A unilateral lesion in the brain stem (e.g., stroke, multiple sclerosis, abscess, tumor) may produce dissociated weakness, with ipsilateral weakness of the SCM and contralateral weakness of the trapezius. '11' In addition, the supranuclear fibers for the SCM muscle probably decussate twice. A lesion in the right side of the pons may result in left-sided weakness (i.e., involvement of the corticospinal tract fibers before their decussation in the medulla) and possible deviation of the head to the left (i.e., involvement of supranuclear input to the left SCM muscle after the first but before the second decussation). Involvement of the paramedian pontine reticular formation may also result in deviation of the eyes toward the weakness.
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