Don't Have a Fit: An Introduction to Seizures in SMALL ANIMALS
Seizures occur in dogs and cats the same as they do in humans but happen more frequently and have been researched more thoroughly in dogs than cats. At some point in their careers, veterinary technicians are likely to witness a seizure and participate in the consequent treatment of a seizure disorder in a client's pet.
Seizures are a sign of abnormal brain function
Seizures can result from intra-or extracranial causes or may have no definable underlying cause
The goal of therapy for seizure disorders is to reduce the number, frequency , and severity of seizures
A seizure can be very frightening and upsetting to observe, especially for owners who watch it happen to a beloved pet. Most owners are eager to work with the veterinary staff to control their pet's seizures. The goal of therapy is to reduce the number, frequency , and severity of seizures, but therapy often does not completely eliminate them.
This article provides a general overview of seizures in dogs and cats, including what a seizure is, why they occur, and how they are classified. The diagnosis of seizures and commonly used anticonvulsant drugs are also discussed.
A seizure is a transitory disturbance of brain function that has a sudden onset, ceases spontaneously, and tends to recur. A variety of involuntary phenomena may be associated with seizures (see Behavioral Changes Associated with Seizures). A seizure results from a sudden and uncontrolled electric discharge of neurons in the cerebral cortex of the brain. In medical terms, it is a paroxuamal depolarizing shift of neurons. Other names for seizure include convulsion, fit, and epilepsy. Neurons can spontaneously discharge for several reasons, including decreased inhibitory neurotransmitter activity , increased excitatory neurotransmitter activity or combination of both. Neurotransmitter activity can be altered by a change in the cell membrane or the internal cell metabolism. Spontaneous discharge can be triggered by almost any alteration in a neuron's environment. Neuronal changes influence the threshold for depolarization that causes seizure activity .
BEHAVIORAL CHANGES ASSOCIATED WITH SEIZURES
Lack of consciousness
Disturbances of memory conscience Altered muscle tone or movement
Visual, auditory , or olfactory hallucinations or other sensory alterations
Salivation, urination, defecaton, or other autonomic nervous system distruptions
Psychic manifestations, such as unexplained fear or rage
According to Oliver and Lorenz, seizures are always a sign of abnormal brain function. At the beginning of a seizure, only a few highly unstable neurons may spontaneously discharge. This initial discharge can cause surrounding neurons or neurons in the opposite brain hemisphere to discharge as well and spread the seizure activity by processes referred to as kindling and mirroring, respectively.
CLASSIFICATION OF SEIZURES
Seizures are classified as either partial or generalized. The terms petit mal and grand mal are no longer used .
Partial seizures occur when neurons discharge in a specific area or seizure focus in the cerebrum; however, partial seizures can progress into generalized seizures. The clinical manifestations of a partial seizure can help indicate the site of the seizure focus in the in the brain. For example, a seizure focus in the motor area of the cerebrum may manifest as a head turn to the opposite side of the focus, tonus-clonus (rigid-relaxed) movement of one or more legs, and flexion of the body. Partial seizures are further classified as simple or complex based on the absence or presence, respectively, of changes in behavior and/or consciousness. An animal exhibiting such behavior as biting at nonexistent flies, rapidly running in circles, or attacking the base of the tail may be having a partial seizure. These animals sometimes respond to antivconvulsant drugs, confirming that such incidents are actually partial seizures.
Generalized seizures (also referred to as tonic-clonic seizures) involve an animal's entire body. During the first (i.e. tonic) phase, which usually lasts between 10 and 30 seconds, an animal falls to the ground, loses consciousness, and rigidly extends its legs; it may also stop breathing or shake. In the second (i.e. clonic) phase, the animal's legs make running or paddling motions, its mouth makes chewing motions, and it may continue to shake. In addition, an animal may urinate, defecate, have dilated pupils, salivate excessively, vocalize, or vomit during either phase. A seizure may alternate between tonic and clonic phases once or repeatedly for the duration of the seizure. The entire seizure usually lasts between 1 and 2 minutes, but it can seem longer .
PARTS OF A SEIZURE
Seizures are divided into four periods or stages: preictus, ictus, postictus, and interictus. During the preictal stage, dogs and cats about to have a seizure may stare into space, become restless or afraid, or try to get
attention from their owners. Owners are often unaware of this part of a seizure. The entire seizure usually lasts between 1 and 2 minutes, but it can seem much longer .
During the postictal stage (i.e. the recovery period after a seizure), animals may wander, be confused, circle, or sleep for a long period. They may also be temporarily blind. The duration and signs of the postictal period are variable and do not necessarily match the severity or duration of the actual seizure.
This phase usually lasts for less than 1 hour but can continue for up to 7 days. Thus any neurologic findings discovered just after a seizure has occurred might be misleading. The interictal period is the
interval between seizures. Some animals are normal during this period and others are not, depending on the cause of the seizures.
Some seizures have no definable cause, a condition referred to as idiopatic epilepsy and that usually begins when an animal is between 1 and 3 years of age. It is common in dogs but not cats. Animals with idiopathic epilepsy are usually normal between 1 and 3 years of age. It is common in dogs but not cats. Animals with idiopathic epilepsy are usually normal between seizure episodes. Some canine breeds are predisposed to having idiopathic epilepsy. In addition, some canine breeds have a high incidence of seizure disorders because they are predisposed to diseases that can cause seizures. Other seizures have specific causes, which can be either extracranial (from outside the brain) or intracranial (from inside the brain).
Status epilepticus refers to a continuous series of tonic-clonic seizures without any recovery period between seizures. This is a severe medical emergency, and the seizures must be stopped. Technicians must remain calm and decisive during such episodes, despite the emergency nature of the condition. Patients in status epilepticus can die as a result of a combination of hyperthermia, circulatory collapse, acidosis and hypoxia. These conditions are caused by continued muscle exertion and impaired or lack of breathing during the seizure. Patients can also become hypoglycemic or develop aspiration pneumonia. A blood sample should be collected as soon as possible and checked for packed cell volume, total protein, blood urea nitrogen, and glucose.
Diazepam can be administered intravenously (IV) or rectally (if a patient is violent) to stop the seizures. The emergency dose used at Texas A&M University's Small Animal Clinic to stop a seizure or status epilepticus is 0.5 mg/kg IV or rectally. If repeated (three or four) doses of diazepam do not stop the seizures, pentobarbital (2 to 4 mg/kg IV, to effect) should be used. Pentobarbital takes 5 to 10 minutes to take effect compared with 15 to 20 minutes for phenobarbital; thus pentobarbital is the first drug of choice.
If seizures recur, the patient needs to be placed on a continuous IV infusion or anticonvulsant drugs for 8 to 12 hours. Diazepam, pentobarbital plus intramuscular Phenobarbital, or Phenobarbital may be used. There is some evidence that pentobarbital does not actually stop the seizure activity but rather prevents the associated motor manifestations. Patients must be closely monitored. Respiration rate, blood pressure, oxygenation, body temperature, and hydration status need to be maintained within normal limits. A patient
that requires anesthesia rather than sedation to control seizures should be intubated to help prevent aspiration pneumonia. The urinary bladder should be palpated and expressed regularly. Patients also have smoother recoveries when diazepam is administered by slow IV drip for several hours before they are awakened.
Because the causes of some seizures are more common during certain age ranges and because some breeds are predisposed to problems and/or diseases that can cause seizures, the approach to diagnosing seizures begins with an animals signalment (i.e., species, age, breed, sex, use). A complete and detailed patient
history should include information about an animal's potential access to chemicals or poisons, vaccination status, illnesses, injuries, and behavioral changes. A careful description of the seizure itself can also be important. Owners should be asked when the seizures started, how often they occur the nature of the seizures and whether the frequency has changed.
A complete physical examination should include auscultation of the heart and lungs, and the patient should be carefully palpated to search for possible lumps, bumps, and masses and/or anything unusual. Abnormalities, or lack thereof, detected during a neurologic examination can help define the underlying problem. Technicians should remember, however, that transitory neurologic deficits may be present in patients that have recently had a seizure.
Laboratory analysis should include a complete blood count, chemistry panel with electrolytes, and urinalysis. Tests for fungal and rickettsial diseases or more specific liver function tests may also be indicated. Additional diagnostic tests to consider include cerebrospinal fluid analysis, radiography (e.g., to detect neoplasia in the lungs), computed tomography, magnetic resonance imaging, nuclear scintigraphy (e.g., to detect evidence of portosystemic shunts), ophthalmic examination (e.g., to detect distemper lesions in the fundus), and electroencephalography (e.g., to document partial seizure activity).
CAUSES OF SEIZURES
Toxins (e.g., lead, organophosphate, ethylene glycol, strychnine, arsenic)
Kidney disease or failure
Liver disease or failure.
Hypoglycemia (e.g., associated with insulinoma)
Electrolyte imbalances (e.g., low calcium, high potassium)
Hypoxia (e.g., associated with cardiorespiratory disease)
Infection (bacterial, viral, fungal, rickettsial, protozoal)
Congenital malformations (e.g., hydrocephalus)
Neoplasia (primary or metastatic cancer)
The goal of therapy is to reduce the number, frequency , and severity of seizures. Often, seizures are not completely eliminated, even with appropriate therapy. Anticonvulsant drugs should be used to control all seizures. Owners may not be aware of seizures that occurred before the first observed episode or of those that occur subsequently because animals are not normally under constant surveillance. The neurons that trigger seizures can cause other neurons to become unstable (i.e., kindling and mirroring), thus leading to more seizures. Consequently, it is best to treat animals even if seizure episodes are infrequent rather than to wait and see if more seizures occur. Seizures tend to become more difficult to control over time; seizures that are difficult to control despite appropriated anticonvulsant drug levels in the bloodstram are called refractory.
Separate treatment should be directed at the underlying cause of the seizure, if it can be determined. For example, brain tumors should be surgically removed or reduced in size with chemotherapy or radiation therapy, antifungal or antibacterial drugs should be administered if infectious causes are identified, or such malformations as shunts should be surgically repaired. Medications have side effects; for example, Phenobarbital can cause liver enzymes to become elevated and can possibly lead to liver failure. The level of anticonvulsant drug(s) in the patient's blood counts and chemistry panels should be routinely evaluated to detect problems (e.g., elevated liver enzymes) early.
Seizure disorders are seen frequently in dogs and cats and can often be successfully treated. Anticonvulsant drug therapy can be used to effectively control most seizures, but blood levels and side effects need to be monitored closely and doses adjusted accordingly. When an underlying cause of the seizure disorder can be determined, it should be eliminated if possible. The goal of therapy is to reduce the number, frequency, and severity of the seizures, but therapy often cannot completely eliminate seizures.
Aspiration pneumonia -An inflammation of the lung parenchyma in which the affected part is consolidated; it results from foreign material, usually food particles or vomit, in the bronchi.
Autonomic- Not under voluntary control
Cerebrum- The largest part of the brain, comprising the cerebral cortex, cerebral white matter, and basal nuclei; abnormal activity in this portion of the brain can lead to seizures.
Depolarization -The destruction, neutralization or change in direction of polarity (negative versus positive charge).
Hydrocephalus- An enlargement of the cerebral ventricular system caused by an increased amount of cerebrospinal fluid.
Hyperlipoproteinemia- An increase in the lipoprotein concentration of the blood.
Hypoglycemia -Low blood sugar .
Hypoxia- Lack of oxygen in tissue
Neurotransmitter- Any specific chemical agent (e.g., dopamine, epinephrine, norepinephrine) that, upon excitation, is released by a presynaptic cell and crosses the synapse to stimulate or inhibit the postsynaptic cell.
Paroxysmal- Sudden onset.
Poltosystemic shunt -Anomalous portal venous circulation in which blood does not flow properly through the liver .
CANINE BREEDS PREDISPOSED TO SEIZURE DISORDERS :
Breeds known or highly suspected to have a genetic basis for epilepsy
German shepherd Keeshond
Breed with a high incidence of seizure disorders but no proven genetic link
Cocker spaniel Collie
Golden retriever Irish setter
Labrador retriever Siberian husky
Wire fox terrier
CANINE BREEDS PREDISPOSED TO SEIZURE-INDUCING DISEASE :
Boston Terrier- Hydrocephalus, neoplasia
Maltese -Portacaval shunts
Miniature pinscher- Hydrocephalus
Miniature poodle- Unknown
Standard poodle- Unknown
Toy poodle- Hydrocephalus
Saint Bernard- Unknown
Miniature schnauzer- Hyperlipoproteinemia, Portacaval shunts
Yorkshire terrier- Hydrodephalus, portacaval shunts