Chapter 127 Seizures
Any animal may be prone to having a seizure, often with little indication of when, why, or how often the seizures will occur. Moreover, the veterinarian never observes most clinical signs of the disease. This unpredictability and lack of direct connection with the disease is challenging to veterinarians trying to determine the appropriate diagnostic and therapeutic plan. The purpose of this chapter is to provide a perspective on how to identify and treat dogs and cats with a seizure disorder.
TERMINOLOGY
My goal is to piece together a rationale categorization for use in small animal epileptic patients adapted from the recent recommendations of the ILAE Task Force on Classification and Terminology. The purpose is to establish a common mode of communication to allow diagnostic and therapeutic data to be tabulated for clinical outcome measures. The proposed new diagnostic scheme consists of five levels, or axes, as proposed by Engel (Table 127-1; for review see Engel, 2001 and Podell, 2004).
Table 127-1 PROPOSED DIAGNOSTIC SCHEME FOR DOGS AND CATS WITH EPILEPTIC SEIZURES
Adapted from Engel JJ: A proposed diagnostic scheme for people with epileptic seizures and epilepsy: Report of the ILAE Task Force on classification and terminology. Epilepsia 42:796, 2001.
Axis 1: Ictal Phenomenology
• An epileptic seizure is the clinical manifestation of excessive and/or hypersynchronous abnormal neuronal activity in the cerebral cortex. Thus, an epileptic seizure has a specific neural origin.
• Epilepsy is present if the animal has a chronic brain disorder characterized by recurrent epileptic seizures. Note that neither the term seizure nor the term epilepsy connotes the underlying etiology of the disorder.
• Non-epileptic episodes are paroxysmal events with severe consequences to the body but without any epileptic electroencephalographic activity (e.g., syncope).
• Status epilepticus can be defined as a state of continuous seizure activity lasting 30 minutes or longer or repeated seizures with failure to return to normalcy within 30 minutes. Epilepsia partialis continua is a continuous focal seizure involving the motor cortex. Although not well documented by electroencephalography in animals, the typical manifestations include facial muscle movements with “chewing gum” activity, repetitive eye or lip twitching, or myoclonic jerking of limb muscles.
Clinical Stages of Epileptic Seizures
• The prodrome is the time period prior to the onset of seizure activity. Owners report that they can “predict” the onset of their pet’s seizures by behaviors exhibited during this time, such as increased anxiety-related behaviors (attention seeking, whining, etc.), reluctance to do normal activity patterns, or increased hiding (in cats).
• The aura is the initial manifestation of a seizure. During this time period, which can last from minutes to hours, animals can exhibit stereotypic sensory or motor behavior (e.g., pacing and licking), autonomic patterns (e.g., salivating, urinating, and vomiting), or even unusual psychic events (e.g., excessive barking and increased or decreased attention seeking).
• The ictal period is the actual seizure event manifested by involuntary muscle tone or movement and/or abnormal sensations or behavior lasting usually from seconds to minutes.
• The postictal period is the time immediately following the ictal phase and can last from minutes to days. During this time, an animal can exhibit unusual behavior, disorientation, inappropriate bowel or bladder activity, excessive or depressed thirst and appetite, and/or actual neurologic deficits of weakness, blindness, and sensory and motor disturbances. The latter problems are known as Todd’s paralysis and are often an indicator of a contralateral cortical epileptic focus. Often owners only observe the postictal period as evidence that their pet has had a seizure. Thus, careful questioning is needed by the clinician to decide if an animal did experience a seizure.
Axis 2: Epileptic Seizure Types
Self-limiting: Isolated (Single Event Only) Seizures
• Focal seizures are the manifestation of a discrete, epileptogenic event in the cerebral cortex. The focal nature of this seizure type is associated with a higher incidence of focal intracranial pathology.
• Focal elementary motor seizures are either commonly seen as facial muscle twitching or manifested by more abnormal behavioral disorders. Progressive involvement of the facial, neck/shoulder, and/or limb muscles is known as a Jacksonian march seizure event.
• Automatism, or automotor seizures, were previously termed complex partial or psychomotor seizures (Engel, 2001). Animals may show “fly-biting” behavior patterns, become aggressive without provocation, howl incessantly, become restless, or exhibit a variety of motor disturbances. Cats may show a variety of abnormal behaviors and/or motor signs, including drooling, hippus, excessive vocalizations, or random, rapid running behaviors in the house.
• Generalized convulsive seizures are the most common seizure type seen in veterinary medicine. These seizures are characterized by impaired consciousness coupled with bilateral motor signs of a tonic-clonic, tonic, or myoclonic nature.
• Generalized non-convulsive seizures are less common and are either atonic (complete loss of voluntary movement with collapse) or absent (ability to stand or pose but without the ability to perceive or respond). These terms replace the previously used terms convulsive (grand mal) and non-convulsive (petit mal) seizures.
Axis 3: Syndrome
• Epilepsy syndromes are not well defined in veterinary medicine, although familial epilepsies are now being identified with segregation analysis.
• A number of purebred dogs have been identified with either proven or highly suspect familial epilepsy: Belgian Tervuren, keeshonds, retrievers, Shetland sheepdogs, and a variety of other breeds.
Axis 4: Etiology
• A seizure develops when the balance shifts toward excessive excitation. Recruitment of a critical number of areas in the brain with synchronized depolarization will lead to a seizure. Reasons for the progressive nature of epilepsy include the following:
• Conditions leading to excessive excitation or loss of inhibition result in depolarization of neurons without normal regulatory feedback mechanisms.
• The number of cells with an intrinsic pattern of high spontaneous firing activity (pacemaker cells) increases in the epileptic focus.
• A mirror focus of actively firing epileptogenic neurons may develop in a similar region on the opposite hemisphere. This enables the number of epileptic foci to multiply rapidly. As an animal continues to have seizures, an increased number of areas of the brain are randomly and spontaneously able to initiate a seizure.
• The differential diagnosis of epileptic seizures due to underlying brain disease can be divided into three main etiologic categories: Idiopathic, symptomatic, and probably symptomatic (previously termed cryptogenic) epileptic seizures.
• Idiopathic epilepsy implies that no underlying structural brain lesion is present, and the epilepsy is presumed to be genetic in origin.
• Symptomatic, or secondary, epilepsy is the result of an identifiable structural lesion of the brain.
• Probably symptomatic, or cryptogenic, epilepsy is believed to be the result of a structural lesion of the brain that is not identified. While this sounds like a nebulous disease category, it has particular implications when understanding why certain animals may be refractory to therapy. Examples of cases that may fit into this category would be prior head trauma in patients with normal imaging, postencephalitic seizures developing at a later date, undetected hypoxic or vascular events of the brain after anesthesia, and birth trauma.
CLINICAL SIGNS
A thorough and accurate history is essential for diagnosis of seizure patients.
• Obtain historical information concerning pedigree information, vaccination status, travel, trauma and toxin exposure potential, previous medical and surgical problems, and drug history.
• Organize information about the actual seizure episode(s) by stage into the prodrome, aura, ictal, postictal, and interictal periods.
• Record dates, times, duration, and description of each abnormality to assess progression and allow a comparison if medication is started.
• Take a thorough neurologic history. For example, question if the dog is more withdrawn or attention seeking, shows any unusual episodes of aggression or irritability, or fails to follow simple commands. These would suggest a structural cerebral problem. Likewise, determine the presence of subtle gait abnormalities (stumbling up and/or down the stairs), visual disturbances (occasionally bumping into objects on one side), and restless sleep patterns that may indicate cerebral problems.
• Classify seizure type as focal or generalized (see “Axis 2: Epileptic Seizure Types” and Table 127-1).
• Characterize as either isolated (one per 24 hours) or clustered (two or more in 24 hours). As a consequence of severe or prolonged seizure events, animals can develop detectable neurologic deficits during the immediate and extended postictal period. More severe immediate postictal abnormalities include vision loss, circling, paresis, profound disorientation, aggressive personality changes, and other dementing behaviors. Some of these changes may last several days to weeks, a condition known as Todd’s paralysis when sensorimotor localizing deficits are found. Fortunately, practically all of these abnormalities are reversible.
• With chronic seizure activity, changes in cellular physiology of the brain can lead to actual changes in neurologic function, especially persistent behavior disturbances. Dogs with recurrent epileptic seizures may also exhibit changes in personality. Lack of obedience, withdrawal activity, changes in socialization with other animals or people in the household, and unprovoked aggressive behavior can all occur as interictal manifestations of a chronic epileptic condition in the dog. Attributing these changes to the disease or the treatment can be difficult to discern.
DIAGNOSIS
Differential Diagnosis
Dogs
When approaching a differential diagnoses list (Table 127-2), consider the signalment, history, physical and neurologic examination changes, and laboratory abnormalities.
Table 127-2 DIFFERENTIAL DIAGNOSES OF SEIZURES IN THE DOG AND CAT
• For dogs <1 year of age, seizures are most commonly secondary, in particular, caused by developmental, metabolic, and inflammatory diseases (see Table 127-2). Specific diseases to include are hydrocephalus, portosystemic shunts, and canine distemper encephalitis, respectively. Specific breeds prone to congenital hydrocephalus are Maltese dogs, Chihuahuas, Yorkshire terriers, and the brachycephalic breeds. Any breed of dog, however, may suffer from this disease.
• For dogs between 1 and 5 years of age, the most common cause of seizures is idiopathic epilepsy. However, a proportion of dogs in this age category may suffer from a congenital brain anomaly that may be progressive in nature.
Diagnostic Approach
Refer to Chapter 125 for information on diagnostic evaluation of the patient with neurologic disease, and Chapter 126 for diagnostic parameters of specific diseases of the brain.
Dogs
• A diagnosis of secondary epilepsy is more likely when the following are true:
• The interval between the first and the second seizure events (an event being all seizures within a 24-hour period) is brief (<4 weeks).
Diagnostic Testing
Dogs
• Evaluate a minimum database consisting of noninvasive blood pressure, complete blood count (CBC), serum chemistry profile, urinalysis, and thoracic radiographs to rule out possible metabolic causes. In younger dogs, a serum bile acid study is recommended to rule out a portosystemic shunt. Unfortunately, these tests have a very low yield of positive findings in most seizure patients.
• If symptomatic epileptic seizures are suspected, consider MRI of the brain. This test offers the best opportunity to diagnose either an intracranial anomaly or a primary neoplasia, which are the prevalent causes of seizures in dogs <1 or >7 years of age, respectively. If negative scan results are found, then cerebrospinal fluid (CSF) should be collected for analysis.
• Collect CSF first if a multifocal disease or meningeal inflammatory process is suspected after the neurologic examination.
• Perform additional diagnostic testing in any dog with persistent interictal neurologic abnormalities. Select tests according to the history and clinical suspicions—for example, serum bile acids to evaluate liver function, serial fasting blood glucose paired with insulin levels to diagnose hyperinsulinemia, plasma lead concentration, and specific serum antibody titers for infectious diseases.
Cats
Consider the following diagnostic tests in cats:
• Evaluate a minimum database consisting of noninvasive blood pressure, complete blood count, serum chemistry profile, urinalysis, baseline thyroid level, retrovirus testing, Toxoplasma antibody titers, and thoracic radiographs to rule out possible metabolic or infectious causes. Serum feline infectious peritonitis (FIP) antibody titers are poorly correlated with the central nervous system form of infection.
• Perform MRI brain scanning in all cats >7 years of age and/or in those cats with an abnormal neurologic exam. The combination of MRI scanning and CSF analysis is often the most direct approach to a definitive diagnosis in any age cat with normal laboratory testing.
TREATMENT
This section describes seizure control therapy. Refer to Chapter 126 for treatment of specific diseases of the brain.
Goals of Therapy
• Realistic goals are a decrease in the number and severity of seizures, fewer post-ictal complications, and an increase in the interictal period.
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