Cerebrospinal fluid

Introduction

• Cerebrospinal fluid is a clear, colorless body fluid present in and around the central nervous system.
• It fills the ventricles of the brain, surrounds the brain and spinal cord in the subarachnoid space, and also occupies the central canal of the spinal cord.
• CSF is one of the most important protective fluids of the body because it acts as a cushion for delicate nervous tissue.
• It protects the brain and spinal cord from mechanical injury, helps in transport of nutrients, removes waste products, and maintains chemical stability of the nervous system.
• Because many diseases of the nervous system alter the composition of CSF, laboratory examination of CSF provides valuable diagnostic information.

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Formation of Cerebrospinal Fluid

• CSF is mainly produced by the choroid plexus, a specialized vascular structure present in the ventricles of the brain.
• About 70% of CSF is formed by the choroid plexus.
• The remaining small amount is produced by:
  • Ependymal lining of ventricles
  • Brain capillaries

Sites of Formation

• Lateral ventricles
• Third ventricle
• Fourth ventricle

Daily Production

• About 500 mL per day

Total Volume

• Adult body normally contains 120–150 mL of CSF at any given time.

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Circulation of CSF

• CSF formed in the lateral ventricles passes through:
  • Interventricular foramina
  • Third ventricle
  • Cerebral aqueduct
  • Fourth ventricle
• From the fourth ventricle it enters:
  • Subarachnoid space
  • Central canal of spinal cord

Absorption

• CSF is absorbed into venous circulation through arachnoid villi.

Choroid plexus plays the major role in CSF formation.

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Functions of CSF

1. Mechanical Protection

• CSF acts as a shock absorber and cushions the brain and spinal cord against sudden movements or external injury.
• It protects delicate nervous tissue from trauma.

2. Buoyancy of Brain

• The brain has considerable weight, but when suspended in CSF its effective weight is greatly reduced.
• This prevents pressure on the lower parts of the brain.

3. Maintenance of Intracranial Pressure

• CSF helps maintain constant pressure inside the skull and spinal canal.

4. Transport of Nutrients

• CSF carries glucose, electrolytes, and other nutrients required by brain cells.

5. Removal of Waste Products

• Metabolic waste products from brain tissue are removed through CSF circulation.

6. Chemical Stability

• CSF maintains proper ionic balance around neurons, which is essential for nerve impulse conduction.

7. Medium for Exchange

• Acts as a medium for exchange of substances between blood and nervous tissue.

8. Defense Function

• Helps protect central nervous system by diluting harmful substances and participating in immune surveillance.

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Collection of Cerebrospinal Fluid 

• Collection of Cerebrospinal fluid is an important diagnostic procedure used to examine diseases of the central nervous system.
• CSF is collected mainly for:
  • Chemical examination
  • Microscopic examination
  • Microbiological examination
  • Pressure measurement
• The most common method of collection is lumbar puncture.

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Method of Collection

Lumbar Puncture

• Lumbar puncture is the standard method used for CSF collection.
• A sterile needle is introduced into the subarachnoid space of the lumbar region.

Site of Collection

Common Site

• Between L3–L4 vertebrae
• Between L4–L5 vertebrae

Reason

• Spinal cord ends above this level, so risk of injury is minimal.

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Position of Patient

Lateral Position

• Patient lies on side with knees flexed toward chest.

Sitting Position

• Sometimes used when required.

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Procedure

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Number of Tubes Collected

Usually Three Tubes

• Tube 1 → Chemical examination
• Tube 2 → Microbiological examination
• Tube 3 → Cell count and microscopy

Amount Collected

• Usually 2–3 mL per tube
• Total 5–10 mL depending on requirement

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Post-Procedure Care

• Monitor the Patient: The patient is usually observed for 1–2 hours after the procedure, particularly for any signs of complications like headache, bleeding, or neurological changes.
• Post-Lumbar Puncture Headache: A common side effect caused by a leak of CSF through the puncture site. It can typically be managed with bed rest, hydration, caffeine, and analgesics. In severe cases, a blood patch may be needed.

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Potential Complications

• Post-Lumbar Puncture Headache: Occurs in about 10-30% of patients.
• Back Pain: Temporary pain at the site of the needle insertion.
• Bleeding: Rare but may occur, particularly in patients with bleeding disorders.
• Infection: Though rare, introducing infection into the spinal canal (meningitis) is a serious concern.
• Brain Herniation: If a patient has elevated intracranial pressure due to a mass lesion, herniation of the brainstem through the foramen magnum can occur.

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Laboratory Examination of CSF

Physical examination

Colour and Clarity

• Normal CSF: Clear and colourless, resembling water.
• Abnormal Colors:
  • Turbid or cloudy: Indicates an elevated number of cells (pleocytosis), commonly seen in infections such as bacterial meningitis or in cases of a high protein content.
  • Xanthochromia (yellowish discolouration): Results from the breakdown of blood in the CSF.
  • Pink/red (bloody): Suggests the presence of red blood cells. This can be due to a traumatic tap or a subarachnoid haemorrhage.
  • Oily appearance: when fatty substances are present.

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Appearance

Normal Appearance

• Fresh CSF is clear, transparent, and colorless.

Turbid CSF

• Indicates increased cells, bacteria, or protein.

Causes of Turbidity

• Bacterial meningitis
• High leukocyte count
• Severe protein increase

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Turbidity or Cloudiness

• Normal CSF is crystal clear.
• Turbid CSF suggests the presence of white blood cells, red blood cells, bacteria, or fungi.
  • Infections: Bacterial meningitis commonly causes a turbid appearance due to many white blood cells. Tuberculous or fungal meningitis may also cause turbidity, but often to a lesser degree.
  • Increased Protein: Conditions like Guillain-Barré syndrome or tumours may cause CSF to appear turbid due to the accumulation of high levels of proteins.
  • Lipids: Sometimes seen in cases of increased lipids in the blood or after fat embolism.

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Viscosity

• Normal CSF is slightly more viscous than water but flows easily from the needle during a lumbar puncture.
• Increased viscosity is rare but can occur in conditions like metastatic mucinous tumours or rarely in hypoproteinaemia states.

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Presence of Coagulum or Clot

• Normal CSF does not clot.
• Abnormal coagulation in the CSF may occur when there is an increased amount of fibrinogen or protein, which can be seen in:
  • Tuberculous meningitis: CSF may form a fibrin web or clot when allowed to sit in a test tube. This is highly suggestive of tuberculous or fungal meningitis.
  • Traumatic tap: Clotting may occur due to blood contamination from the procedure.

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Opening Pressure

• Normal CSF pressure ranges from 6 to 20 cm H₂O (measured with the patient in a lateral decubitus position).
  • Increased opening pressure May suggest conditions such as:
    • Meningitis (particularly bacterial).
    • Subarachnoid haemorrhage.
    • Cerebral oedema.
    • Tumors or space-occupying lesions.
    • Idiopathic intracranial hypertension.
  • Decreased opening pressure May occur in conditions such as:
    • CSF leak (post-surgery, trauma, or spontaneous).
    • Dehydration.
    • CNS hypotension.

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Chemical Analysis of CSF

Glucose

Principle

• Glucose in Cerebrospinal fluid is commonly estimated by the glucose oxidase–peroxidase method.
• Glucose is oxidized by glucose oxidase to form gluconic acid and hydrogen peroxide.
• Hydrogen peroxide reacts with chromogen in presence of peroxidase to produce a colored compound.
• The intensity of color formed is proportional to glucose concentration.

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Procedure

Tube	Working Reagent	Sample / Standard	Distilled Water
Blank	1.0 mL	—	0.01 mL
Standard	1.0 mL	0.01 mL glucose standard	—
Test	1.0 mL	0.01 mL CSF	—

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Observation

• A colored solution develops.
• Greater color intensity indicates higher glucose concentration.

Normal Value

• 50–80 mg/dL
• Usually about two-thirds of blood glucose level.

Clinical significance

• CSF glucose reflects blood glucose and brain metabolism.

Decreased Glucose Seen In

• Bacterial meningitis
• Tuberculous meningitis
• Fungal infection

Increased Glucose Seen In

• Diabetes mellitus

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Protein

Principle

• Protein in CSF is commonly estimated by the sulfosalicylic acid method or turbidimetric method.
• Sulfosalicylic acid precipitates protein present in CSF, producing turbidity.
• The degree of turbidity is proportional to protein concentration.

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Procedure

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Observation

• Turbidity develops if protein is present.
• Greater turbidity indicates increased protein.

Normal Value

• 15–45 mg/dL

Clinical Significance

• CSF protein is lower than plasma protein because only small amounts normally pass through the blood-brain barrier.

Increased Protein Seen In

• Meningitis
• Brain tumors
• Multiple sclerosis
• Guillain-Barré syndrome

Decreased Protein

• Rare, usually after repeated lumbar puncture

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Lactate

Principle

• Lactate present in CSF reacts enzymatically to produce a colored compound.
• The color intensity produced is directly proportional to lactate concentration.

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Procedure

Steps

1. Label test tubes as Test and Standard.
2. Add 1.0 mL lactate reagent to both tubes.
3. Add 0.1 mL CSF sample to test tube.
4. Add 0.1 mL lactate standard to standard tube.
5. Mix gently.
6. Incubate for recommended time.
7. Measure absorbance using colorimeter.

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Observation

• Colored solution develops.
• Higher color intensity indicates increased lactate.

Normal Range

1.2-2.1 mmol/L.

Abnormal Findings

• Increased Lactate:
  • Bacterial Meningitis: High levels of lactate (>3.5 mmol/L) indicate bacterial infection due to the anaerobic metabolism of the invading organisms and impaired oxygenation in the inflamed tissues.
  • Fungal and Tuberculous Meningitis: Moderate increases in lactate can be seen.
  • Stroke: Elevated lactate may indicate areas of ischemia or reduced oxygen supply to the brain.
• Normal or Slightly Increased Lactate:
  • Viral Meningitis: Lactate levels are usually normal or only slightly elevated in viral infections.

Significance

• Lactate is particularly useful in distinguishing between bacterial and viral meningitis.
• High levels strongly suggest a bacterial cause, which requires more aggressive treatment.

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Chloride

Principle

• Chloride in CSF reacts with silver nitrate to form silver chloride precipitate.
• The amount of chloride is measured by titration or colorimetric method.

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Procedure

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Observation

• White precipitate forms first.
• Endpoint appears as brick-red color.

Normal Range

110-130 mEq/L.

Abnormal Findings

• Decreased Chloride:
  • Tuberculous Meningitis: Chloride levels are often reduced in TB meningitis.
  • Other Infections: Prolonged bacterial or fungal infections may also lower CSF chloride, though it is not a commonly measured parameter today.

Significance

• Chloride levels are rarely used as a primary diagnostic tool in modern practice, as more specific and sensitive markers (such as glucose and lactate) are preferred.

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Cellular Examination (Cytology)

Cell Count: Normal CSF contains very few cells (0–5 white blood cells/µL). Increased white blood cell count (pleocytosis) indicates infection, inflammation, or malignancy.

• Neutrophils: High in bacterial meningitis.
• Lymphocytes: Predominant in viral, fungal, or tuberculous meningitis and autoimmune conditions.

Red Blood Cells (RBCs): RBCs can indicate bleeding (e.g., subarachnoid haemorrhage) or trauma during a lumbar puncture.

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Microbiological Examination

1. Direct Microscopic Examination

Gram Staining

• A drop of centrifuged CSF sediment is stained by Gram stain.
• Helps detect bacteria immediately.

Acid-Fast Staining

• Used when tuberculous meningitis is suspected.

India Ink Preparation

• Used for fungal infection, especially cryptococcus.

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2. Culture Examination

Principle

• CSF is inoculated onto culture media to isolate microorganisms.

Common Media Used

• Blood agar
• Chocolate agar
• MacConkey agar

Incubation

• Incubate at 37°C

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3. Common Organisms Detected

• Neisseria meningitidis
• Streptococcus pneumoniae
• Mycobacterium tuberculosis

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4. Sensitivity Testing

• Performed after culture to select effective antibiotics.

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Serological and Immunological Tests

• Antibody Detection: Useful for diagnosing neurosyphilis or autoimmune diseases.
• Antigen Detection: Helpful in diagnosing specific pathogens (e.g., Cryptococcal antigen in fungal meningitis).

Other Special Tests

• Beta-2 transferrin: A specific marker for CSF, useful in diagnosing CSF leaks.
• Tau Protein and Amyloid Beta: Used to diagnose neurodegenerative diseases like Alzheimer’s.

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Clinical Significance

1. Diagnosis of Meningitis

• CSF examination is essential for diagnosing Meningitis.
• In meningitis:
  • Protein level increases
  • Glucose level decreases
  • Cell count rises

2. Differentiation of Types of Meningitis

• CSF helps distinguish:
  • Bacterial meningitis → high neutrophils, low glucose, high protein
  • Viral meningitis → lymphocytes predominate, glucose usually normal
  • Tuberculous meningitis → high protein, low chloride, cobweb clot formation

3. Detection of Central Nervous System Infections

• Microbiological examination detects bacteria, fungi, and other pathogens directly from CSF.

4. Diagnosis of Hemorrhage

• Presence of blood or xanthochromia suggests Subarachnoid hemorrhage.

5. Evaluation of Neurological Disorders

• Increased protein may occur in:
  • Demyelinating diseases
  • Peripheral neuropathies
  • Nerve root lesions

6. Detection of Raised Intracranial Pressure

• CSF pressure measurement helps identify intracranial hypertension.

7. Monitoring Disease Progress and Treatment

• Repeated CSF examination helps assess treatment response in infections and neurological diseases.

8. Detection of Malignant Cells

• CSF may show malignant cells in meningeal spread of tumors.