What is the differential diagnosis and what questions should be asked in the history for a 13-year-old female presenting with black or dark patches on her neck in a dermatology clinic?

Differential Diagnosis and History for Dark Patches on Neck in 13-Year-Old Female

Most Likely Diagnosis

Acanthosis nigricans (AN) is the most probable diagnosis for dark patches on the neck in a 13-year-old female, particularly given the high prevalence of obesity-associated AN in this age group. 1, 2

Differential Diagnosis

Primary Considerations

• Acanthosis nigricans (obesity-associated): Most common cause in adolescents, characterized by symmetric, hyperpigmented, velvety plaques with ill-defined borders in intertriginous areas 1, 2
• Post-inflammatory hyperpigmentation: Following dermatitis, infection, or trauma 3
• Allergic contact dermatitis: Can present with hyperpigmentation, particularly at neck flexures where jewelry, fragrances, or textiles contact skin 3
• Atopic dermatitis with lichenification: May cause darkening at neck flexures with chronic scratching 3

Less Common but Important Considerations

• Confluent and reticulated papillomatosis: Hyperpigmented papules coalescing into plaques, typically on upper trunk and neck 2
• Tinea versicolor: Fungal infection causing hyperpigmented or hypopigmented patches 2
• Drug-induced hyperpigmentation: Certain medications can cause darkening 2
• Syndromic AN: Associated with genetic syndromes (HAIR-AN syndrome, Berardinelli-Seip syndrome) 2, 4
• Malignant AN: Extremely rare in this age group but must be considered if rapid onset and extensive distribution 1, 2

Essential History Questions

Lesion Characteristics

• Duration and progression: When did the patches first appear? Have they changed in size, color, or texture? Rapid progression suggests malignant AN 1, 2
• Distribution: Are similar patches present in other body folds (axillae, groin, antecubital/popliteal fossae)? Symmetric distribution favors AN 1, 2
• Texture: Does the skin feel thickened, rough, or velvety? This is characteristic of AN 1, 2
• Associated symptoms: Any itching, burning, or stinging? Symptoms suggest inflammatory conditions like atopic dermatitis or contact dermatitis 3

Metabolic and Endocrine History

• Weight history: Recent weight gain? Obesity is the most common cause of AN in adolescents 1, 2
• Family history of diabetes or metabolic syndrome: AN serves as a cutaneous marker of insulin resistance 1, 4
• Menstrual history: Irregular periods, hirsutism, or acne suggest polycystic ovary syndrome (PCOS), commonly associated with AN 4
• Symptoms of insulin resistance: Increased thirst, frequent urination, fatigue 1

Medication and Product Exposure

• Current medications: Oral contraceptives, corticosteroids, niacin, insulin, and growth hormone can induce AN 2, 4
• Topical products: New soaps, lotions, perfumes, or jewelry (nickel) that could cause contact dermatitis 3
• Use of skin-lightening products: Hydroquinone can paradoxically cause darkening (exogenous ochronosis), particularly in darker skin types 5

Systemic Symptoms (Red Flags for Malignancy)

• Constitutional symptoms: Fever, night sweats, unintentional weight loss suggest malignant AN or lymphoma 3, 1
• Gastrointestinal symptoms: Abdominal pain, changes in bowel habits, or dysphagia may indicate underlying malignancy 3
• Mucosal involvement: Darkening of lips, oral mucosa, or palms/soles suggests more extensive or syndromic AN 4

Genetic and Syndromic Features

• Family history: Similar skin findings in family members suggest hereditary benign AN 2
• Developmental history: Intellectual disability, skeletal abnormalities, or other congenital anomalies suggest syndromic AN 2, 4

Skin Care and Environmental History

• Sun exposure: Helps differentiate from post-inflammatory hyperpigmentation or melasma 6
• History of skin trauma or infection: Suggests post-inflammatory hyperpigmentation 3
• Atopic history: Personal or family history of asthma, allergic rhinitis, or eczema suggests atopic dermatitis 3

Physical Examination Focus

Skin Examination

• Assess texture and distribution: Velvety, papillomatous plaques in neck, axillae, groin confirm AN 1, 2
• Examine other flexural areas: Symmetric involvement is typical of AN 1
• Check for atypical features: Unilateral distribution, unusual locations, or rapid changes warrant biopsy 2, 4
• Evaluate for contact dermatitis patterns: Involvement at jewelry contact sites, eyelids, or hands suggests allergic contact dermatitis 3

Systemic Examination

• Calculate BMI: Obesity is present in 70% of AN cases 4
• Signs of hyperandrogenism: Hirsutism, acne, male-pattern hair loss suggest PCOS 4
• Acanthosis nigricans severity: Extensive involvement beyond typical sites raises concern for malignancy 1, 2
• Lymph node examination: Cervical, axillary, or inguinal lymphadenopathy suggests malignancy or lymphoma 3

Diagnostic Workup

Initial Laboratory Tests (for suspected AN)

• Fasting glucose and insulin levels: Screen for insulin resistance 2, 4
• Fasting lipid profile: Often abnormal in metabolic syndrome 2
• Hemoglobin A1c: Assess glycemic control 1
• Liver function tests (ALT): Screen for non-alcoholic fatty liver disease associated with insulin resistance 2

Hormonal Evaluation (if PCOS suspected)

• Free testosterone, DHEAS, 17-OH progesterone: Evaluate for hyperandrogenism 4
• Thyroid function tests: Rule out thyroid disorders 4

When to Consider Biopsy

• Atypical presentation: Unilateral, unusual location, or rapid progression 2, 4
• Diagnostic uncertainty: When clinical diagnosis is unclear 1
• Suspected malignancy: Constitutional symptoms or extensive involvement 1, 2

Common Pitfalls

• Missing insulin resistance: AN in adolescents is a critical marker for metabolic disease requiring intervention 1
• Overlooking contact dermatitis: Patch testing should be considered if history suggests product exposure, especially with neck involvement from jewelry or fragrances 3
• Assuming benign etiology: While malignant AN is rare in adolescents, rapid onset with systemic symptoms requires thorough evaluation 1, 2
• Ignoring darker skin considerations: Hyperpigmentation may be more prominent and distressing in patients with darker skin types, requiring culturally sensitive discussion 6