Cutaneous Mastocytosis

Cutaneous mastocytosis is a clonal accumulation of mast cells in the skin without systemic disease. It is mostly a disease of children, and most paediatric cases resolve by adolescence.

Pathophysiology. Activating variants in KIT (D816V in most adults, other variants in children) cause mast cell proliferation and mediator release. Variants of cutaneous disease are maculopapular cutaneous mastocytosis (urticaria pigmentosa), diffuse cutaneous mastocytosis and mastocytoma.

In children, onset is usually in the first 2 years, and disease is limited to skin in most. In adults, skin lesions often accompany systemic mastocytosis, so adults need a systemic workup.

Mast cell mediators cause itching, flushing, blistering (in infants), and in some systemic symptoms.

• Brown or red brown macules, papules or plaques (maculopapular)
• Darier sign: wheal and erythema when a lesion is stroked
• Itching, flushing and blistering in infants
• Solitary nodule (mastocytoma) in infants
• Diffuse thickened, doughy skin with blistering (diffuse form)
• Systemic symptoms in some: flushing, abdominal pain, diarrhoea, hypotension
• Triggers: friction, heat, fever, infection, vaccination, drugs (opioids, NSAIDs), anxiety

• Clinical examination: skin lesion type, distribution and Darier sign, with lesion photographs.
• Skin biopsy: increased mast cells (toluidine blue, tryptase, CD117) in lesional skin, and KIT testing in adults.
• Serum baseline tryptase: in children, under 20 microgram/L suggests no systemic disease. Over 20 microgram/L needs further review.
• Adults: KIT D816V in blood, bone marrow biopsy when baseline tryptase is raised, with full blood count, liver enzymes, calcium, bone density scan and abdominal imaging for organomegaly.
• Children with suspected systemic disease: hepatosplenomegaly, cytopenia or very high tryptase.

Differential diagnosis.

• Systemic mastocytosis with skin involvement
• Langerhans cell histiocytosis, juvenile xanthogranuloma
• Congenital melanocytic naevus and café-au-lait macules
• Insect bites and papular urticaria
• Bullous impetigo and epidermolysis bullosa (infant blistering)
• Mast cell activation syndrome

• Education and trigger avoidance: gentle handling, avoid friction over lesions, avoid hot baths, and avoid known trigger drugs (NSAIDs, opioids, aspirin) unless needed.
• Antihistamine for itching and flushing: cetirizine 10 mg orally once daily in adults (children 1 to 5 years 2.5 mg twice daily; 6 to 11 years 5 mg twice daily), increased as needed.
• Topical treatment: mometasone 0.1% cream once daily for 2 to 4 weeks for symptomatic lesions and blistering, with a low potency steroid on thin skin.
• Mast cell stabiliser: sodium cromoglicate 200 mg orally four times daily (children 100 mg four times daily from age 2 to 12 years) for gastrointestinal symptoms.
• Phototherapy: narrowband UVB or PUVA in adults with severe itching and widespread lesions, with attention to the risk of skin cancer.
• Mastocytoma blistering: protect with dressings and a topical corticosteroid.
• Anaphylaxis risk plan in children with high tryptase or prior reactions: adrenaline auto injector 150 microgram for 7.5 to 25 kg, 300 microgram for 25 to 50 kg, 500 microgram for over 50 kg, with written plan and training of carers.
• Perioperative and vaccine planning: vaccination can proceed, with antihistamine cover and observation for 30 minutes. Plan anaesthesia carefully with premedication.
• Resolution: most children improve by adolescence, so avoid unnecessary long term medication.

Special groups.

• Adults with skin lesions: work up and treat for possible systemic mastocytosis, including lifelong adrenaline for venom reactions.
• Infants with extensive blistering or diffuse disease: admit for monitoring, since there is a higher risk of systemic reactions.

Monitoring. Review every 6 to 12 months in children, with lesion photographs and tryptase if not previously done. Review adults every 6 to 12 months with tryptase, full blood count and bone density.

Referral and escalation. Refer to paediatric dermatology or allergy for all children with suspected mastocytosis, and to haematology or an adult mastocytosis service for adults. Emergency for anaphylaxis, or collapse with flushing.