FOP is characterized by physical handicap due to bone forming in the wrong places and malformed big toes which often contain only one bone. Other features of FOP can include short thumbs, fifth finger clinodactyly (pinkie curved toward the thumb), malformed cervical (neck) vertebrae, short broad neck of the femur (thighbone), deafness, scalp baldness, and mild mental retardation.
The average age at the onset of misplaced bone formation is 5 years (range: from birth to 25 years). The most common starting sites are the neck, spine, and shoulder girdle. Some restriction of movement is present in most (80%) cases by age 7 years. By the age of 15 years, more than 95% of the patients have severely restricted mobility of the arms.
Although progressive, FOP is not cancerous.