Cerebrospinal fluid C9 in demyelinating disease.

We measured CSF and plasma concentrations of C9, IgG, and albumin in 91 patients with demyelination and 73 controls with other neurologic diseases. The C9 index was reduced and IgG index increased in patients with multiple sclerosis and those with isolated demyelinating lesions, irrespective of disease activity; abnormalities were less marked in patients with isolated lesions than in those with MS. Humoral mechanisms may not be responsible for initiating demyelination, but activation of the complement system could amplify tissue damage and account for some symptomatic recovery.