What is the cause of multiple small black patches on the skin of a healthy 4-year-old child's right leg?

Multiple Small Black Patches on a 4-Year-Old's Right Leg

The most likely diagnosis is urticaria pigmentosa (cutaneous mastocytosis), which presents as multiple red-brown to black macules, papules, or plaques in children and represents 70-90% of pediatric cutaneous mastocytosis cases. 1

Primary Differential Diagnosis

Urticaria Pigmentosa (Most Likely)

• Urticaria pigmentosa is the most common form of cutaneous mastocytosis in children, presenting as multiple red to brown to yellow lesions measuring a few millimeters to 1-2 cm in diameter. 1
• The lesions can appear as macules, plaques, or nodules and commonly affect the trunk and extremities, with less involvement of palms, soles, scalp, and face. 1
• A key diagnostic feature is Darier's sign: stroking or rubbing the lesions produces wheal and flare formation, often accompanied by erythema, swelling, and pruritus. 1
• In 80% of cases, lesions appear by 6 months of age (mean onset 2.5 months), and they typically resolve by age 10 years. 1
• Flushing occurs in up to 36% of patients with urticaria pigmentosa. 1

Congenital Melanocytic Nevi (Alternative Consideration)

• Congenital melanocytic nevi (CMN) have an incidence of <1% to 3.6% and present as shades of brown and black within macules, papules, patches, or plaques. 1
• The "classic" subtype shows brown and black coloration, while the "blue" subtype appears as gray-blue patches or plaques. 1
• Multiple CMN are considered high-risk markers and warrant closer evaluation, particularly when numerous "satellite" nevi are present. 1, 2

Clinical Examination Approach

Essential Physical Findings to Assess

• Test for Darier's sign by firmly stroking one of the lesions with a tongue depressor or fingernail—positive wheal and flare formation within minutes strongly suggests urticaria pigmentosa. 1
• Examine the entire body surface to document the total number and distribution of lesions. 1
• Assess for associated symptoms: pruritus, flushing episodes, or dermatographism. 1
• Look for texture changes: lesions may be flat (macular), raised (papular), or nodular. 1

History to Obtain

• Age of onset: lesions appearing before 6 months favor urticaria pigmentosa; present at birth favors CMN. 1
• Symptoms: itching, blistering, or flushing episodes suggest mastocytosis. 1
• Family history of atopic disease or skin conditions. 1
• Any episodes of wheezing, syncope, or gastrointestinal symptoms (rare but concerning for systemic involvement). 1

Management Recommendations

If Urticaria Pigmentosa is Confirmed

• Reassure the family that pediatric cutaneous mastocytosis typically resolves spontaneously by puberty and rarely involves systemic disease. 1
• Avoid triggers that cause mast cell degranulation: vigorous rubbing, hot baths, certain medications (aspirin, NSAIDs, opioids). 1
• Prescribe antihistamines (H1 blockers) for symptomatic relief of pruritus and flushing. 1
• Educate parents about signs of mast cell activation: severe flushing, wheezing, hypotension, or gastrointestinal symptoms requiring emergency evaluation. 1
• Routine screening for systemic involvement is not indicated in typical urticaria pigmentosa without concerning symptoms. 1

If Multiple Congenital Melanocytic Nevi are Suspected

• Children with multiple CMN (>1 lesion) are at highest risk for neurocutaneous melanosis and warrant MRI screening of the brain and spine. 1, 2
• Document the projected adult size of each lesion, as those >40 cm carry higher melanoma risk. 1
• Establish regular dermatologic surveillance for changes in pigmentation, texture, or development of nodules. 1
• Educate parents about skin care: use bland emollients for xerosis and pruritus, avoid trauma to lesions. 1

Critical Pitfalls to Avoid

• Do not dismiss multiple pigmented lesions as benign without proper examination—failure to test for Darier's sign may delay diagnosis of mastocytosis. 1
• Do not overlook the possibility of multiple CMN, which requires risk stratification and potential neuroimaging. 1, 2
• Avoid aggressive bathing with hot water and harsh soaps, which can trigger mast cell degranulation in mastocytosis or worsen xerosis in CMN. 1
• Do not assume all dark patches are the same diagnosis—examine each lesion's characteristics individually. 1

When to Refer

• Refer to pediatric dermatology if diagnosis is uncertain after initial evaluation. 1
• Immediate referral is warranted if the child experiences systemic symptoms: recurrent flushing, wheezing, syncope, or gastrointestinal bleeding. 1
• Refer for consideration of MRI if multiple CMN are confirmed, particularly with >1 lesion or satellite nevi. 1, 2