By Teri Dickinson, DVM
SEIZURES IN DOGS
By Teri Dickinson, DVM
With special thanks for editorial assistance from Patricia J. Luttgen, DVM, MS, diplomate
ACVIM (neurology), Lakewood CO and Grace Robinson, DVM, Dallas, TX
DEFINITION OF TERMS
A seizure has been defined as "a transitory disturbance of brain function"(1)
which results in alterations to behavior and activity caused by abnormal brain function.
The terms convulsion, fit and ictus(2) are synonymous with the term seizure. A generalized
seizure has been defined as "a period of generalized muscle activity accompanied
by loss of consciousness."(3)
Epilepsy3 refers to recurrent seizures arising from a nonprogressive intracranial
(located within the skull) disorder. Note that the term nonprogressive means that
the cause is nonprogressive, not the severity or frequency of the seizures themselves.
Epilepsy is of acquired or idiopathic origin. Acquired or secondary epilepsy is that
in which a previously active brain disorder has left the brain in a seizure prone
state. An example might be a dog that is hit by a car and suffers head trauma, but
survives. Idiopathic epilepsy is also known as inherited, congenital or primary epilepsy
and involves recurrent seizures for which no cause can be identified.(4)
WHAT HAPPENS DURING A SEIZURE?
What happens during a seizure is complex, and not totally understood.(5) In the field
of human medicine much research has been devoted to how a seizure actually occurs
but many questions remain to be answered.
For our purposes, a simple explanation will suffice. Neurons (nerve cells) in the
brain transmit and process information through the creation and conduction of small
electrical currents. Normal function of a neuron is associated with a different electrical
charge inside the cell than outside. This difference is known as the membrane potential
(MP). It reflects the different charges found inside the cell membrane versus outside.
Under certain circumstances, this difference is turned into an actual electrical
charge, which is passed on to neighboring cells. The neuron is said to have "fired."
Neurons also create special substances, neurotransmitters, which when released, act
on the membranes of nearby cells. Neurotransmitters may act to excite or inhibit
nearby cells. Normal transmission of information requires a finely tuned combination
of excitement and inhibition. A seizure reflects the results of too much excitation,
too little inhibition or neurons that are too sensitive to the neurotransmitters.
During a seizure, certain cells (a seizure focus) begin to fire repeatedly and spread
this behavior to other cells. A normal brain responds with enough inhibitory neurotransmitters
to stop the spread. If a group of neurons "runs away," firing repeatedly
and the brain cannot inhibit them, a seizure results. Seizures are often self-limiting
as the renegade neurons exhaust themselves and stop firing. The period after the
seizure may reflect lower than normal activity among the neurons. How and why the
"renegade" neurons cause a seizure is still a matter of much investigation.
PARTS OF A SEIZURE
A seizure consists of three parts, the aura, the ictus and the postictus. The aura
signals the beginning of the seizure. It is characterized by behavioral changes such
as whining, restlessness, hiding or apprehension. During this stage, abnormal electrical
discharges are taking place among the neurons.
The ictus is the seizure stage and is characterized by all the signs associated with
the seizure: unconsciousness, muscle activity, salivation, etc. Neurons are firing
rapidly and are unresponsive to inhibitory substances.
The postictal phase occurs immediately after the seizure and may last a few minutes
to several days. It may be characterized by confusion, wandering, blindness or unresponsiveness.
The neurons are exhausted and unable to respond.
TYPES OF SEIZURES
Seizures can be divided into types, based on the signs seen during the seizure and
changes to the EEG (a reading of the electrical currents in the brain). Most work
on classifying seizures has been done in humans and may not always apply accurately
to dogs, but at least two types of seizures are documented in dogs: partial (also
known as focal) and generalized.(6)
Partial seizures have signs and EEG evidence that the seizure originates from one
area (focus) of the brain. A partial seizure may progress and become a generalized
seizure, or may be limited to a specific abnormality such as repetitive motions of
a single limb or facial twitches. Partial seizures are commonly associated with acquired
causes and brain tumors.
Generalized seizures are also called grand mal, major motor (affecting movement)
or tonic-clonic(7) and are characterized by signs and EEG abnormalities that are
widespread from the very beginning of the attack. Generalized seizures are usually
bilateral (affect both sides), symmetrical, include loss of consciousness, generalized
motor dysfunction and sometimes urination and defecation. Idiopathic epilepsy is
usually evidenced as generalized seizures.
DETERMINING THE CAUSE OF SEIZURES
In attempting to find the cause of seizures, it is important to remember that seizures
are not a disease, but merely a symptom of a disease process. A process of elimination
or "rule outs" is used to determine the cause.
The normal brain is capable of seizing in response to a variety of stimuli; therefore,
the causes of seizures are many. Causes of seizures can be divided into two major
categories, extracranial and intracranial.3 Extracranial causes are those that arise
outside the nervous system, (i.e., exposure to toxins). Intracranial causes arise
within the nervous system (i.e., brain tumors).
Each of these categories can be subdivided. Extracranial causes can be divided into
external or those that originate outside the body (i.e., toxins) and internal or
those that originate inside the body (i.e., liver disease). Intracranial diseases
can be divided into those that are progressive or nonprogressive. Progressive disorders
involve more and more brain tissue over time (i.e., tumors). Nonprogressive diseases
do not involve additional brain tissue, although the severity of the seizures may
increase over time. Epilepsy, both acquired and idiopathic, is an example of a nonprogressive,
intracranial cause of seizures.
Determining the cause of seizures is a process of ruling out possible causes and
begins with the veterinarian obtaining an accurate history, conducting a physical
exam that includes a thorough neurologic (nervous system) examination, and running
basic laboratory tests. A complete blood count, urinalysis and serum chemistries
to check for kidney and liver function, hypoglycemia, hypothyroidism, etc. are routinely
performed. Any abnormal test results will be followed by more specific tests. Acquired
(secondary) epilepsy is usually diagnosed by taking a thorough history, especially
regarding trauma (hit by car), anoxia (lack of oxygen due to near drowning, some
type of entanglement, etc.) or other such incidents. If the history and subsequent
testing fail to
reveal abnormalities, an extracranial cause can usually be ruled out.
If an intracranial cause is suspected, more specific tests will be conducted. These
may include skull x-rays, spinal fluid analysis, EEG's or possibly CT(8) or MRI(9)
scans of the head. Economics and availability of some tests (EEG, CT or MRI) may
limit diagnostic options.
If the specific testing for intracranial causes fails to identify abnormalities,
the remaining choice is nonprogressive intracranial disease. For the most part, nonprogressive
intracranial disease is classified as idiopathic or inherited epilepsy.
Dogs affected with idiopathic epilepsy are usually healthy and completely normal
between seizures with no abnormal test results (even EEG)(10) between seizures. Onset
of seizures usually occurs between six months and five years of age, although seizures
may not be recognized until the animal is older. In some individuals seizures may
be triggered by some stimulus: fatigue, stress, fear, etc. In others, onset of seizures
seems completely random. Epileptic(11) is the term used to describe individuals afflicted
with epilepsy.
It has not been determined if idiopathic epilepsy is an inherited disease in all
breeds of dogs. Despite the relatively high incidence of canine epilepsy, very little
active research is being conducted in dogs.(12)
A review of the literature indicates that inheritance has been proven or is strongly
suspected in the following breeds; Beagles,(13) German Shepherds (Alsatians-England)(14),
Belgian Tervurens,(15) Keeshonden,(16) Dachshunds(17) and Horak's laboratory dogs
(Czechoslovakia).(18) Many breeds are also reported to have a higher than average
incidence of affected individuals, which may also point to a genetic component. Among
these are Poodles, Cocker Spaniels, and Irish Setters.(19) The incidence of affected
individuals in various dog populations is reported to be between 0.5% and 5.7%10,(20).
No reports appear in which idiopathic epilepsy is proven not to be an inherited disease.
Three selective breeding studies of dogs produced incidences of 38%,(21) 66%(22)
and 100%15 epileptic offspring when both sire and dam were epileptic. Another study
documents an incidence of 63%(23) epileptic offspring produced from an accidental
breeding of two related, epileptic individuals. Two of these studies15,19 also noted
a very early onset (6-8 weeks of age) of seizures in these litters.
Pedigree analysis in several above mentioned breeds demonstrates a familial (occurring
in more members of a family than expected by chance) pattern of inheritance. Studies
of humans, mice, rats and rabbits leave little doubt that epilepsy in these species
is inherited. Mice are common animal models for epilepsy in humans, and work is under
way identifying the specific genes that cause the epilepsy.(24)
Cunningham and Farnbach,18 in a study supported in part by the Irish Setter Club
of America, reached the following conclusions:
"Until the genetic mechanisms involved are more clearly understood, it seems
prudent for veterinarians to advise owners of epileptic dogs that:
1) The idiopathic form of canine epilepsy may have a major genetic component.
2) It generally is impossible, without considerable test breedings,
to determine the degree of involvement of either the sire or the dam in producing
epileptic offspring.
3) It is unwise to breed an epileptic dog or to repeat the breeding that produced
it, and it is probably unwise to breed its litter mates."
TREATMENT OF SEIZURES
Treatment of seizures varies widely according to the cause. External extracranial
causes such as environmental toxins (lead, insecticides, etc.) require removal of
the offending substance from the animal's environment. In addition, specific treatments
may be required to counteract the effects of the toxins and anticonvulsants (drugs
used to control or prevent seizures) may be required to control the seizures.
Internal extracranial diseases require identifying the disease process and providing
the specific treatment. These disease processes include fungal infections, kidney
or liver disease, hypothyroidism, viral infections (i.e., distemper or rabies), and
hypoglycemia.
Intracranial diseases include tumors, congenital defects (i.e., hydrocephalus) and
infarctions (loss of blood supply to an area). Therapy includes specific treatment
of the primary disease (if possible) and anticonvulsants as needed to control seizures.
Anticonvulsants are usually the only treatments given for idiopathic epilepsy. Not
all affected individuals require drug therapy. The decision to treat epileptics is
usually based on several factors: frequency and severity of seizures, likelihood
of owner compliance with a treatment regime, and known side effects of anticonvulsants.
Despite their frightening appearance, infrequent grand mal seizures of 1-3 minutes
duration with no abnormalities remaining after the seizure are generally not dangerous
to the patient.
In treating seizures, the goal is to keep seizures to a minimum, while avoiding serious
side effects. All anticonvulsants have some undesirable side effects and require
regular monitoring of the patient. LeCouteur and Child3 recommend anticonvulsant
therapy for dogs known to have one or more seizures per month, unless animals have
clusters of seizures or episodes of status epilepticus (rapidly repeating seizures
with no period of consciousness between them). These animals should be treated regardless
of the length of the interval between seizures. Status epilepticus is a medical emergency
and requires immediate treatment.
Skerrit2 states that 60-70% of all treated cases have a reasonable measure of control
with proper, monitored, anticonvulsant therapy. A variety of drugs are used to treat
seizures. At present, phenobarbital is considered the drug of choice. Additional
drugs may be added to the treatment protocol if the level of control is not satisfactory.
Bibliography
1. DeLahunta, A: Veterinary Neuroanatomy and Clinical Neurology. WB Saunders Co.,
Philadelphia, 1977.
2. Skerritt, G: Canine Epilepsy. Canine Practice. Bailliere Tindall, London, 1991.
3. LeCouteur RA, et al: Clinical Management of Epilepsy in Dogs and Cats. Problems
in Veterinary Medicine. 1:4, 578-595, 1989.
4. Oliver, JE et al: Handbook of Veterinary Neurology. 2nd Edition. WB Saunders,
Co., Philadelphia, 1993.
5. Fenner, WR et al: Mechanisms of Seizure Disorders. Problems in Veterinary Medicine.
1:4, 501-515 1989.
6. Kay, W: What is Epilepsy? Problems in Veterinary Medicine. 1:4 495-500, 1989.
7. alternate muscle contraction and relaxation
8. computerized tomography, a specialized three dimensional form of X-ray, also known
as a CAT scan
9. magnetic resonance image, a type of brain scan
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Medicine. 1:4 535-555, 1989.
11. Dorland's Illustrated Medical Dictionary, 25th Ed. WB Saunders Co., Philadelphia,
1974.
12. Indrierei, RJ (Editor): Problems in Veterinary Medicine 1:4 preface, 1989.
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Seizures in a Purebred Beagle Colony. American Journal of Veterinary Research. 32:12
2039-2048, 1971.
14. Falco, MJ et al: The Genetics of Epilepsy in the British Alsatian. Journal Small
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Education II:1 77-85, 1980.
20. Koestner A, Rehfeld CE: Idiopathic Epilepsy in a Beagle Colony. Argonne National
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Puppies With Epileptic Parents. Veterinary Medicine 86:1060-61, 1991.
24. Rise ML et al: Genes of Epilepsy Mapped in the Mouse. Science 253:669-73, 1991.
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