Red, itchy skin lesions triggered by pregnancy


You are asked to evaluate a healthy 18-year-old girl with a history of “mosquito bites” on her arms and legs that appeared after her first pregnancy 2 years ago. Although not symptomatic, the lesions become redder and more swollen intermittently, particularly when accidentally scratched or rubbed.

An 18-year-old girl presents with red macules and papules that appeared on her legs, trunk, and arms and that swelled if rubbed.





The Case

You are asked to evaluate a healthy 18-year-old girl with a history of “mosquito bites” on her arms and legs that appeared after her first pregnancy 2 years ago. Although not symptomatic, the lesions become redder and more swollen intermittently, particularly when accidentally scratched or rubbed. She states that these “bites” persisted after her first pregnancy and increased significantly with her second pregnancy this year. She does not recall these bites regressing since then.


Dermcase diagnosis } Urticaria pigmentosa


Although relatively common in infants and young children, onset of mastocytosis in adolescents and adults is unusual, occurring in 1 of 1,000 to 8,000 new patients evaluated by a dermatologist.1,2 The disorder has no predilection for a particular race or sex.2 Because patients typically are healthy and asymptomatic, diagnosis is easily missed and often delayed for years after the onset of the skin lesions. In 1 study, the average time from the onset of skin findings to diagnosis was 10 years.3

Cutaneous mastocytosis is the most common form of this disorder, accounting for 90% of cases. Urticaria pigmentosa (UP) is the most common manifestation of the cutaneous form, characterized by round to oval, golden red-brown, slightly elevated papules and/or plaques.4

The cause of UP is unknown. However, it is characterized by an increased number of normal mast cells in the involved skin.2 When activated by a stimulus, mast cells release chemical mediators such as prostaglandins and histamine that results in swelling, itching, and redness. Rubbing, scratching, and other forms of minor trauma and environmental stress such as extremes of temperature can cause the degranulation of mast cells, resulting in erythematous, indurated, and pruritic lesions occasionally progressing to blistering. The most common causes are physical irritation and heat.5


Urticaria pigmentosa is clinically seen as an asymptomatic and minimally itchy, or rarely as an intensely pruritic, papule or plaque that can occur anywhere on the cutaneous surface, with a predilection for the arms and legs.6 The color is often described as golden brown in light-skinned individuals to dark brown in dark-pigmented patients. 

Skin biopsy is usually unnecessary because the diagnosis can be confirmed by rubbing the skin, resulting in a wheal and flare reaction or Darier sign. Although most skin lesions are not symptomatic, patients occasionally complain of pruritus.5 The differential diagnosis may include histiocytosis; pigmented purpura; sarcoidosis; and disorders associated with hyperpigmentation, such as pigmented nevi, café au lait macules, and postinflammatory hyperpigmentation.

When the diagnosis is unclear, a skin biopsy should be obtained to confirm the clinical suspicion. Routine histologic staining may not clearly show the increased number of mast cells, and therefore stains such as toluidine blue, Giemsa, or fluorescein isothiocynate-avidin are needed.5,6 A count of more than 20 mast cells per high-power field is considered an increased number of mast cells.4

Although extracutaneous symptoms are rare, systemic manifestations most commonly involve the bone marrow, bowel, central nervous system, liver, and spleen. Clinical presentations include abdominal cramps, vomiting, headaches, mental confusion, anemia, and hypotensive episodes.4 Common laboratory tests ordered include complete blood count (CBC) with differential, liver function tests, and serum tryptase.7

The clinical manifestations and prognosis of UP vary depending on the age of onset. In young children, UP usually presents as a cutaneous disorder with moderate-sized lesions, averaging 1 cm to 2 cm in size and varying from solitary to disseminated distribution.8 It is a benign disease in young children, with an expectation that the cutaneous manifestations will completely resolve within the first 2 decades of life. The onset of UP in adults and adolescents is rare but often asymptomatic. Lesions tend to be small (usually 2 mm to 4 mm), red to bronze in color, more persistent, and more likely to be associated with systemic findings.

Although there is no cure for UP, it is important to reassure and educate patients about the general self-limited and innocent course. To prevent future exacerbations, patients are advised to avoid stimuli that may trigger symptoms. Patients with a history of hypotensive episodes may be advised to have epinephrine available, although administration of maintenance nonsedating antihistamines can usually prevent this. Generally, UP can be managed pharmacologically by reducing the release of mast cells or reducing the symptoms. The H1 and H2 antihistamines are useful to manage itching, swelling, and gastrointestinal symptoms.9 Examples include cetirizine (Aller-Tec) and ranitidine (Zantac). In addition, oral cromoglycate (Gastrocrom) can be used to stabilize mast cells. If patients are refractory to antihistamines and cromoglycates, glucocorticoids and phototherapy may be used as a last resort.9,10

Urticaria pigmentosa may rarely progress to systemic mastocytosis and, as such, routine follow-ups are not needed. Indications for a follow-up include presentation of systemic symptoms that should warrant a CBC with differential, liver function tests, and serum tryptase. Patients refractory to the suggested pharmacologic treatments should also be followed.

Our patient

The patient has no family history of skin disease; she is not on any medications; and her pregnancies were uncomplicated.

Examination reveals 44 well-demarcated, 2-mm oval erythematous macules and papules on her legs with a few scattered lesions on her trunk and arms as well. Vigorous rubbing of several lesions results in a wheal.

A 4-mm punch biopsy of a papule on her left leg shows an increased number of spindled, oval, and stellate-shaped mast cells in the papillary dermis that are readily identified with Giemsa stain, with at least 40 per high-power field. Laboratory studies including a CBC, metabolic panel, and tryptase levels are normal.

A diagnosis of pregnancy-induced UP with no systemic manifestations is made based on the history, physical examination, and tests. No further testing is done. The patient is reassured that her condition is benign, but her cutaneous manifestations would likely persist. Given that she has no symptoms affecting her quality of life, treatment is deemed unnecessary. It is recommended that if these lesions become bothersome (pruritic and indurated), antihistamines could be used.


Urticaria pigmentosa in infants and young children is common, asymptomatic, and usually self-limited. The onset of UP during adolescence is rare and usually asymptomatic. However, UP in this age group tends to persist and with a higher risk of association with systemic findings. It is important to be aware that unusual triggers including pregnancy may induce UP.



1. Longley J, Duffy TP, Kohn S. The mast cell and mast cell disease. J Am Acad Dermatol. 1995;32(4):545-561. Erratum in: J Am Acad Dermatol.1995;33(1):52.

2. Alto WA, Clarcq L. Cutaneous and systemic manifestations of mastocytosis. Am Fam Physician. 1999;59(11):3047-3054.

3. Horan RF, Austen KF. Systemic mastocytosis: retrospective review of a decade's clinical experience at the Brigham and Women's Hospital. J Invest Dermatol. 1991;96(3):5S-13S.

4. Madendag IC, Madendag Y, Tarhan I, Altinkaya SO, Danisman N. Mastocytosis in pregnancy. Taiwan J Obstet Gynecol. 2010;49(2):192-196.

5. Kettelhut BV, Metcalfe DD. Pediatric mastocytosis. J Invest Dermatol. 1991;96(3):15S-18S.

6. Teng JMC. Health facts for you: mast cell disease (urticaria pigmentosa). University of Wisconsin School of Medicine and Public Health website. Last updated November 14, 2012. Accessed February 10, 2014.

7. Soter NA. Mastocytoses. In: Lebwohl MG, Heymann WR, Berth-Jones J, Coulson I. Treatment of Skin Disease: Comprehensive Therapeutic Strategies. 4th ed. Philadelphia, PA: Elsevier Saunders; 2014:443.   

8. Liu CM, Vanderhooft SL. Vesiculobullous disorders. In: Osborn LM, DeWitt TG, First LR, Zenel JA. Pediatrics. Philadelphia, PA: Elsevier Mosby; 2005:559-560.

9. Heide R, Beishuizen A, De Groot H, et al; Dutch National Mastocytosis Work Group. Mastocytosis in children: a protocol for management. Pediatr Dermatol. 2008;25(4):493-500.

10. Godt O, Proksch E, Streit V, Christophers E. Short- and long-term effectiveness of oral and bath PUVA therapy in urticaria pigmentosa and systemic mastocytosis. Dermatology. 1997;195(1):35-39. 


Mr Vo is a third-year medical student at the University of Ottawa, Ontario, Canada. Dr Gupta is a third-year medical student, University of Ottawa, Ontario, Canada. Dr Cohen, the section editor for Dermcase, is professor of pediatrics and dermatology, Johns Hopkins University School of Medicine, Baltimore, Maryland. The authors and section editor have nothing to disclose in regard to affiliations with or financial interests in any organizations that may have an interest in any part of this article. Vignettes are based on real cases that have been modified to allow the authors and editor to focus on key teaching points. Images also may be edited or substituted for teaching purposes.

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