FOP flare-ups are sporadic episodes of painful soft tissue swellings that may be mistaken for tumors.
They can be rapidly developing and, in children, typically appear first on the head, neck, and upper back. In infants, they may manifest as scalp lumps. In older children and adults, flare-ups can occur anywhere in the body.
The affected area may be erythematous and warm or hot to the touch and may resolve without treatment.