Monoarthritis, or monoarticular arthritis, is inflammation (arthritis) of one joint at a time (as opposed to oligoarthritis, which affects 2-4 joints, and polyarthritis, which affects more than 4 joints). It is usually caused by trauma, infection, or crystalline arthritis.
Causes
Septic arthritis
Septic arthritis is an inflammatory response to an infection (usually bacterial) in the joint. Usually impacting large joints like the hip or the knee, it is a medical emergency with a mortality rate of about 10%. It is treated with oral and intravenous antibiotics as well as joint drainage.
Gout
Gout is inflammation caused by the deposition of needle-shaped monosodium urate crystals in the joints. It is the most common type of inflammatory arthritis in the United States. The disease typically affects a single joint in earlier stages but can progress into polyarthritis over time. It usually occurs in cooler areas of the body, often starting in the big toe.
Pseudogout
Pseudogout (also referred to as calcium pyrophosphate dihydrate crystal deposition disease) is another type of crystalline arthritis that presents very similarly to gout but is caused by the deposition of rhomboid-shaped calcium pyrophosphate dihydrate (CPPD) crystals. The knees, wrists, and fingers are the most common joints affected by pseudogout.
Diagnosis
X-rays, MRI scans, and other types of imaging can show bone loss and erosion from degenerative arthritis or later stages of inflammatory arthritis. Laboratory testing, specifically microscopic synovial fluid examination, is a more accurate method of diagnosing arthritis. By performing a joint aspiration, physicians can check cell counts and look for signs of infection (bacteria/fungus) or crystal formation to determine which, if any, type of arthritis is present.