Test facial sensation using a cotton wisp and a sharp object. Also test for tactile extinction using double simultaneous stimulation.
The corneal reflex, which involves both CN V and CN VII, is tested by touching each cornea gently with a cotton wisp and observing any asymmetries in the blink response.
Feel the masseter muscles during jaw clench. Test for a jaw jerk reflex by gently tapping on the jaw with the mouth slightly open.
Facial sensation can be impaired by lesions of the trigeminal nerve (CN V), the trigeminal sensory nuclei in the brainstem, or ascending sensory pathways to the thalamus and somatosensory cortex in the postcentral gyrus (see Neuroanatomy through Clinical Cases, Chapters 7 and 12). The corneal blink reflex is mediated by polysynaptic connections in the brainstem between the trigeminal (CN V) and facial (CN VII) nerves and can be impaired by lesions anywhere in this circuit.
Extinction in the presence of intact primary sensation is usually caused by right parietal lesions.
Weakness of the muscles of mastication can be due to lesions in the upper motor neuron (UMN) pathways synapsing onto the trigeminal (CN V) motor nucleus, in the lower motor neurons (LMNs) of the trigeminal motor nucleus in the pons or as they exit the brainstem to reach the muscles of mastication, in the neuromuscular junction, or in the muscles themselves.
Presence of a jaw jerk reflex is abnormal, especially if it is prominent. It is a sign of hyperreflexia associated with lesions of UMN pathways projecting to the trigeminal motor nucleus. Both the afferent and the efferent limbs of the jaw jerk reflex are mediated by CN V.