As a result of a viral infection, a patient has swelling of the left facial nerve within the facial canal. His face appears asymmetrical, and he C/O that saliva drips from his mouth while he is chewing. Paralysis of which of the following muscles accounts for these symptoms?
**Core Concept**
The facial nerve (cranial nerve VII) is responsible for controlling the muscles of facial expression, transmitting taste from the anterior two-thirds of the tongue, and regulating saliva production. Swelling within the facial canal can lead to paralysis of the facial nerve, resulting in weakness or paralysis of the muscles it innervates.
**Why the Correct Answer is Right**
The symptoms described, including facial asymmetry and drooling of saliva while chewing, are characteristic of lower motor neuron (LMN) weakness of the facial nerve. The facial nerve innervates the buccinator muscle, which is responsible for compressing the cheek against the teeth during mastication (chewing). When the buccinator muscle is paralyzed, saliva can escape from the mouth while chewing. The orbicularis oculi and orbicularis oris muscles, also innervated by the facial nerve, are responsible for eye closure and lip closure, respectively. However, these muscles are not primarily responsible for the symptoms described.
**Why Each Wrong Option is Incorrect**
**Option A:** The masseter muscle is innervated by the trigeminal nerve (cranial nerve V), not the facial nerve. Paralysis of the masseter muscle would result in weakness of the jaw muscles, not the symptoms described.
**Option B:** The sternocleidomastoid muscle is innervated by the accessory nerve (cranial nerve XI), not the facial nerve. Paralysis of this muscle would result in weakness of the neck and shoulder muscles, not the symptoms described.
**Option C:** The digastric muscle is innervated by both the trigeminal nerve (cranial nerve V) and the facial nerve. However, its primary function is to depress the mandible (lower jaw), and its paralysis would not primarily result in the symptoms described.
**Clinical Pearl / High-Yield Fact**
When evaluating a patient with facial weakness or paralysis, it is essential to distinguish between upper motor neuron (UMN) and lower motor neuron (LMN) lesions. UMN lesions, such as those caused by stroke or cerebral vasculitis, typically result in contralateral facial weakness, whereas LMN lesions, such as those caused by viral infections or Bell's palsy, typically result in ipsilateral facial weakness.
**Correct Answer: C. Digastric muscle paralysis would not primarily result in the symptoms described.