The trigeminal nerve, designated as Cranial Nerve V (CN V), is the largest of the twelve cranial nerves and serves as the main sensory pathway for the face and head. Its assessment is a fundamental part of any neurological examination, providing insights into the integrity of the brainstem and peripheral nervous system. Clinical evaluation of CN V is performed to identify neurological issues that may manifest as pain, numbness, or weakness in the facial region. A systematic approach to testing its sensory and motor components is necessary for accurate diagnosis.
Dual Role of the Trigeminal Nerve
The trigeminal nerve carries both a sensory component and a motor component. The nerve gets its name from its three major sensory divisions, which distribute sensation across distinct areas of the face. The sensory root relays touch, pain, and temperature information from the face to the brainstem.
The ophthalmic division (V1) innervates the forehead, the upper eyelid, the bridge of the nose, and the cornea of the eye. The maxillary division (V2) covers the middle of the face, including the cheek, the upper lip, the upper teeth, and the area just below the eye. Finally, the mandibular division (V3) supplies sensation to the lower jaw, the lower lip, the lower teeth, and the area over the side of the head.
The motor root of the trigeminal nerve is carried exclusively within the mandibular division (V3). This motor component controls the muscles necessary for chewing (mastication). The muscles controlled include the masseter, temporalis, and the medial and lateral pterygoids, which allow for jaw elevation, depression, and side-to-side movement.
Methods for Testing Sensory Function
Sensory function assessment requires testing each of the three divisions bilaterally and comparing the patient’s perception. The process typically begins by testing light touch sensation across the forehead (V1), cheek (V2), and chin (V3) regions. The examiner uses a soft material, such as a cotton wisp, and asks the patient to close their eyes and report when they feel the sensation.
Following light touch, pain and temperature transmission is tested using a sharp-versus-dull discrimination test. A sterile, disposable item, like a broken cotton swab or a safety pin, is used to lightly touch each of the three divisions. The patient is asked to identify whether the stimulus feels sharp or dull, allowing for a mapping of any sensory deficits.
The corneal reflex tests the integrity of the ophthalmic division (V1). The examiner gently touches the edge of the cornea with a clean, fine wisp of cotton, approaching the eye from the side to avoid a blink response triggered by visual threat. A normal response is a bilateral blink of both eyes, confirming sensory information transmission through CN V. While the trigeminal nerve carries the sensory signal (afferent limb), the facial nerve (CN VII) is responsible for the motor response of the eyelid closure (efferent limb).
Evaluating Jaw and Muscle Movement
Motor function is assessed by evaluating the strength and symmetry of the muscles of mastication. The examiner observes the face for signs of muscle atrophy or asymmetry, particularly in the masseter and temporalis muscles. The patient is then instructed to clench their jaw tightly, allowing the examiner to palpate and feel the contraction of the masseter muscles near the angle of the jaw and the temporalis muscles at the temples.
The strength of jaw closure is tested by attempting to gently force the mouth open. Jaw opening strength is tested against the examiner’s resistance.
Finally, the patient is asked to slowly open their mouth wide. The examiner closely watches the midline of the jaw for any deviation during this movement. If the muscles on one side are weak due to nerve damage, the jaw will deviate toward the side of the weakness. This occurs because the unopposed action of the functioning pterygoid muscle on the healthy side pushes the jaw toward the paralyzed side.
Interpreting Assessment Results
The findings from the sensory and motor assessments provide clues about the location and nature of any neurological issue affecting the trigeminal nerve. A sensory loss confined to a single division, such as only the maxillary region (V2), suggests a peripheral lesion or compression affecting that specific branch. In contrast, sensory loss involving all three divisions on one side of the face may indicate damage to the trigeminal ganglion or the nerve root itself, where all divisions converge.
Motor findings, such as unilateral weakness or jaw deviation upon opening, point to a motor pathway lesion. This type of finding can be associated with a stroke or a tumor pressing on the motor root of CN V. Acute, intense facial pain or hyperalgesia (increased sensitivity to painful stimuli) in the distribution of one or more divisions is associated with conditions like Trigeminal Neuralgia.
An abnormal or absent corneal reflex helps localize a problem. If the patient does not blink, the issue could be with the trigeminal nerve (sensory input) or the facial nerve (motor output).