CSF Findings in Meningitis
Examination of the CSF is essential in the evaluation of meningitis. A positive culture gives the most definitive diagnosis as to the cause of the meningitis but analysis of the CSF profile often can indicate the type of meningitis prior to the culture results. The CSF profile consists of the glucose, protein and cell count. For bacterial meningitis the glucose is low, the protein greatly elevated and the white cells are predominantly polymorphonuclear cells in the 200- 20,000 range. For viral meningitis the glucose is usually normal, the protein mildly elevated, the white cell count is predominately lymphocytic and often less than 200. For fungal meningitis the glucose is low, the protein is elevated, the white cell count is mononuclear and the count is usually in the 200-1,000 range. The 4^th type of meningitis is not infectious but rather a meningitic infiltration by cancerous cells. In carcinomatous meningitis the glucose is low, the protein is elevated, the cell count is usually less then 200 and cytological analysis can identify the cancer type.

Guillain-Barre Syndrome- Acute Idiopathic Polyneuritis

The CSF hallmark of Guillain-Barre syndrome is albuminocytological dissociation. The protein is increased but there is only mild pleocytosis. The white cell count is in the 5-50 range. The protein can be normal early in the course of the disease but by day 7 of symptoms the protein is usually in the 50-1,000 mg/dl range. The protein elevation in the lumbar CSF is thought to be due to an altered blood-nerve-CSF barrier and to leaky spinal root capillaries traversing the subarachnoid space.

CSF in Multiple Sclerosis

The CSF findings in multiple sclerosis include a quantitative increase in IgG as well as a qualitative change. These changes are not specific for MS but can also be seen in certain infectious diseases and inflammatory conditions.

The IgG index and the IgG synthesis rate are used to assess the local production of IgG within the central nervous system. For both formulas, albumin, which can only be produced by the liver, is used as a reference protein. Both formulas use the ratio of the CSF to serum concentrations of IgG and albumin to determine how much of the CSF IgG can be accounted for by passive diffusion across the blood-brain and blood-CSF barrier vs. production of the IgG within the intrathecal space. An elevated index or synthesis rate indicates production of IgG within the CNS.

Electrophoresis of concentrated CSF is used to look for oligoclonal bands. Normally the IgG in CSF and serum have a diffuse appearance in the gamma region on electrophoresis. If there are two or more distinct bands in the gamma region then this is considered as showing oligoclonal bands and indicates that a restricted number of clones of lymphocytes are producing these bands of IgG. The CSF must be compared with the serum to make sure that the oligoclonal bands in the CSF are not just a reflection of what is occurring in the systemic circulation.