Definition - one of a group of diseases that cause problems with the tone and contraction of skeletal muscles

Cause - variety of genetic defects that affect muscle development and function, including defects in the gene for myotubularin, myogenic factor 6 and dynamin 2

Onset - for X-linked, myotubularin-related myotubular myopathy: infancy; for autosomal recessive forms: infancy to young adulthood; for autosomal dominant myogenic factor 6-related form: late childhood to adulthood; for autosomal dominant dynamin 2-related form: adolescence to adulthood

Symptoms - for myotubularin-related form: profound loss of muscle tone, weakness of skeletal muscles, respiratory insufficiency, eye muscle weakness, drooping eyelids; autosomal recessive forms: weakness mainly in muscles closer to the center of the body (proximal muscles), facial weakness with or without eye muscle weakness; myogenic factor 6-related form: weakness mainly in muscles closer to the center of the body (proximal muscles), leg cramps, sometimes facial, eye muscle or shoulder muscle weakness; dynamin 2-related form: weakness mainly in muscles distant from center of body (distal muscles), and in neck and abdominal muscles; drooping eyelids; sometimes eye muscle weakness, facial weakness; normal respiratory function

Progression - myotubularin-related form: if respiratory insufficiency compensated for with ventilation, may survive beyond infancy, after which weakness appears nonprogressive; autosomal recessive forms: slow; usually survive past infancy; myogenic factor 6-related form: slow; dynamin 2-related form: slow

Inheritance - myotubularin form is X-linked recessive, meaning primarily affects boys, who inherit the disease through their mothers; female carriers may exhibit some symptoms; some forms are autosomal recessive, meaning faulty gene must be inherited from each parent; myogenic factor 6-related and dynamin 2-related forms are autosomal dominant, meaning disease results from only one faulty gene

Facts About Myopathies