Symptoms usually appear around 6 years of age and
consist of cartilaginous and osteoid transformation of connective tissue.
This ectopic bone formation leads to skeletal muscle mass displacement and
serious limitation of joint movement, mainly in the elbow, hip, and knee.
There is a characteristic hallux deformity, which consists of shortened and
angulated halluces with associated brachymesodactyly. Cervical spine
involvement is common, with varying degrees of cervical fusion and the
possibility of atlantoaxial subluxation. Temporomandibular joint involvement
may occur. Muscles of the face, larynx, eyes, anterior abdominal wall,
diaphragm, and heart usually escape involvement. Finally, limitation of rib
movement may lead to a restrictive, shallow type of breathing but rarely to
respiratory failure, although pneumonia is a common complication. ST-segment
changes and right bundle branch block are however often seen.