NEUROPHYSIOLOGICAL ABNORMALITIES

NEUROPHYSIOLOGICAL ABNORMALITIES The human nervous system consists of the brain, the spinal cord, and an intricate network of rve fi bers projecting from the brain and spinal cord. Structurally, the brain is differentiated into the two cerebral hemispheres, the brain stem, and the cerebellum. The brain, together with the spinal cord, traditionally has been conceptualized as the central nervous system (CNS). The entire network of nerve fi bers is then referred to as the peripheral nervous system (PNS). The brief discussion regarding normal neurological structure and function that follows is meant as an aid in the appreciation of neurophysiological disorders. The intent here is to offer an overview; for a more detailed account of the nervous system, the reader is referred to one of a number of neurophysiological texts (e.g., Bickerstaff, 1978; Lindsley & Holmes, 1984; Swaiman & Ashwal, 2006). Peripheral nerves are referred to by the direction the impulses fl ow and the site of their termination. Specifi cally, the direction of the impulses carried in relation to the CNS, the originating structure, or fi nal destination of the impulse, and the nature of the impulse itself, are used to classify peripheral nerves. For instance, the PNS contains sensory nerves that carry impulses from the sense organs (eyes, ears, nose, etc.) to the CNS. By way of contrast, the motor nerves travel from the CNS to the periphery, exciting both skeletal (voluntary) and smooth (involuntary) muscle into movement. Included in PNS, the cranial nerves arise from or travel to the brain stem (connecting structure between spinal cord and cerebrum). Similarly, the spinal nerves travel to or from the spinal cord. The group of peripheral nerves that carry impulses to smooth muscle (causing involuntary movements of the intestines, heartbeat, constriction of the pupils, etc.) and those that incite the secretion of glands cause automatic changes in the body. These peripheral nerves are sometimes referred to collectively as the autonomic nervous system. Functionally, the fundamental building block of the nervous system is the neuronal circuit. The simplest neuronal circuit contains only two interconnected nerve cells, involving an input and an output cell (e.g., simple knee jerk refl ex). Local circuits exist at all levels of the nervous system and, in fact, such circuits in the spinal cord connect the cerebral cortex, brain stem, and cerebellum. These connections can function as modules in more complex circuits. Indeed, these integrated networks are capable of sustaining complex behavior (Gaddes, 1985; Kandel, Schwartz, & Jessell, 1991). As an example, sensory impulses traveling from the various sense organs to the brain are integrated, recorded, recognized, stored or remembered, as interpreted by the cerebral cortex. Moreover, skeletal movement may be affected by motor nerves traveling by way of the spinal cord. Generally, the entire system works to regulate and coordinate bodily responses to both internal and external changes in the environment (Taber, 1970). A malfunctioning neurological system results in an impaired capacity for responding adaptively to a changing environment. Neurophysiological abnormality may occur by means of many agents and during various stages of the life process; some stages offer more vulnerability than others. Antenatal agents (occurring before birth) described by Nelson (1969) include genetic factors, chromosomal aberrations, placental disease, maternal complications, number of previous pregnancies, age of both mother and father, intrauterine infection, toxic agents (including certain drugs and alcohol), and radiation. Various organ systems begin and end their prenatal development at different times, therefore their sensitivity to agents varies with maturity of the fetus. The most vulnerable period for the brain is from 15 to 25 days of gestation but, clearly, damage can occur at any time during the development of the nervous system (Hetherington & Parke, 1979). Perinatal (occurring just before or after birth) vulnerability to neurological insult is accentuated by premature birth. Inadequate oxygen during this stage, hemorrhage, trauma, and infection are the principal offenders (Nelson, 1969). Postnatal (occurring after birth) damage to the neurological system may include damage incurred after birth, during childhood, or throughout the various stages of adulthood. Infections, principally meningitis and encephalitis,injuries, and degenerative neurological disease have also been implicated (Nelson, 1969). Weller, Swash, McLellan, and Scholtz (1983) estimated that 40 percent of developmental malformations of the CNS arise from genetic abnormality. The most common genetic abnormality is Down’s syndrome. This disorder is associated with a group of chromosomal aberrations involving the 21st chromosome pair. In the great majority of cases, a failure to join occurs during the meiosis process, resulting in a trisomy (additional chromosome) of the 21st chromosome pair. Translocation and mosaician represent less frequently occurring aberrations of the 21st chromosome pair, also associated with Down’s syndrome (Kopp & Parmelee, 1979). The incidence of Down’s syndrome is between one and two per thousand live births for all races and ethnic groups (Gillberg, 1995; Norman, 1963). Although there is some variability in incidence, most researchers cite an increase in relation to maternal age (Benda, 1960; Lawrence, 1981; Weller et al., 1983). A gradual increase begins with maternal age of 35 and escalates drastically after 40. Metabolic or environmental factors in the mothers’ ovaries have been suggested as causes for the syndrome (Benda, 1960; Lawrence, 1981; Nelson, 1969; Norman, 1963; Weller, Swash, McLellan, & Scholtz, 1983). Structural inspection of the Down’s syndrome brain suggests impairment of both growth and differentiation (Benda, 1960). The brain is generally low in weight and the normal convolutional pattern of the brain is simplifi ed. The density of the nerve cells in the cerebral cortex is reduced (Weller et al., 1983). Rate of mental development is not only slower than normal but also deteriorates progressively with age in Down’s syndrome (Cornwell & Birch, 1969; Dicks- Mireaux, 1972; Gillberg, 1995). Many explanations, including neurophysiologic changes, have been offered as explanation for this progressive deterioration. Weller et al. (1983) noted that the microscopic study of brain tissue of Down’s syndrome victims during autopsy reveals patterns of neurofi brillary tangles, senile plaques, and granulovacular degeneration such as are found in Alzheimer’s disease (deteriorative disease of the elderly involving degeneration of the smaller blood vessels of the brain). Kopp and Parmelee (1979) suggest that the severe limitations in higher level integrative abilities evident in Down’s syndrome may cause defi cits in information processing (e.g., use of language) that could have progressive detrimental effects on the child’s intellectual development over time. The child’s capacity for responding adaptively to changing stimulus conditions, a necessity for proper intellectual development, may be impaired directly by the nature of the syndrome. However, the nature of the environment in which these children fi nd themselves, whether it is enriched or impoverished, also can affect development. In contrast to Down’s syndrome, which is genetically related, spina bifi da seems to be more infl uenced by environmental factors. Although genetic factors are suggested by the higher incidence in infants born to parents with a family history of such lesions, it seems that racial, geographical, and even seasonal factors also may be implicated (Kopp & Parmelee, 1979; Weller et al., 1983). Clearly, the interaction of genetic and environmental factors has recently been given prominence. Genetic predisposition combined with certain environmental factors may be the causal condition for spina bifi da occurrence (Carter, 1974). Spina bifi da represents a malformation of the nervous system that appears to be more localized and variable in effect than that of Down’s syndrome. This defect occurs as a result of faulty prenatal development, in which the lower end of embryotic CNS fails to close. The contents of the spinal column (nerve fi bers, meninges, and fl uid) may protrude from the lower back in a sac (meningomyelocele). Individual defects vary depending on the extent of damage to the nerve fi bers and the existence of other associated conditions (Kleinberg, 1982). The spinal cord is frequently abnormal above and below the level of the spina bifi da (Weller et al., 1983). Hydrocephalus, abnormal accumulation of cerebral spinal fl uid, frequently is associated with spina bifi da. Untreated hydrocephalus creates severe enlargement of the head, increased pressure, and subsequent damage to the brain (Kleinberg, 1982). Intellectual levels of victims with spina bifi da are variable, ranging from an IQ of 137 to severe subnormality (Gillberg, 1995; Hunt, 1981). More specifi cally, Spain (1974) associates mental retardation with protrusion of a portion of the brain (cranial meningocele and cephalocele), whereas infants with other forms are considered to have potentially normal intellect. Many individuals with spina bifi da are incontinent of urine and feces, and have weakness of their legs with sensory loss below the level of the lesion (Kleinberg, 1982). Owing to the presence of the typical locomotor problems in spina bifi da, it is unclear whether some defi cits are due to neurological impairment or environmental infl uence. Spain’s (1974) longitudinal spina bifi da studies have revealed signifi cant defi cits in spatial and manipulative development. The fact that the disorder limits the individual’s experience may, in fact, cause or infl uence the specifi c defi - cits in spatial and manipulative development. Among the educational problems noted are diffi culties with arithmetic and perseveration in language, as well as emotionality and poor motivation (Kopp & Parmelee, 1979). Primary disorders of the CNS, like Down’s syndrome and spina bifi da, represent a relatively small proportion of the neurological problems in infants (Horwitz, 1973). More frequently, the genetic programs for potentially normal neurological development are subverted by adverse prenatal or birthing conditions such as lack of oxygen (hypoxia). Cerebral hemorrhage often occurs during prolonged hypoxia. The accumulation of stagnate blood that follows circulatory collapse may cause bleeding and ultimate damage to brain tissue (Weller et al., 1983). Premature infants are especially vulnerable to hypoxia. Since the respiratory system is notfully perfected until the last four to six weeks of gestation, these infants are often born without an optimally functioning respiratory system. Postmortem studies on premature children show that the bleeding usually occurs within one of the cavities of the brain or the space below the arachnoid membrane that contains cerebrospinal fl uid (subarachnoid space [Horwitz, 1973]). Later complications of such subarachnoid hemorrhage involve epilepsy, dementia, and hydrocephalus (Weller et al., 1983). Full- term infants are more likely to suffer from hemorrhage in the mid- brain stem (pons) and the posterior portion of the cerebral cortex (hippocampus). Cause for these differences are not, as yet, fully understood. The location and size of brain lesions at or soon after birth are the primary determinants of the extent of nervous system impairment. The results may range from a gross alteration of brain organization to more minimal effects such as motor overactivity, shortened attention span, or slight muscle impairment (Pincus & Tucker, 1974; Teberg et al., 1982). Large injuries in infants tend to produce more widespread defi cits in intellectual abilities than similar injuries in adults. Dulling of many areas of intellectual functioning, as opposed to having an effect in specifi c functioning (e.g., language development, visual- spatial relationship comprehension), is also a hallmark effect of the diffuse damage that follows hypoxia (Rapin, 1982). Neurological defi ciencies from early injury are diffi cult to predict. The nervous system of the newborn infant is extremely immature, functioning largely at brain stem and spinal cord level. The neurologic refl exes such as Moro, grasping, and stepping represent primitive neuronal function that is largely uninhibited by higher cerebral control. Changes in these refl exes are usually not helpful in localizing the lesion, and may occur with either cortical or subcortical dysfunction (Horwitz, 1973). Damage to the cerebral cortex, for instance, may not be evident until the age when behavior dependent on the damaged part makes its developmental appearance. Thus, pathology of fi ne motor coordination, speech, and cognition is unlikely to be diagnosed in infancy (Rapin, 1982). However, changes in refl exes and disorganized activity of the subcortical structures expressed as a movement disorder or spasticity continue to be used as indicators of neurological damage. In Teberg et al.’s study of low birth weight infants (1982), spastic quadriplegia did, in fact, emerge as the indicative diagnosis of neurological handicap. Churchill, Masland, Naylor, and Ashworth (1974) support this fi nding. Turkewitz (1974) contended that the standard methods used for the early identifi cation of neurologic handicaps are insensitive to many forms of neurological involvement. Infants who have had diffi culties shortly before or during the birth process frequently appear to recover in a few days. However, abnormalities in motor, language, and intellectual functioning become apparent later in infancy and childhood. Studies using indicators of higher levels of neurological organization (e.g., left / right preference) are being investigated in an effort to identify infants who have experienced neurological damage that is normally not expressed until later in life. However, normative patterns of left / right preference for infants must be established fi rst, before atypical patterns can be interpreted. The possibilities for neurophysiological dysfunction are limitless; the pathologies presented should not be considered as inclusive by any means. However, it is hoped that an appreciation of the complexity of cerebral neural structure and the corresponding intricacies of impairment resulting from neurophysiological dysfunction will encourage the reader to treat each impaired patient as a unique individual, for heterogeneity of outcome is common (Gaddes, 1985; Goldstein & Reynolds, 1999; Kopp & Parmelee, 1979).