Chapter 20 - Basic principles in neurologic disease
Disturbances of the nervous system, both peripheral and central, are manifested in four basic ways:
- Destructive or deficiency phenomena associated with destruction of nerve tissue (e.g., infarction, tumor, trauma) or processes that depress or block nerve function (e.g., anesthesia).
- Irritative phenomena as represented by seizures and the pins-and-needles or burning paresthesias of peripheral neuropathy, both of which represent excessive neuronal firing secondary to pathologic depolarization.
- Release phenomena as represented by the hyperactive reflexes and spasticity of corticospinal system involvement, the tremor of Parkinson's disease, the excessive emotional responses following bilateral corticobulbar system loss, sedative withdrawal hyperactivity and the appearances of primitive reflex responses that may reappear in dementing illnesses (e.g., with generalized cerebral cortical degeneration).
- Compensation phenomena. There may be appropriate or inappropriate compensations. For example, appropriate compensation would include visual-motor compensation for the nystagmus and vertigo of vestibular disease, circumduction of the paretic leg on walking to avoid tripping, a high-stepping gait to avoid tripping with a foot drop, or a broad-based gait to compensate for ataxia. Inappropriate compensations would include tendon contractures, for example.
Combinations of these disturbances are the rule. The person who has cerebral infarction is hemiparetic, develops spasticity, may develop seizures early or late, and compensates for hemiparesis appropriately with a circumducting gait. If physical rehabilitation is not carried out, s/he can develop a functionally inappropriate flexion contracture of the arm.
Disease course and tempo (Figure 20-1)
Momentum of disease is another phenomenon that should be considered. This refers essentially to the rate of involvement of the nervous system. An acute destructive lesion (e.g., infarction) causes an early maximal deficit, whereas a chronic, slowly progressive lesion (e.g., tumor) usually produces considerably less deficit because of compensating mechanisms (such as mechanical adjustment, redundancy of function in other regions) that parallel the destructive forces. For example, we have seen a slow-growing meningioma compressing the frontal lobes reach the size of a lemon over at least 25 years and cause no clear-cut neurologic deficit; the patient was admitted to the hospital for onset of seizures. Malignant glial tumors (glioblastoma) frequently infiltrate neuronal tissue and many neurons lying within the tumor continue to function; this is one reason for the surprisingly small deficit occasionally associated with these very large tumors despite their typically rapid growth. It is not surprising, therefore, that removal of these tumors almost invariably leaves the patient with greater neurologic deficits than before surgery because many functioning neurons are lost. Degenerative processes typically produce slow progression while other conditions (such as multiple sclerosis) can appear as exacerbations or flare-ups. Conditions that produce recurrent injury to the brain (such as repeated strokes) appear as a stepwise, progressive dysfunction.
Recuperation of function following the removal of lesions of the nervous system takes three basic forms: resolution, reorganization and compensation.
Resolution of the lesion. This can occur by the clearing of edema, ischemia, hemorrhage or tumor compression of tissue. It is also seen with the resolution of metabolic suppression of neurons, for example, drugs, uremia, hypoxia, etc. This is the main mechanism of recovery in adults and older children with major dysfunction.
Reorganization of the nervous system does occur especially with chronic injury to the nervous system. This can result from redundancy built into the nervous system and/or from multipotentiality of function in the remaining normal neurons. This is an important mode of recuperation following minor destructive lesions. For example, there is significant resolution of symptoms and recovery of normal function after lesions that damage one vestibular nerve. In young children (five years or younger) this can be the major mode of recuperation following destructive lesions. In adults and older children plasticity or multipotentiality of neuronal function is less able to aid in compensation, whereas in young children it can be very marked for certain major functions. For example, damage to the left hemisphere that would be expected to leave an adult with permanent and severe dysphasia can be well compensated by development of speech in the right hemisphere in young children. Additionally, residual hemiparesis in children following large hemispheric lesion is less than that seen in adults. Even in adults, however, it is known that small cortical lesions may be compensated by increased activity in surrounding areas of preserved cerebral cortex. Redundancy is very important for preservation of some functions. A classic example of this is long-term memory for well-learned events and facts. These are very resistant to being lost by cortical lesions because they are diffusely represented in both cerebral cortices. Reorganization is less important in recuperation following large lesions of the nervous system.
Compensation is the process of using unaffected neuronal systems to replace the functions of damaged ones. This occurs depending on the degree of damage and the particular system affected. This also depends on conscious awareness of a deficit, which can be a problem in patients with parietal lobe (particularly nondominant) lesions. Compensation might include using an unaffected limb to replace the function of one that is impaired or it could include training to check the region of a visual field deficit. Some compensation may be enhanced artificially. For example, prisms can be used to correct double vision or to expand visual field. Various occupational and physical therapy protocols have been developed to allow patients with deficits to perform certain functions that would otherwise be impossible.
- Monrad-Krohn, G.H. and Refsum, S.: The Clinical Examination of the Nervous System, ed. 12, London, H.K. Lewis & Co., 1964.