The glossopharyngeal nerve, Cranial Nerve IX (CN IX), is one of the twelve paired nerves originating from the brain and brainstem. These nerves carry sensory and motor information between the brain and various structures in the head and neck. CN IX begins in the medulla oblongata and extends deep into the neck and pharynx. Assessing its function is a standard part of a neurological examination.
Functional Roles of the Glossopharyngeal Nerve
The glossopharyngeal nerve carries motor, sensory, special sensory, and parasympathetic fibers.
- It provides taste sensation from the posterior one-third of the tongue.
- It carries general sensory information from the middle ear, the internal surface of the eardrum, and the pharynx.
- CN IX provides motor innervation to the stylopharyngeus muscle, which elevates and widens the pharynx during swallowing.
- It supplies parasympathetic input to the parotid gland to regulate saliva production.
- It plays a role in monitoring blood pressure through baroreceptors located in the carotid sinus.
Clinical Assessment Procedures for CN IX
Testing CN IX involves several distinct procedures, though it is often examined alongside the Vagus nerve (CN X) due to their overlapping roles in the throat.
Gag Reflex Assessment
The assessment of the gag reflex is a common test. The clinician gently touches the posterior pharyngeal wall or the arches of the pharynx with a cotton applicator or tongue depressor. CN IX serves as the afferent, or sensory, limb of this reflex. A normal response is a quick, visible contraction of the pharyngeal muscles.
Motor Function and Swallowing
Motor function is evaluated by observing the patient while they swallow and by assessing the pharynx and soft palate. The stylopharyngeus muscle is difficult to examine in isolation. Clinicians look for signs of difficulty swallowing, known as dysphagia, when the patient is asked to drink water or eat a small piece of food. The patient may also be asked to say “aah” while the clinician observes the soft palate and uvula. Although the Vagus nerve (CN X) is the primary motor nerve for the palate, any asymmetry or deviation of the uvula could indicate a combined problem affecting both CN IX and CN X.
Taste and Pharyngeal Sensation
The special sensory function of taste is tested on the posterior one-third of the tongue. This is done by applying a small amount of a taste solution, such as sweet, sour, or bitter, using a swab. The patient is asked to identify the flavor without retracting their tongue or swallowing the solution. This procedure is useful when a deficit is specifically suspected. Pharyngeal sensation can also be checked during the gag reflex assessment by asking the patient if they can feel the spatula touching the back of their throat.
Interpreting Abnormal Findings
An abnormal finding from a CN IX test suggests damage to the nerve, which can manifest in several ways, depending on which nerve fibers are affected. A complete absence or a noticeably weak gag reflex on one side may be an indicator of CN IX dysfunction. Since the glossopharyngeal nerve is the sensory component of the reflex, a lack of response means the brain did not receive the initial signal.
Damage to the motor fibers results in difficulty swallowing, or dysphagia, which increases the risk of choking and aspiration. This is due to the impaired function of the stylopharyngeus muscle. If the taste fibers are compromised, the patient may report a loss or alteration of taste specifically on the back third of the tongue.
Abnormalities can be caused by various issues, including stroke, tumors, or local trauma near the nerve’s pathway. Detecting these deficits is an important step in localizing neurological damage and guiding further diagnostic procedures.