Source:Tracey A. Milligan, MD; Tamara B. Kaplan, MD; Neurology, Brigham and Women's/Massachusetts General Hospital, Boston, Massachusetts, USA

The cranial nerve examination follows the mental status evaluation in a neurological exam. However, the examination begins with observations made upon greeting the patient. For example, weakness of the facial muscles (which are innervated by cranial nerve VII) can be readily apparent during the first encounter with the patient. Cranial nerve VII (the facial nerve) also has sensory branches, which innervate the taste buds on the anterior two-thirds of the tongue and the medial aspect of the external auditory canal. Therefore, finding ipsilateral taste dysfunction in a patient with facial weakness confirms the involvement of cranial nerve VII. In addition, knowledge of the neuroanatomy helps the clinician to localize the level of the lesion: unilateral weakness of the lower facial muscles suggests a supranuclear lesion on the opposite side, while lesions involving the nuclear or infranuclear portion of the facial nerve manifest with an ipsilateral paralysis of all the facial muscles on the involved side. Cranial nerve VIII (the acoustic nerve) has two divisions: the hearing (cochlear) division and the vestibular division, which innervates the semicircular canals and plays an important role in maintaining balance. During a routine neurological examination, special testing of the vestibular nerve is usually not performed.

Cranial nerve IX (the glossopharyngeal nerve) and cranial nerve X (thevagus nerve) arise from the medulla and have laryngeal and pharyngeal function; their function is tested by assessing speech and motility of the soft palate.Because cranial nerves IX and Xform the sensory and motor limbs of the gag reflex, eliciting for gag reflex can also test their function. Cranial nerve XI (the spinal accessory nerve) innervates the sternocleidomastoid muscle and the upper portion of the trapezius muscle. These muscles control side-to-side turning the head and shrugging of the shoulders. The cranial nerve exam concludes by testing cranial nerve XII (the hypoglossal nerve), which provides motor control of the muscles of the tongue.

During the neurological assessment, the clinician should always be trying to tie together the findings of the exam to get insight into the underlying disease. The important diagnostic clues might include signs of multiple cranial nerve involvement and unilateral vs. bilateral cranial nerve dysfunction. It will help the clinician formulate differential diagnoses to know whether the patient's symptoms occurred suddenly (as expected with a stroke), over about a day (as in Bell's palsy), or gradually over weeks to months (as with an expanding mass lesion).

Evaluation of cranial nerves I-VI is covered in another video of this collection. This video demonstrates the systematic examination of cranial nerves VII-XII (Table 1).

Table 1. The 12 cranial nerves and their basic functions