An insulinoma is a tumor that arises from the endocrine tissue of the pancreas, specifically the Beta cells within the Islets of Langerhans. This rare growth is classified as a pancreatic neuroendocrine tumor (pNET) and is defined by its ability to produce and release insulin. Most insulinomas are small and singular, and approximately 90% are benign, meaning they do not spread to other parts of the body. They are a cause of severe and recurrent episodes of low blood sugar, or hypoglycemia, due to their uncontrolled hormone secretion.
What Causes Excessive Insulin Production
The fundamental issue in an insulinoma is the tumor’s unregulated secretion of the hormone insulin. In a healthy pancreas, the Beta cells carefully monitor blood glucose levels and only release insulin when sugar concentration rises. This feedback loop is absent in the tumor cells, which continue to produce and release insulin autonomously, regardless of the current glucose level.
This constant, inappropriate release of insulin drives glucose from the bloodstream into the cells, leading to a state of chronic or severe hypoglycemia. The body’s normal counter-regulatory mechanisms, which should suppress insulin release when blood sugar drops, are ineffective against the tumor. While most cases occur spontaneously, a small percentage are associated with the inherited condition Multiple Endocrine Neoplasia type 1 (MEN1).
How the Tumor Affects the Body
The primary impact of an insulinoma is the array of symptoms caused by the resulting low blood sugar. These effects are broadly categorized based on whether they stem from the brain being starved of its main fuel source (neuroglycopenic symptoms) or from the body’s stress response (adrenergic symptoms).
Neuroglycopenic symptoms include confusion, difficulty concentrating, and blurred or double vision. As hypoglycemia becomes pronounced, patients may experience profound behavioral changes, amnesia, and in severe instances, seizures or a coma. The severity of these symptoms often leads to misdiagnosis as a neurological or psychiatric disorder, causing a significant delay in finding the correct cause.
Adrenergic symptoms are the body’s automatic reaction to a sudden drop in blood sugar, triggered by the release of stress hormones like adrenaline. Patients commonly experience sudden sweating, a rapid or pounding heartbeat (palpitations), trembling, and intense anxiety.
Confirming the Diagnosis
Confirming an insulinoma relies on demonstrating the inappropriate release of insulin during a period of hypoglycemia. The standard diagnostic test is the supervised fasting test, which is performed in a hospital setting and can last up to 72 hours. During this period, the patient is restricted from eating, and blood samples are collected periodically.
The test is halted when the patient shows symptoms of hypoglycemia and their blood glucose drops to a defined low level (typically below 45 mg/dL). At this moment, blood is drawn to simultaneously measure glucose, insulin, and C-peptide levels. In an insulinoma, the blood tests will show inappropriately high levels of insulin and C-peptide alongside the low glucose.
Once the biochemical diagnosis is established, the next step is locating the tumor, as insulinomas are often small (less than two centimeters). Imaging studies such as CT scans, MRI, or endoscopic ultrasound (EUS) are employed to pinpoint the tumor’s exact location within the pancreas. Accurate localization is important for surgical planning, as definitive treatment requires physical removal.
Surgical and Medical Interventions
Surgical removal is the definitive and most effective treatment for insulinoma, offering a cure in the vast majority of cases. The surgical approach depends on the tumor’s size and location, with less invasive procedures preferred when possible. For small, easily accessible tumors, enucleation is often used, which involves carefully removing only the tumor while preserving the surrounding pancreatic tissue.
If the tumor is larger or situated in a difficult area, a partial pancreatectomy may be necessary, where a portion of the pancreas is removed. Patients who are not candidates for surgery, or those with malignant tumors that have spread, are managed with medical therapy to control hypoglycemia. Medications like diazoxide are used to directly inhibit the release of insulin from the tumor cells.