The symptoms tend to be intermittent because the tumours release surges of insulin episodically. The main symptom is the hypoglycaemia resulting directly from the insulin excess. Hypoglycaemia occurs mostly after a period of fasting (e.g. before breakfast or late afternoon) or after sustained exercise. The body's organs require a consistent, normal glucose concentration to function adequately. Some organs are more susceptible to aberrations of this, especially the brain, which is why the majority of symptoms experienced are due totransiently impaired brain function. These are:
Further symptoms arise afterwards due to the release of catecholamines, adrenaline and noradrenaline from the adrenal gland (see the section on the physiology of the adrenal gland). This is a response of the body to the hypoglycaemia (these hormones act to increase blood glucose levels). However in addition they have side effects on the cardiovascular system resulting in the following:
In a few patients (less than 10%) gastrointestinal symptoms are experienced such as:
Most of these symptoms resolve after a short period of time due to the action of compensatory mechanisms. However, it is possible for them to persist and become life threatening. Hypoglycaemia can lead to irreversible coma.