Comprehensive case history important when evaluating involuntary movements
ADogs may develop atypical gait and movement disorders that are difficult to determine the underlying cause. Movement disorders may encompass excess (hyperkinesias) or reduced (bradykinesia) movements.
Abnormal involuntary movements include muscle jerks, twitches, postures and oscillations that have been defined as a tic, chorea, tremor, dystonia and myoclonus. The following article describes abnormal involuntary movements of dogs in clinical practice - Podell M: tremors, shakes and twitches: movement disorders in small animals. Proc 20th Annual Forum ACVIM 20:362-364, 2002.
The visualization of abnormal involuntary movements should be supplemented with a comprehensive case history. Information regarding anatomic distribution, rhythmicity, amplitude, speed of onset and offset of the movement, relationship to posture and activity, situations that alleviate or exacerbate the movement, presence or absence during sleep, and affected littermates are essential observations and history to help determine the neuroanatomic localization and potential cause.
If the abnormal movement disorder is not present at the time of the initial examination, owners should be encouraged to videotape the events for future review. Many times, the initial few minutes of observation will solidify the clinical perspective, allowing an accurate diagnostic and therapeutic course of action to proceed.
Excessive movement disorders
Myotonia is a sustained muscle contraction with delayed relaxation. Hereditary myotonia occurs in the Chow Chow and is seen sporadically in Chow crosses and a number of other dog breeds. Myotonia is due to a failure of normal myocyte chloride conductance resulting in delayed muscle hyperpolarization and, therefore, delayed relaxation.
As an autosomal recessive disease, puppies are affected from birth. Signs include a stiff, "sawhorse" stance on movement, with improvement in gait as exercise time increases. Affected dogs have hypertrophied proximal appendicular muscles that exhibit percussion dimpling when being struck with a percussion hammer.
Electromyographic recordings demonstrate the classic myotonic discharge of a high frequency waxing-waning spontaneous discharge. Muscle biopsy is usually normal. Procainamide or quinine may be used for treatment but usually is unsuccessful in alleviating the myotonia. Many dogs can live a good quality life by avoiding excessive exercise in the cold and maintaining a normal exercise routine.
Tetanus is a continuous sustained extensor muscle contraction. Tetanus most often occurs during the time period of shedding deciduous teeth or from contaminated bite wounds. The cause is the tetanus toxin released by Clostridium tetani. The exotoxin, tetanospasm, travels from the infected site via peripheral nerves to the central nervous system. Toxin prevents the release of the inhibitory neurotransmitter, glycine, resulting in excessive excitation of brain stem and motor neurons. Cats and dogs are fairly resistant to tetanus.
However, when infected, they can exhibit an extreme stiffness progressing to extensor rigidity of all limbs, spastic facial muscles and trismus within 10 days of infection. Many animals are hypersensitive to external stimuli. Occasionally, localized tetanus may affect only a body region such as a limb.
Wound debridement, parenteral anaerobic antibiotic administration, muscle relaxation using acepromazine and nutritional support are used in the treatment. Complete remission of clinical signs will usually occur within several weeks to months.
Tetany is a variable and intermittent extensor muscle contraction. Tetany may accompany both central and peripheral nervous system diseases. In dogs, tetany is most commonly seen with hypocalcemia associated eclampsia or hypoparathyroidism.
Total serum calcium is typically below 5.0 mg/dl. Affected dogs may show inability to rise, extensor muscle contractions and hyperthermia (induced by excessive muscle contractions). Treatment includes initial muscle relaxation with benzodiazepines, followed by calcium and vitamin D supplementation.
Myoclonus is sudden, rapid, involuntary muscle movement of short duration caused by active muscle contractions (positive myoclonus) or pauses in muscle activity (negative myoclonus). Reflex myoclonus to auditory stimuli occurs in the Labrador Retriever breed. Spinal myoclonus arises from abnormal neuronal discharges originating in the spinal cord. Segmental myoclonus occurs in canine distemper, producing a repetitive, myoclonic jerk motion of the one or more limbs.
Tremors may occur either at rest or with action.
· Resting tremors describe an involuntary, rhythmic oscillation of a body part completely supported against gravity. This tremor could be seen in a leg with the animal laying down and not supporting weight.
· Action tremor occurs during voluntary contraction of skeletal muscle and may be postural, kinetic, isometric or task-specific.
· Postural tremors describe oscillation of a body part that is voluntarily maintained against gravity. This tremor type is uncommon in small animals.
· Kinetic tremors describe oscillation during guided voluntary movement. These kinetic (intentional) tremors are the most common type seen with cerebellar disease.
· Isometric tremors and task-specific tremors are seen in primate species that can hold objects and initiate specific movements of the hands and arms.
· Physiologic and essential tremor syndromes may occur in older dogs, and in particular, aging Terrier breeds. These tremors are a pure clinical syndrome characterized by progressive action tremor of pelvic limbs that worsens with activity and excitement. Severity can range from barely perceptible tremor to altered gait and balance problems. Signs can progress as the dog ages.
· Drug-induced tremors occur in small animals. Predictable tremors can be seen with stimulant toxicity, such as caffeine, amphetamines and cocaine. Rhythmic involuntary movements are the most common manifestation resulting from exposure to dopamine receptor blocking agents, such as phenothiazine (e.g., acepromazine) or anti-emetic drugs (e.g., metoclopramide).
· Cerebellar-related tremors are definitely the most common cause of tremors in small animals. Congenital cerebellar diseases included hypoplasia, malformation, hypomyelinogenesis, dysmyelinogenesis, abiotrophy and lysosomal storage disease. Acquired cerebellar diseases are inflammatory, infectious (canine distemper), immune-mediated (granulomatous meningoencephalitis and steroid-responsive tremor syndrome), neoplasia, vascular/traumatic and toxin causes.
Cerebellar diseases are often associated with signs related to abnormal motor activity, including any or all of the following: tremors, bilaterally symmetric ataxia without paresis, dysmetria, vestibular signs (head tilt, nystagmus, falling), absent menace with preservation of vision and pupillary changes.
An altered resting posture is often present with affected animals demonstrating truncal ataxia (swaying of the body back and forth or side to side) and compensatory broad-based stance for balance. Cerebellar tremors are associated with diffuse cerebellar cortical diseases. These intention tremors are characterized by a fine head tremor that worsens with initiation of voluntary head movements. The acute onset diseases affecting the cerebellar cortex usually result in more pronounced tremors.
Severe tremors may affect the entire body with complete loss of all muscular coordination and failure to posture and prehend food. Ensuing hyperthermia, rhabdomyolysis and related complications from continuous muscle activity require that these animals be aggressively treated on an emergency basis.
Fortunately, many pure cerebellar diseases can be treated and/or compensated for by the animal.
Treatment of tremor disorders
Emergency treatment for acute onset tremors in the dog may significantly reduce tremor severity. Such treatment may include diazepam at 0.5 mg/kg IV and if poor control, diazepam as continuous rate infusion (0.2-0.5 mg/kg hourly IV to effect); or phenobarbital 20 mg/kg IV followed by 2 mg/kg PO q12h; or propofol as continuous rate infusion (5-10 mg/kg hourly IV to effect) to stop tremors. It is also important to maintain normal body temperature and provide intravenous fluid therapy to avoid dehydration.