Persistent napkin rash, a violaceous nodule, and thrombocytopenia in an infant
A 7-week-old girl is brought with a 3-week history of a persistent napkin rash. She has erythematous plaques in the groin and perianal area with satellite pustules and a 2.5-cm indurated violaceous nodule in the left inguinal fold. There is greasy scaling behind the ears and in the axillae. She was treated with topical clotrimazole and hydrocortisone 1% for 2 weeks without improvement. Weight has fallen from the 50th to the 10th centile. She is afebrile, feeding poorly, and has a few petechiae on the abdomen. Full blood count shows haemoglobin 9.8 g/dL, white cells 11 × 10⁹/L, platelets 85 × 10⁹/L. A potassium hydroxide mount from a pustule is negative. The mother asks whether this is just stubborn thrush.
1. What is your differential diagnosis, and why is Candida unlikely to explain everything?
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The differential includes Langerhans cell histiocytosis (LCH), congenital syphilis, bullous impetigo, scabies, herpes simplex infection, zinc deficiency, biotin deficiency, and an underlying immunodeficiency. Candida is common in the napkin area and can produce satellite pustules, but it usually responds to appropriate topical therapy and does not explain an indurated violaceous nodule, petechiae, thrombocytopenia, failure to thrive, or seborrhoeic scaling behind the ears and in the axillae.
The combination of a seborrhoeic-like eruption, inguinal nodule, petechiae, thrombocytopenia, and poor weight gain is highly concerning for multisystem LCH. A negative KOH mount does not exclude fungal infection, but it makes Candida a less complete explanation. I would not simply change the antifungal cream; I would investigate for a systemic disease.
2. What investigations would you arrange, and what would you look for on biopsy?
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I would arrange a skin biopsy of the nodule for haematoxylin and eosin staining and immunohistochemistry for CD1a, langerin and S100; BRAF V600E testing may be useful. I would also check full blood count and film, liver and renal function, coagulation, and inflammatory markers. Staging for LCH should include a skeletal survey, chest radiograph, abdominal ultrasound, and careful examination for lymphadenopathy and hepatosplenomegaly. Syphilis serology and bacterial and viral swabs are reasonable in an infant with a persistent napkin rash.
On histology, I would look for a proliferation of mononuclear cells with grooved nuclei and eosinophils, and confirm the diagnosis with CD1a and langerin positivity. The key question is whether there is risk-organ involvement: liver, spleen, bone marrow, or lung. Thrombocytopenia and poor feeding raise concern for multisystem disease rather than skin-limited LCH.
3. Outline your initial management and referral.
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I would refer urgently to paediatric dermatology and paediatric oncology for suspected LCH. Skin-limited disease may be treated with topical corticosteroids or, rarely, topical nitrogen mustard, but multisystem disease requires systemic chemotherapy, often vinblastine and prednisolone with or without cytarabine, under specialist supervision. Supportive care includes nutritional support, infection control, and monitoring of blood counts and liver function.
I would also address the family’s question directly: this is not simply stubborn thrush, and a biopsy is needed because the rash is associated with a nodule and low platelets. I would explain that LCH is not inherited in most cases and that treatment depends on staging. If there are concerns about poor feeding, weight faltering, or delayed presentation, I would involve the paediatric safeguarding team according to local policy, while making clear that the primary concern is a serious medical diagnosis.