What are possible congenital myopathies?

Updated: Mar 11, 2019
  • Author: Matthew Harmelink, MD; Chief Editor: Amy Kao, MD  more...
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Answer

Possible congenital myopathies

  • Myopathy with hexagonally cross-linked tubular arrays: Onset occurs during childhood or adulthood and includes slowly progressive, proximal weakness; fatigue; and exertional myalgia. Inheritance is unknown. EM reveals subsarcolemmal non–membrane-bound inclusions that, on cross-section, are arranged in a 6-spoked hexagonal pattern. The inclusions stain dark purple with modified GT stain and are found in only type-2 muscle fibers.

  • Trilaminar myopathy: Only one case has been reported with presentation at birth and was characterized by minimal movement, rigidity, and contractures. The course was nonprogressive. Light microscopy revealed a trilaminar appearance with 3 concentric zones when reacted with the modified GT or NADH stains. The inner and outer zones were densely stained, and the intermediate zone was unstained. On EM, the innermost zone had densely packed mitochondria, glycogen, electron-dense material, and filaments. The middle zone contained Z-disk streaming and well-organized myofibrils. The outermost zone contained cytoplasm with rare mitochondria, filaments, vesicles, and lipids.

  • Zebra body myopathy: Only two congenital-onset cases have been reported and were characterized by hypotonia and weakness that progressed slowly or was nonprogressive. On EM, zebra bodies are characterized by Z-band material connected by fine filaments in a pseudosarcomeric pattern, resulting in a striped appearance. Zebra bodies are normally present in myotendinous junctions, intrafusal fibers, extrafusal fibers, extraocular muscle, and cardiac muscle. Their function is unknown.

  • Congenital myopathy with mosaic fibers and interlacing sarcomeres: Only 1 case with childhood onset, which was characterized by proximal weakness, scoliosis, and talipes equinovarus, has been reported. Progression was minimal, but cardiomyopathy and respiratory insufficiency developed in adulthood. Light microscopy revealed a mosaic pattern of light and dark staining on myosin ATPase. EM revealed bands of myofibrils at right angles to other myofibrils, resulting in an interlacing appearance.

  • Congenital myopathy with apoptotic changes: Only 1 case has been reported and was characterized by congenital-onset hypotonia, proximal weakness, and severe mental retardation. Light microscopy and EM revealed chromatin condensation and nuclear fragmentation. Some fibers were positive for Bax, caspase-3, or TUNNEL.

  • Broad A-band disease: Two sporadic cases have been described with congenital-onset hypotonia and mild nonprogressive proximal muscle weakness. EM revealed disorganization of the thick filaments, leading to a loss of distinct A-band/I-band demarcation and the appearance of smearing or broadening of the A-band.

  • Lamellar body myopathy: Only one case has been described and was characterized by congenital onset, weakness with progression to respiratory failure, and death at age 5 years. Light microscopy revealed increased connective tissue surrounding muscle fibers that stained positively for laminin and fibronectin. EM revealed that these areas contained concentric lamellar bodies between the 2 layers of basement membrane.

  • Myopathy with muscle spindle excess: Only one case has been described and was characterized by congenital onset, hypotonia, proximal weakness, and arthrogryposis. Cardiomyopathy and respiratory failure led to death at about age 1 year. Light microscopy revealed an excess of muscle fiber clusters within fibrous capsules consistent with muscle spindles.


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