Epilepsy Detection and Monitoring - Quantitative EEG Analysis Methods and Clinical Applications - page 142

should not be understood as a single disorder, but rather as a group of syndromes
with vastly divergent symptoms all involving episodic abnormal electrical activity in
the brain.
Roughly 70% of cases present with no known cause. Of the remaining 30%, the
following are the most frequent causes: brain tumor and/or stroke; head trauma,
especially from automobile accidents, gunshot wounds, sports accidents, and falls
and blows; poisoning, such as lead poisoning, and substance abuse; infection, such
as meningitis, viral encephalitis, lupus erythematosus and, less frequently, mumps,
measles, diphtheria, and others; and maternal injury, infection, or systemic illness
that affects the developing brain of the fetus during pregnancy.
All people inherit varying degrees of susceptibility to seizures. The genetic factor
is assumed to be greater when no specific cause can be identified. Mutations in sev-
eral genes have been linked to some types of epilepsy. Several genes that code for
protein subunits of voltage-gated and ligand-gated ion channels have been associ-
ated with forms of generalized epilepsy and infantile seizure syndromes [4]. One
interesting finding in animals is that repeated low-level electrical stimulation (kind-
ling) to some brain sites can lead to permanent increases in seizure susceptibility.
Certain chemicals can also induce seizures. One mechanism proposed for this is
called excitotoxicity.
Epilepsies are classified in five ways: their etiology; semiology, observable mani-
festations of the seizures; location in the brain where the seizures originate; identifi-
able medical syndromes; and the event that triggers the seizures, such as flashing
lights. This classification is based on observation (clinical and EEG) rather than
underlying pathophysiology or anatomy. In 1989, the International League Against
Epilepsy proposed a classification scheme for epilepsies and epileptic syndromes. It
is broadly described as a two-axis scheme having the cause on one axis and the
extent of localization within the brain on the other.
There are many different epilepsy syndromes, each presenting with its own
unique combination of seizure type, typical age of onset, EEG findings, treatment,
and prognosis. Temporal lobe epilepsy is the most common epilepsy of adults. In
most cases, the epileptogenic region is found in the mesial temporal structures (e.g.,
the hippocampus, amygdala, and parahippocampal gyrus). Seizures begin in late
childhood or adolescence. There is an association with febrile seizures in childhood,
and some studies have shown herpes simplex virus (HSV) DNA in these regions,
suggesting this epilepsy has an infectious etiology. Most of these patients have com-
plex partial seizures sometimes preceded by an aura, and some temporal lobe epi-
lepsy patients also suffer from secondary generalized tonic-clonic seizures. Absence
epilepsy is the most common childhood epilepsy and affects children between the
ages of 4 and 12 years of age. These patients have recurrent absence seizures that can
occur hundreds of times a day. On their EEG, one finds the stereotypical generalized
3-Hz spike and wave discharges.
The first line of epilepsy treatment is anticonvulsant medication. In some cases
the implantation of a vagus nerve stimulator or a special ketogenic diet can be help-
ful. Neurosurgical operations for epilepsy can be palliative, reducing the frequency
or severity of seizures; however, in some patients, an operation can be curative.
Although antiepileptic drug treatment is the standard therapy for epilepsy, one third
of all patients remain unresponsive to currently available medication. There is gen-