Biomedical Engineering Reference
In-Depth Information
technique with which to comprehend these complex changes as
they occur throughout the brain. The discovery of the blood oxy-
genation level dependent (BOLD) signal and the advent of func-
tional magnetic resonance imaging (fMRI) have opened a new
window into the neurological correlates of normal and abnormal
brain activity. Absence seizures, because they are not usually asso-
ciated with movement in either humans or animals, provide a
rare opportunity to study ictal brain metabolism via a dynamic
imaging modality such as fMRI, which is sensitive to patient
movement. Animal models allow scientists to fully utilize the
power of fMRI because researchers can simultaneously
(1)
con-
trol seizure onset and type,
(2)
map epileptic activity, and
(3)
invasively explore the neurological underpinnings and molecu-
lar mechanisms of fMRI and seizure genesis. The prevalence of
fMRI in studying epilepsy reflects the excitement that this imag-
ing modality has generated.
We will begin with a discussion of generalized epilepsy and
SWD. Next, we will review possible relationships between neu-
roenergetics and fMRI signals in human and animal BOLD exper-
iments. We will then move on to human and animal studies of
SWD, review possible mechanisms for fMRI decreases in epilepsy,
and finally, discuss future directions and practical applications of
these findings.
2. Generalized
Spike-Wave
Seizures
Epileptic seizures are usually classified under two categories: Gen-
eralized seizures, which involve widespread regions in both hemi-
spheres of the brain; and partial seizures, which involve focal
brain regions
(1)
. In fact, recent evidence suggests that so-called
“generalized” seizures affect focal brain regions more intensely
while sparing others
(2-4)
, and “partial” seizures often involve
widespread cortical-subcortical networks beyond the seizure
focus
(5, 6)
. Nevertheless, the distinction between seizures that
are predominantly unilateral (“focal”) versus bilateral (“general-
ized”) in origin is helpful in establishing broad syndromes for clin-
ical diagnosis. A variety of patterns of electrical activity can be seen
in both partial and generalized seizures, including rhythmic spike-
wave discharges (SWD). Typical large-amplitude bilateral SWD
are seen most commonly in absence epilepsy (
Fig. 9.1
), but are
found in other forms of epilepsy as well
(2)
.Absence(
petit mal
)
is considered a form of generalized epilepsy, and usually begins
in childhood. Absence seizures consist of brief episodes of staring
and unresponsiveness often accompanied by mild eyelid fluttering
or myoclonic jerks. The duration of these seizures is usually less
than 10 s, and post-ictal deficits are not common.
