• The jugular foramen lesions mentioned above in the section on the ninth and tenth cranial nerves are important.
• Lesions in the region of the foramen magnum, upper spinal cord, lower end of the medulla, both extrinsic (neurofibroma) and intrinsic (poliomyelitis), and amyotrophic lateral sclerosis
• Muscular dystrophy, especially myotonic dystrophy, which may present with almost no visible sternomastoid muscle. Myasthenia gravis and polymyositis are also important.
• An upper motor neuron lesion in the hemisphere may produce weakness of the ipsilateral sternomastoid and contralateral trapezius. With a pontine upper motor neuron lesion the contralateral sternomastoid is weak. Although still somewhat controversial, the upper motor neuron respecting the sternomastoid originates either in both hemispheres or, more likely, in the ipsilateral hemisphere and decussates twice, that is, to the opposite pons and back again to the same side of the cord as the hemisphere of origin. The upper motor neuron fibers respecting the trapezius appear to originate in the contralateral hemisphere and decussate only once.
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