Teri Dickinson, DVM
Causes of Seizures in Dogs
Teri Dickinson, DVM
In trying to determine the incidences of the various causes of seizures
in dogs, researchers are often hampered by several things. One, most research is
done in large referral veterinary hospitals, and those hospitals tend only to see
the more difficult cases, which have been referred by general practitioners. Two
of the dogs may have received various treatments prior to the referral, which might
change results of tests run by the researchers. As a result, the incidence of the
various causes of seizures in the general dog population has been difficult, if not
impossible to determine.
In an effort to generate some valid data about the causes of seizures, the Veterinary
College at the Ohio State University devised a new study. In cooperation with local
general practitioners, they began to have seizuring dogs referred after the first
visit to the local vet, and before any treatment had been initiated. In this manner,
they saw all types of seizure cases, not just the difficult referrals. They also
gained the ability to do diagnostic testing on the dogs prior to any treatment being
started, and they were able to run a well standardized set of tests on all the dogs
in the study.
The results of this study were recently published in the veterinary literature as
Seizure classification in dogs from a non-referral based population, written by Michael
Podell, MSc, DVM; William R. Fenner, DVM; and Jean D. Powers, PhD, in the Journal
of the American Veterinary Medical Association, Vol 206, No 11, June 1, 1995. The
following is an excerpt of pertinent points which might be of interest to IG owners.
A total of 50 previously healthy dogs who had suffered one or more seizures were
entered in to the study. A series of diagnostic tests were performed in an attempt
to classify the seizures into one of three categories:
Primary Epileptic Seizures-those without identifiable cause (idiopathic)
Secondary Epileptic Seizures-those with an identifiable abnormality within the head
(intracranial cause)
Reactive Epileptic Seizures-those due to metabolic disease or exposure to toxins
(extracranial cause)
Each dog underwent standard blood and urine tests to check the function of liver,
kidneys, thyroid, etc. Dogs which had abnormal liver function tests underwent liver
biopsies to help determine the cause of the liver problem. All dogs had EEG's (brain
wave recordings) and CSF (spinal fluid taps) performed. Any dog that did not have
an identifiable metabolic cause for the seizures also underwent an MRI (magnetic
resonance imaging) scan. MRI is similar in nature to an x-ray, but reveals much more
information. Unfortunately, MRI can only be performed at large research institutions,
due to the extremely high cost of the equipment.
Based on the owner's history, the seizures were classified as general (generalized
motor activity) or partial (isolated motor activity with or without loss of consciousness).
Notes were also made about how many seizures the dog had suffered, age at onset,
interval between seizures, etc.
On the basis of all this information, the dogs were placed in one of the three classes
mentioned above. Even with the intensive diagnostic work-up provided, 22 dogs (44%)
were classified as primary (no identifiable cause). 23 (46%) had secondary epilepsy
(intracranial cause) and only 5 (10%) had reactive (extracranial) seizures.
Causes of reactive (extracranial) seizures were identified as hepatic (liver) disease,
hypothyroidism, and an overdose of orally administered thyroid hormone. Causes of
secondary (intracranial) seizures were identified as cerebral developmental anomaly
(one or more congenital brain defects) 52%, neoplasia (tumor) 22%, inflammatory process
(canine distemper virus or some other infection) 17%, trauma 4%, and cerebral vascular
accident, CVA, (stroke) 4%. The most common developmental abnormality was hydrocephalus,
which is commonly known as water on the brain. In this condition, fluid does not
drain properly and builds up in the brain, causing pressure, which in turn destroys
brain cells.
In dogs less than one year old at the first observed seizure, 60% had secondary seizures,
usually due to developmental anomalies. In general, one would expect a dog with a
developmental anomaly to begin to seize at an early age, since the condition is present
at birth. 30% of the dogs who began to seize at less than one year were diagnosed
as primary epileptics (no cause could be determined).
Of the dogs greater than five years of age at the time of first seizure, 63% had
secondary epilepsy, often caused by neoplasia, inflammatory processes or CVA. In
this age group, 25% were diagnosed as primary epileptics.
Of the dogs that were between one and five years of age at the time of first seizures,
63% were diagnosed as having primary epilepsy. There was a greater statistical chance
of primary epilepsy if the interval between the first and second seizures was greater
than four weeks, (in dogs that had experienced more than one seizure), whereas seizures
that occurred more frequently tended to be more indicative of other causes.
One interesting finding in the one to five year old group is that through the use
of MRI, five dogs in this group were diagnosed as hydrocephalic. This is contrary
to popular belief that dogs with hydrocephalus usually begin to seize at less than
a year. Without the availability of the MRI, these dogs would (probably) have been
assumed to have primary epilepsy. Nonetheless, even with MRI, CSF, etc. being performed,
in dogs of this age range, a cause for the seizures could be established in only
about 35% of the dogs.
In summary, dogs that begin to seize at less than one year of age should be suspected
of developmental problems or primary epilepsy. Dogs that begin to seize after five
years of age should be carefully examined for intracranial neoplasms, CVA, etc. Otherwise
healthy dogs who begin to seize at one to five years of age are highly likely to
be primary epileptics, particularly if the seizures occur four or more weeks apart.
Seizures that occur more than once a month, may be indicative of a developmental,
metabolic or infectious process. Neither sex of the dog, nor its neutering status,
was found to be significant in predicting the cause of the seizures.
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