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MPD1 is rare, the prevalence is unknown.

Age at onset varies from 4 to 5 years to the early twenties. The disease initially involves the dorsiflexors of the ankles and great toes and then the finger extensors, especially those of the third and fourth fingers. Early weakness of neck flexion is present in all patients. Mild involvement of the facial musculature (particularly of the orbicularis oculi and oris muscles) is often present. Mild proximal weakness develops more than ten years after the onset of the disease. The pathological features vary, although atrophic type I muscle fibres are found in half the patients.

It is caused by mutation of the MYH7 gene (14q11) that encodes the myosin heavy chain of type 1 fibers of skeletal muscle and cardiac ventricles (mutations have been identified in approximately 50% of individuals with MPD1).

Diagnosis is based on clinical findings. The pathological findings are variable. Serum creatine kinase concentration is usually normal but is sometimes increased by up to 3-fold. Electromyographic findings are nonspecific, muscle biopsy is not diagnostic. Molecular genetic testing is available on a research basis only.

Differential diagnoses include milder congenital myopathies (central core disease and centronuclear myopathy) and distal myopathies (Udd distal myopathy, Nonaka distal myopathy, Markesbery-Griggs distal myopathy) (see these terms). Genetically related (allelic) disorders are familial hypertrophic cardiomyopathy 1 and myosin storage myopathy, however, the clinical picture of MPD1 is distinct from that of these conditions.

Prenatal testing may be available for families in which the disease-causing mutation has been identified in an affected family member.

MPD1 is transmitted as an autosomal dominant trait.

Management includes physiotherapy and lightweight splinting of the ankle. Monitoring (electrocardiogram and echocardiogram) for symptoms of cardiac insufficiency is recommended.

Life expectancy is normal.