Can hyperglycemia cause a diabetic coma?

Yes: If the hyperglycemia is high enough it can lead to diabetic coma. We call this hyperosmolar hyperglycemia coma or syndrome. Diabetes ketoacidosis coma is another form of diabetic coma but this one is caused by not having Insulin rather than the extreme hyperglycemia (which is just a sign for this coma).

Lack of insulin..: Resulting in very high sugars and in some diabetes ketones in blood/urine can cause diabetic coma.