CSF Examination in Meningitis

Overview

CSF formed in ventricles, circulates in subarachnoid space (arachnoid-pia) + ventricles. Adult volume 90-150mL. Major diagnostic value in meningitis, primary/metastatic CNS tumours.

Collection

Lumbar puncture (LP), aseptic, needle+stilette (keeps lumen patent).

Sites: LP — midline lower back, 3rd lumbar space (adults), 4th (children, cord ends higher there — avoids injury). Cisternal puncture — if LP site blocked/deformed/infected. Ventricular puncture — infants, open fontanelle.

3-4 sterile tubes, total 6-8mL: Tube1 (protein/glucose/serology), Tube2 (stains/culture), Tube3 (cell counts), Tube4 (special tests — cryptococcal Ag, syphilis serology, molecular, cytology).

Indications

Diagnostic: infection (pyogenic/TB/syphilitic/viral/fungal meningitis), encephalitis, subarachnoid haemorrhage, CNS malignancy (leukaemia, lymphoma), demyelinating disease (MS, SSPE, GBS), raised ICT from spinal blockage, myelography dye injection. Therapeutic: spinal anaesthesia, intrathecal chemo (ALL/lymphoma CNS prophylaxis).

Contraindications

Raised ICP, local infection at site, disseminated sclerosis, brain tumour, cardiorespiratory compromise, uncorrected bleeding diathesis.

Complications

Cerebellar herniation (foramen magnum), extradural/subdural haematoma, infection introduction, post-puncture headache, dry tap.

Physical exam

Pressure: normal 90-180mm water (adults, lateral position); 10-100mm (infants/children, reaches adult by 6-8yr). ↑ in meningitis/oedema/mass lesion; ↓ in dehydration/circulatory collapse.

Colour: normal clear/watery. Turbid = pus/RBCs. Xanthochromia (yellow) = old haemorrhage, obstructive jaundice, ↑protein, Froin syndrome.

Traumatic tap vs SAH: traumatic — first drops haemorrhagic, later clear, supernatant clear after centrifuge. True SAH — blood uniformly mixed, supernatant XANTHOCHROMIC (haemolysed RBC Hb release).

Clot formation: normal CSF doesn’t clot. Disturbed blood-brain barrier → fibrinogen enters → fibrin clot. TB meningitis: fine/delicate “cob-web” clot. Pyogenic: large clot. Also: CNS tumours, polyneuritis.

Microscopic exam

Normal: 0-5 lymphocytes/µL. ↑cell count = pleocytosis. Count promptly (undiluted) — cells stick/degenerate within hours.

Differential significance:

• Neutrophils↑ = acute PYOGENIC (bacterial) meningitis
• Lymphocytes↑ = VIRAL, syphilitic, TUBERCULOUS, FUNGAL meningitis

Confirm with culture/serology. India-ink = cryptococcal meningitis. Acid-fast stain = TB bacilli.

Biochemical evaluation

Meningitis: ↑protein, ↓glucose (microbial utilisation) — BOTH more marked in pyogenic. Chloride↓ in TB meningitis = general dehydration-related deficiency, NOT specific mycobacterial effect.

Summary table

Clinical relevance

Neutrophil vs lymphocyte split = single most useful first-pass CSF reasoning tool — narrows differential before culture/stain results. Cob-web clot (TB) vs large clot (pyogenic) = distinctive tested finding, direct consequence of how much fibrinogen crosses disturbed BBB (more in pyogenic). Chloride↓ in TB = dehydration mechanism, NOT direct mycobacterial effect — easy point to get backwards. Traumatic tap vs true SAH (clearing drops + clear supernatant) = practical bedside skill with real diagnostic consequences.

Overview

Cerebrospinal fluid (CSF) is formed within the ventricles and circulates through the subarachnoid space (between arachnoid and pia mater) and the ventricles, serving as a medium for transfer of substances from the brain and spinal cord into the blood. Total adult CSF volume is 90–150 mL. CSF analysis is of major diagnostic importance in meningitis and in primary/metastatic CNS tumours with CSF involvement.

Collection

CSF is usually obtained by lumbar puncture (LP) under strict aseptic conditions, using a needle with a stilette (which keeps the needle patent by plugging its lumen during insertion).

Sites:

• Lumbar puncture — midline, lower back; 3rd lumbar space in adults, 4th lumbar space in children (the spinal cord itself ends higher, so this avoids cord injury).
• Cisternal puncture — used when spinal cord block, vertebral deformity, or infection at the usual LP site precludes lumbar puncture.
• Ventricular puncture — in infants with an open fontanelle.

CSF is collected into 3–4 sterile tubes (total volume not exceeding 6–8 mL): Tube 1 for protein/glucose/serology; Tube 2 for Gram/other stains, culture, and sensitivity; Tube 3 for cell counts and differential; Tube 4 (if indicated) for special tests (cryptococcal antigen, syphilis serology, molecular tests, cytology).

Indications for Lumbar Puncture

Diagnostic: suspected infection (bacterial — pyogenic, tuberculous, syphilitic; viral; fungal), encephalitis, subarachnoid haemorrhage, primary/metastatic CNS malignancy (acute leukaemia, lymphoma), demyelinating disease (multiple sclerosis, SSPE, Guillain-Barré syndrome), spinal canal blockage causing raised intracranial tension, and injection of radio-opaque dye for myelography.

Therapeutic: spinal anaesthesia, intrathecal chemotherapy (CNS prophylaxis/relapse treatment in ALL, lymphomas).

Contraindications

Raised intracranial pressure, local infective lesion (at the puncture site), disseminated sclerosis, brain tumour, cardiorespiratory compromise, uncorrected bleeding diathesis.

Complications

Cerebellar herniation through the foramen magnum (from raised intracranial pressure), extradural/subdural haematoma, introduction of infection through the needle track, post-puncture headache, and failure to obtain CSF (dry tap).

Physical Examination

Pressure: normal 90–180 mm water (lateral position, adults); 10–100 mm water in infants/children, reaching adult levels by 6–8 years. Raised by meningitis, cerebral oedema, mass lesions; decreased by dehydration, circulatory collapse.

Colour and appearance: normal CSF is clear and watery. Turbidity results from pus or RBCs. Xanthochromia (yellow CSF) results from old haemorrhage, obstructive jaundice, excess protein, or Froin syndrome.

Distinguishing traumatic tap from subarachnoid haemorrhage: in a traumatic tap, the first few drops are haemorrhagic while subsequent drops clear, and the centrifuged supernatant is clear; in true subarachnoid haemorrhage, blood is uniformly mixed throughout, and the supernatant is xanthochromic (from haemoglobin release by haemolysed red cells).

Clot formation: normal CSF does not clot. A disturbed blood-brain barrier allows fibrinogen into CSF, which converts to fibrin and clots. Causes of fibrin clot: meningitis (in tuberculous meningitis, the clot is fine and delicate, classically described as a “cob-web” appearance; in purulent/pyogenic meningitis, a large clot forms), CNS tumours, polyneuritis.

Microscopic Examination

Total cell count: normal CSF contains few or no cells (0–5 lymphocytes/µL is normal). Increased cell count is termed pleocytosis. Counting must be done promptly on undiluted CSF, since pus cells stick together or degenerate within hours.

Differential significance:

• Neutrophils increased — acute pyogenic (bacterial) meningitis.
• Lymphocytes increased — viral, syphilitic, tuberculous, and fungal meningitis.

Any raised cell count should be confirmed with bacterial culture or serologic tests. India-ink preparation diagnoses cryptococcal meningitis; acid-fast staining detects tuberculous bacilli in tuberculous meningitis.

Biochemical Evaluation

Proteins are elevated and glucose is reduced (from microbial utilisation) in meningitis — both changes are more marked in pyogenic meningitis than in the other types. Chloride reduction in tuberculous meningitis reflects general chloride deficiency from dehydration rather than a specific effect of the mycobacteria themselves — a mechanistic point worth distinguishing from a direct “TB lowers chloride” misconception.

CSF Findings Across the Meningitis Types (Summary)

Clinical Relevance

• The neutrophil-versus-lymphocyte split (pyogenic vs viral/TB/fungal/syphilitic) is the single most useful first-pass reasoning tool in CSF interpretation — it narrows the differential from a broad category of “meningitis” to a specific etiologic class before any culture or stain result is even available.
• The cob-web clot of tuberculous meningitis versus the large clot of pyogenic meningitis is a distinctive, specific, and frequently tested physical finding — recognising it as a direct consequence of how much fibrinogen crosses the disturbed blood-brain barrier (more in pyogenic disease) ties a memorable visual to actual pathophysiology rather than an arbitrary fact.
• The chloride-reduction mechanism in TB meningitis (general dehydration-related chloride deficiency, not a specific mycobacterial effect) is a genuinely easy point to get backwards, and understanding the actual mechanism prevents over-attributing biochemical CSF changes to direct organism activity when the real driver is a systemic effect.
• Distinguishing a traumatic tap from true subarachnoid haemorrhage via the clearing-drops-and-clear-supernatant pattern is a practical bedside skill with real consequences — misreading a traumatic tap as haemorrhage (or vice versa) changes the entire diagnostic and management pathway that follows.

No Rendered Diagram for This Topic

This topic is a laboratory reference/comparison table (CSF findings across the four meningitis types) rather than a disease mechanism — a rendered diagram would only redraw the summary table already in notes.md.

Hand-Draw Suggestion

Draw the summary table from notes.md by hand as an active-recall exercise: list the four meningitis types across the top, and try to fill in cell type / clot appearance / protein / glucose / chloride from memory before checking against the reference. This table is the single highest-yield piece of the topic and is best learned by repeated blind recall.