The last few years has brought recognition that Devic's Disease is not the same as multiple sclerosis. Devic's Disease, now being called neuromyelitis optica (NMO), is a syndrome that involves the optic nerve (optic neuritis or ON), and the spinal cord (longitudinally extensive transverse myelitis - or LETM) and may have additional brain lesions as well.
The defining hallmark of this disease at this time is the identification in the patient's serum of a particular antibody - the NMO-IgG antibody that appears to be against a particular protein - an aquaprotein that is located in these areas that are affected in NMO disease. It appears from a growing body of evidence that the NMO-IgG antibody is actually pathologic. Therefore, it treatments directed specifically against this antibody or the cells that make this antibody are being tested. By contrast, typical MS therapies target Tcells. The exact extent of NMO-IgG patients is not clearly known. Some patients may only have recurrent optic neuritis and some may only have recurent transverse myelitis.
Differentiating MS from NMO in patients without the blood test can be difficult. The MRI findings of NMO can to some degree overlap with MS but in general MS has discrete lesions and the NMO has lesions that extend longitudinally down the spinal cord. What to do with patient's whose MRIs and clinical pictures look more like NMO but lack the antibody is still needing to be worked out. Antibody testing in suspected cases is key.