Knowledge on rare diseases and orphan drugs

MCM is the most common form of CM (accounting for up to 90% of cases) but the prevalence in the general population is unknown. Incidence has been estimated at between 1/1000 and 1/125 births. This entity is most commonly reported among the Caucasian population and affects both sexes, although a slight male predominance (1.7-1.8:1) has been reported in cases with early onset.

The majority of patients present in infancy or childhood but onset may also occur in adulthood. As lesions vary in aspect, several subvariants have been described in the past (plaque form, typical form, telangiectatic form, and nodular form) but are all now grouped under the same entity. The plaque or papular form presents with orange/yellow papules or plaques often appearing during the first few months of life. Typical or classic forms have a more widespread, symmetrical distribution of round or oval red/brown macules. The telangiectatic form (telangiectasia macularis eruptiva perstans; TMEP) is a disputed entity described as a rare variant occurring in adults and characterized by the presence of red/brown telangiectatic macules. The nodular form is rare. In MCM, the size and number of lesions is variable, typically ranging in size from 1 mm to over 1 cm and in number from 10-1000 lesions. MCM may appear on all regions of the body but the trunk and extremities are most frequently involved. The palms and soles are usually spared. Darier's sign, dermographism and pruritus are additional features of MCM. Extensive mechanical manipulation and other factors that trigger mast cell degranulation (non-steroidal anti-inflammatory drugs, physical stimuli, emotional stress, insect venom and certain foods) may lead to systemic symptoms such as flushing, headache, dyspnea, wheezing, rhinorrhea, nausea, vomiting, diarrhea, and syncope.

Mutations in the KIT gene (4q11-q12) have been identified in patients with MCM. However, this mutation is rare in the pediatric population and the etiology and pathogenesis of MCM in these cases remains to be determined. MCM generally occurs sporadically but rare familial cases have been reported.

Diagnosis in children is based on the clinical appearance of the lesions and the positive Darier's sign. Occasionally (generally in cases with presentation after 5 years of age), a skin biopsy may be required for confirmation of the diagnosis. In adults, a bone marrow examination should be performed to exclude the diagnosis of SM.

The diagnosis is usually straightforward but misdiagnosis as chronic urticaria or idiopathic anaphylaxis has been reported.

Trigger factors should be avoided and symptomatic management includes administration of antihistamines, topical steroids and mast cell membrane stabilizers. PUVA or UVA1 therapy may also be used for adolescents or adults who do not respond to other forms of treatment.

The prognosis is good, especially for patients with childhood onset below the age of 5 years, with improvement (around 50% of cases) or complete resolution (30% of cases) of symptoms by adolescence. In contrast, spontaneous resolution is rare in patients with adult-onset forms of the disease and there is a higher risk of systemic involvement.

Last update: June 2013 - Expert reviewer(s): Pr A.P. [Arnold] ORANJE