ASSESSING FOR CRANIAL NERVE disease can be challenging, requiring an excellent grasp of both anatomy and potential disease processes. In this article, I'll review the cranial nerves, how to assess them, and how to promptly recognize common disorders and diseases that can affect their function.

Belonging to the peripheral nervous system, the 12 pairs of cranial nerves are identified by names and Roman numerals. The numbers are assigned in the order that each pair of nerves attach to the central nervous system (for details, see Getting on the cranial nerves). Some of the cranial nerves have general motor or sensory functions; others have specialized functions such as vision, smell, or hearing. See Quick guide to the cranial nerves for a helpful mnemonic.

Cranial nerve I-Olfactory

Located in the nose, the olfactory nerve controls the sense of smell. You won't routinely perform a detailed assessment of this nerve, but you should do so if a patient has sustained a traumatic brain or facial injury.

The optic nerve is found in the retina and controls vision.

Assessing nerve function. To test your patient's visual acuity, use a Snellen eye chart, if possible. Make sure it's well lighted and positioned 20 feet from the patient. If he normally wears contact lenses or glasses (other than for reading), he should wear them during the test. Ask him to cover one eye with a card (to prevent him peeking through his fingers) and to read the smallest line of print possible. You may be able to improve his performance by coaxing him to attempt the next-smallest line. If he can't read even the largest letter, have him move closer to the chart and document the distance.

Determine the smallest line of print from which the patient can identify more than half the letters. Document the visual acuity marked at the side of this line and note whether the patient was wearing corrective lenses. Visual acuity is expressed as two numbers (for example, 20/20) with the top number representing the patient's distance from the chart and the bottom number representing the distance at which a normal eye can read the line of letters.

Next, test the patient's visual field, or the area of vision normally seen with one eye. Each visual field extends 60 degrees on the nasal side, 100 degrees on the temporal side, and 135 degrees vertically.

Use the confrontation test to estimate the range of each visual field and identify gross defects. Sit 2 feet in front of the patient at eye level. Ask him to cover one eye lightly and look at your eye directly opposite (for example, tell him to look at your right eye with his left eye). You'll use your visual field as the norm, so close your opposite eye to isolate the visual field in the eye being used for the test.

What can go wrong. Diseases associated with the optic nerve include tumors and vascular abnormalities in or around the nerve and the optic chiasma. Lesions found in the frontal, temporal, parietal, and occipital lobes will cause optic nerve dysfunction. Other diseases such as cerebral aneurysms, migraines, and multiple sclerosis can affect optic nerve function. Patients with optic nerve dysfunction have decreased visual acuity, visual field defects, or both.

Your role is to perform an assessment and note any obvious abnormalities. A patient with visual defects needs further evaluation by an ophthalmologist or other eye care professional. If the patient has significant visual impairment, refer him to community resources specializing in help for the visually impaired.

A patient with decreased visual acuity needs help adjusting to his environment. When you enter his room, introduce yourself and explain what will be happening. Tell him to call for help before getting out of bed, so he won't fall or injure himself. Provide assistive devices such as glasses, large print books, and magnifying glasses, and make sure the room is well lighted.

Teach a patient with a visual field deficit to compensate for the diminished visual field by turning his head toward the "blind" direction. This deficit can present problems with certain daily activities, such as driving, reading, and playing sports. Warn him not to drive until he's had a driving evaluation. Assess his limitations and direct him to individualized training and education.

Cranial nerves III, IV, and VI-Oculomotor, trochlear, and abducens

These three cranial nerves, located in the superior orbital fissure, work together to control extraocular movements. The oculomotor nerve controls pupillary reactions, elevation of the upper eyelid, and most extraocular movements. The trochlear nerve controls downward, internal rotation of the eye, the abducens nerve controls lateral deviation of the eye.

To test pupillary reaction to light, darken the room and ask the patient to look into the distance. Then shine a bright light (such as from a pen light) obliquely into each pupil in turn. Both pupils should constrict-a direct reaction in the eye in which you shone the light, and a consensual reaction in the opposite eye. Always darken the room and use a bright light before concluding that the patient's pupillary reaction to light is absent.

Normally, about one-third of the iris is covered by the upper eyelid. If more of the iris is covered, the patient may have ptosis (an inability to raise the upper eyelid) secondary to oculomotor nerve palsy, Horner's syndrome, or myasthenia gravis.

Assess extraocular movement by telling the patient to follow your finger or pencil without moving his head. With your hand about 18 inches from his eyes, move your finger in a wide H shape, through the six cardinal directions of gaze. Lead your patient's gaze (1) to his extreme right, (2) to the right and upward, and (3) down on the right. Without pausing in the middle, move your finger (4) to the extreme left, (5) to the left and upward, and (6) down on the left. Pause during upward and lateral gaze to detect nystagmus, a fine rhythmic oscillation of the eyes.

What can go wrong. Diseases associated with dysfunction of these three cranial nerves include lesions in the brainstem, myasthenia gravis, and Guillain-Barre syndrome. Causes of pupillary defects include trauma or oculomotor nerve paralysis or palsy. Complete oculomotor nerve palsy results in a dilated, nonreactive pupil.

If the patient has ptosis, the healthcare provider may patch the affected eye and prescribe an eye lubricant, because loss of the ability to blink means the eye isn't lubricated normally. Surgery may be needed to correct permanent or severe ptosis.

Trochlear nerve palsy usually is caused by a head injury, and rarely, by a tumor. The affected eye can't turn inward and downward, resulting in vertical diplopia. The patient tends subconsciously to tilt his head and use eye muscles that aren't affected by the palsy. This position can eliminate the double vision.

Internuclear ophthalmoplegia (INO) is a weakness or paralysis of horizontal eye movements caused by damage to the nerve fibers connecting collections of nerve cells that give rise to the oculomotor and abducens nerves. Horizontal eye movements are impaired, but vertical ones aren't. The eye on the affected side can't turn inward when looking to the opposite side but can turn outward. Nystagmus occurs in the eye on the opposite side when it turns outward. Causes of INO include stroke, tumors, and multiple sclerosis.

The trigeminal nerve controls the muscles of mastication and the three sensory divisions of the face: ophthalmic, maxillary, and mandibular. The sensory limb of this nerve also is responsible for the corneal reflex, along with the facial nerve (cranial nerve VII), which controls the motor response. Because the trigeminal nerve has motor and sensory fibers, you must assess both aspects.

Assessing nerve function. To assess this nerve's motor function, ask the patient to clench his teeth while you palpate the temporal muscles, followed by the masseter muscles. Note the strength of muscle contraction. Ask the patient to move his jaw side to side. If he has difficulty clenching the jaw or moving it to the opposite side, he has masseter and lateral pterygoid muscle weakness.

To assess the sensory function of light touch, tell the patient to close his eyes, then use a wisp of cotton to gently touch various parts of his face on each side. Have him tell you when he feels the cotton on his skin.

To minimize the risk of an attack, advise the patient to use a soft toothbrush and mild mouthwashes. Recommend that he eat easily chewed or pureed food served at room temperature. Refer him for a nutritional assessment, so he can learn which foods to eat to provide enough calories. Advise him to avoid environmental triggers such as wind and cool air.

Remind patients with trigeminal sensory nerve dysfunction to check the affected eye frequently for foreign bodies and to protect the eye by wearing glasses in windy weather or in dusty places.

The facial nerve is located in the internal acoustic meatus, facial canal, and stylomastoid foramen. This nerve has motor and sensory functions, controlling facial movements and innervating taste fibers in the anterior two-thirds of the tongue, as well as innervating sensation in the external ear.

Assessing nerve function. The tongue can distinguish four basic tastes: sweet (tip of tongue), sour (sides of tongue), salty (over most of tongue but concentrated on the sides), and bitter (back of the tongue, controlled by the glossopharyngeal nerve, cranial nerve IX). Although not routine, you can assess taste by having the patient stick out his tongue and testing for each taste. Place sugar on the tip of his tongue and ask him to identify the taste. Then give him a sip of water and test with sour and salty substances on the appropriate area of the tongue. (If you test bitter on the posterior third of the tongue, remember that this portion of the tongue isn't controlled by the facial nerve.)

Test the facial nerve's motor function by asking the patient to raise both eyebrows, frown, close his eyes tightly, show both upper and lower teeth, smile, and puff out both cheeks. Observe for asymmetry, weakness, or abnormal movements.

What can go wrong. Facial nerve dysfunction can occur with basilar skull fractures, strokes, and tumors in or around the brainstem and cerebellopontine angle. This dysfunction can range from mild facial weakness to complete facial paralysis. Some patients also lose the sense of taste in the anterior two-thirds of the tongue. Rehabilitation staff can recommend exercises to strengthen the weak or paralyzed side of the face. Teach patients with facial nerve palsy to maintain oral hygiene and to eat bite-sized pieces of food chewing slowly and thoroughly.

The most common facial nerve dysfunction is Bell's palsy. The cause is unknown, but may involve facial nerve inflammation associated with an immune disorder or a viral infection. Pain behind the ear is usually the first symptom, developing over several hours or a day or two before the facial muscles weaken. This facial weakness, which ranges from mild weakness to complete paralysis, can occur suddenly and reaches its maximum within 48 hours. The affected side of the face becomes expressionless, and the patient can't close his eye.

More serious cases of Bell's palsy can be treated with acyclovir and anti-inflammatory drugs such as prednisone. Teach the patient to talk to his healthcare provider before taking over-the-counter analgesics such as aspirin, acetaminophen, or ibuprofen to relieve pain.

Before steroid therapy starts, obtain the patient's baseline BP; if he has diabetes, also obtain a baseline fasting blood glucose level because possible adverse reactions to corticosteroids include hypertension and hyperglycemia. Patients with hypertension who are taking antihypertensive medications must have their BP checked daily and notify their healthcare provider if it becomes markedly elevated (typically 140/90 mm Hg or greater). Patients with diabetes should monitor their blood glucose at least twice daily until they complete the course of steroids.

Cranial nerve VIII-Acoustic

The acoustic nerve (also known as the vestibulocochlear nerve) is located in the internal auditory meatus and controls hearing (cochlear division) and balance (vestibular division).

Assessing nerve function. To evaluate the patient's hearing, test one ear at a time. Ask the patient to occlude one ear with a finger. Stand 1 to 2 feet away and cover your mouth or ask the patient to close his eyes (so he doesn't read your lips). Exhale fully to minimize the intensity of your voice, and whisper softly toward the patient's unoccluded ear. Choose numbers or other words with two equally accented syllables, such "nine-four" or "baseball." If necessary, increase the intensity of your voice to a medium whisper, a loud whisper, and then a soft, medium, and loud voice. If the patient's hearing is diminished, he may need additional testing to determine if the loss is conductive or sensorineural.

Located in the jugular foramen, these nerves control swallowing, the gag reflex, articulation, and phonation. The glossopharyngeal nerve also is responsible for taste in the posterior third of the tongue. Because these nerves innervate many of the same structures, you can test them together.

Assessing nerve function. Listen to the patient's voice. Is it hoarse (indicating vocal cord paralysis), or does it have a nasal quality (indicating paralysis of the palate)? Does he have difficulty swallowing, indicating weakness of the pharynx or palate? Have him open his mouth and say "ah," or ask him to yawn as you watch the movements of the soft palate and pharynx. The soft palate normally rises symmetrically, the uvula remains in the midline, and each side of the posterior pharynx moves medially, like a curtain. Observe for any asymmetry or deviation of the uvula.

Next, tell the patient you're going to evaluate his gag reflex. Ask him to open his mouth while you use the tip of a long cotton-tipped applicator to very gently stimulate the back of the pharynx on each side. Perform this maneuver very gently so he doesn't vomit.

Evaluate swallowing by observing the patient for dysphagia while he drinks a clear fluid. (Don't do this test with a patient with an impaired gag reflex-he could aspirate.)

What can go wrong. Brainstem tumors are a common cause of dysfunction of these two cranial nerves. The patient may have decreased taste sensation in the posterior third of the tongue, poor speech quality, hoarseness, diminished or absent gag reflex, and dysphagia. Remember that a patient with poor speech quality secondary to cranial nerve damage is more susceptible to aspiration.

A patient with speech difficulty can use other methods of communication, such as writing or communication boards. Refer him for speech therapy as appropriate.

Cranial nerve XI-Spinal accessory

The spinal accessory nerve, which controls neck (sternomastoid) and shoulder (upper portion of the trapezius) movements, enters the foramen magnum and exits by the jugular foramen.

The hypoglossal nerve, located in the hypoglossal canal, controls tongue movements.

What can go wrong. Hypoglossal nerve function can be impaired by tumors at the base of the skull, stroke, amyotrophic lateral sclerosis, brainstem infections, or an injury to the neck (for example, during carotid artery surgery). The tongue becomes weak on the affected side and eventually atrophies, causing difficulty speaking, chewing, and swallowing.

By understanding how to test the 12 cranial nerves, you can identify potential cranial nerve dysfunction promptly and help your patient get the treatment he needs.

Quick guide to the cranial nerves

The familiar mnemonic for the 12 cranial nerves, On old Olympus towering tops, a Finn and German viewed some hops, breaks down as follows:

* I- Olfactory

* II- Optic

* III- Oculomotor

* IV- Trochlear

* V- Trigeminal

* VI- Abducens

* VII- Facial

* VIII- Acoustic

* IX- Glossopharyngeal

* X- Vagus

* XI- Spinal accessory

* XII- Hypoglossal.

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