Differential Diagnosis for Painful Lumps Under the Armpit

The patient, a 33-year-old female, presents with painful lumps under her right armpit. The differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Cat Scratch Disease or Infectious Lymphadenitis: This is often the most common cause of a painful lump under the armpit, especially if the patient has been exposed to cats or has a history of recent infection. The lymph nodes become inflamed and painful due to the body's response to the infection.
• Other Likely Diagnoses
  • Folliculitis or Abscess: These are infections of the hair follicles or skin that can cause painful lumps. They are common and can occur under the armpit, especially in areas where shaving or friction occurs.
  • Lipoma: Although typically painless, a lipoma (a benign tumor composed of fat tissue) can become painful if it becomes inflamed or infected. However, the pain is usually mild.
  • Cyst or Epidermoid Cyst: These can become inflamed and painful, presenting as lumps under the skin.
• Do Not Miss Diagnoses
  • Breast Cancer with Axillary Lymph Node Involvement: Although less common in younger women, breast cancer can present with axillary lymphadenopathy (enlarged lymph nodes under the arm). It's crucial to examine the breast and consider imaging or referral if there's any suspicion.
  • Lymphoma: This is a type of cancer that affects the immune system and can cause lymph nodes to become enlarged and painful. It's less common but critical to diagnose early.
  • Tuberculosis (TB): TB can cause lymphadenitis, which might present as painful lumps under the armpit, especially in individuals who have been exposed to TB or have a weakened immune system.
• Rare Diagnoses
  • Hydradenitis Suppurativa: A chronic skin condition that can cause painful lumps in the armpits due to blocked apocrine glands.
  • Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the lymph nodes, leading to painful lumps.
  • Kikuchi Disease: A rare condition that causes lymph node inflammation, typically in the neck but can occur under the arm, and is often accompanied by fever and fatigue.

Next Steps

• History and Physical Examination: A thorough history to identify potential exposures, recent infections, or systemic symptoms, along with a physical examination to assess the characteristics of the lump and check for other signs of illness.
• Laboratory Tests: May include complete blood count (CBC), blood cultures, or specific tests based on the suspected diagnosis (e.g., TB testing).
• Imaging: Ultrasound or other imaging modalities may be used to evaluate the nature of the lump and to assess for any underlying conditions such as breast cancer.
• Referral: Depending on the diagnosis, referral to a specialist such as a surgeon, oncologist, or dermatologist may be necessary.