Centronuclear myopathy (CNM) is an umbrella term for a group of rare genetic muscle disorders. These disorders are characterized by muscle weakness that can range from mild to profound. Symptoms are often present at birth in the severe forms, but may first develop at any point during life, although onset in adulthood is unusual. There are several genetic forms of CNM including an X-linked form known as myotubular myopathy (XLMTM) and a few autosomal forms, usually referred to as centronuclear myopathy. Autosomal refers to genes that are found on autosomes, or chromosomes other than the X or Y chromosomes (sex chromosomes). Generally, the autosomal forms are less severe than XLMTM, however, in rare cases, individuals with an autosomal form can develop severe complications that are similar to those seen in XLMTM. CNMs derive their name from the abnormal location of the nucleus in the center of the muscle fiber (muscle cell) rather than the normal position on the edge. Centronuclear myopathies can be further classified into the larger, broader category of congenital myopathy, a group of genetic muscle disorders that are present at birth.
Common symptoms include mild to profound muscle weakness and diminished muscle tone (hypotonia or "floppiness"). In more severe cases, feeding difficulties and potentially severe breathing complications (respiratory distress) may occur. Feeding difficulties and respiratory distress develop because of weakness of the muscles that are involved in swallowing and breathing. Involvement of the muscles controlling eye movements is common in all different forms. The overall severity of the disorder can range from mildly affected individuals to individuals who develop severe, life-threatening complications during infancy and early childhood. Three different genes, DNM2, BIN1, and RYR1, have been identified that cause autosomal forms of CNM. XLMTM is caused by mutations to the myotubularin (MTM1) gene.
In the medical literature, centronuclear myopathy (CNM) is generally used for the autosomal forms of the disorder and myotubular myopathy is generally used for the X-linked form (XLMTM). Distinguishing between the X-linked myotubular form and the autosomal forms of CNM is essential as the symptoms are usually more severe in the X-linked form. NORD has a separate report on X-linked myotubular myopathy that describes that form in greater detail. This report specifically deals with the autosomal forms of centronuclear myopathy.