Vaso-Occlusive Crisis

Last updated Jan. 4, 2022, by Marisa Wexler, MS

✅ Fact-checked by Joana Carvalho, PhD

Sickle cell disease (SCD) is an inherited disorder caused by a mutation in the gene encoding a subunit of hemoglobin, the protein that carries oxygen in red blood cells. Mutant hemoglobin — called sickle hemoglobin or hemoglobin S — tends to clump together in red blood cells, causing them to take on an irregular “sickle-like” shape that gives the disease its name.

In addition to their irregular shape, sickled red blood cells tend to have abnormalities in their membranes that make them “stickier” than normal. The combination of their unusual shape and stickiness means that sickled red blood cells can easily stick to blood vessel walls and block blood flow, which in turn can lead to a vaso-occlusive crisis.

What is a vaso-occlusive crisis?

A vaso-occlusive crisis, or VOC, occurs when sickled red blood cells block blood flow to the point that tissues become deprived of oxygen. This in turn sets in motion an inflammatory response as the body tries to rectify the problem. The result is substantial pain, which can affect any part of the body, but most commonly occurs in the back, chest, or extremities.

In most cases, it is not known what triggers a VOC. However, some factors are known to increase the likelihood of these pain crises, such as cold temperatures, dehydration, infection, and stress.

VOCs are sometimes described as occurring in four phases: the first phase, which lasts around three days, is associated with a low-intensity, aching pain that may be accompanied by tingling or numbness as the affected tissue starts to lose oxygen. In the second phase, aching rapidly increases to an acute pain as affected tissue starts to die.

The third phase is characterized by the body’s inflammatory response to tissue death, with constant, severe pain that may be accompanied by fever and usually lasts several days. The fourth and final phase is the resolution of the crisis, which usually takes place over one to two days.

Importantly, these phases do not always go neatly in order, and it is possible to experience multiple VOCs in a row without resolution in between.

Additionally, various approaches for trying to shorten the length of a VOC have thus far yielded poor results.

VOC is the most common cause of hospitalization related to SCD. In addition to causing acute pain, these crises can set the stage for further health complications, such as arthritis, kidney failure, and strokes.

Management of VOCs

VOCs can be extremely painful and are considered a medical emergency. Short-term or acute management largely focuses on controlling pain with pain-relieving medications (analgesics). Opioids are commonly used for this purpose.

Acute management also may involve administering supplementary oxygen to combat low bodily oxygen levels, and/or fluids to manage dehydration. A blood transfusion also may be necessary in some cases.

Until recently, the only medication approved in the U.S. for long-term prevention of VOCs was hydroxyurea, a cancer medication. In recent years, however, new VOC-preventing therapies have garnered approval: Adakveo (crizanlizumab) and Endari (L-glutamine).

Further therapies for the treatment of VOCs and other SCD symptoms are in development.


Sickle Cell Disease News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.