Neurogenic disorders of the muscle

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Neurogenic disorders may present as muscle disease.

Clinically, weakness of muscle may be caused by denervation due to disease either of peripheral nerves or of anterior horn cells.
Denervation is easily recognized on muscle biopsy as it causes atrophy of large groups of fibres. If re-innervation occurs, the normal random distribution of Type 1 and Type 2 fibres is replaced by large groups of a single fibre types, termed fibre-type grouping.

Spinal muscular atrophy is a heritable degeneration of spinal motor neurons

The childhood spinal muscular atrophies (SMA) constitute the second most common autosomal recessive disorder after cystic fibrosis. Disease is manifest by muscle weakness and wasting due to degeneration of the anterior horn cells in the spinal cord, resulting in denervation of muscle. Inheritance is autosomal recessive due to a defective gene located on chromosome 5. Based on age of onset and severity of disease, three main clinical patterns of disease are recognized, thought to be due to different mutations in the gene.

Type I and II Werdnig-Hoffmann disease presents as a floppy baby at birth. In the severe acute form, affected children usually die in the first year of life; an intermediate form is associated with longer survival. Muscle biopsy shows large groups of abnormally small muscle fibres that have never been innervated.

Type III Kugelberg-Welander syndrome is a less severe form of infantile SMA, with onset in the first 2 months of life. Affected babies have proximal limb girdle weakness with slow deterioration over time. Muscle biopsy shows large groups of atrophic fibres in keeping with the denervation.
Juvenile- and adult-onset spinal muscular atrophies are uncommon, presenting in later life, with limb girdle weakness. There is a dominant pattern of inheritance, not linked to chromosome 5. These must be distinguished from other causes of a limb girdle syndrome by investigation. Muscle biopsy shows features of neurogenic atrophy, with fibre-type grouping indicative of re-innervation.
Spinal muscular atrophy is a heritable degeneration of spinal motor neurons.
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