To
understand the pathophysiology of seizures in brief, lets take the following
case.
“Two brothers,
Ram and Shyam were playing chess. When all of a sudden Ram noticed that his 40-year old
brother seemed to be daydreaming and seemed to be confused and having a petrified stare for about 20 seconds. Then suddenly his right hand began to bend into
an awkward position and then to shake. The shaking grew worse, progressing
gradually from the hand to the arm and then to the entire right side of the body.
Then his body stiffened for about 15 seconds, followed by shaking movements of
all four limbs that lasted another 30 seconds or so. Then he became limp and
unconscious.”
1.
Now here Rob first showed the symptoms of daydreaming, confusion and petrified stare, this is known as Aura. Since its a behavioural
disturbance, we can guess that the foci of this seizure activity is in the
temporal lobe(hippocampus, amygdala etc).
2. So
there is abnormal synchronous electrical activity(ASET) somewhere in the
temporal lobe which could have been due to any pathology like tumour,
tuberculoma, stroke, viral encephalitis, neurocysticercosis etc.
3. It
took the seizure activity 20 seconds to override the Surround inhibition of the
temporal lobe and then spread to the neighbouring areas.
4.
Next there is contraction of muscles followed by shaking first in his right
hand, then arm, then the complete right side. This means now the abnormal
synchronous electrical activity(ASET) has spread to the left motor cortex and
progressively involved the entire homunculus.
5.
Next this tonic-clonic activity involves the entire body. This means that ASET
has spread to the contralateral hemishphere via corpus callosum and other
commissures and that it has involved Thalamus, which is the gateway to the
entire cerebral cortex. Now since it is bilateral, we can call it a Generalized seizure.
6.
Lets learn the mechanism of Tonic-Clonic seizure(Grand Mal seizure) activity.
A: First there is sudden inhibition of all
GABAergic activity leading to overriding of the Surround Inhibition, causing
contraction of both agonist and antagonist group of muscles, referred to as the
Tonic phase.
B: Then the GABA-mediated inhibition is
gradually restored and while increasing it starts oscillating with the
excitatory activity mediated by Glutamate via its NMDA and AMPA receptors. If
this oscillation involves the motor cortex, there is shaking movements, referred to as Clonic phase.
C: Ultimately the
GABA-mediated inhibition prevails, resulting in all the muscles becoming
flaccid and the patient becomes unconscious until normal brain function is restored.
-VM
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