Classification of Movement Disorders

Introduction

Movement disorders are neurological syndromes that involve impaired performance of voluntary movements, dysfunction of posture, the presence of abnormal involuntary movements, or the performance of normal-appearing movements at inappropriate or unintended times. The abnormalities of movement are not due to weakness or abnormal muscle tone but may be accompanied by weakness or abnormal tone. By convention, movement disorders are divided into two major categories. The first category is hyperkinetic movement disorders, sometimes referred to as dyskinesias. This category refers to abnormal, repetitive involuntary movements and includes most of the childhood movement disorders including tics, stereotypies, chorea, dystonia, myoclonus, and tremor. The second category is hypokinetic movement disorders, sometimes referred to as akinetic/rigid disorders. The primary movement disorder in this category is parkinsonism, manifest primarily in adulthood as Parkinson disease or one of many forms of secondary parkinsonism. Hypokinetic disorders are relatively uncommon in children. Although weakness, spasticity, and ataxia are characterized by motor dysfunction, common convention considers these entities separate from the classic “movement disorders.” The terms “basal ganglia disorders” or “extrapyramidal disorders” should be abandoned as they lack specificity, and most movement disorders cannot be accurately ascribed to any particular anatomical abnormality.

When faced with a movement disorder, the first step is to characterize the movement phenomena. Is the pattern of movements normal or abnormal? Are there excessive movements or is there a paucity of movement? Is there decomposition or disorder of voluntary movement trajectories? Is the movement paroxysmal (sudden onset and offset), continual (repeated over and over), or continuous (without stop)? Has the movement disorder changed over time? Do environmental stimuli or emotional states modulate the movement disorder? Can the movements be suppressed voluntarily? Is the abnormal movement heralded by a premonitory sensation or urge? Are there findings on the examination suggestive of focal neurologic deficit or systemic disease? Is there a family history of a similar or related condition? Does the movement disorder abate with sleep?

In clinical practice, the diagnosis of a movement disorder requires a qualitative appreciation of the movement type and context. Abnormal movements can be difficult to define. To best classify the disorder phenomenologically, one should describe the characteristics of the movements. Even under the best circumstances, movement disorders may be difficult to characterize. However, careful observation of the spatial and temporal properties of the movement, often with the aid of video, can usually lead to appropriate identification of the phenomenology.

Movements in some contexts may be normal and in others may indicate underlying pathology. Movements that are worrisome for a degenerative disorder in adolescents (myoclonus) may be completely normal in an infant (benign neonatal myoclonus). It can be quite difficult to specifically diagnose a movement disorder without seeing the abnormal movements. Thus, obtaining video examples of the child's movement may be essential to making a correct diagnosis. The video atlas accompanying this book provides examples of the different types of movement disorders.

Many classification schemes have been used to provide a taxonomy for the wide variety of movement disorders. Disorders can be classified by phenomenology, based on the observed temporal and spatial features of the movements themselves, along with characteristic clinical features ( Tables 3.1 and 3.2 ). They can also be classified based on presumed etiology, anatomic localization, or neuropathological features, by disease course, by genetic or molecular criteria, or by other biological factors. In response to advances in genetics, the Movement Disorders Society proposed a new classification of movement disorders but the nomenclature is constantly evolving. Even in the era of “next generation” DNA sequencing, classification of movement disorders according to phenomenology provides an important framework for diagnostic interpretation, for therapeutic decisions, and for development of new treatments. This chapter focuses on phenomenologic definitions and classification. Subsequent chapters discuss the etiologies, localization, and differential diagnosis of these disorders.

Ataxia ( Chapter 14 )

Ataxia literally means “without order.” It is defined as an inability to generate a normal or expected voluntary movement trajectory that cannot be attributed to weakness or involuntary muscle activity about the affected joints. Ataxia can result from impairment of spatial pattern of muscle activity, of the timing of that activity, or both. Specific associated deficits include dysmetria (inaccurate movement to a target (undershoot or overshoot)), dyssynergia (decomposition of multijoint movements), and dysdiadochokinesis (impaired rhythmicity of rapid alternating movements). Ataxia does not occur at rest, but it may be associated with hypotonia of affected body parts at rest. Although ataxia is classically associated with dysfunction of the cerebellum, it can result from lesions in the cerebellar afferent or efferent pathways (see Chapter 2 ).

Athetosis ( Chapter 10 )

Athetosis literally means “without position or place.” It is defined as slow, writhing, continuous, involuntary movements. In athetosis, the same regions of the body are repeatedly involved, unlike chorea where individual involuntary movements appear to move from one body part to another. Athetosis may worsen with attempts at movement or posture, but it can also occur at rest. Athetosis typically involves the distal extremities (hands or feet) more than the proximal extremities and it can also involve the face, neck, and trunk.

Athetosis is distinguished from dystonia by the lack of sustained postures, although it may accompany dystonia. Athetosis differs from chorea by the lack of identifiable movement fragments. Athetosis is not rhythmic or stereotyped. There is some debate as to whether athetosis should be considered as a discrete entity, or whether it is part of the spectrum of dystonia or simply a form of “slow chorea.” Although cerebral palsy is one of the most common causes of childhood athetosis, athetosis may be associated with multiple etiologies.

Ballismus ( Chapter 10 )

Ballismus or ballism refers to involuntary, high amplitude, flinging movements typically occurring proximally. These movements may be brief or continual and may occur in conjunction with chorea. Often, one side of the body is affected, that is, hemiballism . In many cases, hemiballism gets milder with time and evolves into chorea. It has been suggested that because the same lesions can produce both ballismus and chorea, ballismus is likely to part of the spectrum of chorea and not a separate entity.