FIRST CRANIAL NERVE---OLFACTORY NERVE
The olfactory nerve is a special afferent cranial nerve composed of sensory fibers only. Its sole function is to discern smells. Olfaction depends on the integrity of the olfactory neurons in the roof of the nasal cavity and their connections through the olfactory bulb, tract, and stria to the olfactory cortex of the medial frontal and temporal lobes. To test olfaction, an odorant, such as concentrated vanilla, peppermint or coffee extract, is presented to each nostril in turn. The patient is asked to sniff (with eyes closed) and identify each smell. Olfaction is frequently not tested because of unreliable patient responses and lack of objective signs.

SECOND CRANIAL NERVE---OPTIC NERVE
The optic nerve contains special sensory afferent fibers that convey visual information from the retina to the occipital lobe via the visual pathway. Evaluation gives important information about the nerves, optic chiasm, tracts, thalamus, optic radiations, and visual cortex.

CN 2 is also the afferent limb of the pupillary light reflex.

The optic nerve is tested in the office by visual acuity measurement, color vision testing, pupil evaluation, visual field testing, and optic nerve evaluation via ophthalmoscopy and/or stereo biomicroscopy.

Intracranial lesions affecting the visual pathway can be localized based on the pattern of visual field defects. Prechiasmal lesions usually cause monocular field defects. Chiasmal lesions produce heteronymous hemianopsias, and postchiasmal lesions produce homonymous hemianopsias. The further posterior the lesion, the more alike (congruous) the two fields appear.

THIRD CRANIAL NERVE---OCULOMOTOR NERVE
The oculomotor nerve contains somatic efferent and visceral efferent motor fibers. The somatic efferent fibers innervate the levator palpebral superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles of the eye. The visceral efferent fibers convey parasympathetic innervation for pupillary constriction and accommodation.

To begin testing CN 3, it is important to first inspect the eyes. Look for ptosis, note the appearance of the eyes, and check for ocular alignment (the light source reflection should fall at the same location on each eyeball).

Next, test extraocular range of motion by having the patient follow a near target through the six principal positions of gaze ("H" pattern). Note any misalignment of the eyes or complaints of diplopia. When specifically evaluating CN 3 during testing, note adduction (medial rectus), depression while abducting (inferior oblique), and elevation (superior rectus and inferior oblique).

Pupillary constriction is tested by the light reflex, and having the patient focus on a near target can test accommodation. Loss of CN 3 function may cause diplopia, and an eye that is "down and out" with ptosis and mydriasis.

FOURTH CRANIAL NERVE---TROCHLEAR NERVE
The trochlear nerve supplies somatic efferent motor fibers that innervate the superior oblique muscle. The superior oblique is tested, as previously described, by inspection followed by having the patient track a near target moved in an "H" pattern.

Having the patient adduct and look downward to the nose best isolates this nerve. The trochlear nerve is the only nerve to exit from the dorsal aspect of the brain, and it has the longest intracranial course of any cranial nerve. For this reason, it is often the most susceptible to damage from intracranial lesions caused by trauma, inflammatory disease, and compression. Loss of CN 4 function causes diplopia with a compensating head tilt.

FIFTH CRANIAL NERVE---TRIGEMINAL NERVE
The trigeminal nerve supplies both sensory and motor fibers to the face and periorbital area. The afferent sensory fibers separate into three divisions and carry touch, pressure, pain, and temperature sense from the oral and nasal cavities, and the face. Motor efferent fibers function to innervate several facial muscles, including the muscles of mastication.

The sensory portion of the trigeminal nerve is commonly tested by examining the integrity and symmetry of pain and light touch sensation from all areas of the face (forehead, cheek, and jaw). After asking the patient to close his/her eyes, a tissue is lightly touched to one side of the forehead. The tissue is then touched to the opposite side and the patient is asked to compare sensations. A sharp object can be used in the same manner when testing for pain symmetry. The test is then repeated on the cheek (Fig. 8) and jaw line to assess the second and third divisions.

An additional test used to evaluate the trigeminal nerve is the corneal reflex test. Evaluate the reflex by gently touching each cornea with a cotton wisp and observing any asymmetries in the blink response. This tests both the sensory fifth nerve and the motor portion of the seventh nerve, which is responsible for lid closure.

To test the motor component of CN 5, feel and compare the tone of the masseter muscles during jaw clench. Next, have the patient open his/her mouth and resist the examiner's attempt to close it. If there is weakness of the pterygoids, the jaw will deviate towards the side of the weakness. Sensory loss of CN 5 is usually due to trauma while vascular damage, tumors, and trauma can result in damage to the motor aspect of the system.

SIXTH CRANIAL NERVE---ABDUCENS NERVE
The abducens nerve supplies somatic efferent motor fibers to the lateral rectus muscle, which functions to abduct the eye. As in prior discussions regarding extraocular muscles, having the patient follow a near target and tracing an "H" pattern tests the abducens nerve. Inability to abduct the eye indicates a possible deficit. Aneurysms, tumors, meningitis, trauma, and cavernous sinus pathology are all potential causes of abducens deficits. Loss of CN 6 function can elicit complaints of horizontal diplopia and may cause patients to appear esotropic.

SEVENTH CRANIAL NERVE---FACIAL NERVE
The facial nerve supplies efferent motor innervation to the muscles of facial expression, and carries sensory afferent fibers from the anterior two thirds of the tongue for taste. To test the motor division of the facial nerve, start from the top and work down. First, have the patient wrinkle the forehead and check for asymmetry. Next, have the patient shut the eyes tightly while the examiner attempts to open them. Note any weakness on one side. Finally, have the patient show his/her teeth or smile and compare the nasolabial folds on either side of the face.

Bell's palsy is a common lower motor neuron lesion of the facial nucleus or its axon. The result is facial asymmetry with drooping of the eyebrow, a smooth nasolabial fold, drooping of the corner of the mouth, and a reduced blink reflex on the affected side.

To test the sensory fibers of the facial nerve, apply sugar, salt, or lemon juice on a cotton swab to the lateral aspect of each side of the tongue and have the patient identify the taste. Taste is often tested only when specific pathology of the facial nerve is suspected.

EIGHTH CRANIAL NERVE---VESTIBULOCOCHLEAR NERVE
The eighth cranial nerve carries two special sensory afferent fibers, one for audition (hearing) and one for vestibular function (balance). The cochlear division of CN 8 is tested by screening for auditory acuity. This can be done by the examiner lightly rubbing his/her fingers together next to each of the patient's ears and comparing the left and right side responses.

In addition, the Rinne and Weber tests are easy to perform and can help differentiate conductive deficits from neurosensory lesions. The Weber test consists of placing a vibrating tuning fork on the middle of the forehead and asking if the patient feels or hears it best on one side or the other (Fig. 13). The normal patient will say that it is the same on both sides. The patient with unilateral neurosensory hearing loss will hear it best in the normal ear, and the patient with unilateral conductive hearing loss will hear it best in the abnormal ear.

The Rinne test (Fig. 14) consists of comparing bone conduction, assessed by placing the tuning fork on the mastoid process behind the ear, versus air conduction, assessed by holding the tuning fork in air near the front of the ear. Normally, air conduction volume is greater than bone conduction sound volume. For neurosensory hearing loss, air conduction volume is still greater than bone conduction, but for conduction hearing loss, bone conduction sound volume will be greater than air conduction volume.

Vestibular testing is can be used to assess brainstem function in comatose patients or in patients who report vertigo/dizziness.

Damage to CN 8 can be caused by trauma, tumors, or infection and can lead to hearing loss, dizziness, loss of balance, tinnitis, and deafness.

(More information on this topic is provided in another Pacific University On-Line CE course: Sensory Conflict and Other Causes of Dizziness: Etiology Differential Diagnosis, and Management by Robert L. Yolton, PhD, OD.)

NINTH CRANIAL NERVE---GLOSSOPHARYNGEAL NERVE
The ninth cranial nerve supplies motor fibers to the parotid gland and the pharynx. It also carries sensory fibers from the carotid body and taste sensation fibers from the posterior third of the tongue.

The gag reflex tests both the sensory and motor components of CN 9 and CN 10. This involuntary reflex is obtained by stroking the back of the pharynx with a tongue depressor and watching the elevation of the palate (as well as causing the patient to gag).

The motor division of CN 9 and CN 10 is tested by having the patient say "ahh" or "kah". The palate should rise symmetrically in the back of the oral cavity. Paralysis of the ninth nerve causes a pulling of the uvula to the unaffected side. The ninth, tenth, and eleventh cranial nerve pathways are physically so close together that isolated lesions are rarely seen.

TENTH CRANIAL NERVE---VAGUS NERVE
The vagus nerve carries sensory afferent fibers from the larynx, trachea, esophagus, pharynx, and abdominal viscera. It also sends efferent motor fibers to the pharynx, tongue, thoracic and abdominal viscera, and the larynx. Testing of the vagus nerve is performed by the gag reflex and "ahh" test as described above.

A unilateral lesion affecting the vagus nerve can produce hoarseness and difficulty swallowing due to a loss of laryngeal function. Causes of unilateral lesions include trauma from surgical procedures of the neck, aortic aneurysm, and compression due to enlarged paratracheal lymph nodes caused by metastatic carcinoma.

ELEVENTH CRANIAL NERVE---ACCESSORY NERVE
The accessory nerve carries efferent motor fibers to innervate the sternomastoid and trapezius muscles. The accessory nerve is tested by asking the patient to shrug the shoulders (trapezius muscles) and turn the head (sternomastoid muscles) against resistance. Palpate the patient's sternocleidomastoid muscles and feel for tension as the patient attempts to turn his/her head.

Loss of CN 11 function causes a drooping of the ipsilateral shoulder and trapezius on the affected side. The patient may also have difficulty turning his head to the side opposite the lesion. Damage usually occurs secondary to surgery or trauma.

TWELFTH CRANIAL NERVE---HYPOGLOSSAL NERVE
The twelfth cranial nerve supplies efferent motor fibers to the muscles of the tongue. To test the hypoglossal nerve, have the patient stick out their tongue and move it side to side. If there is unilateral weakness, the protruded tongue will deviate towards the side of the weakness.

Further testing includes moving the tongue right to left against resistance, or having the patient say "la, la, la."

Summary of Cranial Nerve Functions

Table 1 summarizes the cranial nerves and their functions.

Cranial Nerve	Number	Innervation(s)	Primary Function(s)	Test(s)
Olfactory	I	Sensory	Smell	Identify odors
Optic	II	Sensory	Vision	Visual acuity, fields, color, nerve head
Oculomotor	III	Motor	Upper lid elevation, extraocular eye movement, pupil constriction, accommodation	Physiologic "H" and near point response
Trochlear	IV	Motor	Superior oblique muscle	Physiologic "H"
Trigeminal	V	Motor	Muscles of mastication	Corneal reflex
Trigeminal	V	Sensory	Scalp, conjunctiva, teeth	Clench jaw/palpate, light touch comparison
Abducens	VI	Motor	Lateral rectus muscle	Abduction, physiologic "H"
Facial	VII	Motor	Muscles of facial expression	Smile, puff cheeks, wrinkle forehead, pry open closed lids
Facial	VII	Sensory	Taste-anterior two thirds of tongue
Vestibulocochlear	VIII	Sensory	Hearing and balance	Rinne test for hearing, Weber test for balance
Glossopharyngeal	IX	Motor	Tongue and pharynx	Gag reflex
Glossopharyngeal	IX	Sensory	Taste-posterior one third of tongue
Vagus	X	Motor	Pharynx, tongue, larynx, thoracic and abdominal viscera	Gag reflex
Vagus	X	Sensory	Larynx, trachea, esophagus
Accessory	XI	Motor	Sternomastoid and trapezius muscles	Shrug, head turn against resistance
Hypoglossal	XII	Motor	Muscles of tongue	Tongue deviation

Table 1: Cranial Nerves Summary. From Muchnick, B.G.: Clinical Medicine in Optometric Practice, St. Louis, 1994, Mosby