The most common cause for cranial nerve XI injury is iatrogenic, such as lymph node biopsies and neck surgeries, the risk is particularly high with procedures that involve the posterior triangle of the neck.
Sternocleidomastoid muscle is innervated by cranial nerve XI ; paralysis of this muscle results in the inability to move the head to the contralateral side, for example, left sternocleidomastoid muscle contracts to turn the head to the right, hence the lesion of this nerve on the left side results in the inability to turn the head to the right.
Trapezius muscle is also innervated by CN XI; paralysis of this muscle results in shoulder droop on the side of the lesion and inability to shrug the shoulder on a physical exam.
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