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04 What is a "seizure threshold"?


This article is from the Canine Epilepsy FAQ, by Alicia Wiersma-Aylward with numerous contributions by others.

04 What is a "seizure threshold"?

Dr. Alexander de Lahunta of Cornell University and others suggest that
each animal inherits a "genetically determined predisposition to
seizures", and that seizures occur when this threshold is exceeded.
(Cunningham, Inherited). In other words, a physical condition (see
examples under section on secondary epilepsy above) which may cause
seizures in a low-threshold animal may not cause seizures in a
"normal" animal.

The seizure threshold is apparently exceptionally low in animals that
suffer from idiopathic (primary) epilepsy. (de Lahunta). An animal's
threshold can also be altered by other means. Certain types of
tranquilizers (e.g. acepromazine) may induce seizures in animal with a
low threshold. The medical condition of alkalosis is reported to
decrease the threshold. (Shell, Differential)

Karen R. Dyer, DVM, PH.D, and Linda G. Shell, DVM, Dilp. ACVIM, note
that there is "convincing experimental evidence" that repetitive
seizures can "irreversibly lower the seizure threshold" in a process
called kindling. William Fenner, DVM and Julie Haas, DVM, describe
kindling as a mechanism in which epileptic neurons in the brain
"recruit" normal neurons into the original seizure focus, enlarging
the area of the brain that can produce seizures. Linda Shell, DVM
describes kindling as the "increased excitability of neurons", and
notes that normal neurons, sufficiently stimulated, become
increasingly able to cause seizures independent of outside

The mirror focus phenomenon also deserves mention. Each hemisphere of
the brain is a "mirror image" of the other. A seizure focus on one
side of the brain will show itself as abnormal wave forms on EEG
recordings. Within a period of weeks, the "normal" side of the brain
will start to show similar EEG abnormalities. In time, the mirror
focus becomes capable of causing seizure activity on its own. Thus,
repetitive, uncontrolled seizures also lower the seizure threshold in
any given animal. That is why early intervention is so important in
the control of seizures.


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