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]

SAQ 2 (10 marks)

A 4-month-old male infant is brought in with a tender, fluctuant 1.5 cm swelling at the right anal margin that has been present for two days. He is afebrile, feeding normally, and systemically well. The parents are anxious about surgery. [5]

1. What is the most likely diagnosis, and what is the natural history of this condition in this age group? (3) [5]
2. Outline the management plan, including the role of drainage and the indication for surgical referral. (4) [5]
3. The mother of the infant asks whether the swelling could be a sign of non-accidental injury. Outline how you would address this concern. (3) [1]

Model answer — SAQ 2

(1) Diagnosis and natural history (3). The most likely diagnosis is a perianal abscess of infancy, which occurs predominantly in male infants under one year of age and is thought to arise from congenital anal crypt abnormalities. The natural history is favourable: the majority of perianal abscesses in well infants resolve or heal after a single simple drainage at the mucocutaneous junction, and only a minority develop a communicating fistula-in-ano. The 2026 narrative review by Alligood and colleagues summarises the evidence that conservative or minimal-intervention management reduces the fistula rate without increasing recurrence, and is the modern default for the well infant. [5]

(2) Management plan and the role of surgical referral (4). The conservative-first approach is the modern teaching. I confirm the diagnosis clinically — a tender, fluctuant swelling at the anal margin in a systemically well male infant — and exclude systemic toxicity and signs of a more complex deep abscess. For a small, well-circumscribed abscess in a well infant I arrange simple drainage at the mucocutaneous junction under topical anaesthesia and oral sucrose, send pus for culture, and review at 24 to 48 hours. I do not arrange a fistulotomy or operative exploration at first presentation, because most fistulae close spontaneously or with conservative measures and unnecessary surgery risks iatrogenic sphincter injury. I refer to paediatric surgery for the minority that develop a persistent or recurrent fistula-in-ano after the initial drainage, or for an atypical, deep, or recurrent abscess in an older or unwell infant. The family is counselled that a perianal fistula in an otherwise well male infant is a different entity from adult fistula-in-ano and is usually managed expectantly. [5]

(3) Addressing the concern about non-accidental injury (3). I take the concern seriously and take a careful, non-judgemental history from both parents, including the onset and progression of the swelling, any witnessed trauma, prior episodes, and the social context. Perianal abscess in a male infant under one year is a well-described entity with a characteristic benign natural history, and on its own is not a marker of abuse; however, atypical perianal findings — particularly an anal fissure with delaying in presentation, an unexplained perianal laceration, a fissure in an older non-constipated child, or signs inconsistent with the offered history — should prompt consideration of non-accidental injury and a safeguarding assessment. I examine the infant fully for other unexplained bruises, fissures, or injuries; if the findings are consistent with a simple perianal abscess in a well-cared-for infant I reassure the family and proceed with the conservative plan, but if any feature is atypical I follow the local safeguarding pathway and discuss with the child-protection team. [1]