Neuromyelitis Optica, Part 1
Multiple sclerosis is by far the most common demyelinating disease of the central nervous system, but certainly not the only or even the most severe such illness. In this post, I would like to discuss a much rarer illness known as either neuromyelitis optica (NMO) or Devic’s disease. There is some controversy about whether or not NMO is simply a variant of MS or a different disease entirely. Most neurologists today believe that NMO is a different disease from MS, but the debate is ongoing.
The diagnostic criteria for NMO were established only in 2006. According to these criteria, to be diagnosed with NMO patient must have:
- Optic neuritis,
- Spinal cord inflammation(termed myelitis)
And at least two of three supportive criteria:
- MRI evidence of a contiguous spinal cord lesion 3 or more segments in length,
- Brain MRI at onset nondiagnostic for multiple sclerosis,
- NMO-IgG antibody detected in the blood.
The main features of NMO are recurrent episodes of optic neuritis combined with inflammation of the spinal cord that is 3 or more segments in length, much longer than spinal inflammation in MS. The optic neuritis in NMO is often in both eyes. Additionally, while the brain MRI in NMO may show some lesions, it typically shows many fewer lesions than in MS and the lesions are not in the same location in the brain. According to the guidelines, the MRI for NMO patients would not meet the diagnostic criteria for MS at the time the illness is diagnosed. Finally, unlike MS, there is a blood test for and antibody in NMO. The NMO antibody is directed against a water channel on certain cells in the nervous system known as astrocytes. It is positive in about 75% of patients with NMO.
Because the test is negative in about 25% of patients with NMO, it is a far from perfect test. If the blood test comes back positive for NMO, it almost certainly means that the person has this illness. However, it is still possible for a person to have NMO, yet not have this antibody in detected in their blood. It may not be possible to differentiate MS from NMO early in the disease course, especially if a patient presents with a symptom such as unilateral optic neuritis that could be a feature of either disease. Since MS is much more common than NMO, it is the illness that should be thought of first. However doctors must always have an open mind and be willing to reconsider this diagnosis if new evidence emerges. In my next post, I will further compare and contrast MS and NMO.