The brain depends on glucose as a fuel. As glucose levels drop below 65 mg/dL (3.2 mmol/L) counterregulatory hormones (eg, glucagon, cortisol, epinephrine) are released, and symptoms of hypoglycemia develop. These symptoms include sweatiness, shaking, confusion, behavioral changes, and, eventually, coma when blood glucose levels fall below 30-40 mg/dL.

"Brittle" diabetes, also known as unstable diabetes or labile diabetes, is a term that was traditionally used to describe the dramatic and recurrent swings in glucose levels, often occurring for no apparent reason in insulin-dependent diabetes. This term, however, has no biologic basis and should not be used.[39] Still, type 1 diabetes can be accompanied by irregular and unpredictable high blood sugar levels, frequently with ketosis, and sometimes with serious low blood sugar levels. Other complications include an impaired counterregulatory response to low blood sugar, infection, gastroparesis (which leads to erratic absorption of dietary carbohydrates), and endocrinopathies (e.g., Addison's disease).[39] These phenomena are believed to occur no more frequently than in 1% to 2% of persons with type 1 diabetes.[40]
The term "diabetes" or "to pass through" was first used in 230 BCE by the Greek Apollonius of Memphis.[108] The disease was considered rare during the time of the Roman empire, with Galen commenting he had only seen two cases during his career.[108] This is possibly due to the diet and lifestyle of the ancients, or because the clinical symptoms were observed during the advanced stage of the disease. Galen named the disease "diarrhea of the urine" (diarrhea urinosa).[110]
Complications of diabetes are responsible for considerable morbidity and mortality. The acute complications of diabetes are hypo- and hyperglycemic coma and infections. The chronic complications include microvascular complications such as retinopathy and nephropathy, and the macrovascular complications of heart disease and stroke. Diabetes mellitus is the commonest cause of blindness and renal failure in the UK and the USA. Other common complications include autonomic and peripheral neuropathy. A combination of vascular and neuropathic disturbances results in a high prevalence of impotence in men with diabetes. Peripheral neuropathy causes lack of sensation in the feet which can cause minor injuries to go unnoticed, become infected and, with circulatory problems obstructing healing, ulceration and gangrene are serious risks and amputation is not uncommon. Evidence from meta-analysis of studies of the relationship between glycemic control and microvascular complications (Wang, Lau, & Chalmers, 1993), and from the longitudinal multicenter Diabetes Control and Complications Trial (DCCT) in the USA (DCCT Research Group, 1993), have established a clear relationship between improved blood glucose control and reduction of risk of retinopathy and other microvascular complications in insulin-dependent diabetes mellitus (IDDM). It is likely that there would be similar findings for noninsulin-dependent diabetes mellitus (NIDDM) though the studies did not include NIDDM patients. However, the DCCT included highly selected, well-motivated, well-educated and well-supported patients, cared for by well-staffed diabetes care teams involving educators and psychologists as well as diabetologists and diabetes specialist nurses.
"Secondary" diabetes refers to elevated blood sugar levels from another medical condition. Secondary diabetes may develop when the pancreatic tissue responsible for the production of insulin is destroyed by disease, such as chronic pancreatitis (inflammation of the pancreas by toxins like excessive alcohol), trauma, or surgical removal of the pancreas.
Several other signs and symptoms can mark the onset of diabetes although they are not specific to the disease. In addition to the known ones above, they include blurred vision, headache, fatigue, slow healing of cuts, and itchy skin. Prolonged high blood glucose can cause glucose absorption in the lens of the eye, which leads to changes in its shape, resulting in vision changes. Long-term vision loss can also be caused by diabetic retinopathy. A number of skin rashes that can occur in diabetes are collectively known as diabetic dermadromes.[23]
Type 2 diabetes, which is often diagnosed when a person has an A1C of at least 7 on two separate occasions, can lead to potentially serious issues, like neuropathy, or nerve damage; vision problems; an increased risk of heart disease; and other diabetes complications. A person’s A1C is the two- to three-month average of his or her blood sugar levels.