A flare-up occurs when the body starts to generate new bone. No one knows what initiates this process, but once it begins, it leads to tissue swelling and much discomfort. Sometimes the individual will not feel well and may develop a low-grade fever. While there is no medication or therapy that can stop the process of bone formation once it has begun, a physician can prescribe medicine to help relieve the pain. A single flare-up may continue for as long as 6-8 weeks. Multiple flare-ups may also occur during an active period of FOP. When a flare-up is in progress, joint stiffness may occur overnight. The stiffness comes from swelling and pressure inside the muscle during the earliest stages of new bone formation.
While flare-ups are usually painful, the degree of pain can vary. Most people find that the pain subsides when a flare-up is over; it appears that it is the process of extra bone formation, rather than the extra bone itself, that usually leads to pain.