1. Surgery on the parotid gland may damage the auriculotemporal nerve of CN V
It causes loss of sensation in the auriculotemporal area of the head. Since the auriculotemporal nerve also carries postganglionic sympathetic nerve fibers to the sweat glands of the head and postganglionic parasympathetic nerve fibers to the parotid gland for salivation, if this nerve is severed, aberrant regeneration may result in a person sweating during eating (Frey syndrome).
2. Surgery on the Parotid Gland or Bell Palsy.
Both of these conditions cause a lower motor neuron lesion of the facial nerve (CN VIII). This results in an ipsilateral paralysis of muscles of facial expression of the upper and lower face, loss of corneal reflex (efferent limb), loss of taste from the anterior two thirds of the tongue, and hyperacusis (increased acuity to sound). Clinical signs include inability to blink the eye or raise the eyebrow (upper face deficit involving orbicularis oculi and frontalis muscles, respectively) and inability to seal the lips or smile properly (lower face deficit involving orbicularis oris muscle) on the affected side.
A stroke within the internal capsule affecting the corticobulbar tract causes an upper motor neuron lesion of the facial nerve (CN VII). This results in a contralateral paralysis of the lower face but spares the upper face. Clinical signs include inability to seal the lips or smile properly (lower face deficit involving orbicularis oris muscle) on the contralateral side.
4. Facial Laceration.
A facial laceration near the anterior border of the masseter muscle will cut the parotid duct of Stensen and the buccal branch of CN VII.