CHAPTER 10 Simon Platt Involuntary movement abnormalities result in some of the most dramatic clinical presentations in veterinary medicine. Classically, involuntary movement disorders are present during periods of inactivity rather than during voluntary movement. Some involuntary movements are persistent, while others are episodic. Certain involuntary movements have characteristics that allow for the identification of specific causes, whereas others are only a reflection of dysfunction of the nervous or musculoskeletal systems. Clinically, it is important to first identify the type of involuntary movement present. Subsequently, a more directed approach can be used to establish the cause of the movement disorder. Paroxysmal events are characterized by the sudden and reversible onset of neurologic dysfunction in an otherwise normal animal. Some movement disorders can be paroxysmal. The animals do not lose consciousness and rarely have a structural lesion identifiable within the central nervous system (CNS). The underlying cause of many of these events may be a functional abnormality related to neurotransmitter imbalances or receptor abnormalities and dysfunction. Several stereotypical events have been described in specific breeds and are discussed below. Confirmation of the specific syndrome is difficult or impossible in the clinical setting but depends heavily on the exclusion of structural CNS abnormalities such as neoplasia, inflammation, and cerebrovascular disease. Terms such as tics, twitches, shivering, shuddering, and fasciculation are often used to describe episodic, irregular muscle movements or depolarization associated with muscle contractions (Table 10.1). Involuntary movements, however, are usually manifested through abnormal motion of the limbs, trunk, or head. Table 10.1 Types of involuntary movement and paroxysmal disorders. Myoclonus6, 7, 22 Myoclonus is a shocklike contraction of a muscle, or muscles, that tends to occur repeatedly in a rhythmic pattern and may persist during sleep. It is akin to the rhythmic depolarization and contraction that occurs in the heart with each beat. Myoclonus can be focal, multifocal, or generalized; it often presents in a thoracic limb; however, a pelvic limb or the facial muscles including the tongue may also be involved. Myoclonus may be physiological (such as that seen when falling asleep or during sleep), epileptic, or symptomatic associated with CNS disease. An idiopathic, essential, myoclonus has been recognized in people but has not been described in veterinary medicine. Myoclonus in dogs is usually the result of distemper infection, which establishes a pacemaker-like depolarization of local motor neurons; however, it has been associated with lead toxicity and other causes of CNS inflammation. Myoclonus is often described to originate from spinal disease causing a localized persistent movement abnormality due to abnormal lower motor neuronal discharges. This usually affects one or two limbs and occasionally the jaw. The muscle contractions occur rhythmically and are most obvious in resting animals; they are present throughout activity and do not disappear during sleep. It can also arise from apparent cerebral disease as a type of seizure. Tremor6, 7, 16, 22, 24 Tremor is one of the most common involuntary movement disorders in humans, and is also surprisingly common as a clinical abnormality in dogs. Tremor is an involuntary rhythmic, oscillating movement of fixed frequency resulting from alternate or synchronous contraction of reciprocally innervated antagonistic muscles. It can be focal, affecting just one limb or the head, for example, or generalized. Electromyographically, tremor is characterized by rhythmic bursts of motor neuron activity occurring in opposing muscle groups. The contraction of muscles with opposing function gives tremor a biphasic nature. This biphasic character differentiates tremor from other abnormalities of movement. While seen during the awake state, true tremor should cease with sleep. As for myoclonus, tremors may be physiological; idiopathic (or essential), such as that seen in senile tremor of dogs; or pathological, due to a nervous system disease. Tremor is ultimately a disorder of movement. Therefore, lesions in any of the regions of the CNS and peripheral nervous system (PNS) and musculoskeletal system primarily responsible for normal movement, may generate a tremor. This makes localization challenging when considering the clinical signs alone. In humans, important motor areas include the basal nuclei and other components of the extrapyramidal system, the cerebellum, diffuse neuronal cell bodies involved in segmental and supraspinal reflex mechanisms, components of the lower motor neuron, and the interconnecting pathways. Additionally, abnormalities of the mechanical apparatus of the limbs (e.g. bones, joints, tendons) may also result in tremor as a result of pain and weakness. However, species differences do exist, and it is important to note that lesions involving the basal nuclei and substantia nigra commonly result in tremor in human beings but not in dogs. Tremors that occur or worsen when an animal is trying to perform purposeful movements (intention tremors) are most often associated with cerebellar disease. Fine tremor (decreased amplitude and increased frequency) is more often associated with diffuse neuronal disease or muscle weakness. The causative lesion may give rise to other signs of neurologic dysfunction that can help further define the localization, such as dysmetria associated with cerebellar disease. Myokymia and neuromyotonia3, 6, 22, 27, 28, 31, 33 These refer to the involuntary rippling of muscles that persists even during sleep and under anesthesia. The disorders represent a continuum of signs that result from motor axon or terminal hyperexcitability. This hyperexcitability can be caused by a wide variety of disorders of the CNS and PNS but is particularly related to changes in ion channel function. Electromyography (EMG) in myokymia reveals short bursts of ectopically generated motor unit potentials, firing at rates of 5–62 Hz and appearing as doublets, triplets, or multiplets (these bursts fire rhythmically or semi-rhythmically, and sound like soldiers marching). Neuromyotonia is characterized by muscle stiffness and persistent contraction related to an underlying spontaneous repetitive firing of motor unit potentials. On EMG there are prolonged bursts of motor unit potentials, firing at rapid rates of 150–300 Hz, which begin and end abruptly, do not occur repetitively in a rhythmic fashion, and have characteristic waning amplitude. There are few descriptions in companion animals, but it appears to be an emerging problem in Jack Russell Terriers. Fasciculations arise from ectopic electrical activity in the distal axon and are typically the manifestation of irritability of the neuronal cell body or its associated axons. Myokymia is a result of spontaneous discharges of large motor units and indicates neuronal disease, followed by a sprouting of the motor unit territory in response to denervation. The term neuromyokymia has been used to implicate the role of the neuronal axon in this disorder. Dyskinesia19, 20, 23, 25, 30 Dyskinesia is defined as impairment of the power of voluntary movements resulting in fragmented or incomplete movements. Dogs reported with these abnormalities may exhibit abnormal postures, such as holding up a limb in an attempt to move or adopting a kyphotic posture of the spine without being able to initiate movement. The pathophysiologic mechanisms underlying these movements are poorly understood but may represent a central neurotransmitter or pathway abnormality, or possibly a local muscular abnormality. The impaired movement can appear as and have been termed muscle “cramps,” which are defined as paroxysmal—prolonged and severe contraction of muscles that may be painful and can be either focal or generalized. Examples of diseases associated with cramps which may be dyskinesias include Scotty cramp, episodic falling of Cavalier King Charles Spaniels, “epileptoid cramping” of Border Terriers, and extreme generalized muscular stiffness in male Labrador Retrievers. Muscle cramps have also been described secondary to systemic diseases such as hypoadrenocor-ticism. Dyskinesias are movement disorders that occur spontaneously during activity or at rest, causing involuntary contractions of groups of muscles in a conscious animal. The descriptions of these conditions indicate that the most common clinical sign is that of dystonia causing increased muscle tone in one or several limbs, possibly leading to collapse. The movements can be triggered by excitement or exercise. The localization of the purported functional neurotransmitter-based abnormalities responsible for these disorders may be CNS or PNS. In general, movement disorders may have origins in the cerebrocortical neurons, basal nuclei, or PNS. The clinical presentation of movement disorders is complex, often variable, and sometimes even bizarre. Establishing the correct diagnosis can, therefore, be difficult. Obtaining an accurate history of the patient is important to define the onset and progression of the condition in addition to elucidating any underlying systemic health problems that could be causing the disorder. Physical examination is essential as some tremor disorders may be associated with systemic disease. Many tremor syndromes may also be associated with neurologic deficits; therefore a neurologic examination can help to localize the causative lesion or associated deficits and determine the next stages necessary in the diagnostic work-up. The following tests should be considered in most patients with movement disorders: The clinical characteristics of the abnormality may not only suggest that it is a movement disorder rather than one of the many mimics but also aid in the underlying etiology. Various classification schemes have been proposed based on the presence of activity at the time of the disorder, whether they are continuous or episodic, involve the muscle or nerves, and whether there is too much or too little movement. Ultimately, these schemes can be overcomplicated in veterinary medicine when so few of these disorders are seen and definitively diagnosed. More simply, the abnormalities can be investigated by dividing them into localized or generalized syndromes. Localized limb tremors/myoclonus Recent classification schemes have suggested calling tremors a form of myoclonus (often action related, indicating they are more pronounced with activity). In this text, tremors and myoclonus are kept as separate entities with “myoclonus” referring to rhythmic activity of large groups of muscles causing flexion and extension of limbs as opposed to diseases causing more “fine” movement abnormalities of small muscle groups (tremor).
Involuntary Movements and Paroxysmal Disorders
Introduction
Types of involuntary movements
Clinical sign or syndrome
Definition
Cramp
Muscle cramps are involuntarily and forcibly contracted muscles that do not relax
Dyskinesia
Difficulty or distortion in performing voluntary movements
Dystonia
Sustained muscle contractions cause twisting and repetitive movements or abnormal postures
Fasciculation
Involuntary contractions or twitching of groups of muscle fibers
Myoclonus
Rhythmic movement of a portion of the body resulting from sudden involuntary contraction and relaxation of muscle groups
Myokymia
Continuous involuntary muscle twitching that give the appearance of wormlike rippling of muscle
Myotonia
Sustained muscular contraction following an initiating stimulus
Rigidity
Increased resistance to change in position or angle of joint(s)
Spasm
A brief, automatic jerking movement
Spasticity
A state of increased tone of a muscle
Tetanus
Sustained muscular contraction without a period of relaxation
Tetany
Intermittent tonic muscular contractions
Tremor
Any abnormal repetitive shaking movement of the body
Diagnostic approach
Establishing the etiology using clinical characteristics
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