Lumbar Puncture - Neurology & Neurosurgery

Overview

• Adams and Victor's Principles of Neurology, 12th Ed.
• Roberts and Hedges' Clinical Procedures in Emergency Medicine
• Pye's Surgical Handicraft, 22nd Ed.
• Plum and Posner's Diagnosis and Treatment of Stupor and Coma

Anatomy

1. Skin and subcutaneous fat
2. Supraspinal ligament
3. Interspinous ligament
4. Ligamentum flavum (a distinct "give" is felt here)
5. Epidural space
6. Dura mater + arachnoid mater (another "give" felt)
7. Subarachnoid space - CSF obtained here (cauda equina floats freely)

Indications

• Suspected meningitis (bacterial, viral, tuberculous, fungal)
• Suspected subarachnoid hemorrhage (SAH) when CT is negative but clinical suspicion is high
• Encephalitis (viral, autoimmune)
• Carcinomatous meningitis / leptomeningeal spread
• CNS lymphoma / leukemia (including CNS staging)
• Neurosyphilis, neuroborreliosis
• Demyelinating disease (multiple sclerosis - oligoclonal bands)
• Guillain-Barre syndrome (albuminocytologic dissociation)
• Normal pressure hydrocephalus (opening pressure + gait/cognition response to drainage)
• Myelography / radionuclide cisternography (contrast instillation)

• Intrathecal antibiotics, antifungals, or antitumor agents (e.g., methotrexate)
• Spinal anesthesia instillation
• CSF drainage for pseudotumor cerebri (idiopathic intracranial hypertension) - repeated LP may be therapeutic
• Reduce drug concentration in CSF (e.g., inadvertent intrathecal drug overdose - additional 30-40 mL CSF withdrawn)

Patient Positioning

• Patient lies on their left side with hips, knees, and neck fully flexed (knees to chest, chin tucked)
• Maximally opens the interspinous spaces
• Allows accurate opening pressure measurement

• Patient sits upright, hunched forward with arms resting on a pillow
• Useful in obese patients or when midline identification is difficult
• Better for procedures in infants
• Note: opening pressure measurement is unreliable in this position

Pre-Procedure Assessment

1. Signs of raised ICP: headache, papilledema, altered consciousness, focal neurological signs
2. Coagulation status: INR ≤1.4 and platelets >50,000/mm³ are generally acceptable; aspirin alone at conventional doses is not a contraindication
3. Infection at the proposed skin site: cellulitis is a contraindication
4. Spinal anatomy: prior surgery, spinal stenosis, or deformity may complicate access

• Age ≥60 years
• Immunocompromised state
• History of CNS disease (mass, stroke, focal infection)
• Seizure within 1 week
• Altered level of consciousness
• Focal neurological findings (papilledema, gaze palsy, arm drift, leg drift, facial palsy, abnormal visual fields)

Technique (Step-by-Step)

1. Explain the procedure to the patient and obtain consent
2. Position the patient (lateral decubitus, hips and neck flexed)
3. Identify the L3-L4 or L4-L5 interspace using the posterior superior iliac crest as a surface landmark (Tuffier's line = line connecting the iliac crests = approximately L4 spinous process)
4. Prepare the skin with antiseptic solution using aseptic technique, drape the area
5. Infiltrate ~2 mL of 1% lidocaine (lignocaine) into the skin and subcutaneous tissue
6. Insert the LP needle (with stylet in place) in the midline, aimed slightly cephalad (toward the umbilicus), advancing through each layer
7. A "give" (loss of resistance) is felt as the needle passes through the ligamentum flavum; a second give as it pierces the dura
8. Remove the stylet - CSF should drip freely
9. Attach a manometer and measure opening pressure (normal: 70-180 mm H₂O in lateral decubitus)
10. Collect CSF into numbered tubes (typically 4 tubes, 1-2 mL each)
11. Measure closing pressure if desired
12. Replace stylet before removing the needle (reduces post-LP headache)
13. Apply firm dressing; patient kept supine

CSF Analysis - Normal Values & Formulas

Characteristic CSF Profiles (from Adams & Victor, Table 2-1)

LP for Subarachnoid Hemorrhage

• RBC count in CSF - counts <500/mm³ make SAH exceedingly unlikely
• Xanthochromia (yellow tint due to hemoglobin breakdown): appears 2-4 hours after SAH, persists up to 4 weeks; indicates true blood in subarachnoid space, not traumatic tap
• Traumatic tap vs. true SAH:
  • Traumatic tap: RBCs clear between tube 1 and tube 4 (≥25-30% reduction); blood may streak into clear CSF; clot may form in tube (defibrination occurs at hemorrhage site in SAH, so clot does NOT form)
  • True SAH: uniform blood across all tubes; xanthochromia present; no clot formation
• D-dimer in CSF can identify fibrinolysis as evidence of SAH (false positives with DIC, prior traumatic tap, or thrombolytics)
• Fluoroscopically guided LP reduces traumatic tap rate but requires radiology suite

Contraindications

• Infection (cellulitis) overlying the LP site
• Clinical evidence of herniation (Cushing's triad, blown pupil, decorticate/decerebrate posturing)
• Suspected intracranial mass with mass effect (asymmetric lesions near tentorium or foramen magnum are particularly dangerous)

• Raised ICP without known cause (get CT first)
• Coagulopathy (INR >1.4, platelets <50,000)
• Anticoagulant therapy (timing depends on agent - see below)
• Spinal deformity, prior lumbar surgery

Anticoagulation and LP

• Aspirin + any other anticoagulant: do NOT proceed without risk-benefit assessment
• Spinal epidural hematoma (SEH) is the most feared complication in anticoagulated patients - presents with back pain, progressive leg weakness, bowel/bladder dysfunction; treat with emergency MRI and laminectomy

Complications

Special Situations

LP in Meningitis

• Do not delay empiric antibiotics to wait for LP if there is clinical concern - draw blood cultures and treat
• Only a large brain abscess or substantial brain swelling absolutely contraindicates LP in suspected bacterial meningitis
• CSF protein >45 mg/dL in >90% of bacterial meningitis cases
• CSF glucose <40 mg/dL or <50% of blood glucose suggests bacterial meningitis
• WBC >50,000/mm³ raises possibility of brain abscess ruptured into a ventricle
• Normal CSF pressure in bacterial meningitis should raise suspicion of needle occlusion or spinal block

LP in Infants

• Sitting position preferred (better midline identification)
• Avoid tight neck flexion - can cause airway obstruction
• Preoxygenation is recommended, especially in neonates with respiratory distress
• Use stylet during advancement and on withdrawal to prevent epidermoid implantation
• Opening pressure measurement unreliable in a struggling infant

Normal Pressure Hydrocephalus (NPH)

• Classic triad: gait apraxia, dementia, urinary incontinence ("wet, wacky, wobbly")
• LP serves both diagnostically (opening pressure typically 150-200 mm H₂O, within normal but in upper range) and therapeutically (large-volume LP - 30-50 mL removed; improvement in gait predicts response to VP shunt)

Pseudotumor Cerebri (IIH)

• Elevated opening pressure (>250 mm H₂O); therapeutic LP removes enough CSF to lower pressure and relieve headache/visual symptoms
• Repeated LPs are used as a temporizing measure while medical therapy (acetazolamide, weight loss) or surgical shunting is arranged
• LP is safer here than in mass lesions because ventricles communicate freely

Post-LP Headache Management

• Orthostatic headache beginning 24-48 hours after LP, lasting up to 2 weeks
• Mechanism: persistent dural leak → CSF hypotension → downward displacement of brain → traction on pain-sensitive structures
• Cranial nerve palsies (especially CN VI) may accompany severe PLPH due to traction from intracranial hypotension
• Conservative: supine rest, oral/IV hydration, caffeine (300-500 mg)
• Definitive: Epidural blood patch (autologous blood 10-20 mL injected into epidural space at same level) - highly effective (>85% success rate), considered gold standard for refractory PLPH

Summary

• Adams and Victor's Principles of Neurology, 12th Ed., p. 25 (Table 2-1 CSF formulas; LP technique)
• Roberts and Hedges' Clinical Procedures in Emergency Medicine (Chapter 60 - Lumbar Puncture, pp. 1436-1444)
• Pye's Surgical Handicraft, 22nd Ed., p. 2587 (procedure description)
• Plum and Posner's Diagnosis and Treatment of Stupor and Coma, p. 414 (herniation risk, post-LP headache)