Differential Diagnosis for Monoarthritis

The differential diagnosis for monoarthritis, which refers to inflammation of a single joint, can be extensive and varies based on the patient's presentation, history, and physical examination findings. Here's a categorized approach to the differential diagnosis:

• Single Most Likely Diagnosis
  • Gout: This is often considered the most likely diagnosis for acute monoarthritis, especially in the absence of a clear traumatic cause. Gout typically affects the base of the big toe (first metatarsophalangeal joint) but can affect any joint. The sudden onset of intense pain, redness, and swelling is characteristic.
• Other Likely Diagnoses
  • Septic Arthritis: An infection within the joint space, which can be caused by bacteria, viruses, or fungi. It's a medical emergency requiring prompt diagnosis and treatment. Risk factors include recent joint surgery, immunocompromised state, or intravenous drug use.
  • Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): Similar to gout but caused by the deposition of calcium pyrophosphate dihydrate crystals within the joint. It often affects the knee.
  • Traumatic Injury: Including fractures, ligament sprains, or meniscal tears, which can cause monoarthritis, especially if there's an associated hemarthrosis (blood in the joint).
  • Osteoarthritis (OA): While OA typically affects multiple joints, it can start in one joint, especially after trauma or with significant overuse.
• Do Not Miss Diagnoses
  • Lyme Arthritis: Caused by Borrelia burgdorferi, the bacteria responsible for Lyme disease. It can cause intermittent monoarthritis, typically affecting the knee.
  • Tuberculous Arthritis: A form of extrapulmonary tuberculosis that can affect any joint, often presenting with chronic monoarthritis.
  • Gonococcal Arthritis: A sexually transmitted infection caused by Neisseria gonorrhoeae, which can cause septic arthritis, often in multiple joints but can start as monoarthritis.
  • Rheumatoid Arthritis (RA): Although RA is typically a polyarthritis, it can occasionally start in a single joint before progressing to multiple joints.
• Rare Diagnoses
  • Hemochromatosis: A genetic disorder leading to iron overload, which can cause arthropathy, typically affecting the second and third metacarpophalangeal joints.
  • Charcot Joint (Neuropathic Arthropathy): Seen in patients with significant sensory neuropathy, leading to repeated trauma and degeneration of a single joint, often the foot in diabetic patients.
  • Sarcoid Arthritis: Part of sarcoidosis, a systemic disease that can cause monoarthritis, among other manifestations.
  • Amyloid Arthropathy: Associated with amyloidosis, where amyloid deposits in the joint can cause chronic monoarthritis.

Each of these diagnoses has distinct features and risk factors, and a thorough history, physical examination, and appropriate diagnostic tests (such as joint aspiration, blood tests, and imaging) are crucial for making an accurate diagnosis.