Differential Diagnosis for a Painful Subcutaneous Nodule

When approaching a painful subcutaneous nodule, it's crucial to consider a wide range of potential diagnoses due to the diverse etiologies that can present with similar symptoms. The differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis
  • Epidermoid Cyst: This is often the most common diagnosis for a painful subcutaneous nodule. Epidermoid cysts are benign cysts that can become inflamed and painful, especially if they become infected. They are usually found on the face, neck, or trunk and can be tender to the touch.
• Other Likely Diagnoses
  • Lipoma: Although typically painless, a lipoma can become painful if it becomes large enough to press on surrounding nerves or if it undergoes fatty necrosis. Lipomas are benign tumors composed of fat tissue and are very common.
  • Furuncle or Carbuncle: These are infections of the hair follicle and surrounding tissue, often caused by Staphylococcus aureus. They can present as painful, swollen nodules and may require antibiotic treatment.
  • Rheumatoid Nodule: In patients with rheumatoid arthritis, subcutaneous nodules can form, which are usually firm, painless, and found over bony prominences. However, they can occasionally be painful.
• Do Not Miss Diagnoses
  • Malignancy (e.g., Metastatic Disease): Although less common, a painful subcutaneous nodule could be a sign of metastatic cancer. Certain types of cancer, like breast, lung, or gastrointestinal cancers, can metastasize to the skin.
  • Infectious Diseases (e.g., Erythema Induratum, Sporotrichosis): These conditions can present with painful subcutaneous nodules and have significant implications for treatment and prognosis.
  • Vasculitis (e.g., Erythema Nodosum, Polyarteritis Nodosa): Vasculitis can cause painful nodules and is associated with inflammation of blood vessels, which can have systemic implications.
• Rare Diagnoses
  • Panniculitis: This is inflammation of the subcutaneous fat and can present with painful nodules. It has various causes, including autoimmune diseases, infections, and trauma.
  • Subcutaneous Mycoses (e.g., Mycetoma): These are chronic infections caused by fungi or bacteria that can cause painful subcutaneous nodules, usually after a traumatic inoculation.
  • Histiocytosis: A group of diseases involving abnormal proliferation of histiocytes, which can present with painful subcutaneous nodules among other systemic symptoms.

Each of these diagnoses has distinct clinical features, and a thorough history, physical examination, and sometimes diagnostic tests (like imaging or biopsy) are necessary to determine the correct diagnosis and guide appropriate treatment.