Solitary Mastocytoma

Solitary mastocytoma is a single, localised nodule or plaque of mast cells in the skin, usually in infants. It is benign and tends to resolve spontaneously, and systemic mastocytosis is very uncommon.

Pathophysiology. A localised clone of mast cells with an activating KIT variant forms a nodule. Rubbing releases mediators and causes swelling, redness and sometimes a blister.

Most appear in the first months of life and fade over months to years. A few children have 2 or 3 lesions.

Systemic symptoms from mediator release occur only occasionally and rarely in solitary lesions.

• A single pink, yellow brown or orange nodule or plaque, 1 to 5 cm, on the trunk or limbs
• Darier sign: swelling and redness after rubbing the lesion
• Blistering over the lesion
• Itching and flushing when the lesion is rubbed
• Rarely systemic symptoms (flushing, diarrhoea, wheeze)
• Spontaneous flattening and fading

• Clinical examination: typical lesion in an infant with a Darier sign. Avoid vigorous rubbing at the examination.
• Biopsy is needed only if the diagnosis is uncertain (dense mast cell infiltrate, tryptase and CD117 positive).
• Serum baseline tryptase: a normal or mildly raised level supports disease limited to the skin. Over 20 microgram/L needs further review.
• Examination for other lesions and for organomegaly or lymph nodes.
• Systemic workup only if tryptase is high or there are systemic symptoms.

Differential diagnosis.

• Juvenile xanthogranuloma
• Spitz naevus and congenital melanocytic naevus
• Langerhans cell histiocytosis
• Bullous impetigo and insect bite
• Urticaria pigmentosa and diffuse cutaneous mastocytosis
• Infantile haemangioma and dermatofibroma

• Reassurance: the nodule usually resolves within months to years, and no treatment is needed if the child is well.
• Avoid rubbing and friction, and protect the area with a soft dressing.
• Antihistamine for itching or flushing: cetirizine (children 1 to 5 years 2.5 mg twice daily; 6 to 11 years 5 mg twice daily).
• Topical corticosteroid for a symptomatic or blistering lesion: mometasone 0.1% cream once daily for 2 to 4 weeks, or under occlusion for a short period.
• Excision for a persistent or symptomatic lesion at a site of constant friction, with antihistamine cover, since the area can release mediators.
• Trigger avoidance: avoid opioids, NSAIDs and aspirin where possible, and use antihistamine cover for procedures.
• Anaphylaxis plan only when there have been systemic reactions: adrenaline 0.01 mg/kg intramuscularly (maximum 0.5 mg) and an auto injector (150 microgram for 7.5 to 25 kg).

Special groups.

• Children with several lesions: check baseline tryptase and consider evaluating for maculopapular disease.
• Adults with a new solitary lesion: take a biopsy and consider a systemic workup.

Monitoring. Review every 6 to 12 months until resolution, with photographs and measurements. Tryptase if not done before.

Referral and escalation. Refer to paediatric dermatology for diagnostic doubt, repeated blistering or multiple lesions. Urgent for systemic symptoms, and emergency for anaphylaxis.