Fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP), previously known as myositis ossificans progressiva (MOP) and also known as Munchmeyer’s disease, is a rare, inherited disorder characterised by progressive fibrosis and ossification of muscles, tendons, fasciae, aponeuroses, and ligaments of multiple sites. It is disabling and ultimately fatal. 

FOP should not be confused with myositis ossificans circumscripta, which is usually posttraumatic.

Epidemiology

FOP is a very rare disease, with an estimated prevalence of ~1 per 2 million. There is no racial/ethnic, or gender predilection ³. The process and symptoms of heterotopic ossification start between 2 and 5 years old.

Pathology

Most cases arise from sporadic mutations, although some are inherited in an autosomal dominant fashion ³.

Radiographic features

FOP can be evaluated using plain radiographs, CT and/or MRI. MRI is mostly useful for more subtle oedema which would not be seen on the other modalities. Characteristic features include:

• congenital hallux valgus
• monophalangic first toe
• shortened metacarpals
• pseudoexostoses (ossification of ligamentous insertions)
• microdactyly of the first metacarpal/metatarsal
• neck muscle oedema
• C2-C7 facet joint fusion

Treatment and prognosis

This is a progressive, fatal disease with the median survival being 45 years.

Differential diagnosis

Since the process of heterotopic ossification is quite unique, differential diagnosis on a long term basis is quite limited. However, for punctual abnormalities, the following differential diagnosis should be considered:

• scleroderma
• CREST syndrome
• juvenile fibromatosis
• dermatomyositis