Lumbar puncture in infants and children — formative SAQs

SAQ 1 (10 marks)

A 4-month-old previously well infant presents with a 12-hour history of fever, irritability, and poor feeding. On examination the temperature is 38.9°C, heart rate 170, capillary refill 2 seconds, the anterior fontanelle is full but not bulging, and there are no focal neurological signs. The team is planning a septic workup including a lumbar puncture. ^{[1] [5]}

1. Outline the structured pre-lumbar-puncture assessment you will perform at the bedside before draping. (4) ^[1]
2. Describe the positioning and needle selection that maximise the chance of a successful diagnostic lumbar puncture and a reliable opening pressure in this infant. (3) ^{[2] [3]}
3. List the CSF tests you will request on the four collection tubes, and state the normal CSF values that apply to this age group. (3) ^[1]

Model answer — SAQ 1

(1) Pre-LP assessment (4). I run a structured pre-LP check asking four questions. First, raised pressure: I assess the conscious state using a paediatric GCS, examine the pupils and look for any focal neurology, and examine the fontanelle and (where feasible) the fundi for papilloedema; the full fontanelle here without focal signs does not mandate CT, but I would image first if any of these were abnormal. Second, stability: I confirm the observations show no shock, no Cushing's triad, and no respiratory compromise; the capillary refill of 2 seconds and the perfusion are acceptable. Third, bleeding: I ask about a bleeding history, check a platelet count and INR where relevant, and confirm the practical thresholds of platelets above 50 × 10⁹ per litre and INR under 1.5. Fourth, skin: I inspect the puncture site for cellulitis or a spinal dysraphism. I also obtain and document consent, plan the analgesia (topical anaesthesia plus oral sucrose and a parent holding the infant), and confirm the right child and right procedure. ^{[1] [4]}

(2) Positioning and needle selection (3). I position the infant in the left lateral decubitus position curled into a fetal position with hips and knees flexed, taking care not to over-flex the neck because an infant can obstruct their airway; I keep the shoulders and hips perpendicular to the bed because rotation narrows the interspinous space and is a common cause of a failed LP. I select an atraumatic pencil-point spinal needle (Whitacre or Sprotte) of an appropriate gauge (a 22-gauge paediatric needle in this age group), which reduces the rate of post-dural puncture headache compared with a cutting Quincke needle without reducing success. The lateral decubitus position is the one that allows a reliable opening pressure, read with the child calm and the legs extended at moment manometer is read. ^{[2] [3]}

(3) CSF tests and normal values for this age (3). I collect four numbered tubes and send them for: tube 1, cell count and differential; tube 2, protein and glucose with a paired plasma glucose taken at the same time; tube 3, Gram stain, microscopy and culture, with bacterial and viral multiplex polymerase chain reaction if clinically indicated; tube 4, any additional studies including viral PCR, cytology, or mycobacterial studies as indicated. For this 4-month-old the accepted normal CSF values are a white cell count under 5 per microlitre in the older infant (with a higher tolerance in the neonate, up to 20 to 30), protein around 0.15 to 0.45 g/L (higher accepted in the neonate), and a CSF glucose above 60% of a paired serum glucose (typically over 2.5 mmol/L). ^[1]