Lumbar puncture and CSF analysis
To obtain cerebrospinal fluid (liquor cerebrospinalis), a lumbar puncture is performed. The patient lies on their side with knees maximally drawn to the abdomen and head bent forward. The needle is inserted between the spinous processes of the lower lumbar vertebrae after skin antisepsis and local anesthesia.
Fluid pressure is measured immediately upon its appearance in the needle (normal is 100–200 mm H₂O). The procedure is strictly contraindicated in case of elevated intracranial pressure due to the high risk of brain herniation.
Normal CSF parameters:
- Color and transparency: colorless, completely transparent fluid.
- Density: 1.005–1.008.
- Cytosis (cell count): up to 5 per 1 µL (predominantly lymphocytes).
- Protein: no more than 0.45 g/L.
- Glucose: 2.2–3.9 mmol/L.
Pathological changes:
- Xanthochromia (yellow, brown, or green discoloration) occurs due to hemoglobin degradation following intracranial hemorrhage.
- Pleocytosis (elevated cell count) indicates inflammation. A neutrophilic profile is a marker of bacterial infection, while a lymphocytic profile indicates viral.
- Protein-cell dissociation (elevated protein without a significant increase in cells) is characteristic of tumors and demyelinating polyradiculoneuropathies.
Morphological studies (biopsy)
Morphological analysis is used for the differential diagnosis of nervous system and muscular disorders.
- Muscle biopsy. It allows distinguishing primary muscle damage (tissue replaced by adipose or connective tissue) from neurogenic lesions. In spinal cord or nerve damage, fibers in the affected motor units decrease sharply, while adjacent intact areas undergo compensatory enlargement. Signs of inflammation are also detected (e.g., in polymyositis).
- Peripheral nerve biopsy. The sural nerve (n. suralis) is most frequently examined. It is a sensory nerve, and its damage does not cause significant motor complications. The method reveals demyelination, axonal degeneration, inflammatory infiltrates, vasculitis of nutrient arteries (vasa nervorum), and amyloid deposits.
- Skin biopsy. Used in the early stages of small-fiber neuropathy (e.g., in diabetic neuropathy). The main sign of pathology is the complete absence of terminal nerve fibers in the epidermis.
Ultrasound diagnostic methods
Ultrasound diagnostics of the main cerebral arteries are based on the Doppler effect (evaluation of blood flow parameters) and echotomography (visualization of the vessel wall and structures).
The modern standard is duplex scanning of the neck vessels. It carries no risk of complications and is comparable in diagnostic accuracy to angiography. The greatest diagnostic value lies in examining the bifurcation of the common carotid artery (CCA) and the origin of the internal carotid artery (ICA) — typical sites for atherosclerotic plaque localization.
Ultrasound signs of pathology:
- Stenosis: color Doppler shows a filling defect, while spectral Doppler records a significant increase in blood flow velocity.
- ICA occlusion (blockage): blood flow in the affected area is completely absent, while reduced blood flow is recorded in the CCA.
Transcranial Dopplerography (TCD) is performed through the cranial bones. It is essential for detecting cerebral artery spasm, assessing collateral circulation, and long-term monitoring of microembolism signs.
Neuroimaging: SPECT and MRI
Various tomography methods address different clinical tasks, complementing each other when examining patients with dementia, epilepsy, or traumatic brain injury.
- MRI (including FLAIR and T2 modes) can detect local structural and morphological changes, such as signs of past infarction in brain structures (e.g., in the thalamus).
- SPECT (single-photon emission computed tomography) reveals functional disorders, demonstrating zones of significantly reduced metabolism in the cerebral cortex, which is especially important in early stages of diseases.
Briefly: answers to topical questions
What are the absolute and relative contraindications for lumbar puncture?
Absolute and relative contraindications for lumbar puncture include a wide range of neurological, somatic, and hemodynamic disorders.
These include:
- Infectious processes — local skin and soft tissue infection at the puncture site.
- Neurological pathologies — brain mass effect, elevated intracranial pressure, intraventricular pathway occlusion, focal neurological symptoms, seizures, altered level of consciousness (GCS less than 9) or fluctuating consciousness, inadequate pupillary light reflex ("doll's eye" sign).
- Somatic disorders — unstable hemodynamics, septic shock, relative bradycardia and hypertension, respiratory failure.
- Hemostasis disorders — platelet count less than 100 × 10⁹/L, ongoing anticoagulant therapy, progressive purpuric rash.
Between which specific vertebrae is the needle inserted during a lumbar puncture in adults?
During a spinal puncture, the needle is inserted into the space between the fourth and fifth lumbar vertebrae.
The anatomical landmark for determining the puncture site is the spinous process of the fourth lumbar vertebra (L4). To locate it, Tuffier's line is used — a horizontal line connecting the iliac crests and passing precisely through the L4 spinous process. The puncture is performed strictly between the spinous processes of the lower lumbar vertebrae.
To remember CSF cytosis (up to 5) and pressure (100–200): "One or two hundred drops of pure water, and only five little cells inside."
Class and exam questions
What are the normal cytosis parameters in CSF?
Normally, the cell count in cerebrospinal fluid should not exceed 5 per 1 µL, predominantly consisting of lymphocytes.
What is protein-cell dissociation?
This is a pathological condition of the CSF characterized by a predominant increase in protein content with a normal cell count. It occurs in tumors and certain polyradiculoneuropathies.
Why is the sural nerve (n. suralis) preferred for nerve biopsy?
The sural nerve is exclusively sensory. Harvesting tissue from it does not lead to significant motor or trophic complications for the patient.
What is the difference between MRI and SPECT in brain examination?
Structural MRI visualizes local morphological changes (e.g., a focus of prior infarction), whereas SPECT reveals functional disorders — zones of cortical hypometabolism.
What else to study on this topic
- Therapeutic goals of lumbar puncture (sanation, intrathecal drug administration)
- Evaluation of collateral circulation in occlusive lesions
- Arterio-arterial and cardiac embolism in TCD
- Indications for surgical intervention (carotid endarterectomy, stenting)
- Ultrastructural muscle changes in metabolic myopathies (electron microscopy)
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Other topics in "Chapter 18. Instrumental and Laboratory Diagnostic Methods"
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