Differential Diagnosis for Painful Lumps on Arms and Legs

Single Most Likely Diagnosis

• Erythema nodosum: This condition is characterized by the sudden onset of painful, erythematous nodules or lumps, typically on the lower extremities but can also occur on the arms. The mobility at the fat tissue level and the increasing pain are consistent with erythema nodosum, which often results from an inflammatory process and can be associated with various underlying conditions, including infections, autoimmune diseases, or drug reactions.

Other Likely Diagnoses

• Lipoma: Although typically painless, lipomas can become painful if they grow rapidly or become inflamed. Their mobility within the subcutaneous fat is consistent with the description provided.
• Furuncles or Carbuncles: These are skin infections of the hair follicle that can form painful lumps. They are usually localized but can occur in multiple areas, especially if caused by a bacterial infection like Staphylococcus aureus.
• Nodular vasculitis: Also known as erythema induratum, this condition presents with painful nodules on the legs, which can be mobile. It is often associated with tuberculosis or other chronic infections.

Do Not Miss Diagnoses

• Subcutaneous panniculitis-like T-cell lymphoma: A rare form of lymphoma that presents with subcutaneous nodules, which can be painful and mimic benign conditions. Early diagnosis is crucial for treatment.
• Deep vein thrombosis (DVT): Although DVT typically presents with swelling, warmth, and discoloration of the affected limb, in some cases, it can cause a painful lump, especially if there is associated inflammation or a superficial thrombophlebitis.
• Infections like cellulitis or abscesses: These can present with painful, mobile lumps, especially if they are located in the subcutaneous tissue.

Rare Diagnoses

• Weber-Christian disease: A rare inflammatory disorder of the fatty tissues under the skin, characterized by recurring episodes of painful inflammation in the subcutaneous fat.
• Alpha-1 antitrypsin deficiency panniculitis: A rare condition associated with deficiency of the alpha-1 antitrypsin enzyme, leading to painful subcutaneous nodules.
• Scleroderma: While primarily known for its effects on the skin and internal organs, scleroderma can also cause subcutaneous nodules, though this is less common.

Each of these diagnoses requires careful consideration of the patient's full clinical presentation, including history, physical examination, and potentially, diagnostic tests such as imaging or biopsy, to determine the underlying cause of the painful lumps.