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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.
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.
Assessing nerve function. First make sure that each nasal passage is open by compressing one side of the nose and asking the patient to breathe through the other. Then tell him to close both eyes, occlude one nostril, and identify the odor from a common and nonirritating odor placed under the opposite nostril, such as coffee, cloves, lemon, vanilla, or soap. Test the other nostril in the same manner. The patient doesn't have to identify the odor, but he should be able to perceive it. Don't test him with a noxious odor such as acetone, alcohol, or ammonia-these odors can irritate nasal mucosa.1
What can go wrong. Olfactory hallucinations are a common symptom of temporal epilepsy. Anosmia (loss of the sense of smell) is most commonly caused by head injury, but also can be caused by nasal obstruction or acute or chronic rhinitis. Patients who lose the sense of smell typically also lose their sense of taste. Anosmia, hyposmia (a decreased sensation of smell), or dysosmia (a distortion of smell) can be an early sign of a tumor on the floor of the anterior fossa.1
Teach a patient with olfactory nerve dysfunction to take extra precautions to protect himself from danger, because odors normally warn people about certain hazards. Advise him to have smoke detectors in his home and to test them yearly. Also tell him to label and date food, and to follow food-safety guidelines for storing and discarding food.2
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.
If your patient is confined to bed, use the Rosenbaum Pocket Vision Screener, which is adapted from the Snellen chart. Hold it 14 inches from the patient and ask him to read down from the largest letters to the smallest letters he can see. Normal vision in this instance is 14/14.3
If you don't have vision-testing charts, you can use any available print to screen the patient's visual acuity. If he can't read even the largest letters, test his ability to count your upraised fingers and to tell you when you shine a flashlight in his eye. Test each eye individually, having the patient cover the opposite eye.4
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.
Starting from the periphery, slowly move a pencil or finger into the patient's field of vision and ask him to indicate when he first sees the object. Check each quadrant (upper and lower) individually. If you note any defects, refer the patient to an ophthalmologist for further assessment.3
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.
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.
Assessing nerve function. To assess the oculomotor nerve, check the size and shape of the pupils, and compare them. Remember that up to 38% of healthy patients have anisocoria, or a difference of more than 0.4 mm in the diameter of one pupil compared with the other.4
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.
If the pupillary reaction is impaired or questionable, test the patient's near reaction in normal room light. Hold your finger or a pencil about 4 inches (10 cm) from the patient's eye. Ask him to look alternately at it and into the distance directly behind it. Watch for the pupil to constrict when he's focusing on the near object. Test one eye at a time.4
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.
Also look for normal conjugate movements of the eyes in each direction-they should move in the same direction, at the same speed, and remain in alignment. Watch for any deviation from normal and for lid lag (a rim of sclera visible above the iris as the patient looks down) as the eyes move from up to down.4
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.
Disorders associated with the abducens nerve include multiple sclerosis and elevated intracranial pressure. In these cases, the affected eye can't fully turn outward and may be turned inward. Diplopia results when the person looks in the same direction as the affected eye (for example, if the left eye is affected, diplopia results when the patient looks to the left).5
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.
One-and-a-half syndrome is a disorder of extraocular movements characterized by conjugate horizontal gaze palsy in one direction and INO in the other. The eye on the affected side remains fixed in the middle and the other eye can turn outward but not inward. Vertical eye movements aren't affected.6
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 test the corneal reflex, ask the patient to look up and away from you. (If he wears contact lenses, he should remove them before this test.) Approach him from the other side, out of his line of vision. Avoiding the eyelashes, touch the cornea (not just the conjunctiva) lightly with a fine wisp of cotton. The normal reaction is to blink, but patients who wear contact lenses may have a diminished reflex or no reflex.4
What can go wrong. Trigeminal neuralgia, once called tic douloureux, is the disorder most commonly associated with this cranial nerve. Characterized by severe intermittent facial pain, it usually occurs in adults who are middle-aged and older (although it can affect people of all ages) and is more common in women. The cause is unknown, but is thought to be a blood vessel pressing on the trigeminal nerve as it exits the brainstem. Trigeminal neuralgia also may be associated with disorders such as multiple sclerosis and other diseases that damage the myelin sheath that covers certain nerves.6
Pain in trigeminal neuralgia can occur spontaneously or be triggered by touching a particular spot (known as a trigger point) on the face, lips, or tongue. Certain activities, such as brushing the teeth or chewing, may also trigger an attack. The patient may feel repeated short, lightning-like bursts of excruciating stabbing pain in any part of the lower portion of the face, typically in the cheek next to the nose or in the jaw. (Trigeminal neuralgia sometimes is misdiagnosed as dental pain.) Usually one side of the face is affected. The pain can last up to 2 minutes and can recur hundreds of times a day. Episodes can last for days to months at a time, then not recur for months or years. But more typically, episodes worsen over time, with shorter pain-free intervals in between.6
Treatment for trigeminal neuralgia includes medications such as antiepileptic drugs and tricyclic antidepressants, surgery, and complementary therapies such as acupuncture, biofeedback, vitamin therapy, and electrical nerve stimulation. Opioids and other analgesics are usually ineffective. Surgery may be indicated for patients whose pain isn't relieved by medication, or who suffer intolerable adverse reactions such as excess fatigue.6
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.
Teach a patient with decreased facial sensation from trigeminal nerve dysfunction how to protect his face from injury. For example, he should shave with an electric shaver. Advise him to have regular dental check-ups.2 He should use sunblock when outdoors, and wear a face mask if he plays contact sports.
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.
The prognosis for most patients with Bell's palsy is good. Mild cases may not require treatment and symptoms may self-resolve in 2 weeks. Even without treatment, most patients start to recover within 2 weeks and recover fully in 3 to 6 months.6
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.
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.
A routine cranial nerve assessment rarely includes specific tests of the vestibular function of the acoustic nerve.4 However, if your patient complains of vertigo or other balance-related problems, refer him for further evaluation.
What can go wrong. Acoustic dysfunction most often is caused by basilar skull fractures or tumors, and can result in hearing loss or vestibular impairment. Age-related changes in older adults can lead to vestibular dysfunction and increased fall risk.7 Patients with signs and symptoms of these impairments should have a neuro-otologic evaluation. Use letter boards or sign language to communicate with patients with hearing loss. Give the patient and his family information on hearing aids and other assistive devices as appropriate.2
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.
If the patient has difficulty swallowing, consult a speech therapist for a bedside speech and swallow evaluation, and further testing as indicated. The results of these tests will determine the proper consistency of foods for him. If he still has swallowing difficulties, he'll need to be evaluated for enteral tube feeding or other intervention.2
A patient with speech difficulty can use other methods of communication, such as writing or communication boards. Refer him for speech therapy as appropriate.
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.
Assessing nerve function. Have the patient remove his shirt. Stand behind him and look for atrophy or fasciculations (fine, irregular movements) in the trapezius muscles. Compare the sides. Place your hands on his shoulders and ask him to raise or shrug both his shoulders against your hands. Assess the strength of this movement. To evaluate the sternocleidomastoid muscle, place your hand against the patient's cheek and ask him to turn his head against your hand. Repeat this on the other side. Note the force of the movement against your hand.1
What can go wrong. Stroke can affect the spinal accessory nerve, causing paresis or paralysis. Because of weakness and atrophy of the trapezius and sternocleidomastoid muscles, the patient will have difficulty turning his head or raising his shoulders on the affected side. Help him follow the exercise plan outlined by the physical therapist to strengthen these muscles and help to restore function.2
The hypoglossal nerve, located in the hypoglossal canal, controls tongue movements.
Assessing nerve function. Listen to the patient's articulation, which depends on the hypoglossal nerve as well as the vagus, facial, and trigeminal nerves. Ask the patient to stick out his tongue and inspect it for lateral deviation, atrophy, and fasciculations. Ask him to move his tongue from side to side, and note the symmetry of the movement.4
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.
Treatment depends on the underlying cause. A speech therapist can help the patient learn to speak more clearly. Monitor him closely during meals because of the aspiration risk. Also monitor his caloric intake and daily weight. If appropriate, refer him to a dietitian and speech therapist for strategies to improve nutrition.2
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.
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.
1. Fuller G. Neurological Examination Made Easy. 3rd ed. New York, NY: Churchill Livingstone; 2004. [Context Link]
2. Cox CL, Boswell GM, McGrath A, Reynolds T, Cole E. Cranial nerve damage. Emerg Nurse. 2004;12(2):14-21. [Context Link]
3. Hickey JV. The Clinical Practice of Neurological & Neurosurgical Nursing. 6th ed. Lippincott Williams & Wilkins; 2008. [Context Link]
4. Bickley L, ed. Bates' Guide to Physical Assessment. 10th ed. Lippincott Williams & Wilkins; 2008. [Context Link]
5. Merck Manual, second home edition. http://www.merck.com. [Context Link]
6. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov. [Context Link]
7. Agrawal Y, Carey JP, Della Santina CC, Schubert MC, Minor LB. Disorders of balance and vestibular function in U.S. adults. Arch Intern Med. 2009;169(10):938-944. [Context Link]
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