How low does a patient's blood glucose have to be before they start showing neurological symptoms?

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".