Source:Tracey A. Milligan, MD; Tamara B. Kaplan, MD; Neurology, Brigham and Women's/Massachusetts General Hospital, Boston, Massachusetts, USA During each section of the neurological testing, the examiner uses the powers of observation to assess the patient. In some cases, cranial nerve dysfunction is readily apparent: a patient might mention a characteristic chief complaint (such as loss of smell or diplopia), or a visually evident physical sign of cranial nerve involvement, such as in facial nerve palsy. However, in many cases a patient's history doesn't directly suggest cranial nerve pathologies, as some of them (such as sixth nerve palsy) may have subtle manifestations and can only be uncovered by a careful neurological exam. Importantly, a variety of pathological conditions that are associated with alterations in mental status (such as some neurodegenerative disorders or brain lesions) can also cause cranial nerve dysfunction; therefore, any abnormal findings during a mental status exam should prompt a careful and complete neurological exam. The cranial nerve examination is applied neuroanatomy. The cranial nerves are symmetrical; therefore, while performing the examination, the examiner should compare each side to the other. A physician should approach the examination in a systematic fashion and go through the cranial nerves in their numerical order. I Olfactory Smell II Optic Visual acuity, afferent pupillary response III Oculomotor Horizontal eye movements (adduction), efferent pupillary response IV Trochlear Downward vertical eye movement, internal rotation of eye V Trigeminal Facial sensation, jaw movement VI Abducens Horizontal eye movement (abduction) VII Facial Facial movement and strength, taste, dampening of loud sounds, sensation; anterior wall of external ear canal VIII Acoustic Hearing, vestibular functioning IX Glossopharyngeal Movement of pharynx, sensation of pharynx, posterior tongue (including taste of posterior tongue), and most of ear canal X Vagal Movement and sensation of palate, pharynx, gag reflex, guttural sounds XI Spinal accessory Strength of sternocleidomastoids and trapezius muscles XII Hypoglossal Tongue protrusion and lateral movements Table 1. The 12 Cranial Nerves and Their Basic Functions Cranial nerve I (the olfactory nerve) is a purely sensory nerve that conveys the sense of smell, and is not routinely tested during most examinations. Cranial nerve II (the optic nerve) is the only cranial nerve that can be directly visualized as it exits from the central nervous system. Its axons convey visual information and compose the afferent limb of the pupillary reflex. Testing of the pupillary responses also assesses the function of cranial nerve III (the oculomotor nerve), parasympathetic fibers of which form the efferent limb of the pupillary reflex. Cranial nerve exam includes assessment of the extraocular movements, which are controlled by cranial nerves III, IV and VI. Cranial nerve III innervates the superior, medial, and inferior rectus muscles, as well as the inferior oblique muscle, which together function to move eyes medially and in the vertical plane. Cranial nerve IV (the trochlear nerve_ innervates the superior oblique muscles, which moves the eye downward and outward. Cranial nerve VI (the abducens nerve) innervates the lateral rectus muscles, which abducts the eyes. The function of the medial and lateral rectus muscles is straightforward: Lateral rectus is involved in abduction, meaning lateral movement along the horizontal plane. Medial rectus adducts moving the eye medially movement along the horizontal plane. The remaining muscles each causes movement in more than one direction and some combination of elevation/depression, abduction/adduction, intorsion/extorsion. Muscle Innervation Primary action Secondary action Tertiary action Medial rectus CN III Adduction -- -- Superior rectus CN III Elevation Intortion Adduction Inferior rectus CN III Depression Extortion Adduction Inferior oblique CN III Extorsion Elevation Abduction Superior oblique CN IV Intorsion Depression Abduction Lateral rectus CN VI Abduction -- -- Table 2. The function of the six extraocular muscles. This first part of the cranial nerve exam concludes by testing cranial nerve V (the trigeminal nerve). This nerve has both motor and sensory components. It controls facial sensation, masticatory movements and forms the afferent limb of the corneal reflex. There are 3 major sensory branches of the trigeminal nerve - the ophthalmic, maxillary, and mandibular (also labeled V1, V2, and V3, respectively).