Seizures have been reported as the most common neurologic problem encountered in small animal practice. While idiopathic epilepsy is the most common cause of seizures in dogs, other causes must also be considered. A brief overview of the diagnostic approach and fundamentals of seizures and epilepsy is presented below.
Seizure (ictus) is a paroxysmal period of abnormal cerebral function associated with a variety of clinical manifestations. Clinical manifestations seen with seizure activity are altered levels of consciousness, change in muscle tone (usually increased tonus with tonic and/or clonic movements), contractions of facial muscles (e.g., eyelid twitching, chewing gum), and autonomic disturbances (salivation, urination, defecation). Pre-ictal changes can be seen and are called prodrome when they occur days or hours before a seizure or aura when they immediately precede a seizure. During these periods dogs may display abnormal behaviors such as restless and either hide or seek out the owner.
It is important to differentiate the term seizure from epilepsy. Epilepsy is the disease characterized by recurrent seizures. The author prefers to restrict the use of the term epilepsy to recurrent seizures of intracranial origin, although others have used it also to characterize recurrent seizures of extracranial origin.
Seizures can be broadly classified as focal, generalized, or focal with secondary generalization. Focal seizures can be simple or complex. It is advisable to avoid using the terms "grand or petit mal" when classifying seizures because they are rather archaic and because petit mal is often incorrectly used in reference to focal seizures (petit mal seizure was the term coined by Samuel Tissot in 1770 to describe a form of generalized seizure known as absence seizures, which is rare in animals). Focal seizures are often thought to be associated with structural cortical abnormalities, either congenital or acquired. This concept has been recently questioned in some breeds, and it is still open to debate. Generalized seizures are typical of idiopathic epilepsy, and are also seen with metabolic or toxic diseases.
Seizures can be caused by extracranial and intracranial diseases. Extracranial diseases are mainly metabolic (e.g., hypoglycemia, hypocalcemia) or toxic (e.g., organophosphate, lead) disturbances. Seizures caused by extracranial diseases tend to be associated with other signs such as vomiting, weakness or tremors. An acute onset of cluster seizures with or without other neurological signs is common with metabolic or toxic disease. In the author's experience the extracranial disease that most commonly resembles idiopathic epilepsy is insulinoma, because dogs are often clinically normal during the interictal period and it may be difficult to document hypoglycemia initially. Intracranial causes of seizure are functional diseases such as idiopathic epilepsy, or structural (active or inactive) diseases, such as brain tumors or glial scars caused by a past brain injury. Overall, the majority of seizures in dogs are caused by intracranial diseases.
Epilepsies can be classified as idiopathic (primary), symptomatic or probable symptomatic. Idiopathic epilepsy is the most common type of epilepsy, affecting several breeds. Idiopathic epilepsy is a functional disease of the brain, usually manifested between 1 and 5 years of age, with no other neurological sign and a normal neurological examination. Symptomatic epilepsy is used to describe seizures caused by a known intracranial disease such as neoplasia or encephalitis. Probable symptomatic epilepsy (formerly cryptogenic) is used in cases where symptomatic epilepsy is suspected but cannot be confirmed with the available diagnostic methods. Probable symptomatic epilepsy is often suspected in dogs with focal seizures where a cause cannot be identified.
The first step is to confirm that the event described by the owner is actually a seizure. Veterinarians rarely see patients having seizures; therefore a well-taken clinical history is essential. One must remember that the seizures can be either focal or generalized and their presentation and severity can vary considerably. The owner should be asked to describe step by step the entire episodic event. There are several episodic events which can be mistaken for seizure activity. The most common ones are vestibular attacks, syncopal events due to cardiac disease, spinal pain (usually cervical pain), tremors, behavioral disorders, and narcolepsy/cataplexy. A well-taken clinical history, associated with physical and neurological examinations should be able to differentiate these episodic events from seizure activity.
Once it is ascertained that the patient is having seizures, further information regarding the frequency, duration, clinical status between seizures, and present and past medical history should be obtained. This information is invaluable in establishing the cause of the seizures. A detailed physical examination focused on cardiac and musculoskeletal function should be performed. The neurological examination is the single most important tool to assist the clinician in presumptively differentiate between idiopathic and symptomatic epilepsy. Special attention should be to the components of the neurological examination that specifically evaluate the thalamocortical function. These components are: detailed history of the mentation and behavior in the interictal period; menace response; nasal sensation; and postural reactions, mainly proprioception. The patient's mentation and behavior should also be assessed during examination. The clinician should seek other signs of cortical dysfunction such as blindness, disorientation, circling, pacing and head pressing. If the physical and neurological examinations are normal, complete blood count and biochemistry profile should be performed. Bile acids and serum lead level can also be performed if the history suggests portosystemic shunts or lead exposure. The goal of these tests is to rule out an extracranial disease. If the neurological examination is abnormal between seizures, it is likely that the patient likely has an intracranial structural disease and further diagnostic studies should be directed at evaluating the cerebrum. Specific neurodiagnostic tests are advanced imaging, such as computed tomography (CT), or preferably magnetic resonance imaging (MRI), and cerebrospinal fluid (CSF) analysis. The decision on which test should be done first should be dictated by the differential diagnoses being considered. If the results of physical and neurological examinations are unremarkable, results of CBC and biochemistry profile are normal, and the dog is a young adult dog of a breed predisposed to idiopathic epilepsy, the most likely cause is idiopathic epilepsy.
The diagnostic approach should allow the clinician to establish the cause of the seizures, which will then enable the selection of the most appropriate therapy and increase the likelihood of therapeutic success.
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