What are the possible causes and recommended evaluation and treatment for a dark hyperpigmented rash in the axillae?

Dark Hyperpigmented Rash in the Armpits: Evaluation and Management

The most likely diagnosis is acanthosis nigricans, which presents as thickened, hyperpigmented plaques in intertriginous areas including the axillae and requires evaluation for underlying endocrinologic disorders (especially insulin resistance and diabetes) or, less commonly, internal malignancy. 1, 2

Differential Diagnosis and Clinical Assessment

The key differential diagnoses for dark hyperpigmented rash in the axillae include:

• Acanthosis nigricans: Characterized by velvety, hyperpigmented, thickened plaques in body folds (axillae, neck, groin). This is the most common cause and is strongly associated with insulin resistance, obesity, diabetes mellitus, and endocrine disorders. In rare cases, it may herald internal malignancy, particularly gastric adenocarcinoma. 1, 2

• Intertrigo with post-inflammatory hyperpigmentation: Superficial inflammatory disorder caused by skin-on-skin friction in opposing skin surfaces, facilitated by moisture and heat. Secondary hyperpigmentation can develop after chronic inflammation. 3

• Seborrheic dermatitis: Can present with symmetric, well-demarcated patches with greasy yellowish scales in intertriginous areas, though typically less hyperpigmented than acanthosis nigricans. 4

• Chronic eczema: May result in lichenification and hyperpigmentation in chronic cases, with dermal pigment deposition contributing to darkening. 5

Essential Clinical Features to Identify

For acanthosis nigricans, look for:

• Velvety texture with thickened plaques (not just flat discoloration) 1
• Symmetric distribution in neck, axillae, groin, and other intertriginous areas 1, 2
• Absence of scale (unlike seborrheic dermatitis or eczema) 1

For intertrigo, look for:

• Maceration and moisture in skin folds 3
• Evidence of friction or chafing 3
• Possible secondary bacterial or fungal infection (crusting, weeping, satellite lesions) 3

Recommended Evaluation

Laboratory workup for acanthosis nigricans should include:

• Fasting glucose and hemoglobin A1c to screen for diabetes mellitus 1
• Fasting insulin levels to assess for insulin resistance 1
• Lipid panel (often abnormal in metabolic syndrome) 1
• Consider thyroid function tests if other endocrine symptoms present 1
• In adults with rapid onset or extensive disease without obesity/diabetes, consider age-appropriate cancer screening (especially upper gastrointestinal endoscopy for gastric cancer) 2

Treatment Approach

For Acanthosis Nigricans:

The primary treatment is addressing the underlying cause:

• Weight loss and lifestyle modification for obesity-associated cases 1
• Optimize glycemic control in diabetic patients 1
• Treat underlying endocrinopathy if identified 1

Topical treatments for cosmetic improvement:

• Topical retinoids (tretinoin 0.05-0.1%) may reduce hyperpigmentation and thickness 1
• Keratolytics such as salicylic acid 2-6% or urea 10-20% to reduce hyperkeratosis 1
• Avoid irritating the area with harsh soaps or friction 4

For Intertrigo with Hyperpigmentation:

• Use mild, pH-neutral non-soap cleansers to preserve skin barrier 4
• Apply barrier creams or emollients to reduce friction 4
• Keep area dry; consider absorbent powders in skin folds 3
• If secondary infection suspected (crusting, weeping, odor), obtain bacterial and fungal cultures 3, 6
• Treat bacterial superinfection with appropriate antibiotics (e.g., flucloxacillin for Staphylococcus aureus) 4
• Low-potency topical corticosteroids (hydrocortisone 1%) for 1-2 weeks maximum if significant inflammation present 4

For Seborrheic Dermatitis:

• Ketoconazole 2% cream applied to affected areas 4
• Avoid alcohol-containing preparations that worsen dryness 4
• Low-potency topical corticosteroids (hydrocortisone 1%) for significant erythema, limited to 2-4 weeks maximum 4

Critical Pitfalls to Avoid

• Do not dismiss acanthosis nigricans as simply "dirty skin" – it is a marker of systemic disease requiring metabolic evaluation 1, 2
• Do not use medium- or high-potency topical corticosteroids in axillae due to high risk of skin atrophy, striae, and telangiectasia in intertriginous areas 4
• Do not overlook malignancy-associated acanthosis nigricans in non-obese adults with rapid onset disease – this requires urgent cancer screening 2
• Avoid greasy or occlusive products in intertriginous areas as they can promote secondary bacterial or fungal infections 4, 3

When to Refer

Refer to dermatology if:

• Diagnostic uncertainty between acanthosis nigricans and other conditions 1
• Rapid progression or extensive disease suggesting malignancy-associated acanthosis nigricans 2
• Failure to improve with treatment of underlying metabolic disorder 1
• Need for biopsy to confirm diagnosis or rule out other conditions 1