Neuromyelitis optica

What it is

NMO = optic neuritis + acute myelitis, the acute myelitis being longitudinally extensive spanning myelitis at least 3 vertebral segments on MRI.

Its whole weight in the exam is that it has to be told apart from MS, because the treatments part ways.

The optic neuritis is bilateral and severe. The spinal cord lesions are longitudinally extensive, at least 3 segments.

An area postrema syndrome characterized by intractable hiccups, nausea and vomiting occurs in ~ 40% of patients with NMO.

Antibody: Anti-AQP4 (Aquaporin-4 IgG) in NMO; no specific antibody in MS (OCBs in CSF). Aquaporin-4 IgG is usually present in NMO and absent in MS, and any case suspected with NMO is checked for Aquaporin-4 antibodies.
Optic neuritis: bilateral, severe - against unilateral, often milder.
Spinal cord lesions: longitudinally extensive, at least 3 segments - against short lesions, less than 2 segments.
MRI brain: often normal or nonspecific early - against periventricular Dawson's fingers.
CSF cell count: elevated in relapse - against >50 WCC very rare.
CSF neutrophils and eosinophils: often present - against usually absent.
CSF protein: often very elevated in relapse - against usually mild to mild elevated.
CSF OCB: rarely positive, often absent, present in ~25% - against usually present, given as ~90% on one page and positive in ~95% on another.
Treatment: rituximab, eculizumab, avoiding MS drugs like interferon - against interferon-B, glatiramer, fingolimod.
Prognosis: worse, more severe attacks - against variable, often milder relapses.