Abscess drainage and minor procedures — formative SAQs

SAQ 1 (10 marks)

A previously well 6-year-old presents with a four-day history of a tender, red, domed lump on the right thigh. On examination the temperature is 37.2°C, the lump is 4 cm across and fluctuant with a central pointing pustule, and there is surrounding cellulitis extending about 3 cm beyond the margin. The team plans to drain it in the treatment room. ^{[1] [2]}

1. Outline the structured bedside assessment you will perform before draining this abscess, including the explicit check for the red flags of a necrotising infection. (4) ^[1]
2. Describe the analgesia plan, including the weight-based maximum dose of plain lidocaine for this 22 kg child and the corresponding volume of one-percent lidocaine. (3) ^[4]
3. Describe the drainage technique you would use for this 4 cm abscess and the aftercare plan, including when you would give adjuvant antibiotics. (3) ^{[2] [3]}

Model answer — SAQ 1

(1) Structured bedside assessment (4). I run a structured pre-drainage check asking four questions. First, is this a drainable abscess or something else: I confirm fluctuance on palpation, look for the central pointing pustule that marks the natural incision site, mark the margins of the surrounding cellulitis with a pen, and palpate for regional lymphadenopathy; I use a bedside ultrasound to confirm a hypoechoic collection if there is any doubt. Second, are there red flags of necrotising fasciitis that mandate escalation rather than treatment-room drainage — I 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 (tachycardia, hypotension, altered conscious state), and skin changes of grey or dusky discolouration, haemorrhagic bullae or necrosis; none is present here, so the abscess is appropriate for treatment-room drainage. Third, is the child systemically well enough for a treatment-room procedure: the temperature and observations are normal. Fourth, is the child's developmental stage matched to the analgesia plan, and have I calculated the weight-based local anaesthetic dose and documented consent. ^[1]

(2) Analgesia plan and weight-based lidocaine dose (3). I apply a topical anaesthetic early — EMLA under an occlusive dressing for at least 60 minutes, or amethocaine 4% (Ametop) for 30 to 45 minutes — over the planned incision site. For a cooperative school-age child I then infiltrate the abscess wall with plain one-percent lidocaine, aspirating before each injection and injecting slowly to avoid intravascular delivery. The weight-based maximum dose of plain lidocaine is 3 mg/kg, so for a 22 kg child the maximum is 66 mg, which corresponds to 6.6 mL of one-percent lidocaine (one-percent lidocaine contains 10 mg/mL). I write the dose in milligrams and the maximum volume on the drape and do not exceed it; if I needed more volume I would switch to a more dilute concentration (0.5%) or add adrenaline (which raises the maximum to 7 mg/kg). If the child were anxious or uncooperative I would add nitrous 50/50 or dissociative sedation by the local protocol with full monitoring. ^[4]

(3) Drainage technique, aftercare and antibiotics (3). For a 4 cm abscess with surrounding cellulitis I use the loop drainage technique: under sterile conditions I make two small stab incisions at the poles of the abscess along the skin tension lines, express the pus and send a sample in a sterile container for Gram stain, culture and susceptibility, then break the loculations by passing a finger or a blunt haemostat around the cavity to divide every septum. I pass a silicone vessel loop through the cavity with a haemostat, bring it out through the other stab incision, and tie it with light tension over the skin; the loop keeps the cavity open as it heals by secondary intention and is removed at the one-week review. Aftercare is a simple dressing, simple analgesia, and a 24 to 48 hour review with a safety-net for worsening pain, spreading redness or fever. Adjuvant oral antibiotics are indicated here because of the surrounding cellulitis: I give cephalexin first-line, with clindamycin or trimethoprim-sulfamethoxazole if community-acquired MRSA is suspected locally, guided by the pus culture result. ^{[2] [3]}