Cranial nerves examination

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There are 12 pairs of cranial nerves although the optic nerve is really an extension of the brain rather than a peripheral nerve. The ability to test them swiftly, efficiently and to interpret the findings should be a core competency for general practitioncer.



I). Cranial nerve I – Olfactory nerve:

1). Anatomy:

The olfactory nerves (CN I) have sensory fibers concerned with the special sense of smell. The olfactory receptor neurons are in the olfactory epithelium (olfactory mucosa) in the roof of the nasal cavity. The central processes of the olfactory receptor neurons ascend through foramina in the cribriform plate of the ethmoid to reach the olfactory bulbs in the anterior cranial fossa. These nerves synapse on neurons in the bulbs, and the processes of these neurons follow the olfactory tracts to the primary and associated areas of the cerebral cortex.

2). Examination:

Test the sense of smell by presenting the patient with familiar and nonirritating odors. First be sure that each nasal passage is open by compressing one side of the nose and asking the patient to sniff through the other. The patient should then close both eyes. Occlude one nostril and test smell in the other with such substances as cloves, coffee, soap, or vanilla. Ask if the patient smells anything and, if so, what. Test the other side. A person should normally perceive odor on each side, and can often identify it.

3). Olfactory disorder:

Hyperosmia is an increased ability to smell.

Hyposmia is a reduced ability to smell and to detect odours

Anosmia - loss of smell has many causes, including nasal disease, head trauma, smoking, aging, and the use of cocaine. It may be congenital.

Dysomia is the impairment of olfactory stimuli processing leading to an altered sense of smell.


II). Cranial nerve II – Optic nerve:

1). Anatomy:

The optic nerves (CN II) have sensory fibers concerned with the special sense of vision. The nerve fibers arise from ganglion cells in the retina. They exit the orbit via the optic canals; fibers from the nasal half of the retina cross to the contralateral side at the optic chiasm. The fibers then pass via the optic tracts to the geniculate bodies of the thalamus, where they synapse on neurons whose processes form the optic radiations to the primary visual cortex of the occipital lobe.

2). Examination:

Visual acuity:

To test the acuity of central vision use a Snellen eye chart, if possible, and light it well. Position the patient 20 feet from the chart. Patients who use glasses other than for reading should put them on. Ask the patient to cover one eye with a card (to prevent peeking through the fingers) and to read the smallest line of print possible. Coaxing to attempt the next line may improve performance. A patient who cannot read the largest letter should be positioned closer to the chart; note the intervening distance. Determine the smallest line of print from which the patient can identify more than half the letters. Record the visual acuity designated at the side of this line, along with use of glasses, if any. Visual acuity is expressed as two numbers (e.g., 20/30): the first indicates the distance of patient from chart, and the second, the distance at which a normal eye can read the line of letters.

Testing near vision with a special handheld card helps to identify the need for reading glasses or bifocals in patients over age 45. You can also use this card to test visual acuity at the bedside. Held 14 inches from the patient’s eyes, the card simulates a Snellen chart. You may, however, let patients choose their own distance.

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Visual field:

Screening starts in the temporal fields because most defects involve these areas. Imagine the patient’s visual fields projected onto a glass bowl that encircles the front of the patient’s head. Ask the patient to look with both eyes into your eyes. While you return the patient’s gaze, place your hands about 2 feet apart, lateral to the patient’s ears. Instruct the patient to point to your fingers as soon as they are seen. Then slowly move the wiggling fingers of both your hands along the imaginary bowl and toward the line of gaze until the patient identifies them. Repeat this pattern in the upper and lower temporal quadrants.

Normally, a person sees both sets of fingers at the same time. If so, fields areusually normal.

Further Testing. If you find a defect, try to establish its boundaries. Test one eye at a time. If you suspect a temporal defect in the left visual field, for example, ask the patient to cover the right eye and, with the left one, to look into your eye directly opposite. Then slowly move your wiggling fingers from the defective area toward the better vision, noting where the patient first responds. Repeat this at several levels to define the border.

A temporal defect in the visual field of one eye suggests a nasal defect in the other eye. To test this hypothesis, examine the other eye in a similar way, again moving from the anticipated defect toward the better vision.

Use of the ophthalmoscope

Many people find use of the ophthalmoscope daunting. It can only be mastered with practice and this means using it over the course of years. Do not bother clicking through the lenses until you are adept in its use but use the 0 dioptre lens to view the retina. It is very difficult to see much through a small pupil in a bright room. It should be possible to take the patient to a darker room or to dim the light with curtains or a blind. Better still, a mydriatic agent such as 1% homatropine eyedrops can be used, provided that the patient does not have to drive for the rest of the day. The effect can last up to 24 hours.

Take the ophthalmoscope and ask the patient to fix his gaze on something in the distance such as a picture on the wall and to ignore you. Do the pupils look equal? First shine the light on the eye and then remove it. The pupil should be brisk in its response of both constriction and dilatation. This is testing both the ophthalmic and oculomotor nerves as the response is dependent upon appreciation of light and the motor response of the muscles of the iris. There is also a consensual response in that the contralateral pupil will also respond but less markedly.

Then use the ophthalmoscope to examine the eye. First hold it away from the patient and look through it at the eye. There should be an orange reflex from light reflected from the retina. This means that the lens is clear and there is no significant cataract. Check that the optic disc is clear. Note the vessels of the retina and try to see the periphery. Repeat on the other side. It is not reasonable to assume that because one side is normal that the other side will be too.

Only by seeing many normal discs is it possible to be confident about abnormality. Try to learn:

The normal colour of a disc. It is usually pink but in people with dark skin it appears grey. People with blond hair tend to have paler retinae than those with darker hair. Hair may go grey with the passage of time or blond or coloured with the contents of a bottle but the retina is constant.

A clear and distinct optic disc. Only by being familiar with a normal disc is it possible to recognisepapilloedema when it occurs. Cupping of the disc, as with glaucoma, brings the vessels up over the disc.

Learn to follow the 4 arteries and veins. Follow then to the periphery. Note tortuosity. Note any silver wiring or A-V nipping. The appearance of central retinal artery thrombosis and retinal vein thrombosis are described in the respective articles. A detached retina is avascular and may appear as a crescent or a grey cloud.

Only by learning the normal appearance of a retina will such abnormalities as neovascularisation, cotton wool spots and haemorrhages be recognised.

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III). Cranial Nerves III, IV, and VI—Oculomotor, Trochlear, and Abducens.
1). Anatomy:

The oculomotor nerves (CN III) send somatic motor fibers to all extraocular muscles, except the superior oblique and lateral rectus. They also send presynaptic parasympathetic fibers to the ciliary ganglion for innervation of the ciliary body and sphincter pupillae. These nerves originate from the brainstem, emerging medial to the cerebral peduncles, and run in the lateral wall of the cavernous sinus. They enter the orbit through the superior orbital fissures and divide into superior and inferior branches.

The trochlear nerves (CN IV) supply somatic motor fibers to the superior oblique muscles and send proprioceptive fibers to this muscle, which abducts, depresses, and medially rotates the eyeball. The nerves emerge from the posterior aspect of the brainstem. They run a long intracranial course, passing around the brainstem to enter the dura mater in the free edge of the cerebellar tentorium close to the posterior clinoid process. They then run in the lateral wall of the cavernous sinus, entering the orbit via the superior orbital fissures.

The abducens nerves (CN VI) supply somatic motor fibers to the lateral rectus muscles of the eyeballs and proprioceptive fibers to these muscles. The nerves originate from the pons, pierce the dura on the clivis, traverse the cavernous sinus and superior orbital fissures, and enter the orbits.

2). Examination:

Test the extraocular movements in the six cardinal directions of gaze, and look for loss of conjugate movements in any of the six directions. Check convergence of the eyes. Identify any nystagmus, noting the direction of gaze in which it appears, the plane in which movements occur (horizontal, vertical, rotary, or mixed), and the direction of the quick and slow components.

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IV). Cranial Nerve V—Trigeminal
1). Anatomy:

The trigeminal nerves (CN V) supply somatic motor fibers to the muscles of mastication, mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini muscles. They also distribute postsynaptic parasympathetic fibers of the head to their destinations. CN V is sensory to the dura of the anterior and middle cranial fossae, skin of the face, teeth, gingiva, mucous membrane of the nasal cavity, paranasal sinuses, and mouth. CN V originates from the lateral surface of the pons by two roots: motor and sensory. These roots cross the medial part of the crest of the petrous part of the temporal bone. They enter the trigeminal cave of the dura lateral to the body of the sphenoid and cavernous sinus. The sensory root leads to the trigeminal ganglion; the motor root runs parallel to the sensory root, then bypasses the ganglion and becomes part of the mandibular nerve (CN V3).

2). Examination

Motor. While palpating the temporal and masseter muscles in turn, ask the patient to clench his or her teeth. Note the strength of muscle contraction.

Sensory. After explaining what you plan to do, test the forehead, cheeks, and jaw on each side for pain sensation. Suggested areas are indicated by the circles. The patient’s eyes should be closed. Use a safety pin or other suitable sharp object,* occasionally substituting the blunt end for the point as a stimulus. Ask the patient to report whether it is “sharp” or “dull” and to compare sides.

If you find an abnormality, confirm it by testing temperature sensation. Two test tubes, filled with hot and ice-cold water, are the traditional stimuli. A tuning fork may also be used. It usually feels cool. If you are near running water, the fork is easily made colder or warm. Dry it before use. Touch the skin and ask the patient to identify “hot” or “cold.”

Then test for light touch, using a fine wisp of cotton. Ask the patient to respond whenever you touch the skin.

Test the corneal reflex. Ask the patient to look up and away from you. Approaching from the other side, out of the patient’s line of vision, and avoiding the eyelashes, touch the cornea (not just the conjunctiva) lightly with a fine wisp of cotton. If the patient is apprehensive, however, first touching the conjunctiva may allay fear.

Look for blinking of the eyes, the normal reaction to this stimulus. (The sensory limb of this reflex is carried in CN V, the motor response in CN VII.) Use of contact lenses frequently diminishes or abolishes this reflex.

V). Cranial Nerve VII—Facial.

Inspect the face, both at rest and during conversation with the patient. Note any asymmetry (e.g., of the nasolabial folds), and observe any tics or other abnormal movements.

Ask the patient to:
1. Raise both eyebrows.
2. Frown.
3. Close both eyes tightly so that you cannot open them. Test muscular strength by trying to open them, as illustrated.
4. Show both upper and lower teeth.
5. Smile.
6. Puff out both cheeks.

Note any weakness or asymmetry.

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Cranial Nerve VIII—Acoustic.

Assess hearing. If hearing loss is present, (1) test for lateralization, and (2) compare air and bone conduction.

Specific tests of vestibular function are seldom included in the usual neurologic examination. Consult textbooks of neurology or otolaryngology as the need arises.

Cranial Nerves IX and X—Glossopharyngeal and Vagus.

Listen to the patient’s voice. Is it hoarse or does it have a nasal quality? Is there difficulty in swallowing?

Ask the patient to say “ah” or to yawn as you watch the movements of the soft palate and the 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. The slightly curved uvula seen occasionally in a normal person should not be mistaken for a uvula deviated by a 10th nerve lesion.

Warn the patient that you are going to test the gag reflex. Stimulate the back of the throat lightly on each side in turn and note the gag reflex. It may be symmetrically diminished or absent in some normal people.

Cranial Nerve XI—Spinal Accessory.

From behind, look for atrophy or fasciculations in the trapezius muscles, and compare one side with the other. Ask the patient to shrug both shoulders upward against your hands. Note the strength and contraction of the trapezii.

Ask the patient to turn his or her head to each side against your hand. Observe the contraction of the opposite sternomastoid and note the force of the movement against your hand.

Cranial Nerve XII—Hypoglossal.

Listen to the articulation of the patient’s words. This depends on Cranial Nerves V, VII, and X as well as XII. Inspect the patient’s tongue as it lies on the floor of the mouth. Look for any atrophy or fasciculations (fine, flickering, irregular movements in small groups of muscle fibers). Some coarser restless movements are often seen in a normal tongue. Then, with the patient’s tongue protruded, look for asymmetry, atrophy, or deviation from the midline. Ask the patient to move the tongue from side to side, and note the symmetry of the movement. In ambiguous cases, ask the patient to push the tongue against the inside of each cheek in turn as you palpate externally for strength.

Sources: Clinically Oriented Anatomy
Bate's guide to physical examination