Seizures in dogs and cats are abnormal brain activity. They can be very subtle or they can cause violent convulsions. Seizures occur when nerve cells (neurons) in the brain malfunction and become excited, firing without control. Most cat and dog seizures occur at night or when they are resting. Seizures can occur once and never occur again—for example because of heatstroke or fever—or they can occur repeatedly. Repeated seizures are called epilepsy. Unfortunately, if seizures are not treated, they occur more frequently and with more force because increasingly larger areas of the brain become affected.
Because discussing seizures is almost as complicated as the brain itself, doctors developed several categories to simplify the discussion. Seizures are categorized by:
Elements in the blood directly influence the brain. Among the most significant are sodium, potassium, chloride, calcium, glucose, and blood pH. For example, Chihuahua puppies are inclined to develop seizures because their bodies don't have reserves to maintain normal blood sugar levels. Lactating mothers develop seizures because their blood calcium levels are depleted by nursing young. This is most common when the puppies are 3-4 weeks old and have ravenous appetites.
Nerve cell membranes are like woven walls with gates. The membranes provide structure, but they are built so that some molecules can pass right through them. Most molecules, though, pass through the gates. These gates have receptors that are open and active or closed and inactive. For seizures to occur, molecules must either penetrate the walls or the gate receptor molecules must be open. Faulty construction of walls or gates increases the likelihood that molecules that should not enter the nerve cell can enter and stimulate seizure activity. Because the membrane walls of nerve cells are 60% fat, pets receiving good Omega 3 fatty acids from flax or fish oils have the healthiest nerve cell walls.
There is a system that prevents nerve cells from being too open to excitatory signals, but just as a friend who is wildly enthusiastic about something can entice other friends to become excited, nerve cells can excite each other so they fire repeatedly and create seizure activity. The process of exciting other neurons is called kindling, and it is one of the reasons that partial seizures become generalized seizures and that petit mal seizures can become grand mal seizures. Kindling is most likely when seizures occur often or with increasing force and when pets don't receive anti-seizure medication.
Neurotransmitters are small molecules that travel between nerves and change the nerve cell's receptivity. For example, glutamate is a neurotransmitter that excites nerve cells and opens them to activity. Gamma amino benzoic acid (GABA) is a neurotransmitter than excites nerve cells in the young animal, but inhibits nerve cells and closes them to activity in older animals. Anything that changes the concentration of neurotransmitters, including glutamate and GABA, changes the brain's sensitivity to developing seizures. Molecules in the blood, such as calcium, sodium, sugar (glucose) and potassium, affect neurotransmitter activity.
Seizures can cause muscle contractions called convulsions or they may occur without the muscles being involved. Seizures accompanied by convulsions are called grand mal seizures; seizures that occur without convulsions are called petit mal seizures.
With grand mal seizures, pets lose consciousness, fall to the floor, and their legs convulse or paddle spastically. Normally, grand mal seizures last between 45 and 90 seconds. Some pets involuntarily urinate or defecate. During convulsions, blood and oxygen do not circulate to the brain and vital organs, so if convulsions continue for several minutes (status epilepticus), pets die of anoxia (loss of oxygen).
For some time before the seizure, which is called the preictal period, your pet's brain functions abnormally. During this period, which is called an aura, pets may be anxious, seek out the family, or hide under beds. After the seizure, known as the postictal period, the brain recovers in a process that may take minutes, hours, or days. During the recovery period, your pet may be blind, dazed, ravenously hungry, or dangerously aggressive.
Pets that have seizures but don't have convulsions have petit mal seizures. They may appear dazed or disoriented, stop what they are doing and stare into space or up at imaginary stars, but they don't fall down and don't have convulsions. These events are called simple, partial petit mal seizures. Many pet guardians don't notice when their pets have these seizures, and don't seek veterinary care. Unfortunately, untreated petit mal seizures can become generalized convulsive seizures.
There is another form of petit mal seizure in which pets snap at imaginary flies, chew imaginary gum, or bite the skin on their flanks. These behaviors are called complex partial seizures or psychomotor seizures. Because these behaviors are unusual, pet guardians ask their veterinarians about them and their pets receive medical treatment. Without treatment, complex partial seizures can progress to convulsive seizures.
Seizures can involve the entire brain or involve a limited area of the brain. These are respectively called generalized and partial or focal seizures.
When the entire brain is involved, your pet has "generalized" seizures. These can be obvious convulsive grand mal seizures or the subtle petit mal seizures. Because the entire brain is involved, effects occur on both sides of the body.
When only part of the brain is involved, your pet has "partial" or focal seizures. These can be obvious convulsive grand mal seizures or the subtle petit mal seizures. Because only a part of the brain is involved, symptoms are localized to one side of the body. Partial or focal seizures can progress to generalized seizures. Progression is most likely if seizures are not treated.
Epilepsy that has no cause is called primary or idiopathic epilepsy. If the anatomy or function of the brain causes seizures, it is called secondary epilepsy. If your pet has a problem outside the brain causing seizures, it has reactive epilepsy.
When no problems exist within the brain and the cause of epilepsy cannot be found, your pet has "primary" or "idiopathic" epilepsy. Because pets with idiopathic epilepsy have no unusual brain characteristics and their nerves work properly—except when they are having seizures—the cause is assumed to be within the DNA. Thus, this form of epilepsy is genetic and can be inherited. Primary or idiopathic epilepsy is the most common form of epilepsy in dogs, but it is rare in cats. It usually affects dogs of middle age; that is, not young puppies, and not senior pets. Most dogs that develop primary epilepsy are between one and five years of age. Their seizures occur when sleeping or resting.
When the cause of seizures is a structural or chemical problem within the brain, the disease is said to be secondary. Secondary seizures are caused by hydrocephalus, brain cancer, brain infections, brain inflammation, toxins, aneurysms, and stroke damage. With secondary or structural seizures, problems within the nervous system or cranial nerves are obvious even when your pet is not having a seizure. Secondary seizures usually occur in pets less than a year old or more than five years old.
Reactive seizures occur in pets with perfectly healthy brains but with problems originating outside the brain, such as low blood sugar or heart disease. Pets that develop reactive seizures are usually less than a year old or more than five years old. When pets less than a year old have reactive seizures, they usually have infections. When older pets have reactive seizures, they often have cancer.
Like drum beats, seizures can occur with different rhythms. Normally seizures are widely separated so that brain cells can recover completely. With some abnormalities, though, seizures occur much closer together than they normally would. These are cluster seizures. When seizures occur so rapidly that they are continuous, this is an emergency called status epilepticus.
When seizures occur more than once in 24 hours, they are called cluster seizures. Cluster seizures are usually caused by toxins or structural problems within the brain that continue to excite neurons even when they are exhausted and would normally be recuperating.
When seizures last longer than five minutes or occur so quickly that your pet's brain doesn't recover between seizures, your pet is said to be in status epilepticus. This is a medical emergency. Status epilepticus is usually caused by toxins or structural problems within the brain and is difficult to treat with normal anti-seizure medications.
Refractory epilepsy occurs when anticonvulsant medications don't control the seizures. Usually these seizures are caused by structural problems within the brain, such as tumors. Thus, refractory epilepsy is usually a form of secondary epilepsy.
Dogs are more prone to develop seizures than cats. Among the dog breeds most often affected are:
Some large-breed dogs develop seizures because they are genetically predisposed to epilepsy. Among them are the Shepherds and Retrievers: German Shepherd, Belgian Tervuren, Australian Shepherd, Labrador Retriever, and Golden Retriever.
Herding dogs with multi-drug resistant 1 (MDR1) gene maydevelop seizures when exposed to particular drugs, such as Ivermectin. Veterinarians in these cases generally prescribe drugs given daily for a specified period of time or a combination of lower dose medication in an effort to reduce the risk of seizures or other adverse reactions.
Dog breeds that may have the MDR1 gene include the Australian Shepherd, Border Collie, Collie, German Shepherd, Longhaired Whippet, McNab, Old English Sheepdog, Shetland Sheepdog, and Silken Windhound. To learn more about which drugs cause seizures and how to have your dog tested for the MDR1 gene, visit the Agility and Working Dog Section.
Seizures occur in small dogs that have malformed skulls, malformed brains, or hypoglycemia (a condition that occurs when blood sugar is too low). Newborn and infant pets of small-breed dogs, especially Chihuahuas and Yorkshire Terriers, are prone to seizures because they have little body fat and no significant carbohydrate stores to metabolize when their blood sugar is low (hypoglycemia). These pets also have little stomachs so they cannot eat enough in a single meal to sustain them for a very long time.
Dogs with short, flat noses (brachycephalic breeds—Pug, Boston Terrier, and English Bulldog) may choke on saliva in their throat and airway and have difficulty breathing so that oxygen does not reach their brain. They also have difficulty keeping their throats open while sleeping or while recovering from anesthesia. This is what causes them to snore and to have sleep apnea. Many brachycephalic dogs also have tiny nostrils so that not much air can enter the nose. Together, these anatomical features predispose brachycephalic breeds to seizures caused by anoxia (absence of oxygen).
Bull Terriers have an inherited form of epilepsy that includes tail chasing, irrational fear, and unprovoked aggression.
Outdoor pets develop seizures from eating poisonous plants or toxic materials, such as mushrooms, Chinaberry berries, Coriaria leaves and berries, and Moonweed. Outdoor pets may lick old batteries and develop lead toxicity, lap up dangerous chemicals in garages, such as antifreeze, and eat rotted garbage that can cause severe gastric infections with diarrhea and vomiting. Diarrhea and vomiting alter the concentrations of sodium and potassium in the blood, and the alteration can cause seizures.