Rodney S. Bagley, DVM, DACVIM (Neurology and Internal Medicine)
Involuntary movement disorders may occur at rest or during activity.
Tremor is an involuntary, rhythmic, oscillatory movement of all or part of the body. It results from the alternate or synchronous contraction of reciprocally innervated, antagonistic muscles. The equal to and fro movements are characteristic, however, can be difficult to appreciate when the tremor is high frequency and low amplitude (fine). True tremor ceases with sleep. Tremor can be localized to one body area or be generalized (whole body). Localized tremor usually involves the head or pelvic limbs.
Myoclonus is a shock like contraction of a muscle or muscles. This most often occurs in a thoracic limb, however, a pelvic limb or the facial muscles may also be involved. Myoclonus in dogs is usually the result of distemper infection which establishes a pacemaker-like depolarization of local motor neurons.
Other apparently involuntary movements are less well classified in animals.
Seizures are one the two most common admitting complaints of a neurologic nature in small animals. Due to the numerous causes, clinical ramifications, and treatments, a logical evaluation of affected animals is imperative. An initial but important decision is whether, in fact, the animal is having a seizure, or another episodic abnormality. The diagnosis of a seizure disorder often requires eliminating structural and metabolic intracranial causes for the seizure. Finally, client communication regarding possible treatments and realistic future expectations for the animal affected with a seizure disorder is one of the most important aspects of dealing with the seizuring animal.
As with all clinical evaluation, important decisions and determinations influence the ultimate diagnosis and management of the animal's problem. Questions arise during evaluations that help to clarify the disease. Similar questions occur with seizure disorders. Following logic thought pattern is most apt to result in the appropriate diagnosis and subsequent management of affected animals. It may help to record observations in some form to ensure all information is collected and assessed.
Is the Animal Having a True Seizure, or Something Else?
A seizure is defined as a paroxysmal, transitory disturbance of brain function that has a sudden onset, ceases spontaneously, and has a tendency to recur. Seizures are usually associated with autonomic disturbances such as urination, salivation and defecation. Epilepsy is a recurrent seizure disorder irrespective of cause.
Unfortunately, "seizure" is a common word that non-medical individuals are familiar with, and apply to episodic disturbances that affect their animals. This may lead to inappropriate diagnoses and treatments if the actual cause of the "episodic disturbance" is not appropriately identified and treated.
History and owner descriptions of the events in question are important aspects of clinical evaluation of animals with seizure disorders, as often the events are not witnessed in the hospital. The owner should not be asked leading questions to avoid introducing bias that may affect later interpretations. One way to avoid this is to simply ask the owner to describe what happens to the animal.
Generalized, tonic clonic seizures have characteristic features in most dogs and cats allowing for their differentiation from other events. Important clues that a generalized seizure is occurring include falling over or becoming laterally recumbent, paddling or running movements of the limbs, urination, defecation, or salivation. If may be difficult to ascertain if an animal is conscious or aware of the event, but this is often an important aspect of determining that a seizure has occurred. Animals with generalized seizure often have their eyes open which may confuse an owner into believing that the animal is conscious. If the animal is responsive to external stimuli such as touching or talking, this may indicate that at least some consciousness is preserved. With focal seizures, however, consciousness may remain intact adding confusion to the clinical evaluation.
Generalized seizures often last a short period of time. Single seizure events often are only 30 to 90 seconds in duration. There may be a preictal phase of varying lengths. Often, during this period, the animals have behavioral abnormally such as hiding or becoming apprehensive. Following a seizure, there is often a phase (postictal) where the animals may also behave abnormally, and most importantly, have focal neurologic examination abnormalities. This feature becomes important when formulating a differential diagnosis for seizuring animals.
Events that are often confused with true seizures include syncope, narcolepsy, cataplexy, behavior disorders, weakness episodes, vestibular disorders, cervical painful responses, tremors, and other episodic movement disorders. If the description of the event is not clear, and if the event occurs frequently at home, asking the owners to videotape the event for further review is often helpful in the clinical diagnosis.
Seizures can be classified in numerous ways depending upon where the seizure activity originates, which areas of the brain are affected, or the actual cause of the seizure. A simplistic yet realistic classification system initially categorizes seizures as having either a generalized or focal onset.
Generalized seizures affect the entire body simultaneously. The generalized tonic, clonic seizure is the most common seizure seen in animals.
Focal seizures remain localized to one body region. Focal seizures may become generalized, and are of value in localization of the seizure focus to one or the other side of the brain. Focal seizures are more often associated with structural brain disease.
Bizarre behaviors may be manifestations of seizure disorders, possibly initiated in components of the limbic system. "Fly biting" would be one example. Other behavior disorders such as flank sucking or tail biting may result from a seizure focus, but definitive evidence is lacking.
Others ways to classify seizures depend upon the actual cause of the seizure disorder. These categorize seizures into those without cause (idiopathic) and those with a specific cause (symptomatic). Cryptogenic seizures are those seizures that presumably have a specific cause but it cannot be identified.
Neuroanatomically, seizures are most often associated with supratentorial or forebrain abnormalities. While this is the area of the nervous system where the seizure activity originates, the disease that results in a seizure may be present in other systems besides the nervous system.
After the clinician is convinced that a seizure disorder is present, a differential diagnosis list should be generated. The most important question to be answered is whether the seizure focus is the result of primary intracranial disease, the result of an extracranial disturbance, or is occurring without either a metabolic or structural derangement (idiopathic). Generally the causes for seizures can be categorized into those secondary to an intracranial structural cause (structural), an extracranial cause (metabolic), or idiopathic epilepsy (idiopathic).
Remember that the more diagnostic information that is collected from the animal and if no abnormalities are found the stronger the support for an idiopathic cause. If, for example, you only do a CBC on all dogs with seizures, since in many this test will be normal, you will be diagnosing a lot of idiopathic seizures.
Metabolic (Extracranial) disease resulting in seizures can be separated into those causes arising within the dog itself (endogenous or intradog; e.g., Hepatic encephalopathy) or those arising external to the dog (exogenous, extradog; e.g., toxins). There are numerous metabolic diseases that can result in seizures. These include hypo-hyperglycemia, liver disease, renal disease, electrolyte disturbances, many toxins, anemias, and cardiopulmonary diseases. In most instances, seizures result due to an abnormality of nerve cell metabolic function or neurotransmission. Multiple factors are required for effective nerve cell function including energy (oxygen, glucose), electrical current (electrolytes, electrolyte pumps), and synaptic transmission (energy, neurotransmitters). Alterations of this physiologic environment secondary to systemic metabolic derangements or toxicity can result in seizures. More often, it may be combination of metabolic derangements rather than a single factor that results in the seizure.
Structural diseases resulting in seizures include hydrocephalus, storage diseases, neoplastic (primary and secondary brain tumor), inflammatory diseases (infectious and non-infectious encephalitis), trauma (injury), and vascular-based diseases. Intracranial disease can alter normal physiological relationships through numerous mechanisms which result in seizures. These include, but are not limited to, physical invasion and/or destruction of neurons, metabolic alterations in neuronal or glial cells, impairment of vascular supply to normal tissue (ischemia or hypoxia), impairment of autoregulation, hemorrhage (intraparenchymal or intraventricular), irritation (seizure generation), obstruction of the ventricular system, edema formation, production of physiologically active substances, and increases intracranial pressure (ICP). These processes may initiate and perpetuate each other.
Idiopathic epilepsy is the primary disease in this category. There is a significant population of dogs (>100,000 dogs?) who have a seizure disorder without an obvious metabolic or structural cause. A diagnosis of idiopathic epilepsy is always suspected, as there is no definitive diagnostic test that confirms this disease. Assuredly there are multiple causes for idiopathic seizures that are not identified and therefore obligate a diagnosis of idiopathic epilepsy. Possibly the initiating event has come and gone, and the only after effect is alterations in neuronal depolarization that persists. Conversely, there most likely are some dogs that have abnormal neuronal depolarizations independent of another disease process.
Consideration of the signalment and history may aid in your initial categorization. Seizures resulting from idiopathic epilepsy usually begin between 1 year and 4 years of age of the animal's life. Therefore, a dog that begins having seizures at 9 years of age almost always does not have idiopathic epilepsy. Certain breeds of dogs are known or thought to have an increased incidence of idiopathic epilepsy such as Beagles. High incidence breeds include the Saint Bernard, German shepherd, Golden retriever, Irish setter, American cocker spaniel, Wire-haired fox terriers, Alaskan malamutes, Siberian Huskies, and miniature Poodles. An obvious question from owners is whether this disease is inherited or genetic. Without extensive breeding studies this question is impossible to answer, however, there is often a suspicion that idiopathic epilepsy has a genetic component.
History of systemic illness or toxin exposure may be important when establishing a differential diagnosis. Previous head trauma, often times months or even years prior to the onset, may result in a seizure disorder (posttraumatic epilepsy).
In some animals, their first seizure is not witnessed and therefore is undocumented. If, for example, an animal is left alone for long periods of time (at night, or during the day when the owners are working), and the seizure occurs without causing damage to the animal or the environment, the existence of the seizure disorder may not be obvious to the owner. Another situation is where a new family adopts the animal in adulthood, and any history of previous seizure activity is not known. In these instances, when the seizure is witnessed for the first time (i.e., in adulthood or later age) the owners will assume that this is the onset of the seizure disorder. This is how a small percentage of animals with idiopathic epilepsy have seizures that escape detection in earlier life, and only come to be evaluated later in life.
Neurological examination can be extremely informative in animals with seizures. Idiopathic seizures are not associated with interictal neurological deficits. The caveat is that some dogs may have neurological deficits in the post-ictal period. Some will have abnormalities that last for days after the seizure. Therefore, knowledge of when the seizure(s) occurred in relationship to the examination is important. Often, with severe seizures (multiples, clusters, or status epilepticus), deficits will remain for extended periods of time following the seizure.
Metabolic causes may result in persistent neurological deficits, especially if the mental status is altered. These deficits are usually symmetric, however, very rarely may be asymmetric. Also, while the initiating cause of the seizure may be metabolic in origin, the consequences of the seizure may result in a structural change in the brain. This may be due to edema, hypoxia, infarction, ischemia, or neuronal cell body damage. These focal structural changes may be reflected as deficits on the clinical neurologic examination.