Focal seizure originates from one side of the brain, it could be localized or distributed over a larger area of that side. while the focal to bilateral tonic clonic is a case when the seizure start focally( in one side) and then spread to involve the both sides of the brain.
How it could be explained - from a neurophysiological point of view -to evolve a bilateral seizure from a focal seizure?
Because activation of centra diencephalic structures (centrencphalic region).
Epilepsy is all about neuroplasticity or brain change. Check out old Website that's still limping along (looks crappy 'cause still running on Frames, which is out of voque some time ago). Regardless, need to look at the Articles on Kindling (model of focal epilepsy) and Long Term Potentiation (LTP).
Main page:
http://hargreaves.swong.webfactional.com/index.htm
Kindling link:
http://hargreaves.swong.webfactional.com/kindle.htm
LTP link:
http://hargreaves.swong.webfactional.com/ltp.htm
The neurophysiological basis of epileptic seizures is hypersynchronous electrical activity of neural networks.
In focal seizures, this would involve circumscribed brain regions. Through repetitive activity, however, synaptic connections within distant cortico-cortical and cortico-thalamocortical networks become strengthened - this is underpinned by different forms of synaptic plasticity (long-term potentiation, spike timing-dependent plasticity, structural plasticity amongst others, driven by changes in gene expression that favour excitatory electrical and neurochemical profiles). This also explains why uncontrolled seizure activity during refractory status epilepticus becomes increasingly difficult to control.
Precisely what determines the 'decision' of whether a focal seizure generalises, I think, is probabilistic - and that is determined by historical activity and associated plasticity.
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