CHAPTER ONE part two

The facial nerve, cranial nerve VII, innervates all the muscles of facial expression, ie, the muscles around the eyes, mouth, nose, ears and neck. It also innervates the stapedius muscle in the ear, which dampens excessive movement of the ossicles when subject to loud sounds. The facial nerve subserves taste to the anterior two thirds of the tongue and sensation to the outer ear.

The motor nucleus of VII sits in the pons while its axons loop around the nucleus of the abducens nerve and emerges from the pontomedullary junction. The facial nerve then courses through the internal auditory meatus where it is joined by the auditory nerve, and enters the facial canal of the temporal bone wherein lies the geniculate ganglion. Distal to the geniculate ganglion, the facial nerve gives off the chorda tympani, which supplies taste to the anterior two thirds of the tongue via the lingual nerve. The facial nerve exits the facial canal through the stylomastoid foramen, passing through the parotid gland, before innervating the muscles of the face, the posterior belly of the digastric, the stylohyoid, the buccinator, and the platysma. A branch of the facial nerve runs in the facial canal to innervate the stapedius muscle.

The sensory component of the facial nerve consists of the lingual nerve, which innervates the anterior two thirds of the tongue and sensory branches, which in turn innervate the external auditory meatus.

The facial nerve also mediates parasympathetic innervation to the lacrimal, sublingual and submaxillary glands as well as the vessels of the mucous membranes of the palate, nasopharynx and nasal cavity.

To test the facial nerve, first observe the patient's face for symmetry by paying close attention to the nasolabial folds, forehead wrinkles, spontaneous smiling and blinking. Then, ask the patient to show his teeth, raise his eyebrows, squeeze his eyes shut tightly and hold air in his cheeks. Facial weakness may be due to upper motor neuron or lower motor neuron facial palsy. Upper motor neuron palsy implies that there is a lesion contralateral to the side of facial weakness which is disrupting the face motor fibers somewhere in its course from the primary motor cortex to the facial nucleus within the pons (ie, upper motor neuron to the facial nerve nucleus, Figure 2-12). A typical presentation of an upper motor neuron palsy is a patient with a right subcortical lacunar infarct resulting in flattened left nasolabial fold, decreased up turning of the left corner of the mouth on smiling, and symmetric wrinkling of forehead bilaterally, in addition to a left hemiparesis. Lower motor neuron palsy implies a lesion involving the facial nerve at the nucleus in the pons or along the course of the facial nerve ipsilateral to the side of facial weakness (Figure 2-13). Bell's palsy is a lower motor neuron facial palsy whereby the patient has unilateral flattening of the nasolabial fold with inability to upturn the corner of the mouth upon smiling, inability to wrinkle his forehead, delayed or absent blinking due to weakness of the eyelid, and inability to hold air in the cheeks due to escape of air through the corner of the mouth which is weak. In addition, patients with Bell's palsy may complain of dry eye from disruption of parasympathetic innervation of the lacrimal gland, hyperacusis or augmented hearing in the ear ipsilateral to the lesion from paralysis of the stapedius muscle and diminished taste from a lesion proximal to the lingual nerve, which inhibits afferent signals concerning taste from reaching the brainstem. Taste should be evaluated in any patient suspected of a lower motor neuron facial palsy by applying a wet cotton applicator dipped in sugar or salt on the patient's protruded tongue and asking the patient to identify the substance. (Do not allow the patient to stick his tongue back into his mouth, as unilateral taste can then not be accurately tested.) Be sure to look at the external auditory meatus where vesicles from herpes zoster may erupt from infection of the sensory branches of the peripheral facial nerve.

In summary upper motor neuron facial weakness spares the frontalis (forehead muscle) so the patient can wrinkle his brow. Lower motor neuron facial weakness involves the forehead muscle and the patient can’t wrinkle the brow and in addition has unilateral hyperacusis and loss of taste.

Facial diplegia, or bilateral lower motor neuron facial weakness, is seen in such conditions as Guillain-Barré syndrome or sarcoidosis.

Cranial Nerve VIII - Acoustic Nerve

The auditory nerve, cranial nerve VIII, is composed of two divisions, the cochlear nerve, which subserves hearing, and the vestibular nerve, which provides sense of balance. The cochlear nerve carries fibers from the spiral ganglion of the cochlea in the petrous portion of the temporal bone, through the internal auditory meatus, and to the cochlear nucleus, which sits in the lower pons, near the cerebellopontine angle. Each cochlear nucleus has connections to bilateral primary auditory cortices in the temporal lobes. Lesions of the cochlear nerve commonly present with ipsilateral decreased hearing and sometimes tinnitus.

The vestibular nerve is composed of nerve fibers from the labyrinth of the inner ear, which converge on the vestibular ganglion within the internal auditory meatus and travel alongside the cochlear nerve to terminate on the vestibular nuclei within the lower pons. The vestibular nuclei have connections to:

the vestibulospinal tract of the spinal cord which allows reflex movements of the limbs in response to stimulation of the inner ear vestibular fibers
the medial longitudinal fasciculus which courses between the pons and midbrain and allows conjugate eye movements in relationship to head movements
the cerebellum that regulates muscle tone in relation to changes in posture. Acute vestibular nerve disease commonly presents with vertigo, nausea and ataxia.

To test the auditory nerve, first check gross hearing in each ear by rubbing your fingers about 30 inches from the patient's ear, with the contralateral ear covered. If hearing in one ear is impaired, perform Rinne and Weber tests. Both tests employ the use of a 256 Hz tuning fork. In the Rinne test, the vibrating tuning fork is placed over the mastoid process, behind the ear to test bone conduction (BC). Ask the patient to tell you when he no longer hears the vibrating fork, after which the tuning fork is placed in front of the ear and the patient asked if he can hear it (air conduction = AC). Next perform the Weber test by placing a vibrating tuning fork over the middle of the forehead and ask the patient if the sound is louder in one ear compared to the other. With conductive hearing loss, from middle ear disease or obstruction of the external auditory meatus with wax, BC will be greater than AC and Weber test will lateralize to the deaf ear. However, with sensorineural hearing loss AC is better than BC and Weber test will lateralize to the good ear. Sensorineural hearing loss may result from lesions of:

the cochlea secondary to otosclerosis, Meniere's disease and noise- or drug-induced hearing loss
the auditory nerve from cerebellopontine angle tumors such as acoustic neuroma or trauma
the cochlear nerve in the pons from stroke or demyelinating disease.

Vestibular nerve function can be tested with calorics or postural maneuvers. In patients suspected of vestibular nerve damage such as vestibular neuronitis or lesions of the inner ear, as seen with Meniere's disease, about 250 cc of cold water is injected into one ear with the patient lying down in a bed angled at 30 degrees at the head. The patient is asked to look straight ahead and the eyes observed for nystagmus. In the normal patient, nausea, nystagmus with the fast phase away from the stimulated ear, past pointing to the contralateral side, and falling to the injected side will ensue. With complete peripheral vestibular nerve lesions, no symptoms will be elicited. With partial peripheral vestibular nerve lesions, nystagmus will appear but at reduced amplitudes and velocity.

In patients suspected of benign positional vertigo, presenting with vertigo or dizziness associated with changes in head position, the Hallpike maneuver should be attempted when not contraindicated due to severe cervical spine disease. To perform the Hallpike maneuver, the patient sits up in bed and then quickly lies back on command so that his head hangs over the edge of the bed. The head is tilted backward below the plane of his body and turned to one side by the examiner who holds the patient's head in his hands. The patient is asked to look in the direction that his head is turned (Figure 2-14). Watch for nystagmus in the direction of gaze and ask the patient if he feels vertigo. If no nystagmus is observed after 15 seconds, have the patient sit up and repeat the maneuver turning the patients head and directing his gaze in the contralateral direction. The absence of nystagmus suggests normal vestibular nerve function. However, with peripheral vestibular nerve dysfunction, such as benign positional vertigo, the patient will complain of vertigo and rotary nystagmus will appear after a 1-5 second latency toward the direction in which the eyes are deviated. With repeated maneuvers, the nystagmus and sensation of vertigo will fatigue, and disappear, a sign of peripheral vestibular disease, in contrast to central vestibular disease from stroke or other intrinsic brainstem lesions, which manifests as nonfatigable nystagmus without delay in onset.

Cranial Nerves IX and X - The Glossopharyngeal and Vagus Nerves

The glossopharyngeal nerve (cranial nerve IX) contains sensory and motor fibers as well as autonomic innervation to the parotid glands. It mediates taste to the posterior one third of the tongue and sensation to the pharynx and middle ear. These sensory fibers end in the nucleus solitarius of the medulla and connect with the superior salivary nucleus to allow the reflex of salivation. Nerves to the parotid gland originate in the inferior salivary nucleus, from which fibers travel through the middle ear to the lesser petrosal nerve and then on to the otic ganglion, which sends parasympathetic nerves to the parotid gland. The motor nucleus of the glossopharyngeal nerve originates in the medulla and innervates the stylopharyngeus muscle.

Like the glossopharyngeal nerve, the vagus nerve (cranial nerve X) contains sensory, motor and autonomic fibers. It mediates sensation in the tympanic membrane, external auditory canal, and external ear (as do cranial nerves V, VII and IX) via the jugular ganglion and travels centrally into the spinal tract of the fifth nerve. Visceral sensations from the pharynx, larynx, bronchi, esophagus and the abdomen are carried by the vagus nerve to the tractus solitarius of the medulla. Motor innervation to the muscles of the soft palate, pharynx and larynx originates in the nucleus ambiguus of the medulla. Autonomic fibers arise from the dorsal motor nucleus of vagus and synapse at peripheral ganglia to provide parasympathetic innervation to the trachea, esophagus, heart, stomach, and small intestine.

To test glossopharyngeal and vagus nerve function, examine the position of the uvula and its movement by asking the patient to say "Ah." the soft palate should elevate symmetrically and the uvula should remain in the midline. The gag reflex can be tested by touching the pharyngeal wall on each side with a cotton tip applicator. This reflex relies on an intact sensory arc, as mediated by sensory fibers of the glossopharyngeal nerve to the soft palate, and an intact motor arc, as mediated by the motor fibers of the vagus nerve to the soft palate and pharynx. Deviation of the uvula to one side implies a lower motor lesion of the vagus nerve contralateral to the side the uvula is deviating to (Figure 2-15). An upper motor neuron vagus nerve lesion will present with the uvula deviating toward the side of the lesion. The presence of a gag reflex does not necessarily imply that the patient can swallow without aspiration after a stroke. Impairment of swallowing is usually due to bilateral vagus nerve lesions. On the other hand, the absence of a gag reflex does not imply inability to swallow. Dysarthria and dysphagia, when associated with emotional lability, is suggestive of pseudobulbar palsy, a condition characterized by weakness of muscles innervated by the medulla (palate, pharynx, and larynx) because of interruption of corticobulbar fibers, as may be seen with multiple bilateral strokes. Hoarseness may be seen with tumors encroaching on the recurrent laryngeal nerve, a branch of the vagus nerve. This results in unilateral vocal cord paralysis.

Cranial Nerve XI - The Accessory Nerve

The spinal accessory nerve, cranial nerve XI, innervates the sternocleidomastoid and trapezius muscles. It is composed of spinal fibers originating in the anterior horn cells of the first five cervical cord segments and an accessory component, which travels briefly alongside the vagus nerve. The dorsal and ventral roots from the first five cervical cord segments unite to enter the skull through the foramen magnum and exit through the jugular foramen.

To test the strength of the sternocleidomastoids ask the patient to turn his head against your hand, which is placed over the mandible. Repeat this maneuver with your hand on the contralateral mandible. Observe the sternocleidomastoid, which is contralateral to the side to which the patient is turning his head. Weakness detected when the patient turns his head to the left implies that the right sternocleidomastoid is weak. To test the trapezius, ask the patient to shrug his shoulders and press down on the shoulders. Trapezius weakness is manifest as difficulty in elevating the shoulders. When the sternocleidomastoid and trapezius are weak on the same side, an ipsilateral peripheral accessory palsy, involving cranial nerves X and XI, is implied as may be seen with a jugular foramen tumor, ie, glomus tumor or neurofibroma. Because the cerebral hemisphere innervates the contralateral trapezius and ipsilateral sternocleidomastoid, a large right hemisphere stroke will result in weakness of the left trapezius and right sternocleidomastoid. Bilateral wasting of the sternocleidomastoid may be seen with myopathic conditions such as myotonic dystrophy and polymyositis or motor neuron disease, the latter usually associated with fasciculations.

Cranial Nerve XII - Hypoglossal Nerve

The hypoglossal nerve, cranial nerve XII, is a pure motor nerve, innervating the muscles of the tongue. It obtains supranuclear innervation from the contralateral motor cortex. The nucleus of the hypoglossal nerve sits in the medial aspect of the medulla, near the floor of the fourth ventricle and exits the skull through the hypoglossal canal.

To test the function of the hypoglossal nerve, ask the patient to protrude his tongue and wiggle it from side to side. Look for deviation and atrophy. To check for subtle weakness, ask the patient to push his tongue against the wall of his cheek while you push against it through the outer cheek. Like the forehead, each side of the tongue receives upper motor neuron innervation from bilateral motor cortices. Each half of the tongue pushes the tongue in the contralateral direction, ie, left half of tongue pushes to the right (Figure 2-16). Thus, if the tongue deviates to one side, it is pointing to the side that is weak. Tongue deviation, combined with wasting on the side to which it is deviated, implies a unilateral, lower motor neuron, hypoglossal nucleus or nerve lesion as may be seen with syringobulbia (a degenerative cavity within the brainstem), with basilar meningitis, or foramen magnum tumor. If the tongue deviates and is of normal bulk, one should consider an upper motor neuron lesion, such as stroke or tumor in the hemisphere contralateral to the side of deviation, and look for associated hemiparesis on the side of tongue deviation.

The Mental Status Examination
As previously noted, the neurologic exam begins with an assessment of the patient's mental status. In most cases, a large part of the mental status exam may be ascertained from observation of the patient as history is provided. A more detailed mental status exam can be divided into the following components:

Level of consciousness
Intellectual performance
Thought processes
Psychomotor function or Praxis
Psychosensory function or Gnosia
Language

Level of Consciousness
Level of consciousness implies awareness of surroundings. If one is examining a patient who is somnolent or comatose, it is important to determine the degree of stimulation that is required to alert the patient, ie, voice, light touch, sternal rub. Consciousness is dependent on the normal functioning of the reticular activating system, which originates in the pons and projects to the cortex of bilateral hemispheres via the thalamus. The reticular activating system activates the cortex when one awakens and inhibits the cortex when in sleep. The hypothalamus is also important in maintaining level of alertness. Structural damage to the reticular activating system, thalamus or hypothalamus from stroke or hemorrhage may impair consciousness to the same degree as large destructive lesions of bilateral cerebral hemispheres. During brain herniation, compression of the reticular activating system may produce profound coma. Similarly, metabolic abnormalities such as hyperglycemia or drugs may produce coma by impairing neuronal function diffusely within the brain.

Evaluation of a comatose patient requires examination of four systems: the motor system, pupils and fundi, ocular movements, and respiratory pattern. The main task is to determine whether the etiology of coma is due to metabolic-toxic causes or structural damage. Metabolic-toxic causes should have nonfocal exams while structural injury to the cerebral hemispheres or brainstem will result in focal neurologic signs. Bilateral cortical dysfunction or disease of the reticular activating system in the brainstem is necessary to produce coma.

Motor system. In evaluating the motor system, look for lateralizing signs such as asymmetry of movement either spontaneously or to painful stimulation and asymmetric reflexes. Describe any spontaneous posturing. Decorticate posturing is characterized by tonic flexion of the arms and extension of the legs and implies a lesion at the level of the midbrain (Figure 2-17). Decerebrate posturing is manifest as tonic adduction and extension of the arms and legs and suggests a lesion at the level of the pons. In general, metabolic disturbances do not result in posturing, although anoxia and hypoglycemia can produce posturing. A mass lesion, which previously produced lateralized signs, may result in decorticate or decerebrate posturing when it expands and compresses the brainstem.

Pupils and fundi. Papilledema suggests increased intracranial pressure from a mass lesion or cerebral edema. Check the pupils for size and reactivity to direct light. With metabolic disease the pupils tend to be small and sluggishly reactive. Asymmetry of pupil size and reactivity, particularly the unilateral dilated pupil, suggests mass effect with herniation. Thalamic lesions usually produce a 2 mm nonreactive pupils, 4-5 mm fixed pupils suggest a midbrain lesion and pinpoint pupils suggest pontine dysfunction. Any nonmetabolic sign requires emergent CT scan for evaluation of possible mass lesion.

Ocular movement. Eye movements should be intact with metabolic disease, as noted with spontaneous movement or with Doll's eye maneuver. Doll's eye maneuver should be performed once severe cervical spine disease or fracture has been ruled out (Figure 2-18). The patient's head is moved swiftly from side to side, with the eyes held open. An intact Doll's eye reflex is characterized by the eyes moving conjugately in the direction opposite to which the head is being turned, ie, head turn to the left should swing both eyes across the midline to the right. This maneuver checks the integrity of the brainstem between the midbrain and pons. If the Doll's eye maneuver does not produce eye movements, cold caloric testing is necessary (Figure 2-18A). The head of the bed is raised by 30 degrees. Examination of the tympanic membranes for perforation should be ruled out before cold water is injected into each ear. If the brainstem is intact, injection of cold water into the ear should elicit tonic conjugate deviation of the eyes toward the side of injection. Nystagmus away from the side injected may or may not be present, but is not necessary to assess the integrity of the brainstem. Inability to produce the full range of eye movements with either the Doll's eye maneuver or cold calorics suggests brainstem pathology from pressure on the brainstem (herniation from a subdural hematoma) or from direct brainstem injury (basilar artery stroke). The unilateral third nerve palsy, manifest as a fixed, dilated pupil in an eye, which is "down and out" in position, is the classic example of a hemisphere lesion producing brainstem signs of oculomotor and pupillary dysfunction. A mass in one hemisphere causes the uncus of the temporal lobe to herniate over the edge of the tentorium, where it impinges on the third nerve. Compression of the parasympathetic fibers on the outer portion of the nerve, results in ipsilateral pupillary dilation and is early sign of the uncal herniation syndrome.

Respiratory patterns. The respiratory pattern of metabolic disease characteristically produces Cheyne-Stokes respirations. However, early mass lesions may also produce Cheyne-Stokes respirations. Central neurogenic hyperventilation, which is manifest as rapid shallow breathing, indicates midbrain dysfunction. Cluster or apneustic breathing suggests pontine injury. Ataxic, shallow breathing is characteristic of agonal respirations from medullary lesion.

In the patient who is somnolent, but arousable to stimulation, or confused, the etiology is most likely metabolic or toxic causes unless there are focal neurologic signs to suggest a structural lesion.

Intellectual Performance
Intellectual performance provides the best evidence of organic brain damage and its extent. Diffuse involvement of the brain results in deterioration of general intellectual functions while a structural lesion results in impairment of specific intellectual functions. Difficulties with maintaining attention, and perseveration of thought, manifest as slowness to shift from one topic to another. These, and poor memory are examples of specific intellectual deficits which should lead the examiner to more specific testing of memory, calculations, and judgment.

Memory depends on the ability to store and retrieve information both on a short and long-term basis. It is critical for learning. When evaluating memory function, it is important to realize that inattention, decreased motivation and poor cooperation, all symptoms of depression, can appear to impair memory. However, in depression, memory deficits may be overcome by improving the patient's cooperation and concentration, while organic deficits in memory are not altered with increased effort.

The temporal stages of memory include sensory store, short-term memory and long-term memory. Sensory store is the stage in which sensory input is converted to perceived sensation. This occurs within 250 milliseconds after stimulus onset. Short-term memory implies storage of memory for a few seconds up to 1-2 minutes. Information may be recycled in short term memory if it is of interest or stored in long-term memory. Clinical disorders of memory are not defects in sensory store or short-term memory. Patients can usually store items for 15-45 seconds even with the most severe degree of anterograde memory loss. If the sensory store of short-term memory is impaired it is usually due to poor attention. Attention can be assessed with the digit span test.

Formation of long-term memory requires both intact sensory store, short-term memory and the consolidation of short-term memory into long-term memory. Most clinical memory deficits involve transfer of information from short term to long-term memory. This deficit is referred to as anterograde amnesia or the inability to form new long-term memory and is classically seen in Korsakoff's psychosis from thiamine deficiency. Once information has been stored in long-term memory it can decay if not rehearsed. Long-term memory is the last memory to be lost in organic disease, with the most remote events, ie, childhood, retained the longest. This phenomenon is observed in Alzheimer's dementia. The loss of remote memory is referred to as retrograde amnesia and is always accompanied with severe anterograde amnesia. A classic cause of this condition is head trauma with the memory deficit proportional to the severity of the blow.

The neuroanatomy of memory involves the hippocampus, the dorsomedial nucleus of the thalamus, mammillary bodies, fornix, and entorhinal cortex. The hippocampus and the nearby temporal stem, which carries fibers from the middle and inferior temporal gyri to the dorsal medial nucleus of the thalamus, plays an important role in transferring memories from short term to long term memory. Lesions of the dorsomedial nucleus of the thalamus, mammillary bodies, fornix, and entorhinal cortex also cause memory deficits. Interestingly, unilateral lesions of these structures result in minimal memory impairment. Bilateral lesions are necessary to produce clinically significant memory deficits.

To test memory, check digit span to make sure attention capacity is intact. The patient is asked to repeat a gradually increasing sequence of numbers, eg, 2-3-7-4, 5-8-4-6-1, 2-0-5-1-6-9, etc. The normal patient should be able to repeat at least 7 digits. Present the patient with three words (baseball, tree, car) and three complex shapes that are drawn for the patient. Have the patient recall the words and shapes after five minutes. This procedure checks short-term to long-term memory transfer and is an effective screen for anterograde amnesia. Ask the patient about the remote and recent past to check for retrograde amnesia.

Disturbances in calculations are seen with diffuse brain lesions and in lesions of the angular gyrus of the dominant hemisphere. The patient is asked to perform simple calculations of addition, subtraction, multiplication or division. In the presence of speech difficulty, such as expressive aphasia, a calculation in written form may be provided and the patient asked to point to the correct answer among several choices provided.

To assess judgment, the patient is asked to interpret several simple proverbs, such as "A stitch in time saves nine," and "A rolling stone gathers no moss." These phrases allow one to assess the patient's retention, comprehension and formulation of abstract material. However, one should be cautious to account for cultural differences in interpreting the results of this test.

Thought Processes
The testing of thought processes assesses the patient's subjective experiences and interpretation of them. Mood is evaluated by observing the patient and asking him directly about how he is feeling. The patient's insight into his problems may be ascertained by asking him what brought him into the hospital and what might have caused his illness. Testing for delusions, hallucinations and illusions is generally not necessary, unless warranted by patient behavior.

Psychomotor Function
Praxis is the ability to conceive, formulate and execute a complex, skilled, volitional movement. Apraxia is an acquired disorder resulting in the inability to perform a learned movement in the presence of normal muscle strength, sensation, coordination, comprehension and attention. The left hemisphere is dominant for single motor acts (make a fist) and for sequencing of motor acts (make a fist, open your hand, place your hand on its edge). Lesions in the dorsolateral frontal and parietal lobes on the left produce apraxia. The patient often cannot perform a single or sequential motor act upon command or by imitation, although he may be observed to do it spontaneously. Performance improves when an object is held in the hand.

With ideomotor apraxia, the patient can imitate motor acts but cannot perform the same acts on command. To assess for ideomotor apraxia, ask the patient to show you how to blow out a match or comb his hair. If he cannot do so, have him imitate you performing these acts. In ideomotor apraxia, the lesion is in the left frontal lobe with involvement of the anterior corpus callosum (connects the two hemispheres and enables cross communication). The patient usually has a right hemiparesis. There is disconnection of verbally activated motor engrams in the left hemisphere from the right motor cortex due to the callosal lesion, such that comprehension of a verbal command (in the left hemisphere) cannot reach the right hemisphere for execution of an act with the left hand. The syndrome resembles that seen in split-brain patients.

With ideational apraxia, the patient cannot produce or imitate simple motor acts to command. The ability to synthesize a motor program is impaired. The lesion is usually bilateral in the superior parietal regions. Misreaching in space often accompanies the syndrome. To test for apraxia, the patient may be asked to show you how to fold a letter, place it in an envelope, seal it and place a stamp on the envelope.

Psychosensory Function
Psychosensory functions include the ability to recognize objects via visual, auditory or tactile sensation. Agnosia is an acquired disorder of recognition of previously familiar objects in the face of normal vision, hearing and somatic sensation. The patient may be shown a set of keys and asked to name it and describe its function. With visual agnosia, he is able to do neither. There is disconnection of visual information from memory stores. The patient can recognize and name the keys, however, if he is allowed to rely on other senses such as touching the keys or hearing them jingle together. Anosognosia is the denial of illness. The most common example is the hemiplegic patient who denies his weakness. This condition is observed with lesions of the inferior parietal lobe near the supramarginal gyrus.

The Sensory System
Performing this part of the examination may be time consuming because of misunderstanding or lack of patient cooperation. With some experience and practice, useful information can be obtained. If the patient has no sensory symptoms a routine sensory examination is usually performed. If, however, the patient has sensory symptoms, an examination tailored to the symptoms is performed in addition to the usual survey.

We will first go over some basic neuroanatomical pathways so that abnormal findings on the examination can be translated into useful clinical information.

The sensory modalities usually tested are superficial sensation and deep sensation. Superficial sensation encompasses light touch, pain and temperature sensibility. Deep sensation includes joint and vibratory sensibility and pain from deep muscle and ligamentous structures.

Sensory stimuli are picked up at their origin by specialized receptors whose unique firing patterns enable the brain to identify different types of stimuli. The information is relayed upwards to its ultimate destination, the primary sensory cortex of the parietal lobe (post-central gyrus). Here, sensory information is integrated into meaning (eg, feeling an object and being able to identify it, or experiencing pain and then undergoing suffering and anguish as a result of it). All sensory modality fibers are grouped together in peripheral nerves but once they reach the spinal cord, they split and travel to their ultimate destination over different routes. It is awareness of these pathways and how they are distributed at different levels of the neuraxis that enables the examiner to localize the level of a lesion based on the clinical findings of the sensory examination.

Peripheral nerve lesions can produce sensory deficits, motor deficits, autonomic dysfunction, or all of these. The sensory loss characteristically has sharp borders, and if it is a mixed nerve, for example, the median nerve, sensory, motor and autonomic fibers are affected. Sensory nerves have no motor fibers and lesions produce sensory loss for all modalities. Partial lesions may produce a disquieting burning or lancinating pain as well. An example of this type of nerve is the lateral femoral cutaneous nerve supplying the skin of the lateral thigh (Figure 2-19). Examples of findings secondary to common peripheral nerve lesions are found at the end of this chapter.

The peripheral nerve cell body is located in the dorsal root ganglion near the spinal cord. It is a bipolar cell with peripheral and central connections. As the specific central processes of these cells enter the spinal cord (Figure 2-19A) they either synapse and cross in one or two segments to enter the spinothalamic tract (pain and temperature) or remain ipsilateral and travel upwards in the dorsal columns or lateral spinocerebellar tracts (proprioception, joint receptor sensation). The dorsal columns convey information that will ultimately reach consciousness and the spinocerebellar tracts send sensory information to the cerebellum for its use in coordinating motor activity.

If a patient has a lesion involving the nerve root itself there will be sensory and motor loss characterized by the nerve fibers present in the root. Root sensory distribution follows a dermatomal distribution. A dermatome map is shown in Figure 2-20.

Things to remember about root lesions are the following:

most frequent in the cervical and lumbo-sacral regions
associated with pain.
commonly caused by intervertebral disc herniations and spondylosis
can also occur secondary to metastatic disease, metabolic or inflammatory/infectious disorders
sensory loss found on exam is not always dramatic because of overlap of sensation with the roots above and below
muscle weakness is characteristic for the root in question

Once the sensory fibers enter the spinal cord they begin their upward ascent. Pain and temperature fibers travel upwards on the side opposite to their origin in the lateral spinothalamic tract (Figure 2-21). Fibers for facial pain and temperature sensation originate in the Gasserian ganglion and then travel downward in the descending root of V before they cross over to join the contralateral spinothalamic tract. They travel in the ventral central trigeminal tract and terminate in the ventromedial thalamic nucleus. As a result of this unusual arrangement the lateral medulla is characterized by having ipsilateral facial and contralateral body pain and temperature fibers on the same side. For example, a unilateral lesion in the lateral medulla (Wallenberg’s syndrome) demonstrates loss of pain and temperature on the ipsilateral side of the face, and contralateral side of the body.

The medial and lateral spinothalamic tracts travel side by side above the medulla. They are arranged topographically with the facial fibers being more medial.

All sensory fibers ultimately converge and synapse in the ventrolateral and ventromedial thalamic nuclei. Here, as well as in the spinal cord tracts, fibers are laminated topographically with cervical, thoracic, lumbar and sacral fibers and cell bodies having characteristic locations. The thalamic neurons project their connections to neurons in the post central gyrus, which also has topographic representation (Figure 2-22).

Proprioceptive fibers and touch fibers travel in the dorsal columns ipsilateral to the side of their origin until they reach the lower medulla. Fibers from T-7 and below travel in the fasciculus gracilis and synapse in the nucleus gracilis. T-6 and higher afferent fibers travel upward in the fasciculus cuneatus, and synapse in the nucleus cuneatus. There is some modulation for sensory discrimination at this nuclear level and then the fibers cross to the opposite side in the internal arcuate fibers to then form the medial lemniscus. As the fibers travel up to the thalamus they are joined by facial sensory fibers from the opposite trigeminal sensory nucleus. Unlike pain and temperature fibers, proprioceptive and light touch fibers do not have a location where body and facial representation are on opposite sides. After synapsing in the thalamus the fibers project to the primary sensory cortex of the parietal lobe where all sensory modalities are processed and interpreted.

Some light touch fibers travel in the anterior spinothalamic tract (Figure 2-23) and some vibratory fibers travel in the lateral columns. For this reason there may be sparing of some light touch and vibration sensation with dorsal column lesions.

Sensory information is also conveyed to the cerebellum via the dorsal and ventral spinocerebellar tracts (Figure 2-24). Typically, however lesions of these tracts in the spinal cord do not produce significant cerebellar ataxia.

The preceding is a cursory anatomy review and the interested reader is referred to numerous more detailed neuroanatomy references.

Having gained knowledge of the above pathways the examiner must now obtain historical and clinical information from the patient. This information will then be analyzed to localize the level of dysfunction.

The Sensory Examination
The patient should be in a comfortable position and undressed except for a gown. Exposure of the feet, abdomen and trunk as well as the perineum is necessary to perform an adequate sensory examination.

Primary Modalities to be Tested
· Light touch
Test item: Cotton wisp. Touch patient lightly with eyes closed and have them say “yes” when touched. Compare sensation on right and left side of body. Ascend from the foot upward and ask the patient to identify the level where touch is first appreciated or becomes more pronounced.

· Vibration
Test item: 256 Hz Tuning fork. Strike the fork and hold it to a bony prominence such as the first toe, ankle malleous, tibial plateau, or ileum. Having to increase the vibration and apply more proximal stimulation implies that the deficit is more pronounced.

· Pain
Test item: Sterile pin. Touch the patient with the sharp or dull end and ask them to identify “sharp or dull” with the eyes closed. One can also ascend from the foot upwards and ask the patient to identify the level where appreciation of sharpness occurs or where an appreciable increase in sensation occurs.

· Temperature
Test item: Cold tuning fork; hot and cold water in a test tube or flask. With eyes closed ask them to identify when touched with hot or cold. Levels and laterality can also be tested as described for pain and light touch.

· Position sense
Move patient’s finger and, later, toe up or down with the patient’s eyes closed, and ask them to identify the direction of the motion. Greater deficits are characterized by having to move a more proximal joint such as ankle, knee or hip for the patient to appreciate the movement.

Cortical Discrimination Testing (Combined sensation)
Simple sensations can be appreciated and poorly localized at the thalamic level. It is at the cortical level that sensations are combined and integrated into meaningful and symbolic information. A cortical lesion is usually recognized if there is not a significant absence or loss of primary sensory modalities, and the patient is unable to integrate the appreciated sensations into symbolic information. When sensory recognition functions are impaired a lesion is implied in the contralateral parietal lobe. Basic tests for these modalities are:

Two point discrimination. Test item: small calipers. These may be applied to the face, fingertips, palms and tibial regions. The usual sensory thresholds are: face 2-5 mm; finger tips 3-6 mm; palms 10-15 mm; and shins 30-40 mm. Increased distance threshold or loss of this ability implies a contralateral parietal lobe lesion.

Stereognosis. This is the ability to identify an object only by feeling it. The patient is asked to close their eyes. A test object is placed in the hand being tested. The patient can manipulate and feel the object with the test hand only and is asked to identify it. Test items can include a key, thimble, coin or bolt. The side suspected to be abnormal is usually tested first.

Traced figure identification. Numbers (1-9) are traced on the fingertips or palms of the hands while the patient’s eyes are closed. The examiner orients himself so that the numbers are upright to the patient. The patient is asked to identify each number.

Double simultaneous stimulation. Homologous parts of the body are touched simultaneously or separately (eg, right hand, both hands, left hand). The patient is asked to answer right, left or both hands. With a parietal lobe lesion the patient may not identify being touched on the side opposite the lesion when right and left sides are simultaneously stimulated. This phenomenon is termed sensory extinction.

It will be through repetition and clinical correlation that one becomes proficient at doing the sensory examination. The more commonly seen sensory loss patterns are listed below.

1. Isolated nerve lesions (mononeuropathy)
            Median nerve (Carpal tunnel syndrome) Figure 2-25.
            Ulnar nerve (Elbow entrapment) Figure 2-26.
            Lateral femoral cutaneous nerve (meralgia paresthetica) Figure 2-19.

2. Mononeuritis multiplex
Combinations of peripheral nerve lesions occur, usually caused by nerve infarcts secondary to vasculitis or diabetic vasculopathy.

3. Sensory peripheral neuropathy
Disease affecting peripheral nerves may affect the Schwann cell myelin sheath (demyelinating neuropathy) or the nerve axons (axonal neuropathy). These two types are usually clinically indistinguishable in sensory neuropathies. Motor axonal neuropathy is associated with muscle atrophy. Peripheral neuropathy characteristically starts in the feet and is symmetrical. Progression is characterized by rising deficit levels in the legs and eventual involvement of the fingers. In any peripheral nerve or root lesion the sensory or motor arc of the deep tendon reflex can be interrupted leading to diminished or absent deep tendon reflexes. Distal reflexes (ankle) are diminished more than proximal reflexes (biceps).

4. Root lesion
The dermatome maps for the sensory distribution of individual roots are shown in Figure 2-20. Root lesions may manifest a vague sensory alteration or loss following the corresponding dermatome, or no objective sensory loss. Often the patient will have paresthesias in the root distribution. The location of common root paresthesias are C-5, shoulder region; C-6, thumb; C-7, middle finger; C-8, 5th finger; L-3, anterior thigh; L-5, great toe; and S-1, medial sole of the foot. If a patient cannot appreciate the sensation of bladder fullness, passing stools or sexual sensations, it may imply deficits of the S-3,4,5 sensory roots.

Sensory loss or characteristic paresthesias, when combined with a root pattern of muscle weakness, will confirm the presence of radiculopathy. Root lesions are also, usually characterized by the presence of pain, especially if the root is being compressed.

5. Spinal cord

Lesions of the spinal cord are usually of two different types. External (compressive) lesions and intrinsic lesions.

External compressive lesions affect the spinal cord as a whole, even though one side may be compressed more. As a result all tracts are affected to some degree. Because the corresponding nerve root is also compressed or stretched, pain is a prominent symptom. Ascending and descending pathways are interrupted and sensation is usually diminished distal to the lesion. Localizing signs would be localized root pain, sensory loss below the level of the lesion, an absent root reflex at the level of the lesion, and generally increased reflexes below this level. Compressive lesions can be caused by herniated discs, tumors or abscess, among others.

Because sensory fibers separate into distinct tracts when they enter the spinal cord some are affected by intrinsic spinal cord lesions while others are completely spared. This produces a characteristic finding of intrinsic cord lesions termed sensory dissociation. These lesions may be caused by infarction, tumor or a syrinyx. Some common cord syndromes are:

Brown-Séquard syndrome (Figure 2-27)

ipsilateral plegia below the lesion
ipsilateral proprioception and light touch loss below the lesion
contralateral pain and temperature loss below the lesion

Anterior spinal artery infarction (Figure 2-28)

paraplegia below the lesion
pain and temperature loss below the lesion
sparing of dorsal column sensation

Central cord syndrome (cervical) (Figure 2-29)

shawl distribution pain and temperature loss
sparing of light touch and proprioception
lower motor neuron weakness of the affected cord levels (anterior horn cell involvement)

Complete cord transection. (Figure 2-30)
-loss of all modalities below the level of the lesion

6. Brainstem
Brainstem lesions at the level of the medulla have ipsilateral loss of pain and temperature of the face and contralateral loss on the body. Light touch and proprioceptive loss is contralateral. Above this level all sensory modality findings are contralateral to the side of the lesion because all pathways have crossed.

7. Thalamus
Thalamic lesions produce contralateral loss of all sensory modalities in the face, extremities and trunk. In addition, stimulation may be perceived as uncomfortable and painful (dysesthesia).

8. Cortical lesions
Lesions of the cerebral cortex cause diminution of all sensory modalities on the contralateral side of the body. In addition, higher integrative sensory functions are impaired causing defects in stereognosis, two-point discrimination, double simultaneous stimulation and traced figure identification as previously discussed. The extent of the sensory loss parallels the size of the lesion. The pattern of cortical sensory representation in the cerebral cortex is illustrated in Figure 2-22.

The foregoing contains essentials of the sensory examination and should become easier to perform and interpret with continued use. The video on how to perform the neurological examination should be watched as well.

Summary
Characteristics of sensory system lesions:

Peripheral nerve           All sensory modalities are affected.
                                    The borders are sharply demarcated.
                                    There may be hyperesthesia, discomfort and pain.

Root                             All sensory modalities are affected.
                                    Sensory loss is vague but in a dermatomal distribution.
                                    Pain is present and may radiate in the dermatome distribution.

Spinal cord There is sensory dissociation.
A unilateral lesion produces ipsilateral loss of light touch and proprioception and contralateral loss of pain and temperature.

Medulla                        There is sensory dissociation.
                                    Pain and temperature are lost on the ipsilateral side of the face and                                     contralateral side of the body.
                                    Light touch and proprioception are lost on the contralateral side of the                                     body.

Upper brainstem There is sensory dissociation.
All sensory modalities are now crossed and on the same side. Unilateral lesions cause contralateral loss of sensory modalities.

Thalamus Sensory dissociation is no longer present.
Ipsilateral lesions produce contralateral loss of all modalities.

Cerebral cortex.           Sensory dissociation is absent.
                                    Ipsilateral lesions produce contralateral loss of all modalities.
                                    Discriminative sensory functions are lost.

Continue to Chapter 1 part 3

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