Which of the following would NOT impair elevation of the upper lip?

Correct answer: C. A block of the nerve within the infraorbital canal

• A. The FACIAL nerve (CN VII) exits the skull at the STYLOMASTOID FORAMEN and supplies every muscle of facial expression, including levator labii superioris, levator anguli oris and zygomaticus major and minor. A lesion there paralyses the whole half of the face and abolishes lip elevation.
• B. A vestibular infiltration deposits solution among those same terminal facial nerve branches as they run superficially, producing a transient motor weakness of the lip — exactly what alarmed this patient. It is harmless and resolves with the anaesthetic.
• C. CORRECT as the exception. The nerve running in the infraorbital canal is the INFRAORBITAL nerve, the terminal continuation of the maxillary division of the trigeminal nerve (V2). It is purely SENSORY, supplying the lower eyelid, the side of the nose and the skin and mucosa of the upper lip, and giving off the zygomaticotemporal and zygomaticofacial branches along its course. Blocking it NUMBS the upper lip but leaves the motor supply entirely untouched — which is why a patient with a profound infraorbital block still smiles normally on that side.
• D. Within the parotid gland the facial nerve divides into its five terminal branches. The zygomatic and buccal branches are the ones that elevate the upper lip, and injuring them during parotid surgery is a recognised and permanent complication.

Note: The whole trick of this item is SENSORY versus MOTOR. V2 SENSES the upper lip; CN VII MOVES it. The same logic explains the far more frequently examined complication of a misplaced inferior alveolar block — solution deposited too deep and posterior enters the parotid capsule, anaesthetises the facial nerve trunk and produces a transient unilateral facial palsy with an inability to close the eye, which must be protected until sensation returns.