Clinical Evaluation of the Nervous System

Neurologic History

It is a common medical school teaching that acquiring an accurate medical history alone can help the clinician secure the correct diagnosis in approximately 90% of all patients. Historical information obtained by a skilled clinician, more often than not, will uncover a patient's entire anatomic and etiologic illness. The history can then inform the neurologic examination to confirm dysfunction prior to reliance on sophisticated neuroimaging. It is paramount for the astute clinician to master the skill of anatomic localization in the nervous system.

This complex but beautiful system is composed of 10 subsystems (from a practical standpoint): cortex, pyramidal tracts, basal ganglia, brainstem, cranial nerves, cerebellum, spinal cord, nerve roots, peripheral nerves, and muscle. Understanding each subsystem of the nervous system is equivalent to mastering the anatomy of one entire internal organ. Many of the subsystems stretch over long distances either vertically (ie, pyramidal tracts, dorsal columns) or horizontally (ie, cortex, cranial nerves, brainstem), which can complicate accurate anatomic localization. To evaluate the functional state of the nervous system, the neurosurgeon requires a basic knowledge of the pertinent anatomy as well as an understanding of the role of ancillary imaging and laboratory tests. Apart from the optic nerve head, which can be evaluated by a funduscopic examination, the rest of the nervous system is hidden from direct observation, and therefore, at the clinical level, disease usually must be inferred from a disorder of normal function.

Focal Cortical Signs

We will begin this tour at the top with the cerebral cortex and then continue down the line. In general, conversation with the patient during the course of the examination will elicit the cortical deficits that are obvious. The ability to talk and respond to questions in a sensible and coherent fashion reveals a great deal about the cerebral cortices. More subtle cortical deficits require meticulous testing, often by neuropsychological examinations, the interpretation of which requires specific training. Neuropsychological examinations are performed more commonly in the pre- and postoperative stages of modern neurosurgical intervention. These tests can give detailed information on executive function, memory, language, visuospatial processing, academic function, processing speed, motivation, and personality. It is important to understand what subtle deficits existed preoperatively and how well the deficits improved postoperatively, or which new deficits will require active rehabilitative intervention to improve after surgery.

In broad strokes, the examiner must understand two major types of pathognomonic cortical signs: focal and bihemispheric. Focal cortical signs direct the examiner to a specific area of cortex in one hemisphere, bihemispheric in both hemispheres. Certain portions of the cerebral hemispheres are also termed silent areas, because the localizing evidence for lesions here may be absent.

Left frontal lobe dysfunction can result in Broca aphasia, also known as motor or expressive aphasia, and is characterized by halting, slow, and nonfluent speech. Speech lesions in the arcuate fasciculus, a dense bundle of fibers connecting the Wernicke area to the Broca area, prevent patients from repeating phrases but does not impair comprehension ( Table 3.1 ).

The right frontal lobe, despite its size, is a relatively silent lobe, other than loss of speech intonation (inflection and emotion in speech). The areas of major clinical importance are the motor strip (area 4), the supplementary motor area (area 6), the frontal eye fields (area 8), and the cortical center for micturition (medial surface of the frontal lobe). Frontal lobes play a major role in personality and acquired social behavior. Frontal lobe dysfunction may result in loss of drive, apathy, loss of personal hygiene, inability to manage one's family affairs or business, and disinhibition.

Pathognomonic signs of left parietal dysfunction include right-sided cortical sensory loss, right-sided sensory-motor seizures, or a Gerstmann syndrome, characterized by finger agnosia (inability to recognize one's fingers), acalculia (inability to calculate numbers), right/left confusion, and agraphia without alexia (an ability to read but not write). Another sign of left parietal cortical dysfunction is cortical sensory loss and results in agraphesthesia (inability to identify numbers written on his/her skin). Sensory seizures may spread up or down the sensory strip and have been described as the Jacksonian march. The movement, usually clonic, begins in one portion of the body, for example, the thumb or fingers, and spreads to involve the wrist, arm, face, and leg on the same side along the stereotypical pattern of cortical organization termed the homunculus ( Fig. 3.1 ). A Todd paralysis may then occur following the attack, with the same distribution. Right parietal lesions cause a characteristic disturbance of space perception and left-side neglect.

In 98% of right-handed people, the Wernicke area is located in the left temporal lobe. In most left-handed people, the Wernicke area is still located in either the left temporal lobe alone or in both temporal lobes. In only a minority of left-handed people is the Wernicke area confined to the right temporal lobe. A lesion in the Wernicke area results in a sensory or receptive aphasia characterized by fluent speech filled with gibberish words. Written words come from the occipital cortex, while spoken words may come from both temporal lobes. A mistake in naming results in a paraphasia and is often the result of a lesion in the posterosuperior temporal lobe, but can have quite variable localization. Adjacent to the Wernicke area in the temporal lobe is another area called the “dysnomia center,” which shows variable localization from person to person. Another pathognomonic sign of temporal lobe dysfunction is a focal, temporal lobe seizure, described as fits consisting of a sense of fear, smell, pleasure, or déjà vu. Another common manifestation of temporal lobe seizures is the automatism, a brief episode of automatic behavior during which the patient is unaware of his or her surroundings and is unable to communicate with others. Patients with complex partial seizures may experience sudden unpleasant smells (eg, burning rubber) of brief duration, which constitute olfactory auras. Temporal lobe dysfunction may also cause a superior quadrantanopia (loss of a quarter of the visual field), described as a “pie in the sky,” as a result of a disruption of the optic radiations, called the Meyer loop, which dip into the temporal lobe.

Left occipital lobe dysfunction produces a right homonomous hemianopia (loss of the right half of a visual field), although loss of this field can theoretically result from a lesion of the left optic tract or left thalamic lateral geniculate body. A right or left hemianopia can therefore result from any retrochiasmal lesion (behind the chiasm). Color dysnomia (inability to name colors) is the result of an interruption of fibers streaming from the occipital lobe to the Wernicke area, the comprehension center in the left temporal lobe.

Lesions of the corpus callosum prevent the interhemispheric transfer of information, so a patient cannot follow instructions with his or her left hand but retains the ability to perform these same instructions with the right hand. Another syndrome of the corpus callosum is alexia without agraphia (inability to read but retained ability to write) and is caused by a lesion extending from the left occipital lobe and into the splenium of the corpus callosum.

Signs such as lethargy, stupor, coma, disorientation, confusion, amnesia, dementia, and delirium often result from bihemispheric dysfunction and are not derived from a simple focal cortical lesion.

Pyramidal Tract

The pyramidal tract begins in the motor strip of the cortex and courses downward through the brain and into the spinal cord. In the hemispheres it is called the coronal radiata and then becomes the internal capsule, cerebral peduncle, and pyramidal tract, which crosses at the medulla–spinal cord junction, and finally in the spinal cord becomes the corticospinal tract. Functionally, a lesion anywhere along this tract can produce the same long tract signs. Signs of pyramidal tract dysfunction include spasticity, weakness, slowing of rapid alternating movements, hyperreflexia, and a Babinski sign. Muscle tone is examined by manipulating the major joints and determining the degree of resistance. Muscle strength is commonly graded from 0 to 5 using the grading system shown in Table 3.2A , and spasticity is one type of increased tone (resistance of a relaxed limb to flexion and extension) graded as outlined in Table 3.2C .

Acute lesions anywhere along the pyramidal tract may also produce flaccid hemiparesis, at least initially, with spasticity developing later. If the whole area of cortex supplying a limb is damaged, the extrapyramidal pathways may be unable to take over and an acute global flaccid weakness of the limb can occur. Intraoperative monitoring has been used to mitigate injury to the corticospinal tract. Pyramidal tract lesions typically produce weakness of an arm and leg, or face and arm, or all three together. Facial weakness may manifest with a slight flattening of the nasolabial fold; however, the forehead will not be weak (frontalis muscle) because the muscles on each side of the forehead have dual innervation by both cerebral hemispheres (corticopontine fibers). The less affected muscles are the antigravity muscles (wrist flexors, biceps, gluteus maximus, quadriceps, and gastrocnemius). Specific tests of grouped muscle strength can also be quite useful ( Table 3.3 ): pronator drift (arms outstretched with the palms up), standing on each foot, hopping on one foot, walking on toes (gastrocnemius), walking on heels (tibialis anterior), and deep knee bend (proximal hip muscles). Typically, pyramidal lesions often cause rapid alternating movements to become slowed but accuracy is preserved. This is in contrast to cerebellar lesions (see later discussion), which can result in fast but inaccurate, sloppy movements.

Reflexes can also be quite important in detecting subtle pyramidal tract lesions, especially if asymmetrical. Reflexes are graded by a numerical system as shown in Table 3.2B : 0 indicates an absent reflex, trace describes a reflex that is palpable but not visible, 1+ is hypoactive but present, 2+ is normal, 3+ is hyperactive, 4+ implies unsustained clonus, and 5+ is sustained clonus. Clonus is a series of rhythmic involuntary muscle contractions induced by sudden stretching of a spastic muscle such as at the ankle. The cutaneous reflex (abdominal twitch obtained when you gently stroke someone's abdomen) and the cremasteric reflex (L1, L2 innervation; retraction of the testicle upward with a brush along the inner thigh) may also be lost in pyramidal tract lesions. The abdominal cutaneous reflexes in the upper quadrant of the abdomen are mediated by segments T8 and T9, the lower by T10 to T12. If, for example, the lower abdominal reflexes are absent but the upper are preserved, the lesion may be between T9 and L1. The Hoffmann reflex is reflective of hyperreflexia and spasticity on that side and suggests pyramidal tract involvement. It is elicited by snapping the distal phalanx of the middle finger; a pathologic response consists of thumb flexion. The Babinski reflex is the best-known sign of disturbed pyramidal tract function. The Babinski reflex is an important sign of upper motor neuron disease, but it should not be confused with a more delayed voluntary knee and toe withdrawal due to oversensitive soles of the feet. The Babinski reflex is sought by stroking the lateral border of the sole of the foot, beginning at the heel and moving toward the toes. The stimulus should be firm but not painful. The abnormal response, referred to as the Babinski sign , consists of immediate dorsiflexion of the big toe and subsequent separation (fanning) of the other toes. The Babinski sign is present in infancy but usually disappears at about 10 months of age (range 6–12 months). When plantar responses produce equivocal results, a related reflex may be tested by stroking the lateral aspect of the dorsum of the foot and is known as the Chaddock sign.

In general, the more spasticity is present, the more likely the pyramidal tract lesion is in the spinal cord, especially if the spasticity is bilateral. Conversely, it is unusual for a pyramidal tract lesion in the spinal cord to produce a hemiparesis or monoparesis. A hemiparesis that involves the face places the lesion somewhere above the facial nucleus, although if the hemiparesis spares the face, the lesion need not be below the facial nucleus. Mild or more chronic hydrocephalus may also cause impressive pyramidal tract dysfunction in the legs more than in the arm fibers. Bladder axons also become stretched by the dilated ventricles associated with hydrocephalus and cause urinary urgency and incontinence. Finally, it should be remembered that the spinal cord terminates normally at the level of the L1–L2 vertebral body, and, therefore, neurologically L5 is anatomically in the lower thoracic region.

Extrapyramidal System

Unlike the pyramidal tracts, which govern strength and fine dexterity, the basal ganglia govern the speed and spontaneity of movements. Two basic patterns emerge with basal ganglia dysfunction: either too much or not enough movement. The number one characteristic of a basal ganglia tremor is its presence at rest and disappearance with movement, in contrast to a cerebellar tremor , which is minimal at rest and exaggerated with movement (intention tremor). The strength and deep tendon reflexes are normal in extrapyramidal diseases and there is no Babinski sign. However, the tone is either hypotonic, as occurs in choreiform disorders, or increased (rigid), as in the bradykinetic (slowness of movements) varieties with ratchety rigidity appropriately called cogwheeling. Choreiform movements are involuntary random jerky movements of small muscles of the hands, feet, or face and may be proximal enough to cause the whole arm to jerk gently. If instead of the small distal muscles, the larger more proximal muscles involuntarily flinch, the patient may have ballismus. Ballismus can be unilateral, but chorea is almost always bilateral. Athetoid movements are slower, more continuous, and sustained, and they may involve the head, neck, limb girdles, and distal extremities. Dystonic movements resemble a fixation of athetoid movements involving larger portions of the body. Torticollis, or torsion of the neck, is an example of a neck dystonia that is the result of the continuous contraction of the sternocleidomastoid muscle on one side. Postural and gait abnormalities of extrapyramidal disease are most diagnostic in patients with Parkinson disease (tremor, bradykinesia, and rigidity). A blank expression and infrequent blinking, walking with a leaning forward posture, and a festinating gait (running, shuffling feet) are typical findings of a Parkinson patient. Once in gear, the initially bradykinetic patient may have difficulty stopping. At the same time, the patient's hand is coarsely shaking at three times a second and the patient's speech is also devoid of normal changes in pitch and cadence.

Cranial Nerves

There are 12 cranial nerves but only nerves III to XII enter the brainstem (I and II do not). Diagnosing a cranial neuropathy is only the beginning, because the lesion may lie anywhere along the course of the cranial nerve.