Perform a lumbar puncture on an infant — OSCE

Candidate brief

You have this station to assess a 4-month-old infant before a diagnostic lumbar puncture, perform the structured pre-LP safety check, position the child and choose the needle, outline the technique and the opening pressure measurement, and discuss the complications and aftercare. Treat this as a calm, planned procedure in a stable child, with an explicit safety check before any drape goes on. [1] [4]

Key teaching and management objectives

Begin by performing the structured pre-LP check, asking four explicit questions. Raised pressure: assess the conscious state with a paediatric GCS, examine the pupils and look for any focal neurology, examine the fontanelle and the fundi, and image first if any of these are abnormal. Stability: confirm the observations show no shock, no Cushing's triad, and no respiratory compromise. Bleeding: ask about a bleeding history, check a platelet count and INR where relevant, and confirm the pragmatic thresholds of platelets above 50 × 10⁹ per litre and INR under 1.5. Skin: inspect the puncture site for cellulitis or a spinal dysraphism. Document consent and plan the analgesia. [1] [3]

Next, position the child correctly. The default for a reliable opening pressure is the left lateral decubitus position, curled into a fetal position with hips and knees flexed; in the infant take care not to over-flex the neck, because an infant can obstruct their airway. Keep the shoulders and hips perpendicular to the bed, because rotation narrows the interspinous space and is a common cause of a failed LP. [1]

Choose the atraumatic pencil-point spinal needle (Whitacre or Sprotte) of an appropriate gauge (a 22-gauge paediatric needle in this age group), which reduces post-dural puncture headache compared with a cutting Quincke needle without compromising successful CSF collection. The needle must be styletted throughout, with the stylet removed only to check for CSF flow and replaced before withdrawal to prevent a delayed epidermoid tumour. [2]

Identify the correct interspace using Tuffier's line drawn between the iliac crests, which crosses the L4 spinous process or the L4-L5 interspace. In the very young infant choose L4-L5 or L5-S1 because the conus medullaris sits at L3 at birth; in the older child choose L3-L4 or L4-L5. Apply the topical anaesthetic (EMLA for at least 60 minutes, or amethocaine for 30 to 45 minutes) and infiltrate 1% lidocaine subcutaneously (max 3 mg/kg) before the pass. [1] [2]

Once CSF flows, attach the manometer and measure the opening pressure with the child calm and the legs extended at the moment the manometer is read. Collect four numbered tubes for cell count and differential, protein and glucose with a paired serum glucose, Gram stain and culture with bacterial and viral multiplex PCR, and any additional studies. Replace the stylet before withdrawing the needle to avoid arachnoid strand traction and a nerve root injury. Label the tubes at the bedside and send them promptly. [1]

Close with complications and aftercare. The common complications are local back pain, transient radicular pain during the procedure, and post-dural puncture headache, which is characteristically postural and managed with rest, analgesia and hydration, with an epidural blood patch reserved for severe or persistent cases. The serious complications — spinal epidural haematoma in the coagulopathic child, epidural abscess, iatrogenic meningitis, and cerebral herniation in raised intracranial pressure — are prevented by the structured pre-LP check and the imaging-first rule. In suspected bacterial meningitis the rule is antibiotics first, LP when the child is stable, with dexamethasone given with or before the first antibiotic dose in suspected pneumococcal disease. [3] [4]

Marking domains

• Patient safety and pre-LP check (3 marks). Explicitly screens for raised pressure, neurology, stability, bleeding and skin; identifies the imaging-first criteria; documents consent.
• Procedural knowledge (4 marks). Correct interspace for age using Tuffier's line; atraumatic needle and stylet discipline; correct position with hips and shoulders perpendicular; opening pressure measured in lateral decubitus with the child calm.
• Analgesia and communication (2 marks). Plans age-appropriate topical anaesthesia and oral sucrose; communicates honestly with the parent and, where appropriate, the child.
• Sample handling and interpretation (2 marks). Pairs a serum glucose; sends four numbered tubes; recognises the traumatic tap and the corrected white-cell count.
• Complications, aftercare and escalation (3 marks). Names the common and the serious complications; gives the post-dural puncture headache plan; states the antibiotics-first rule in suspected bacterial meningitis and the role of dexamethasone. [3] [4]