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The three most common causes of coma in diabetic patients are:

Very low blood sugar or severe hypoglycaemia

If the blood sugar level in the brain drops to a critical level (below 3.5 mmol/l), a person is at risk of losing consciousness and falling into a diabetic coma. Normally, as blood sugar begins to fall, the body compensates for this by releasing glucagon, a hormone that opposes the actions of insulin by raising blood glucose. A coma is more likely to occur if an individual has taken an overdose of insulin or other anti-diabetic medications, consumed a lot of alcohol or taken part in an unusual amount of exercise (especially if they are also on insulin).

Diabetic ketoacidosis

This is mostly seen in people with type 1 diabetes or those who are taking insulin. If there is shortage of insulin, the body fails to use the glucose in the blood for energy and instead fats are broken down to provide it, which forms acidic compounds called ketones. A build up ketones in the body causes a condition called ketoacidosis. Ketoacidosis may occur as a result of a missed dose of insulin or as a result of acute infection, injury or surgery causing a rise in hormones that counteract the effects of insulin.

Hyperosmolar hyperglycemic state (HHS)

This condition is characterized by severe dehydration and very high blood glucose. A missed dose of insulin, acute infection or injury, and heavy food or sugar intake are the most commonly causes of HHS severe enough to lead to coma. This type of coma develops slowly over several days or weeks.

Diabetic coma is a condition in which a patient loses consciousness because of excessively high or excessively low glucose (blood sugar). A person in a diabetic coma is still breathing but is in a profound state of unconsciousness and cannot be aroused by stimuli.

A coma is a deep, prolonged state of unconsciousness caused by an injury or disease. A region deep within the brain stem stimulates the brain to various levels of wakefulness and alertness, depending on signals received from the eyes, ears, skin and other sensory organs. When this area of the brain is disturbed, a person's consciousness may be impaired.

In most cases, a diabetic coma lasts a few days. Rarely, some patients may remain in a persistent coma (sometimes called "awake coma"). Patients in this state may have open eyes, and they may make grunting sounds or other vocalizations. However, the higher brain functions are absent.

Insulin Shock
Insulin is a hormone your pancreas normally produces in response to rising glucose levels. Many people with diabetes must take insulin to prevent their blood glucose from rising too high. If you take more insulin than your body needs, you could suddenly develop hypoglycemia, or low blood glucose. People who take insulin and exercise without eating or drink too much alcohol are particularly susceptible to hypoglycemia. Insulin shock, which is a form of diabetic coma, may occur if your blood glucose falls too low to support your brain's metabolic demands - usually below 50 mg/dL. Seizures may occur before the onset of coma.

Diabetic Ketoacidosis
Just as a low blood glucose level can trigger unconsciousness, extremely high blood glucose levels can alter brain function and lead to coma. Diabetic ketoacidosis, which usually affects people with type 1 diabetes, happens when you don't have enough insulin in your system. DKA may be the initial manifestation of newly developed diabetes, or it may result from a skipped insulin dose in a person who has already been diagnosed with diabetes. Infections and alcohol abuse can also trigger DKA.

Blood glucose levels in people with DKA are typically above 250 mg/dL, and ketones - the byproducts of fat breakdown - can be detected in the urine and blood of people suffering from DKA. The primary cause of coma in people with DKA is brain swelling due to severe dehydration and loss of electrolytes, which result from excessive urination caused by elevated blood glucose and ketone levels. If untreated, DKA is fatal.

Hyperglycemic Hyperosmolar Syndrome
Just as people with type 1 diabetes are susceptible to DKA, those with type 2 diabetes are at risk for hyperglycemic hyperosmolar syndrome if their diabetes is not adequately controlled. This refers to the extremely high blood glucose levels - often above 500 mg/dL - and dehydration that characterize this disorder. Prolonged or severe high blood glucose triggers excessive urination, which in turn leads to dehydration and loss of electrolytes. If this situation persists, your blood becomes progressively more concentrated and, mirroring the brain injury seen in DKA, you may go into a coma. Because most people with type 2 diabetes still make some insulin, they usually do not break down fat tissue or produce large amounts of ketones when their diabetes is uncontrolled. So, hyperglycemic hyperosmolar syndrome is often called nonketotic hyperosmolar syndrome. The fatality rate in nonketotic hyperosmolar syndrome can be as high as 40 percent.