INTRODUCTION

Movement disorders encompass a large range of conditions that are characterized by abnormalities in volitional movement or by the presence of excessive involuntary movements. Over recent years, the field has witnessed an explosion in knowledge, including descriptions of new disorders, expansion of epidemiologic investigations, identification of genetic subtypes and molecular genetic mechanisms, revision of pathophysiologic theories, visualization of brain function with neuroimaging techniques, and development of new therapeutic strategies encompassing both symptom control and neuroprotection. The traditional approach to movement disorders emphasized their division into two categories: hypokinetic and hyperkinetic disorders (Table 1-1). According to this scheme, the hypokinetic disorders (parkinsonisms) feature delayed, slow, and low-amplitude voluntary movements with rigidity, and impairments in gait and postural reflexes. The hyperkinetic disorders are characterized by involuntary movements that occur spontaneously or are superimposed on volitional movement. Examples of the hyperkinetic disorders include chorea, hemiballismus, dystonia, tremor, tics, ataxia, and myoclonus. Based on pathologic findings or clinical resemblance to pathologically defined conditions, these disorders were collectively attributed to primary degeneration or dysfunction of the basal ganglia, an area of the brain that includes the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra, with extensive efferent and afferent connections to other brain areas. Although these general categorizations are still useful clinically, over the past decade, evidence has confirmed that many movement disorders encompass both hypokinetic and hyperkinetic features. For example, youngonset Parkinson’s disease may present with dystonia; certain of the spinocerebellar ataxias (SCAs) may show pure parkinsonian signs; Huntington’s disease may present with parkinsonism and dystonia; bradykinesia typically coexists with chorea; and dystonia and parkinsonism tend to predominate in advanced Huntington’s disease. Patients with primary dystonia have electrophysiologic evidence of bradykinesia. The notion that movement disorders could be simplistically divided has been replaced by the concept that these syndromes reflect complex interactions among basal ganglionic structures and between the basal ganglia and other central nervous system areas. Advances in the number, scope, and methodology of epidemiologic studies of movement disorders have expanded our understanding of the relative roles of genetic and environmental factors associated with disease. In Parkinson’s disease, twin studies have indicated that young-onset disease may have a large genetic contribution, while olderonset disease may be more dependent on exposure to specific environmental toxins, such as rotenone or paraquat; or lack of exposure to protective agents (Tanner, 2003). In the future, information gleaned from these studies in conjunction with investigations of basic mechanisms may more definitively explain the causes of specific diseases, thus providing the rationale for more effective treatments. 11 KEY POINTS: