Differential Diagnosis for a Cyst on the Back that Began to Drain and Became Painful

• Single Most Likely Diagnosis
  • Infected Epidermoid Cyst (Sebaceous Cyst): This is the most likely diagnosis given the history of a long-standing cyst that became painful and started draining after massage. Epidermoid cysts can become infected, leading to pain, swelling, and drainage.
• Other Likely Diagnoses
  • Furuncle or Carbuncle: These are infections of the hair follicle and surrounding tissue, which can present similarly to an infected cyst, especially if the cyst has become secondarily infected.
  • Pilonidal Cyst: Although more common in the sacral area, a pilonidal cyst could present on the back and become infected, leading to pain and drainage.
  • Abscess: A collection of pus that can occur anywhere on the body, including the back, and could be mistaken for a draining cyst.
• Do Not Miss Diagnoses
  • Malignant Tumor: Although rare, a malignant tumor (such as a squamous cell carcinoma or basal cell carcinoma) could initially present as a cystic lesion that becomes painful and drains. Missing this diagnosis could have severe consequences.
  • Osteomyelitis: An infection of the bone that could present with localized pain, swelling, and drainage, especially if the infection has spread from a superficial cyst to the underlying bone.
  • Actinomycosis: A rare bacterial infection that can cause abscesses or cystic lesions that drain, often with a history of trauma or manipulation of the area.
• Rare Diagnoses
  • Hydatid Cyst: Caused by the Echinococcus parasite, this type of cyst is rare in many parts of the world but can occur in the skin and soft tissues, including the back, and may become infected or rupture.
  • Cystic Metastasis: In rare cases, metastatic disease from internal cancers can present as cystic lesions on the skin, including the back. This would be an unusual presentation but should be considered in patients with a known history of cancer.