Biopsy diagnosis of acute MS, especially with stereotactic needle biopsies, may be tricky because cellularity and reactive astrocytes in the lesions may be misinterpreted as a neoplasm. CSF protein is moderately elevated, and there is mild mononuclear pleocytosis. The latter is a measure of the activity of the disease. Oligoclonal IgG bands are detected on agarose electrophoresis in 90 percent of patients. This pattern may be present even when the total amount of IgG is normal.
Oligoclonal bands indicate that IgG represents antibodies to specific antigens.
About 70 percent of MS patients and only 5 percent of controls have antibodies to measles. A smaller number have antibodies to rubella, mumps, and herpes simplex. Similar CSF changes are seen in some chronic CNS infections such as chronic measles encephalitis and syphilis. Myelin proteins such as myelin basic protein leak from plaques into the CSF and can be detected by radioimmunoassay. MS is probably an autoimmune disorder.