Drain a paediatric skin abscess — OSCE

Candidate brief

You have this station to assess a 6-year-old before a treatment-room abscess drainage, perform the structured pre-drainage safety check including the red flags of necrotising fasciitis, calculate the weight-based local anaesthetic dose, choose between packing and the loop drainage technique, outline the drainage procedure and the aftercare, and discuss the complications. Treat this as a calm, planned procedure in a stable child, with an explicit safety check before any drape goes on. [1] [2]

Key teaching and management objectives

Begin by performing the structured pre-drainage check, asking four explicit questions. Drainable abscess versus something else: confirm fluctuance, look for the pointing pustule that marks the incision site, mark the margins of the surrounding cellulitis, and use a bedside ultrasound to confirm a hypoechoic collection if there is any doubt. Red flags of necrotising fasciitis that mandate escalation rather than treatment-room drainage: explicitly ask about pain out of proportion to the visible findings, induration extending beyond the visible margins, skin anaesthesia, crepitus, a rapidly evolving course over hours, systemic toxicity, and skin changes of grey or dusky discolouration, haemorrhagic bullae or necrosis; none is present here. Stability: confirm the child is systemically well. Consent, the child's developmental stage, and the analgesia plan. [1]

Next, calculate the weight-based local anaesthetic dose. For a 22 kg child the maximum dose of plain lidocaine is 3 mg/kg, which is 66 mg, corresponding to 6.6 mL of one-percent lidocaine (10 mg/mL). Write the dose in milligrams and the maximum volume on the drape and do not exceed it; aspirate before every injection and inject slowly to avoid intravascular delivery and local anaesthetic systemic toxicity. Apply a topical anaesthetic (EMLA for at least 60 minutes, or amethocaine for 30 to 45 minutes) before infiltration, and plan procedural sedation with full monitoring if the child is anxious. [3]

Choose between packing and the loop drainage technique. For this 4 cm abscess with surrounding cellulitis the loop technique is preferred: under sterile conditions, make two small stab incisions at the poles of the abscess along the skin tension lines, express the pus and send a sample for Gram stain, culture and susceptibility, break the loculations by passing a finger or a blunt haemostat around the cavity to divide every septum, pass a silicone vessel loop through the cavity, bring it out through the other stab incision, and tie it with light tension over the skin. The systematic review evidence shows the loop technique reduces the need for a secondary procedure and the recurrence rate compared with packing, particularly in abscesses over about 5 cm. The loop is removed at the one-week review. [2]

Close with aftercare, antibiotics, complications, and escalation. Aftercare is a simple dressing, simple analgesia, and a 24 to 48 hour review with a safety-net for worsening pain, spreading redness, fever, or recurrence. Adjuvant oral antibiotics are indicated here because of the surrounding cellulitis over 5 cm total extent: cephalexin first-line, with clindamycin or trimethoprim-sulfamethoxazole for suspected community-acquired MRSA, guided by the pus culture result. The common complications are local pain, mild bleeding, scarring, and recurrence from an incomplete drainage; the serious complications are local anaesthetic systemic toxicity from exceeding the weight-based dose, damage to a neurovascular structure from poor technique, and progression to a necrotising infection from a delayed diagnosis. The Talan 2016 trial and the network meta-analysis support the modest benefit of adjuvant antibiotics in abscesses of this size and with surrounding cellulitis. [3] [4]

Marking domains

• Patient safety and the pre-drainage check (3 marks). Explicitly screens for drainable abscess versus cellulitis versus necrotising infection; lists the red flags of necrotising fasciitis without prompting; documents consent and a safety-net.
• Local anaesthetic dose and pharmacology (4 marks). Calculates the weight-based dose correctly (22 kg × 3 mg/kg = 66 mg = 6.6 mL of one-percent lidocaine); states the dose with adrenaline (7 mg/kg) and bupivacaine (2 mg/kg); describes aspiration and slow injection to prevent LAST.
• Procedural technique (3 marks). Chooses the loop drainage technique for this 4 cm abscess; describes incision over the pointing pustule, loculation breakdown with a finger or haemostat, vessel loop insertion and tension; sends pus for culture.
• Antibiotics and aftercare (2 marks). Gives adjuvant antibiotics because of the surrounding cellulitis; names cephalexin first-line with a MRSA alternative; plans a 24 to 48 hour review.
• Complications and escalation (3 marks). Names the common and the serious complications; recognises the early and late signs of LAST and the intralipid rescue; states when to refer for surgical exploration. [2] [3]