What is the difference between a severe hypoglycemic event and a tonic seizure?

Different: Severe hypoglycemia can cause a lot of symptoms, including a seizure. But it does not have to lead to a seizure. It can manifest as confusion, headaches, irritability, syncope(fainting), and even coma's. A tonic seizure is a seizure where a person's body makes sudden stiffening movements.

Seizure : A hypoglycemic 'event' means because of severely low blood sugar you lose consciousness.But, thats not a seizure. A seizure is a burst of abnormal electrica activity. You might or might not totally lose consciousness. (There are several kinds of seizures as well.) Lastly, ultralow blood sugar can be a cause of seizure.