DermDx: Dark Spots on Trunk and Extremities


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A 61-year-old Hispanic woman with a history of seasonal allergies, seizure disorder, chronic migraines, and osteoarthritis presents for her annual physical and evaluation of an “embarrassing rash.” She reports first noticing the rash approximately 6 years ago; within the past 2 years, the rash has become more noticeable and spread to cover her chest, back, bilateral arms, and upper thighs. She describes crops of red lesions that present with pruritus and then turn dark with resolution of the itching. She has not noticed any specific triggers for the lesions and tried topical lotions, which were ineffective. She is concerned about the permanence of the darkened lesions.

On physical examination, she has erythematous and hyperpigmented, monomorphous 2- to 3-mm papules over the chest, arms, legs, and back (Figure). Her preliminary laboratory work shows an elevated total protein level at 8.5 g/dL, globulin at 4.3 g/dL, and HbA1c of 5.7%. She is referred to dermatology where a punch biopsy is obtained and pathology reveals an increased number of mast cells in the superficial dermis with dilated blood vessels and few lymphocytes and eosinophils. The mast cells have a spindle morphology and show tryptase highlighted the mast cells.

Mastocytosis is a heterogeneous disorder characterized by accumulation of mast cells in the dermis and occasionally other organs. Cutaneous mastocytosis, mastocytosis in the absence of systemic symptoms, constitutes 90% of cases not associated with a hematologic disorder.1 The characteristic skin...

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Mastocytosis is a heterogeneous disorder characterized by accumulation of mast cells in the dermis and occasionally other organs. Cutaneous mastocytosis, mastocytosis in the absence of systemic symptoms, constitutes 90% of cases not associated with a hematologic disorder.1

The characteristic skin lesion of the most common subtype of cutaneous mastocytosis, called urticaria pigmentosa, is seen in Figure.1 The lesion presents as tan to red-brown macules that may emerge on the chest, abdomen, upper or lower extremities with sparing of the palms, soles, face, and scalp.1 Lesions may be mistaken for freckles initially but can evolve into papules, nodules, or plaques.2

Darier sign, the development of local urticaria and erythema within minutes of physical irritation such as rubbing, stroking, or itching, is often present but is not pathognomonic. Additionally, urticaria may be exacerbated by changes in temperature, ingestion of hot beverages or spicy foods, alcohol, emotional stress, or certain medications (opioids, nonsteroidal anti-inflammatory drugs, muscle relaxants, and iodinated contrast media). In both cutaneous and systemic mastocytosis, patients may also experience symptoms secondary to explosive release of mast cell mediators that may present as anaphylaxis, flushing, syncope, gastrointestinal symptoms, or vascular collapse.

The prevalence of mastocytosis in the general population is unknown; however, it is estimated to occur in 1 in 10,000 patients who present to a dermatology clinic for evaluation.1 No race or sex predilection is observed for the disease. Mastocytosis has a bimodal distribution with a peak incidence in infants, with resolution of lesions by adult years, and again in older adults, with persistence of lesions long-term.1 In patients with cutaneous mastocytosis, thorough review of systems and workup to rule out hematologic disorder/systemic involvement should start with serum tryptase measurement. Serum total tryptase greater than 20 ng/mL is suggestive of systemic disease and warrants referral to hematology-oncology for additional workup.3

The differential diagnosis should include follicular eczema, lymphomatoid papulosis, papular urticaria, and pityriasis lichenoides chronica (Table).4,5

Table. Selected Differential Diagnosis of Cutaneous Mastocytosis4,5

Follicular eczemaForm of atopic dermatitis Individual papules that include a hair follicle Associated signs of inflammation (eg, erythema, edema, pruritus, and warmth)
Lymphomatoid papulosisChronic, recurrent eruption of papules and nodules Originally small erythematous to violaceous papules Evolve to larger papules and nodules with central crusting, necrosis, or hemorrhage then spontaneously regress
Papular urticariaChronic or recurrent pruritic eruption believed to be an allergic (hypersensitivity) reaction to insect bites Can last weeks to months and often recurs at the same time each year Associated pruritus

Treatments for cutaneous mastocytosis include sedating and nonsedating antihistamines, cromolyn sodium, and psoralen and UV-A therapy.2

Kimberly Aparicio, MD, is assistant professor in the Department of Family and Community Medicine at UT Southwestern Medical Center in Dallas; Cynthia F. Griffith, MPAS, PA-C, is a physician assistant in UT Southwestern’s Department of Dermatology; Ashika Angirekula is a medical student at UT Southwestern Medical Center.

Ms Griffith has served on the steering committee for Potomac Center for Medical Education and worked on medical education supported by an educational grant from Castle Biosciences, Inc.


1. Di Raimondo C, Del Duca E, Silvaggio D, et al. Cutaneous mastocytosis: a dermatological perspective. Australas J Dermatol. 2021;62(1):e1-e7. doi:10.1111/ajd.13443

2. Krishnaprasadh D, Levasseur K. Mastocytosis. Pediatr Rev. 2021;42(8):468-470. doi:10.1542/PIR.2020-0108

3. Matito A, Azaña JM, Torrelo A, Alvarez-Twose I. Cutaneous mastocytosis in adults and children: new classification and prognostic factors. Immunol Allergy Clin North Am. 2018;38(3):351-363. doi:10.1016/j.iac.2018.04.001

4. Toumi A, Fazal S, Litaiem N. Lymphomatoid papulosis. In: StatPearls. StatPearls Publishing; Updated June 19, 2022. Available from:

5. Dabija D, Tadi P, Danosos GN. Chronic urticaria. In: StatPearls. StatPearls Publishing; May 1, 2022. Available from:

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