There are cytotoxic T cells and muscle cell necrosis, but autophagy may be the main pathogenetic mechanism. These are autophagic vacuoles which are acid phosphatase positive. 16-18 nm filamentous inclusions on EM are seen in OPMD and other rare chronic wasting muscle disorders. IBM is occasionally associated with abnormal mitochondria. In normal motor units, impulses from the motor neuron cause simultaneous contraction of all fibers.
Denervated fibers cease to contract and shrink.
MND is a progressive disease of middle to old age with widespread degeneration of motor neurons. Bulbar involvement carries a particularly bad prognosis. Extraocular and cardiac muscles are spared. Increasing incidence and geographical variation imply environmental factors. MND is usually rapidly progressive and may involve type I grouping and small some compensatory hypertrophy.