In taking a history and examining the patient, the neurologic clinician elicits symptoms and signs. Symptoms are subjective experiences resulting from the disorder (ie, "I have a headache"; "The vision in my right eye became blurry a month ago"). Signs are objective abnormalities detected on examination (eg, a hyperactive reflex or abnormal eye movements).
The history may provide crucial information about diagnosis. For example, a patient was admitted to the hospital in a coma. His wife told the admitting physician that "my husband has high blood pressure but doesn't like to take his medicine. This morning he complained of the worst headache in his life. Then he passed out." On the basis of this history and a brief (but careful) examination, the physician rapidly reached a tentative diagnosis of subarachnoid hemorrhage (bleeding from an aneurysm, ie, a defect in a cerebral artery into the subarachnoid space). He confirmed this diagnostic impression with appropriate (but focused) imaging and laboratory tests and instituted appropriate therapy.
The astute clinical observer may be able to detect signs of neurologic disease by carefully observing the patients' spontaneous behavior as they walk into the room and tell their story. Even before touching the patient, the clinician may observe the "festinating" (shuffling, small-stepped) gait of Parkinson's disease, hemiparesis (weakness of one side of the body) resulting from a hemispheric lesion such as a stroke, or a third nerve palsy suggesting an intracranial mass. The way patients tell their story also may be informative; for example, it may reveal aphasia (difficulty with language), confusion, or impaired memory. Details of history taking and the neurologic examination are included in Appendix A.
In synthesizing the information obtained from the history and examination, the clinician usually keeps asking the questions, "Where is the lesion? What is the lesion?" This thinking process will usually result in the correct diagnosis. Several points should be kept in mind while one is going through the diagnostic process.
Neurologic Signs and Symptoms Often Reflect Focal Pathology of the Nervous System
It is well established that, with respect to many functions. Different parts of the nervous system subserve different functions. In turn, in many parts of the brain or spinal cord, even relatively small well-circumscribed lesions produce loss or severe impairment of a specific function. This effect reflects the principle of localized function within the nervous system.
There are numerous examples of localized function. (1) Aphasia (difficulty producing or understanding language) often results from damage to well-localized speech areas within the left cerebral hemisphere. (2) Control of fine movements of each hand is dependent on signals sent from a hand area within the motor cortex in the contralateral cerebral hemisphere. The motor cortex is organized in the form of a map, or "homunculus," reflecting control of different parts of the body by different parts of the motor cortex (see Chapter 10, especially Fig 10–14). A lesion affecting the hand area or the pathways that descend from it to the spinal cord can result in loss of skilled movements or even paralysis of the hand. (3) At a more basic level, many reflexes, which are tested as part of the neurologic examination, depend on circuits that run through particular parts of the nervous system. For example, the patellar reflex (knee jerk) depends on afferent and efferent nerve fibers in the femoral nerve and L3 and L4 spinal roots and the L3 and L4 spinal segments, where afferent Ia axons synapse with motor neurons that subserve the reflex. Damage to any part of this circuit (nerve, spinal roots, or L3 or L4 spinal segments) can interfere with the reflex.
As a corollary of the principle of localized function, it is often possible to predict, from neurologic signs and symptoms, which parts of the nervous system are involved. An accurate history and a careful examination can provide important clues about the localization of dysfunction in the nervous system.
Manifestations of Neurologic Disease May Be Negative or Positive
Negative manifestations result from loss of function (eg, hemiparesis, weakness of an eye muscle, impaired sensation, or loss of memory). Negative manifestations of neurologic disease may reflect damage to neurons (eg, in stroke, where there is often loss of neurons located within a particular vascular territory, and in Parkinson's disease, where there is degeneration of neurons in the substantia nigra) or to glial cells or myelin (eg, in multiple sclerosis, in which there is inflammatory damage to myelin). Positive abnormalities result from inappropriate excitation. These include, for example, seizures (caused by abnormal cortical discharge) and spasticity (from the loss of inhibition of motor neurons).
Lesions of Spinal Roots and Peripheral Nerves Can Cause Neurological Dysfunction
Injury to spinal roots, or to peripheral nerves, can produce characteristic patterns of clinical abnormality. Each dorsal root, and each peripheral nerve, provides sensory innervation for a particular part of the body (Appendix C). An irritative lesion, such as compression of a spinal dorsal root or of a peripheral nerve, can thus produce pain in a particular area of the body. Similarly, each ventral root, and each peripheral nerve, innervates a particular set of muscles; these muscles have well-defined actions that can be assessed in the clinic or at the bedside (Appendix B). Focal lesions of spinal roots (which can occur with spinal disc disease) or of peripheral nerve (which can occur as a result of localized penetrating injuries due to trauma, or to localized compression due to disorders such as carpal tunnel syndrome) can produce very characteristic patterns of pain, sensory loss, or weakness.
Lesions of White and Gray Matter Cause Neurologic Dysfunction
Damage to gray or white matter (or both) interferes with normal neurologic function. Lesions in gray matter interfere with the function of neuronal cell bodies and synapses, thereby leading to negative or positive abnormalities, as previously described. Lesions in white matter, on the other hand, interfere with axonal conduction and produce disconnection syndromes, which usually cause negative manifestations. Examples of these syndromes include optic neuritis (demyelination of the optic nerve), which interferes with vision; and infarction affecting pyramidal tract axons, which descend from the motor cortex in regions such as the internal capsule, which can cause "pure motor stroke" (Fig 4–1).
Magnetic resonance image (MRI) of a 51-year-old patient with hypertension. The patient complained of weakness of the right side of the face and the right arm and leg, which had developed over a 5-h period. There was no sensory loss or problems with language or cognition. The MRI revealed a small infarction in the internal capsule (arrow), which destroyed axons descending from the motor cortex, thus causing a "pure motor stroke" in this patient.
Some neurologic disorders affect primarily gray matter (eg, amyotrophic lateral sclerosis, a degenerative disease leading to the death of motor neurons in the cerebral cortex and gray matter of the spinal cord). Others primarily affect white matter (eg, multiple sclerosis). Still other disorders affect both gray and white matter (eg, large strokes, which lead to necrosis of the cerebral cortex and underlying white matter).
Neurologic Disease Can Result in Syndromes
A syndrome is a constellation of signs and symptoms frequently associated with each other and suggests that the signs and symptoms have a common origin. An example is Wallenberg's syndrome, which is characterized by vertigo, nausea, hoarseness, and dysphagia (difficulty swallowing). Other signs and symptoms include ipsilateral ataxia, ptosis, and meiosis; impairment of all sensory modalities over the ipsilateral face; and loss of pain and temperature sensitivity over the contralateral torso and limbs. This syndrome results from dysfunction of clustered nuclei and tracts in the lateral medulla and is usually due to infarction resulting from occlusion of the posterior inferior cerebellar artery, which irrigates these neighboring structures.
Neighborhood Signs May Help to Localize the Lesion
The brain and spinal cord contain many tracts and nuclei that are intimately associated with each other or are anatomic neighbors of each other. Particularly in the brain stem and spinal cord, where there is not much room, there is crowding of nuclei and fiber tracts. Many pathologic processes result in lesions that are larger than any single nucleus or tract. Combinations of signs and symptoms may help to localize the lesion. Figure 4–2 shows a section through the medulla of a patient with multiple sclerosis. The patient had a sensory loss in the legs (impaired touch-pressure sense and position sense) and weakness of the tongue. As an alternative to positing the presence of two separate lesions to account for these two abnormalities, the clinician should pose the question, "Might a single lesion account for both abnormalities?" Knowledge of brain stem neuroanatomy allowed the clinician to localize the lesion in the medial part of the medulla.
A: Section through the medulla, stained for myelin, from a patient with multiple sclerosis. Notice the multiple demyelinated plaques (labeled 1–4) that are disseminated throughout the central nervous system (CNS). B: Even a single lesion can interfere with function in multiple neighboring parts of the CNS. Notice that plaque 3 involves the hypoglossal root (producing weakness of the tongue) and the medial lemnisci (causing an impairment of vibratory and touch-pressure sense). Figure 7–7B shows, for comparison, a diagram of the normal medulla at this level.
Dysfunction of the Nervous System Can Be due to Destruction or Compression of Neural Tissue or Compromise of the Ventricles or Vasculature
Several types of pathologic conditions can lead to dysfunction of the nervous system (Table 4–1). Destruction of neurons (or associated glial cells) occurs in disorders such as stroke (in which neurons are injured as a result of ischemia) and Parkinson's disease (in which degeneration of neurons occurs in one region of the brain stem, the substantia nigra). Destruction of axons secondary to trauma causes much of the dysfunction in spinal cord injury, and destruction of myelin as a result of inflammatory processes leads to the abnormal function in multiple sclerosis.
TABLE 4–1Mechanisms Leading to Dysfunction in Typical Neurologic Diseases. ||Download (.pdf) TABLE 4–1 Mechanisms Leading to Dysfunction in Typical Neurologic Diseases.
|Mechanism ||Disease Example ||Target ||Comments |
|Destruction ||Stroke ||Neurons (often cortical) ||Acute destruction, within hours of loss of blood flow |
|Destruction ||Parkinson's disease ||Neurons (subcortical) ||Chronic degeneration of neurons in substantia nigra |
|Destruction ||Spinal cord injury ||Ascending and descending axons ||Injury to fiber tracts from trauma |
|Destruction ||Multiple sclerosis ||Myelin ||Inflammatory damage to myelin sheaths in CNS |
|Compression ||Subdural hematoma ||Cerebral hemisphere ||Expanding blood clot injures underlying brain tissue |
|Compromise of ventricular pathways ||Cerebellar tumor ||Fourth ventricle ||Expanding mass compresses ventricle, impairs CSF outflow |
Compression can also cause dysfunction, without the invasion of the brain and spinal cord per se. This occurs, for example, in subdural hematoma, when an expanding blood clot, contained by the skull vault, compresses the adjacent brain, initially causing reversible dysfunction, before triggering the death of neural tissue. Early recognition and surgical drainage of the clot can lead to full recovery.
Finally, compromise of ventricular pathways or of the vasculature can lead to neurologic signs and symptoms. For example, a small cerebellar astrocytoma, critically located above the fourth ventricle, may compress the ventricle and obstruct the outflow of cerebrospinal fluid. The tumor may lead to obstructive hydrocephalus with widespread destructive effects on both cerebral hemispheres. In this case, a small, critically placed mass produces widespread neural dysfunction as a result of its effect on the outflow tracts for cerebrospinal fluid.
Critically placed vascular lesions can also produce devastating effects on the nervous system. Because certain cerebral arteries nourish the same parts of the brain in all humans, occlusion of these arteries produces characteristic clinical syndromes. For example, occlusion of the carotid artery, owing to atherosclerosis in the neck, can lead to infarction of much of the cerebral hemisphere which it supplies. Occlusion of the posterior cerebral artery produces infarction of the occipital lobe which depends on it for nourishment.