The Trigeminal Nerve (Cranial Nerve V or CN V) is the largest of the twelve cranial nerves, providing both the primary sensory input from the face and motor control for the powerful muscles used in chewing. Its name, meaning “triplets,” refers to its three major divisions, which cover distinct regions of the face. Assessing the Trigeminal Nerve is a standard part of a comprehensive neurological examination because damage can lead to significant issues, including chronic facial pain or difficulty with mastication. Evaluating CN V’s sensory, motor, and reflex pathways is essential for localizing any potential neurological dysfunction.
Sensory Division Testing
The Trigeminal Nerve transmits sensations like touch, pain, and temperature from the face to the brain. Its sensory component divides into three distinct branches, each innervating a specific area. The Ophthalmic division (V1) covers the forehead and upper eyelid. The Maxillary division (V2) is responsible for sensation in the middle of the face, including the cheek. The Mandibular division (V3) provides sensation to the lower jaw and lower lip.
Sensory testing begins with light touch, typically using a wisp of cotton. The patient is instructed to close their eyes and report when they feel the touch. The examiner lightly touches areas within V1, V2, and V3 on one side of the face, then repeats the process on the corresponding area of the opposite side. Sensation must be compared bilaterally, asking the patient if the feeling is the same on both sides.
To assess pain and temperature sensation, a sharp stimulus is used, along with a rounded end for comparison. The sharp stimulus is gently applied to the dermatomes of V1, V2, and V3 on both sides. The patient is asked to distinguish between sharp and dull sensations and compare the intensity of the feeling bilaterally. Any area of decreased or absent sensation (hypesthesia or anesthesia) should be noted as it helps localize a potential lesion.
Motor Function Examination
The motor component of the Trigeminal Nerve travels exclusively with the Mandibular division (V3) and controls the muscles of mastication necessary for chewing. These muscles include the temporalis, masseter, and the medial and lateral pterygoids. The examination starts with a visual inspection of the face, looking for asymmetry, atrophy (decrease in muscle bulk), or involuntary movements. Atrophy can suggest a lower motor neuron lesion.
Next, the patient is asked to clench their teeth tightly. The examiner then palpates the masseter muscles just above the angle of the jaw and the temporalis muscles at the temples. The strength and bulk of the muscle contraction should be assessed and compared on both sides to check for symmetry and tone. Weakness on one side results in a diminished firmness of contraction.
To further test muscle power, the patient is asked to open their mouth, and the examiner applies downward pressure on the chin, which the patient must resist. The pterygoid muscles, which are responsible for lateral jaw movement, are assessed by having the patient move the jaw from side to side against the resistance of the examiner’s hand. Unilateral weakness of the pterygoid muscle causes the jaw to deviate toward the side of the lesion when the mouth is opened.
Reflexes and Clinical Significance
Two important reflexes are associated with the Trigeminal Nerve: the Corneal Reflex and the Jaw Jerk Reflex. The Corneal Reflex is a protective mechanism that involves the rapid, involuntary blinking of both eyes when the cornea is lightly touched. The ophthalmic division (V1) serves as the afferent (sensory) pathway, carrying the stimulus signal. The efferent (motor) pathway is mediated by the Facial Nerve (Cranial Nerve VII), which causes the orbicularis oculi muscle to contract and close the eyelid.
The Jaw Jerk Reflex is a stretch reflex that is typically absent or very slight in a healthy individual. To test it, the patient’s mouth is held slightly open, and the examiner taps a finger placed on the chin with a reflex hammer. A normal response is a minimal or absent upward jerk of the mandible. Both the sensory and motor components of this monosynaptic reflex are mediated entirely by CN V.
Abnormal findings in the assessment of the Trigeminal Nerve carry significant clinical meaning. Sensory loss following a clear V1, V2, or V3 pattern suggests a peripheral nerve lesion affecting that specific branch. Conversely, a sensory loss pattern that involves the entire face, often described as an “onion-skin” distribution, can point to a more central brainstem lesion.
Muscle weakness or atrophy in the masseter and temporalis muscles indicates a motor pathway dysfunction, which may be due to a lower motor neuron problem. An absent Corneal Reflex indicates a problem with the sensory V1 pathway or the motor CN VII pathway. An exaggerated or hyperactive Jaw Jerk Reflex is a sign of an upper motor neuron lesion, suggesting the pathology is located above the level of the Trigeminal motor nucleus in the brainstem.