SN2 SHORT NOTE 5M ★ High-yield

Describe the normal composition of cerebrospinal fluid and the changes in meningitis.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Cerebrospinal fluid (CSF) = the clear fluid surrounding the brain and spinal cord, secreted chiefly by the choroid plexus.

Volume 130–150 mL; produced at about 500 mL/day, so it is replaced three to four times daily
Functions — mechanical protection (buoyancy), homeostasis of the neuronal environment, removal of metabolites, and a route for some signalling

Normal Composition

Constituent	CSF	Compared with plasma
Appearance	Clear, "crystal clear"	—
Pressure	60–150 mm H2O	—
Glucose	45–80 mg/dL	About two-thirds of blood glucose
Protein	15–45 mg/dL	Far lower — the blood–brain barrier excludes it
Chloride	120–130 mEq/L	Higher than plasma
Cells	0–5 lymphocytes/mm3	No red cells or neutrophils
Sodium, osmolality	Similar to plasma	—
Calcium, potassium, urea	Lower	—

A simultaneous blood glucose must always be taken, since CSF glucose is meaningful only as a ratio

Changes in Meningitis

Feature	Pyogenic	Tubercular	Viral	Fungal
Appearance	Turbid, purulent	Clear; cobweb clot	Clear	Clear or turbid
Cells	1000–5000, neutrophils	100–500, lymphocytes	10–500, lymphocytes	20–500, lymphocytes
Protein	↑↑ (100–500)	↑↑↑ (very high)	Normal or slightly ↑	↑
Glucose	↓↓ (very low)	↓	Normal	↓
Chloride	↓	↓↓ (markedly low)	Normal	↓
Confirmation	Gram stain, culture, latex agglutination	ZN stain, GeneXpert, ADA	PCR	India ink, cryptococcal antigen

CLINICAL PEARL

The two most useful discriminators are glucose and the cell type. Normal glucose with lymphocytes points to a viral cause; low glucose with lymphocytes and a very high protein to tuberculosis; low glucose with neutrophils to a pyogenic infection.

Other Abnormal Findings

Xanthochromia — yellow supernatant after centrifugation, from bilirubin; indicates subarachnoid haemorrhage more than 12 hours old, and distinguishes it from a traumatic tap
Froin syndrome — very high protein with xanthochromia and rapid clotting, in spinal block
Albumino-cytological dissociation — raised protein with normal cell count; characteristic of Guillain–Barré syndrome
Oligoclonal bands on CSF electrophoresis — multiple sclerosis
Raised adenosine deaminase (ADA) supports tuberculous meningitis

Applied Aspects

Lumbar puncture is contraindicated when intracranial pressure is raised with a focal lesion — the risk is coning; image the head first if there are focal signs, papilloedema or altered consciousness
Antibiotics must not be delayed for the lumbar puncture in suspected bacterial meningitis
Partially treated pyogenic meningitis can mimic the tubercular picture, with lymphocytes and a modestly low glucose — a common diagnostic trap
CSF glucose falls in bacterial and tubercular meningitis because organisms and leucocytes consume it and transport is impaired

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