Small Animal Medicine & Surgery, College of Veterinary Medicine, University of Georgia, Athens, GA, USA
A seizure can be defined as a repetitive neurological event that is the clinical manifestation of excessive and/ or synchronous abnormal neuronal activity in the cerebral cortex. The terms “seizures”, “fits”, and “convulsions” are synonymous. The term convulsion is generally reserved for seizures that have a generalized motor component. These latter terms are old terminology which is no longer used.
A seizure can have several components. Many of these components are not always appreciated/occur in clinical cases. The actual seizure is called the ictus.
1. Aura: The period of altered motor, sensory, autonomic, or behavior that may occur before the seizure. Behaviorally, animals may hide, appear nervous, or seek their owners.
2. Ictus: The actual seizure. This usually lasts for 1 to 2 minutes (but the length of time is variable) and can have a variable appearance.
3. Post-ictal: The period after the seizure where animals may return to normal within seconds or may be abnormal for minutes to hours. Examples of abnormal post-ictal behavior include restlessness, an increased appetite, lethargy, confusion, disorientation, aggression, or blindness.
Seizures are always a sign of abnormal brain function. The neuroanatomic diagnosis for seizures is the prosencephalon. However, the underlying etiology or dysfunction may be from a structural (primary) lesion within the brain (an intracranial etiology) or a disturbance outside the brain (an extracranial etiology) that is affecting the prosencephalon. Other clinical signs that help localize a prosencephalic lesion include:
- Abnormal mentation - Helps localize to intracranial. Any lesion affecting either the reticular activating system or the cerebrum or thalamus can result in an abnormal mentation.
- Normal gait - Chronic structural lesions affecting the cerebrum or thalamus do not cause a disturbance in the gait (i.e., the gait is normal). This is not necessarily the case for acute structural lesions or metabolic/toxic disease. Occasionally, animals will walk propulsively and will tend to circle toward the side of the lesion.
- Postural reactions - Ascending GP information projects to the contralateral thalamus and cerebral cortex. Cranial nerve examination - Abnormalities typically affect the cranial nerve involved in responses while those involved in the reflexes are normal.
Causes of Seizures
The three broad categories for the underlying causes of seizures are intracranial causes, extracranial causes, idiopathic. Depending on the species, idiopathic (also referred to idiopathic epilepsy if there are recurring seizures without an underlying cause) may be common such as in dogs or relatively uncommon as in cats and horses.
The term idiopathic epilepsy refers to the syndrome of recurrent seizure activity with an unknown etiology. The recurrent seizure activity has no demonstrable pathologic cause. The seizures are probably due to a number of diverse biochemical defects that may be inherited. Hereditary seizures may be a preferred terminology for recurrent seizure activity of unknown etiology in which a defined pattern of inheritance has been demonstrated. In dogs, a familial predisposition for epilepsy has been reported for the beagle, keeshound, Belgian Tervueren, golden retriever, Labrador retriever, vizla and Shetland sheepdog. In the Bernese mountain dog and in Labrador retrievers a polygenic, recessive mode of inheritance has been suggested. In vizslas an autosomal recessive trait has been suggested. Hereditary epilepsy has not been reported in the cat. Despite this, idiopathic seizures can occur but are typically uncommon in cats. The age of onset is usually between 1 to 5 years of age. Seizures are usually generalized tonic/clonic and last 1 to 2 minutes. Pre-ictal and post-ictal may be seen. Animals are clinically normal in the inter-ictal period (between seizures). The neurological examination is normal. The seizures can occur in clusters (multiple seizures in a 24-hour period with a return to normal consciousness between seizures). If untreated and severe, seizure activity can progress to status epilepticus (1 seizure that lasts longer than 5 minutes; 2 or more seizures that occur without a return to normal consciousness). The diagnosis is suspected based on the history, signalment, but the true diagnosis necessitates the exclusion of all other causes. The physical examination, neurological examination, and diagnostic tests are normal. The diagnostic approach to determining the cause of the seizure activity should start with an evaluation of the animal’s signalment, history, physical examination, and neurological examination. Extracranial disease is ruled out prior to pursuing intracranial disease.
Some rule outs are more common in a given age group. For example, in dogs less than one year of age, we are more likely to consider causes such as congenital anomalies (e.g., hydrocephalus), infectious diseases, metabolic diseases of young dogs (e.g., hypoglycemia or portosystemic shunts), toxins, or trauma. In 1- to 5-year-old dogs, we would be more likely to consider idiopathic epilepsy, inflammatory diseases (infectious and non-infectious), trauma, and metabolic diseases. Finally, in dogs older than 5 years old, causes such as neoplasia, inflammatory disease, metabolic disease, and vascular disease may be more likely. It is important to question the owner about the current episode to ensure that the abnormal activity is indeed seizure activity (and not, for example, syncope, transient vestibular attacks, or narcolepsy). Ask for a description of the abnormal activity, including the duration, severity, and whether autonomic signs of salivation, urination, or defecation were seen. A description of motor movements is often very helpful. Knowledge of what the animal was doing prior to the event can sometimes help differentiate causes. For example, a 2-year-old male boxer dog that comes running out extremely excited to see its owner and suddenly collapses with little rigidity likely is having a syncope. While a 10-year-old Labrador that is calmly laying on the floor, slowly rises to its feet, then falls over paddling is more likely to be having a seizure.
Relevant questions pertaining to the patient’s history should include the following:
1. Has the animal been previously healthy or had a history of systemic illness?
2. What was the age of onset of seizure activity?
3. What is the frequency of seizure activity?
4. How has the condition progressed?
5. Is the animal normal between seizures?
6. Has there been a history of trauma or possible toxin exposure?
7. Is the animal taking any medications (for seizures or forgot her conditions)?
If underlying disease is present, this should be treated appropriately. If seizures are recurrent, anti-convulsant therapy should be considered for the patient.
Goals of Anti-convulsant Therapy
- To maintain seizure “control” and limit unacceptable side effects (seizure control does not necessarily mean elimination of seizure activity)
- To decrease seizure frequency (to less than one seizure every 6 weeks or one cluster every eight weeks)
- To decrease the seizure severity and duration
- Ideally, to eliminate cluster seizures