Hyperglycemia

Hyperglycemia means high blood sugar levels at or beyond 200 mg/dl. It is a serious and life threatening health problem that can be fatal if not properly treated in time. 
In a healthy body, insulin helps glucose (sugar) get from the blood into the body's cells. There, the cells can use the sugar to make energy. However, when there is not enough insulin in the body, or if a person's body does not react to insulin normally (this is called insulin resistance), too much sugar stays in the blood. This can cause serious health problems.
Extremely high blood sugar means diabetes mellitus, a condition that often causes very high blood sugar. Hyperglycemia that comes and goes may mean diabetes of any kind, or may be developing under low insulin levels or resistance to insulin.
Symptoms.
Early Signs.
Three of the early signs of hyperglycemia are "the 3 polys": polydipsia (feeling very thirsty), polyphagia (feeling very hungry), and polyuria (urinating a lot). These things happen because:
Other early signs of high blood sugar include blurred vision, headache, and feeling tired.
Late Signs.
If hyperglycemia gets worse or is not treated, it can cause toxic acids called ketones to build up in the blood and urine. This is called diabetic ketoacidosis, which is a medical emergency.
Symptoms of diabetic ketoacidosis can include: