In a 37-year-old woman with painful, rapidly enlarging, rubbery lumbar subcutaneous nodules that feel fatty on ultrasound, recent idiopathic urticaria, chronic lymphadenopathy, fatigue, and pain that improves with prednisone, what diagnoses should be considered?

Differential Diagnosis: Subcutaneous Sarcoidosis is the Primary Consideration

This patient most likely has subcutaneous sarcoidosis, given the constellation of painful subcutaneous nodules that respond to prednisone, chronic lymphadenopathy, fatigue, and recent urticaria. 1, 2

Primary Diagnosis: Subcutaneous Sarcoidosis

The clinical presentation strongly suggests sarcoidosis based on several key features:

• Subcutaneous nodules are a recognized manifestation of cutaneous sarcoidosis, occurring in up to 30% of sarcoidosis patients 2. These nodules can be painful and are steroid-responsive 3
• Prednisone responsiveness with symptom recurrence after discontinuation is characteristic of sarcoidosis, which typically requires prolonged glucocorticoid therapy 2, 3
• Chronic lymphadenopathy is highly consistent with sarcoidosis, particularly when bilateral or involving multiple stations 1
• Chronic fatigue is a common systemic manifestation of sarcoidosis 1
• Recent urticaria may represent a non-specific inflammatory manifestation, as sarcoidosis can present with various skin lesions including maculopapular and erythematous lesions 1

The ultrasound showing fatty appearance does not exclude sarcoidosis—subcutaneous nodules in sarcoidosis can have variable imaging characteristics 4. The absence of tumors on CT further supports a benign inflammatory process.

Diagnostic Workup for Sarcoidosis

To confirm sarcoidosis, obtain:

1. Chest imaging (CT chest) - Look for bilateral hilar adenopathy or perilymphatic nodules, which are highly probable features 1
2. Serum ACE level - Elevated >50% above upper limit of normal supports diagnosis 1
3. Serum calcium and 24-hour urine calcium - Hypercalcemia/hypercalciuria with abnormal vitamin D metabolism is highly probable for sarcoidosis 1
4. Deep incisional biopsy of nodule - Essential to demonstrate non-caseating granulomas 4. Fine needle aspiration may be insufficient
5. Ophthalmologic examination - Screen for uveitis, a highly probable feature 1

Critical Alternative Diagnoses to Exclude

Lupus Erythematosus Panniculitis (LEP)

This must be considered given:

• Painful subcutaneous nodules that respond to steroids 5
• Can present with fatigue and systemic symptoms
• May coexist with urticaria-like lesions

Exclude with: ANA, anti-dsDNA, complement levels (C3, C4), and deep incisional biopsy showing lymphocytic panniculitis rather than granulomas 5

Idiopathic Multicentric Castleman Disease (iMCD)

Consider this diagnosis because:

• Chronic lymphadenopathy for years 6
• Fatigue and systemic inflammation
• Can present with urticaria-like skin manifestations 6
• Responds to steroids but requires specific therapy

Exclude with: IL-6 levels (markedly elevated in iMCD), serum protein electrophoresis showing hypergammaglobulinemia, and lymph node biopsy showing plasmacytic variant with increased IgG4+ cells 6

Subcutaneous Panniculitis-Like T-Cell Lymphoma

This is a critical diagnosis to exclude given:

• Subcutaneous nodules with systemic symptoms 7
• Chronic fatigue and potential for profound lipoatrophy over time 7
• Initial presentation can mimic benign panniculitis 5

Exclude with: Deep incisional biopsy with immunohistochemistry (CD3, CD8, CD56, granzyme B, perforin) and T-cell receptor gene rearrangement studies 7, 5

Management Algorithm

If sarcoidosis is confirmed:

1. For symptomatic cutaneous sarcoidosis with cosmetically important or painful lesions: Oral glucocorticoids are recommended 2
2. Initial prednisone dosing: Typically 20-40 mg daily, then taper gradually to avoid adrenal insufficiency 8
3. For steroid-dependent or refractory disease: Consider methotrexate as steroid-sparing agent 2
4. Monitor for: Systemic organ involvement (cardiac, pulmonary, neurologic) with baseline ECG, pulmonary function tests, and ophthalmologic exam 1

Critical Pitfalls to Avoid

• Do not rely on superficial biopsy or fine needle aspiration alone - Deep incisional biopsy is required to demonstrate granulomas and exclude lymphoma 4, 5
• Do not abruptly discontinue prednisone - Taper gradually over months to prevent adrenal insufficiency and disease flare 8
• Do not assume isolated cutaneous disease - Sarcoidosis requires comprehensive evaluation for systemic involvement 1
• Do not miss lymphoma - Subcutaneous T-cell lymphoma can initially respond to steroids and mimic benign panniculitis for years before diagnosis 7

The patient requires deep incisional biopsy of a nodule as the next critical step, along with chest CT, serum ACE, calcium studies, and comprehensive autoimmune serologies to differentiate between sarcoidosis, lupus panniculitis, Castleman disease, and lymphoma.