Differential Diagnosis for 79-year-old Male with Worsening Swelling and Joint Pain

Single Most Likely Diagnosis

• Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): The presence of joint pain and swelling, especially with the aspiration results showing a high number of red blood cells (RBC) and white blood cells (WBC), along with the lack of improvement with Lyme disease treatment, suggests a crystalline arthropathy. The normal uric acid level does not rule out pseudogout, as uric acid levels can be normal in gout and pseudogout.

Other Likely Diagnoses

• Gout: Although the uric acid level is normal, gout cannot be ruled out entirely, especially in the context of joint pain and swelling. Gout can occur with normal uric acid levels, especially during an acute attack.
• Septic Arthritis: The high number of WBCs in the aspirate, despite the patient being treated for Lyme disease, raises the concern for a septic joint, which is a medical emergency. The presence of a high number of RBCs and WBCs supports this diagnosis.
• Hemarthrosis: The +4 RBCs in the aspirate could suggest hemarthrosis, especially if the patient is on anticoagulation therapy, which is not mentioned but should be considered.

Do Not Miss Diagnoses

• Infectious Arthritis (including Lyme Arthritis): Despite the patient being treated for Lyme disease, the lack of improvement and the presence of significant joint inflammation suggest that either the treatment is not effective, or the diagnosis of Lyme arthritis is incorrect, or there is a co-infection.
• Malignancy-related Arthropathy: Although rare, certain malignancies can cause arthropathies that mimic other conditions. Given the patient's age, this is a diagnosis that should not be missed, as it could significantly alter management and prognosis.

Rare Diagnoses

• Amyloid Arthropathy: This condition can cause joint pain and swelling, especially in older adults, and can be associated with various types of amyloidosis. It is rare but should be considered in the differential diagnosis of unexplained arthropathy.
• Ochronotic Arthropathy: Associated with alkaptonuria, this condition can cause joint pain and swelling due to the accumulation of homogentisic acid in the joints. It is very rare and typically presents with distinctive darkening of the cartilage and other tissues.