What is a Seizure?
Most people think of seizures as “chaotic” brain activity. In fact, seizures are just the opposite. In a normal animal’s electroencephalogram (EEG), it is easy to recognize that there is no distinct pattern to the electrical activity being recorded from the brain
A seizure occurs when brain cells (neurons), which have a natural tendency to “want” to be active, form oscillating or reverberating, synchronous discharges. The summation of several neurons discharging at one time creates an environment where a local abnormality may influence nearby neurons to spontaneously fire, leading to the further discharge of more neurons. As these electrical signals spread throughout the brain, they travel through the normal conductive pathways of the brain eventually reaching areas such as the movement center, resulting in the manifestation of, in several cases, a generalized motor seizure. A seizure is a sudden, involuntary, synchronous discharge of brain neurons. Seizures are sometimes referred to as convulsions, attacks, fits or an ictus. The condition, known as epilepsy specifically referrers to recurrent seizures associated with a functional disturbance in the brain.
Because chronic seizures (epilepsy) affect nearly 3% of the canine population, most clinical veterinarians will be presented with numerous cases of dogs with a history of seizures every year. Cats are not nearly affected as often as dogs, however, the same approach to the seizuring patient can be made for cats, as well as dogs. The first step in the diagnostic road is to figure out if in fact your pet is having seizures. Other conditions that are commonly mistaken for seizures may include:
- Syncope (not breathing)
- Balance disorders
- Severe muscle weakness
- Neck pain
- Tremor syndromes
- Behavioral abnormalities
- Periodic weakness
- Sleep attacks
It is very important before proceeding with an extensive diagnostic work – up to determine if your pet is experiencing a seizure or a “seizure – like” event. There is no etiological significance to a seizure. A seizure simply represents a sign of brain dysfunction. The brain is a fundamentally excitatory organ and it is in your pets best interest, for survivability, to be able to take in as much information from its environment as possible. Subsequent to a volley of information, the brain will then discharge it’s various responses. These responses, by default, will be in an excitatory fashion. This basal excitation can be seen in several areas of the nervous system. For instance, the activation of antigravity muscles when an animal is falling will result in an excitation of extensor muscles on the side the animal is falling to in order to prevent contact with the ground. The brain is in a constant balance between inhibition and excitation (go or no go). Neurons want to fire and it is up to the inhibitory components of the central nervous system to keep neuronal activation in check. All neurons if isolated from their surroundings will fire spontaneously. When the neuron is placed in an environment such as the brain, its firing potential is controlled by other nearby neurons and components of the fluid that surrounds the neurons. The primary means of “quieting” the brain is through inhibitory neurotransmitters (chemicals that allow neurons to “talk” to each other). One of the long-standing hypotheses for the generation of seizure activity is due to a lack of inhibition mechanisms or inhibitory neurotransmitters.
The majority of therapies used for seizure treatment are directed at the prevention of seizures, not necessarily the cause.
What are the different classifications of seizures?
There are numerous classifications schemes of seizures. Veterinary Neurologists are currently working on a classification scheme to be universally accepted. In general seizures can be broken into several categories. Seizures can broadly be divided into three separate classifications:
- Primary: Seizures for which no cause can be found
- Sometimes referred to as “idiopathic” or “inherited”
- Secondary: Seizures that have a proven cause. Sometimes referred to as “symptomatic”
- Structural: cancer, inflammation, infection, stroke, hydrocephalus, trauma, etc…
- Metabolic: liver disease, kidney disease, low blood sugar, etc…
- Cryptogenic: Seizures that are suspected to be secondary but no proven cause has been found
Seizures can then be subclassified depending upon the type of seizure:
- Generalized (previously referred to as “grand – mal”)
- Rigidity and contractions
- Rigidity alone
- Contraction alone or alternating relaxation and contraction
- Complete loss of muscle tone
- Brief shock – like contractions of individual muscle groups
- Partial or focal
- Simple partial: consciousness is maintained
- Rhythmic contractions of facial muscles
- Fly biting
- Lip smacking
- Tail chasing
- Flank sucking
- Obsessive compulsive behavior
- Complex partial: consciousness impaired
- Bizarre behavior
- Abdominal pain
A seizure usually consists of four separate components in animals; the prodrome, aura, ictus and post ictus. All of these components may not be recognized in animals and the duration and clinical manifestation of each portion may change.
The prodrome is commonly experienced in people. It is described as a behavioral or mood change. Astute owners may recognize a change in their pet’s behavior prior to a seizure episode. The prodrome may occur days or weeks before the actual seizure event.
The aura is described by people as a feeling of anxiety or impending “sense” that a seizure is coming. In animals this may be manifested by restlessness, whining, trembling, periods of affection and autonomic signs such as salivation. The aura may last for hours or days.
The ictus is the generally recognized seizure event. The most common manifestation is a generalized tonic – clonic event that usually lasts for 1 - 3 minutes followed by a period of recovery.
The post ictal period immediately follows the seizure. This event can last from minutes to days and may manifest as blindness, aimless wandering, confusion, salivation, and periods of affection or aggression. There is no correlation between the severity of the ictus and the length or severity of the post ictal period. Many animals with severe seizures will recovery quickly and conversely, many animals with mild seizures will have debilitating post ictal periods.
Current thoughts are that the aura, which is characterized by an increase in neuronal excitability and propagation throughout the brain, may actually be a partial seizure that then continues to develop into a generalized seizure. In one study, 85% of dogs that were diagnosed with partial seizures went on to develop secondary generalized seizures.
In order to be successful at solving the seizure puzzle, it helps to have a plan. The plan starts with the initial determination that the animal is actually having a seizure. Most owners are fairly astute when realizing that their pets are actually having a seizure. Sometimes descriptions of bizarre behaviors may leave the clinician with some question as to whether or not the animal is having a seizure or a “seizure – like” episode. If there is any question, a video taped episode may help determine if further evaluation for seizures is necessary. Once it is determined that a seizure is indeed the problem, your veterinarian can proceed to differentiating between primary or secondary epilepsy.
What can be causing a seizure?
There are basically only three reasons your pet might be having seizures. Number one is an actual structural problem with the brain. Something is irritating the brain either on a gross level (like a brain tumor or blood clot) or on a microscopic level, such as meningitis / encephalitis (infection or inflammation of the brain).
A second cause of seizures is metabolic problem in the body that the brain is reacting to such as severe liver, kidney or heart disease. Toxins and poisons are often included in these causes.
The final reason is…….we don’t know. Those animals that do not have a defined reason for having the seizures, despite looking are often said to have idiopathic, genetic or primary epilepsy. There is likely a cause for the seizure such as an abnormality with particular proteins that the brain makes, but we are unable to identify the cause. Since primary epilepsy most often affects younger animals, we often refer to older animals where a cause cannot be found as having cryptogenic epilepsy.
Published on December 18, 2007.