Anatomy clinical correlates: Glossopharyngeal (CN IX), vagus (X), spinal accessory (CN XI) and hypoglossal (CN XII) nerves

The glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves, also known as cranial nerves IX, X, XI, and XII, respectively, all combine to carry sensory, motor, and parasympathetic innervation to the pharynx, larynx, tongue, and many other regions. Injury of these nerves can affect important functions like swallowing, the gag reflex, breathing, and even cardiac output. Now, we know that cranial nerves can be a hard pill to swallow, but soon you’ll see that by knowing the anatomy and the important functions of these nerves, the clinical presentations and the management of these injuries isn’t so difficult to understand.

First, let’s discuss bulbar palsy, which refers to a unilateral lower motor neuron lesion of cranial nerves IX, X, XI and XII, and it’s caused by a lesion in the medulla that affects the nucleus ambiguus and the hypoglossal nucleus. Symptoms are associated with impaired function of the affected nerves. For example, if the glossopharyngeal nerve is damaged, this causes difficulty with swallowing. Other common symptoms include nasal regurgitation, slurred speech, and difficulty speaking. Also, reflexes like the gag reflex and jaw jerk are usually absent.

Pseudobulbar palsy, on the other hand, refers to a bilateral upper motor neuron lesion of cranial nerves XI, X, XI and XII, caused by more supra nuclear proximal damage to motor fibers somewhere between the cortex and the medulla leading to an upper motor neuron injury. The symptoms are similar to bulbar palsy, however, the gag reflex and jaw jerk are usually overactive, which if you remember, is indicative of an upper motor neuron lesion.

Next, let’s discuss jugular foramen syndrome, which refers to a collection of symptoms that arise when cranial nerves IX, X, and XI, which all pass through the jugular foramen, are damaged in that area. Any obstructions or lesions, most commonly tumors in the area, but also trauma or an abscess, can damage these cranial nerves and cause jugular foramen syndrome.

Let’s talk a bit about an important function of the glossopharyngeal and vagus nerve, which is blood pressure control. The glossopharyngeal nerve innervates baroreceptors in the carotid sinus, while the vagus nerve innervates baroreceptors in the aortic arch. Baroreceptors detect changes in blood pressure and send that information through their corresponding nerve to the solitary nucleus. The solitary nucleus indirectly modulates autonomic control of the heart and blood vessels by decreasing sympathetic impulses and increasing parasympathetic impulses. Simply put, baroreceptors detect changes in the blood pressure, and send that information to the solitary nucleus, which then adjusts the blood pressure accordingly to either raise or lower it.

Now, a hypersensitive carotid sinus reacts too strongly to a rise in blood pressure, which leads to overactivation of the parasympathetic innervation and a drop in blood pressure that can cause dizziness or syncope.

Carotid sinus massage is a maneuver that can tell us if there is carotid sinus hypersensitivity, and can help to diagnose or treat paroxysmal supraventricular tachycardia. The way this works is that by applying external pressure to the carotid sinuses, the baroreceptors become activated as if there is a high blood pressure. This causes them to send their impulses through the glossopharyngeal nerve up to the solitary nucleus. The solitary nucleus then sends impulses down through the parasympathetic nerve fibers of the vagus nerve which tell the sinoatrial and atrioventricular nodes to slow down, leading to a reduction of the heart rate. Since there is a parasympathetic effect mediated through the vagus nerve, a carotid sinus massage is sometimes referred to as a vagal maneuver.

The glossopharyngeal and vagus nerves also play a role in the gag reflex. The gag reflex is an important mechanism that prevents objects in the oral cavity from entering the throat and also helps prevent choking. This reflex has an afferent limb supplied by the glossopharyngeal nerve, and an efferent limb supplied by the vagus nerve. Damage to either of these nerves will result in a diminished or lost gag reflex. If the glossopharyngeal nerve is damaged on one side, there will be no gag reflex when that side of the soft palate or posterior tongue is stimulated. The taste and general somatic sensation on the posterior third of the tongue on that side will also be lost. An example of when the glossopharyngeal nerve is vulnerable to injury is during tonsillectomies. Now, if the vagus nerve on one side is damaged, the sensation and taste will be normal, but pharyngeal muscles will only contract on the healthy side. The main complication of diminished or lost gag reflex is difficulty swallowing as well as a risk of aspiration and choking.

Ok, it’s time for a quiz. Can you recall the differences between the bulbar and the pseudobulbar palsy?

Great! Now let’s talk about each nerve separately. The word “Vagus” comes from the latin word “vagary”, which means wandering. This reflects the vagus nerve’s wide distribution throughout the body - in fact, it’s the most extensively distributed cranial nerve! The vagus nerve has plenty of branches, but let’s focus on just three for now: the group of pharyngeal branches, the superior laryngeal nerve, and the recurrent laryngeal nerve.

Now, to assess for injury to the pharyngeal branches, it’s often enough to just look at the uvula. Seriously! That’s because these branches also innervate the levator veli palatini muscles on either side, which keeps the palatal arches elevated, and the uvula in the center. Now, if the vagus is damaged on one side, the palatal arches of that side will drop, while the arches on the contralateral side keep pulling the uvula, so it deviates to the contralateral, healthy side.

The superior laryngeal nerve, on the other hand, can be injured during skull base surgery, which results in loss of sensation to the superior laryngeal mucosa. Now, remember that the superior branches into an internal and an external branch, and injury to the external branch is more common. The external branch lies deep to the superior thyroid artery and it innervates the cricothyroid muscle, which tenses the vocal folds. If the external branch is damaged, which can happen during a thyroidectomy, the vocal folds won’t be able to stretch tightly, which is required for raising our voice or changing its pitch. So with injury to the external branch, the affected individual can sound more monotone and have poor vocal quality.

Finally, the recurrent laryngeal nerve in the neck passes adjacent to the inferior thyroid artery and its branches. An enlarged thyroid can put pressure on the nerve, and procedures like neck surgeries and thyroidectomies can injure the nerve.