Differential Diagnosis for Lymph Node Swelling under the Arm Pit

Single Most Likely Diagnosis

• Benign Reactive Lymphadenopathy: This is the most common cause of lymph node swelling and is often due to a viral infection or a localized skin infection. The lymph nodes are typically soft, mobile, and tender, which matches the presentation.

Other Likely Diagnoses

• Cat Scratch Disease: Caused by Bartonella henselae, this infection is common in individuals who have been scratched by a cat. The lymph nodes, especially in the axilla (underarm), can become swollen, tender, and may suppurate.
• Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition can lead to lymphadenopathy, including in the axillary region, along with fever, sore throat, and fatigue.
• Breast Cancer or Other Local Malignancies: Although less common, axillary lymph node swelling can be a sign of breast cancer or other malignancies in the vicinity, such as lymphoma.

Do Not Miss Diagnoses

• Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy, which may initially be painless and mobile. Early diagnosis is crucial for effective treatment.
• Metastatic Cancer: Similar to breast cancer, other cancers can metastasize to the axillary lymph nodes, making prompt evaluation critical.
• Tuberculosis: Although less common in some regions, tuberculosis can cause lymphadenopathy, and axillary involvement is possible. It's a critical diagnosis to consider due to its public health implications and the need for specific treatment.

Rare Diagnoses

• Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can lead to lymphadenopathy.
• Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, typically in the cervical region but can also affect axillary nodes.
• Sarcoidosis: Although more commonly associated with pulmonary symptoms, sarcoidosis can cause lymphadenopathy in various locations, including the axilla, due to the formation of granulomas.

Each of these diagnoses has a different set of implications for patient care, ranging from watchful waiting for benign conditions to urgent intervention for malignancies or infectious diseases. A thorough history, physical examination, and diagnostic tests (such as imaging and biopsy) are essential for determining the underlying cause of the lymph node swelling.