Differential Diagnoses for Black Rash in Bilateral Axillae
The most likely diagnosis is acanthosis nigricans, a hyperpigmented velvety skin change strongly associated with insulin resistance in type 2 diabetes, particularly in patients on insulin therapy. 1
Primary Differential: Acanthosis Nigricans
Acanthosis nigricans presents as dark, thickened, velvety patches in body folds and is the most common cause of hyperpigmented axillary rashes in diabetic patients. 1 Key features include:
- Bilateral distribution in intertriginous areas (axillae, neck, groin) 1
- Velvety texture with gradual darkening over weeks to months 1
- Strong association with insulin resistance and hyperinsulinemia, which this patient has given her insulin requirement (18 units daily) 2
- Metformin use (250 mg daily in this case) is actually protective against worsening insulin resistance, but the patient's relatively low dose may be insufficient 2
Secondary Differentials to Consider
Erythrasma
- Bacterial infection (Corynebacterium minutissimum) causing brown-red patches that can appear dark 1
- Distinguished by coral-red fluorescence under Wood's lamp examination 1
- More common in diabetic patients due to altered skin pH and moisture 1
Tinea Corporis (Fungal Infection)
- Can present with hyperpigmentation in chronic cases 1
- Typically has raised, scaly borders with central clearing 1
- Diabetic patients have increased susceptibility to fungal infections 1
Post-Inflammatory Hyperpigmentation
- May follow previous dermatitis, folliculitis, or friction 1
- History of preceding inflammatory condition would be present 1
Critical Clinical Pitfalls
Do not dismiss this as simple "dirty skin" - this is a common error that delays recognition of underlying metabolic disease. 1 The bilateral axillary location and 1.5-month duration in a diabetic patient on suboptimal metformin dosing (250 mg is below the typical therapeutic range of 1500-2550 mg daily) strongly suggests acanthosis nigricans related to inadequate glycemic control. 2, 3
Immediate Management Recommendations
Optimize metformin dosing first: The current dose of 250 mg daily is substantially below therapeutic levels. 2 Metformin should be titrated gradually to minimize gastrointestinal side effects, starting at 500 mg once or twice daily with meals and increasing to a target of 1500-2000 mg daily (maximum 2550 mg/day). 2, 3
Assess glycemic control: Check HbA1c immediately - if ≥8.5%, intensification beyond metformin optimization is needed. 2 The patient's insulin dose of 18 units daily suggests inadequate control with current therapy. 2
Consider adding GLP-1 receptor agonist or SGLT2 inhibitor if HbA1c remains elevated after metformin optimization, as these agents provide cardiovascular and renal protection beyond glycemic control. 2
Monitoring Considerations
- Vitamin B12 levels should be checked given metformin use, even at low doses, as deficiency can develop and worsen neuropathy. 2, 3, 4
- Renal function (eGFR) must be verified before increasing metformin - contraindicated if eGFR <30 mL/min/1.73 m², dose reduction required if eGFR 30-45 mL/min/1.73 m². 2, 3
- Reassess in 3 months - if acanthosis nigricans improves with better glycemic control, this confirms the diagnosis. 2