Neuromyelitis optica (NMO) associated antigen, a water channel protein, known as aquaporin 4 (AQP4) is found both in the central and peripheral tissues. Detection of AQP4 antibody is clinically useful in differentiating between NMO and other optic neuritis or myelitis. Unlike multiple sclerosis, a patient with AQP4 antibody can benefit from plasmapheresis and immunosuppression.
On cerebellum, AQP4 is found in the juxtaposed pial membrane, microvessels in the white matter, molecular layer and granular layer.
Often, AQP4 antibody is also associated with the staining of the cytoplasm of the granular cells: Such reactivity is also found in the CSF.
AQP4 transfected HEK cell system is a newer test with higher sensitivity and has superseded detection by cerebellum.
Clinical: Neuromyelitis optica (also known as Devic's disease, ~65%) is an immune-mediated demyelinated disorder of the optic nerve and spinal cord. The disorder resembles multiple sclerosis (MS) and the two cannot be distinguished from one another.
Treatment is different for each disorder therefore important to distinguish it from MS.