What causes a diabetic coma?

Uncontrolled glucose. Both low and high blood glucose can lead to impairments in consciousness.
Coma in a diabetic. Coma in a person with diabetes has many causes, the two most common ones are low blood sugar, which should be treated promptly with sugar tablets or any drink that has sugar in it if the person is awake and is able to drink or an injection of a medication called glucagon (all diabetics should have an injection of glucagon at all time). The other cause is DKA which should be treated in a hospital.