Acne fulminans — SAQ

Stem

A 16-year-old boy is brought to the dermatology clinic by his mother with a two-week history of a sudden, explosive eruption of painful lesions on his chest, back and shoulders. He had mild acne for the previous year, well controlled with topical adapalene. Over the last two weeks he has developed multiple large, tender, inflamed nodules that have ulcerated and are covered with thick, dark, necrotic haemorrhagic crusts. He is unable to sleep on his back because of the pain.

On systemic enquiry, he reports fever (39.2 degrees C at home), severe arthralgia of both knees and the left hip, generalised myalgia, malaise, and a three-kilogram weight loss. He has missed school for the last week. His mother is very distressed by his appearance and mentions that he has become withdrawn and tearful. He is a keen gym-goer and recently started taking whey protein and a "testosterone booster" supplement purchased online.

On examination: temperature 38.8 degrees C, pulse 96/min, BP 118/72 mmHg. The chest, upper back and shoulders are covered with coalescing, ulcerative, inflammatory nodules (1 to 3 cm) with overlying necrotic, haemorrhagic crusts; several lesions have drained purulent and haemorrhagic material. Comedones are sparse. The surrounding skin is intensely erythematous. There is tenderness over the medial end of the right clavicle and the sternoclavicular joints. Both knees are warm and tender but without effusion. There is no hepatosplenomegaly or lymphadenopathy. He appears unwell and in significant pain.

Blood tests: WBC 16.2 x 10^9/L (neutrophils 13.1 x 10^9/L), Hb 114 g/L, platelets 410 x 10^9/L, ESR 62 mm/h, CRP 88 mg/L. Blood cultures: pending. Skin swab: pending.

Questions

a) What is the clinical diagnosis, and state the three features that define it. (2 marks)

The diagnosis is acne fulminans (acute febrile ulcerative acne conglobata; acne maligna) — the most severe variant of inflammatory acne. It is defined by a diagnostic triad:^[6]

1. Sudden onset of ulcerative, haemorrhagic-crusted inflammatory nodules on the chest, back and shoulders.^[6]
2. Systemic symptoms — fever, arthralgia, myalgia, malaise and weight loss.
3. Laboratory evidence of systemic inflammation — leukocytosis with neutrophilia, markedly elevated ESR and CRP, and mild anaemia of chronic disease.

The abrupt deterioration from mild acne, the dramatic cutaneous ulceration, the clavicular tenderness (suggesting osteolytic bone lesions) and the bodybuilding supplement history are all classic. Acne conglobata is excluded because it lacks systemic symptoms and raised inflammatory markers and is chronic rather than acute.^[6]

b) Outline the proposed pathophysiology, and explain why cultures are typically sterile. (2 marks)

Acne fulminans is best understood as an explosive immune-mediated hypersensitivity reaction to Cutibacterium acnes (formerly Propionibacterium acnes) antigens, not as a primary bacterial infection. Three mechanisms are central:^[6]

1. Type III (immune-complex) hypersensitivity — circulating immune complexes of C. acnes antigen, antibody and complement form and deposit in tissue (skin, synovium, bone), driving the systemic symptoms (fever, arthralgia, myalgia, osteolytic bone lesions).^[6]
2. Intense neutrophilic infiltration and tissue destruction — C5a and IL-8 recruit massive numbers of neutrophils, which degranulate and release lysosomal enzymes (elastase, collagenase) that destroy the follicular wall and overlying epidermis, producing the ulceration with necrotic haemorrhagic crust.
3. Autoinflammatory (IL-1) axis — in a subset, PSTPIP1 mutations drive excess IL-1-beta via inflammasome activation, linking AF to the PAPA/PAPASH/SAPHO family.

Cultures are sterile because the inflammation is driven by the host immune response (immune complexes, complement, neutrophils) to C. acnes antigen, not by bacterial overgrowth or invasive infection. The organism itself remains confined to the follicle.^[6]

c) State the definitive treatment, including the critical sequence, specific drugs, doses and rationale. Explain why isotretinoin must NOT be started alone. (4 marks)

The cardinal principle: systemic corticosteroids FIRST, then low-dose isotretinoin — never isotretinoin alone.^[3][6]

Step 1 — Corticosteroid induction (Weeks 0 to 4):

• Oral prednisolone 0.5 to 1.0 mg/kg/day as a single morning dose.^[6]
• Rationale: corticosteroids broadly suppress the immune-complex cascade, complement activation, neutrophil recruitment and cytokine release; they bring fever and arthralmia under control within 24 to 72 hours and halt new lesions.
• Continue for approximately 4 weeks until systemic symptoms resolved and no new lesions for at least 2 weeks.

Step 2 — Introduce low-dose isotretinoin (Weeks 2 to 4):

• Once inflammation is controlled, add oral isotretinoin at a LOW starting dose, with continued corticosteroid cover.^[6]

Step 3 — Taper corticosteroid, uptitrate isotretinoin (Weeks 4 to 8+):

• Taper prednisolone slowly over months to avoid a rebound flare — see the local protocol for the schedule.
• Uptitrate isotretinoin toward the standard daily range.^[6]

Step 4 — Continue to cumulative target (Months 2 to 6):

• Continue isotretinoin to the standard cumulative target over months — the best predictor of long-term remission; see the local protocol for the exact dose.^[6]

Why isotretinoin must NOT be started alone: isotretinoin can trigger acne fulminans even at low starting doses, so corticosteroid cover must precede or accompany isotretinoin introduction.^[1]

Additional measures: advise cessation of the whey protein and testosterone booster (the trigger must be removed); analgesia with paracetamol; gentle wound care for ulcerated lesions; baseline fasting lipids, LFTs and beta-HCG before isotretinoin; monthly monitoring of lipids and LFTs;^[2][6] urgent adolescent mental-health referral for his withdrawal and tearfulness; X-ray or MRI of the right clavicle for the suspected osteolytic lesion.

d) Name four complications of acne fulminans, including the one that is inevitable and the one that is mandatory to screen for actively. (1 mark)

1. Severe atrophic, keloidal and hypertrophic scarring — this is expected; early treatment limits but does not prevent it; scar revision is deferred until durable disease quiescence.^[6]
2. Depression, anxiety, social isolation, school refusal and suicidal ideation — the psychological impact of sudden disfigurement in an adolescent is devastating; this is mandatory to screen for actively at every visit (PHQ-A or HADS).
3. Osteolytic bone lesions of the clavicle and sternum, with chronic osteitis and (rarely) pathological fracture.
4. Rebound flare if corticosteroids are tapered too quickly or isotretinoin is stopped prematurely.

^[3]e) The patient returns 6 weeks later, after having been started on isotretinoin 1 mg/kg/day by his GP at the same visit, with worsening ulceration, new fever and rigors. What has happened, what is the immediate management, and how is isotretinoin reintroduced? (1 mark)

^[2]This is isotretinoin-induced acne fulminans — the high starting dose of isotretinoin (1 mg/kg/day) without corticosteroid cover has precipitated or worsened the immune-complex flare. Immediate management:

^[1]1. Stop or markedly reduce the isotretinoin. 2. Start oral prednisolone 0.5 to 1.0 mg/kg/day and continue until the inflammation is controlled (afebrile, no new lesions, falling CRP). 3. Reintroduce isotretinoin at a LOW dose (0.25 to 0.5 mg/kg/day) once inflammation is quiescent, with continued corticosteroid cover and gradual uptitration as in the standard regimen above. 4. Screen for and treat any secondary infection (the new rigors mandate blood cultures and empirical anti-staphylococcal cover).

References4ShowHide

1. [1]Fakih A, Goens J, Grozdev I, et al. Acne fulminans induced by a low dose isotretinoin: case report and review of the literature Dermatol Online J, 2020.PMID 33423422
2. [2]Kandhari S, Thomas J, Khunger N, et al. Expert consensus on the rational approach to isotretinoin usage for effective management of acne: ERAISE ACNE recommendations Dermatol Ther (Heidelb), 2026.PMID 41915360
3. [3]Ortonne JP. Oral isotretinoin treatment policy. Do we all agree? Dermatology, 1997.PMID 9310744
4. [6]Greywal T, Zaenglein AL, Baldwin HE, et al. Evidence-based recommendations for the management of acne fulminans and its variants J Am Acad Dermatol, 2017.PMID 28619551