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Extravasation is the leakage of a fluid out of its container. In the case of inflammation, it refers to the movement of white blood cells from the capillaries to the tissues surrounding them (diapedesis). In the case of malignant cancer metastasis it refers to cancer cells exiting the capillaries and entering organs. It is frequently used in medical contexts, either referring to urine, or to blood.
More specifically, it can refer to:
- Extravasation (intravenous)
- Extravasation of infusates
- Extravasation of irrigation fluid
- Extravasation of urine
- Leukocyte extravasation
Extravasation of irrigation fluid 
Extravasation of irrigation fluid is the unintended migration of irrigation fluid (e.g. saline) introduced into a human body. This may occur in a number of types of endoscopic surgery, such as minimally invasive orthopedic surgery, i.e. arthroscopy, TURP (trans-urethral resection of the prostate) and TCRE (trans-cervical resection of the endometrium).
In arthroscopy, fluid under pressure is used to inflate and distend a joint and make a working surgical space. Arthroscopy is typically performed on shoulder and knee joints; however, hip arthroscopy is becoming more popular. Arthroscopy is done by making surgical portals or puncture wounds into the joint. A surgical instrument called an arthroscope is used to introduce irrigation fluid under pressure to distend the joint. The arthroscope includes a small (typically 4mm in diameter) optic scope rod to view the joint. Other portals or puncture wounds are made to introduce surgical instruments to perform cutting or repair procedures.
If the joint is surrounded by soft tissue, as in the shoulder and hip, fluid under pressure may leak out of the joint space through the surgical portals and collect in the patient's soft tissue. A typical arthroscopy can result in 1–3 liters of irrigation fluid being absorbed into the patient's interstitial tissue. This buildup of irrigation fluid in the soft tissue may cause edema. This swelling can interfere with the arthroscopic procedure by collapsing the surgical space, or migrating into the patient's neck and causing airway blockage. In hip arthroscopy, a feared complication is abdominal flooding where the irrigation fluid leaks from the hip joint capsule and drains into the abdominal cavity. Risk factors for fluid extravasaton include procedure length (> 90–120 min), obesity, and age (> 45–50) with accompanying lack of muscle tone.
Shoulder arthroscopy is typically limited to about 90–120 minutes before the swelling from fluid extravasation interferes with the procedure, and presents a potential risk to the patient. Typically, fluid extravasation is managed by controlling fluid pressure, or hastening the procedure.
Arthroscopic instrumentation such as the newer Extravastat devices to drain extravasated fluid from the soft tissue during shoulder and hip arthroscopy has been reported to be beneficial in reducing fluid extravasation and swelling.
Extravasation of infusates 
Extravasation may also refer to the leakage of infused substances into the vasculature into the subcutaneous tissue. The leakage of high osmolarity solutions or chemotherapy agents can result in significant tissue destruction, and significant complications.
See also 
- Complications in Orthopaedics: Shoulder Arthroscopy, chapter "Severe Edema During Shoulder Arthroscopy" pp 9–16, Xavier A. Duralde, MD, editor.
- Immediate postoperative fluid retention and weight gain after shoulder arthroscopy. LoIK, Burkhart SS. Arthroscopy. 2005 May; 21(5):605–610.
- Life Threatening Airway Edema Resuling From Prolonged Shoulder Arthroscopy, Steven L. Orebaugh, M.D. Anesthesiology, V 99, No. 6, December 2003.
- Severe Airway Obstruction During Arthroscopic Shoulder Surgery, Stephan Blumenthal, M.D., Marco Nadig, M.D., Christian Gerber, M.D., Alain Borgeat, M.D. Anesthesiology, V 99, No. 6, December 2003.
- Complete Airway Obstruction During Shoulder Arthroscopy, Hynson, J.M., Tung, A., Guevara, J.E., et al. Anesthesiology Analg, 76:875–878, 1993.
- Complications of hip arthroscopy, Sampson, T.G. Clin Sports Med. 20(4):831–835, October 2001.
- Monograph on Complications in Shoulder Arthroscopy published by the American Association of Orthopedic Surgeons (AAOS)
- British National Health Service page on complications from extravasation of infusates.
- Cannuflow Research archive on extravasation in shoulder arthroscopy
- Cannulae with Outflow Reduces Fluid Gain in Shoulder Arthroscopy Hasan M. Syed, Seth B. Gillham, Christopher M. Jobe, Wesley P. Phipatanakul and Montri D. Wongworawat