The cranial nerves are twelve pairs of nerves that emerge from the brain and brainstem, acting as communication pathways between the brain and the head, neck, and torso. They handle functions including sensation, motor control, and parasympathetic regulation. Cranial Nerve IX (CN IX), the glossopharyngeal nerve, plays a significant role in the head and neck. Assessing CN IX function is a regular part of a neurological examination, offering insights into the health of the nervous system.
Core Functions of Cranial Nerve IX
The Glossopharyngeal nerve (CN IX) is a mixed nerve, carrying sensory, motor, and parasympathetic fibers. Its sensory functions include general sensation from the upper pharynx, the inner surface of the eardrum, and the middle ear cavity. It also conveys the special sense of taste from the posterior one-third of the tongue.
The motor component innervates the stylopharyngeus muscle. This muscle elevates the pharynx and larynx, which is necessary for speech and swallowing. CN IX also carries sensory fibers that relay information from the carotid sinus and carotid body, helping regulate blood pressure and blood gas levels. Finally, the nerve contains parasympathetic fibers that control the secretion of the parotid gland.
Procedural Steps for Sensory Assessment
Testing the sensory functions of CN IX involves evaluating both general sensation and the special sense of taste. General sensation is typically assessed in the pharynx, often alongside the gag reflex. The examiner gently touches the posterior wall of the pharynx or the soft palate with a sterile cotton swab or tongue depressor.
The patient is asked if they can feel the touch, and sensation is compared bilaterally to check for unilateral loss. Testing taste sensation requires using solutions representing the four basic tastes: sweet, sour, salty, and bitter. These solutions are applied to the posterior third of the tongue on each side using a separate cotton applicator for each taste.
Before applying the solution, the patient must keep the tongue protruded and signal the perceived taste without pulling the tongue back. Testing the posterior third of the tongue specifically targets CN IX, as the anterior two-thirds are innervated by a different nerve. This examination step provides specific data on CN IX function, though it is often less routine in a brief neurological screening.
Procedural Steps for Motor Assessment
Assessing the motor function of CN IX focuses on the stylopharyngeus muscle and the act of swallowing. The patient is asked about any difficulty or discomfort when swallowing, known as dysphagia. A simple test involves having the patient take a sip of water and observing for any delay, coughing, or difficulty managing the fluid.
Direct observation of the pharynx is performed by asking the patient to open their mouth and say “Ah.” The examiner uses a tongue depressor, if needed, to visualize the soft palate and the uvula. Normally, the soft palate should elevate symmetrically, and the uvula should remain in a midline position as it moves upward.
The integrity of the pharyngeal reflex, or gag reflex, is a combined test for CN IX and CN X. CN IX is responsible for the sensory (afferent) limb of this reflex. Gently touching the posterior pharyngeal wall or the palatal arch with a sterile object should trigger a reflexive contraction of the pharyngeal muscles. The motor response (efferent limb), which causes pharyngeal constriction, is primarily controlled by the Vagus nerve (CN X).
Interpreting Assessment Findings
Findings from the CN IX assessment can point toward neurological issues, often involving the brainstem or structures near the jugular foramen. A loss of taste sensation on the posterior one-third of the tongue directly indicates a sensory deficit in CN IX. This finding, combined with other deficits, helps localize the lesion.
Motor weakness, particularly unilateral paralysis of the pharyngeal wall, results in an asymmetrical elevation of the soft palate when the patient says “Ah.” The uvula may be pulled toward the unaffected side due to the unopposed action of the normal muscle. This motor finding is often combined with symptoms like a hoarse or nasal voice due to concurrent CN X involvement.
The absence of a gag reflex on one side suggests a problem with the sensory input provided by CN IX. Swallowing difficulties (dysphagia) or speech problems (dysphonia) observed during the assessment suggest pathology affecting the nerve’s motor or sensory pathways. These symptoms may signal a lesion in the lower brainstem or peripheral nerve compression.