which may appear similar to those seen in multiple sclerosis)
3. corticospinal tract involvement by extensive longitudinal lesions; has been reported more frequently in Korean patients ⁵
4. larger "spilled ink pattern" of hemispheric white matter lesions which often vanish with steroid administration; may be more common in children and in patients from the Far East and Africa

Unlike MS, NMO does not appear to involve the cortex and its presence should prompt other diagnoses such as MS ^5,11. 

Treatment and prognosis

Treatment of NMO is evolving, with immunosuppression (e.g. anti-CD20 monoclonal antibody rituximab) appearing effective ⁵.

It is important to distinguish NMO from MS as the treatment not only is different but treating a patient with NMO with MS-specific therapies (e.g. beta-interferon or natalizumab) can actually lead to its exacerbation ⁵. 

Patients with a relapsing course have a poorer prognosis ⁴:

• blind in one or both eyes: monophasic  22% vs. relapsing 60%
• monoplegia or paraplegia: monophasic 31% vs. relapsing 52%

Differential diagnosis

The differential diagnosis depends on the presentation, and when classic, the diagnosis can be made with a fair degree of certainty. 

For patients with spinal cord involvement, the differential is that of a longitudinally extensive spinal cord lesion. 

Involvement of the cerebral white matter and corpus callosum has a broad differential depending on the pattern, but the most important differential is multiple sclerosis (MS). Features helpful in favouring NMO over MS include ^5,7: 

• periventricular/aqueductal distribution (see above)
• absent perivenular orientation of periventricular lesions (no Dawson's fingers)
• more extensive involvement of the corpus callosum
• larger, more confluent lesions
• lack of open ring enhancement
• lack of cortical grey matter involvement