Most textbooks will tell you that hypoglycaemic encephalopathy kicks in when the blood glucose concentration falls below about 1.5 mmol/l. At this point, the lack of a metabolic substrate for the brain starts to show: the patient may get confused, unsteady or drowsy. However, at this stage a full recovery can be expected if the glucose level is returned to normal fairly quickly.
Once the glucose falls to less than 0.5 mmol/l, the patient is usually in a coma. Although recovery can be good if the deficit is rapidly reversed, there is also a substantial danger of permanent sequelae. In particular, the caudate nucleus and the cerebellum seem to get hit the most, and the long-term result can be ataxia, hemiparesis, parkinsonism or even dementia.
Damage to the brain secondary to very low blood glucose levels is known as a neuroglycopenic brain injury, with "neuroglycopenic" basically meaning "low glucose to the brain".