An insulinoma is a rare, typically small tumor that originates in the pancreas. This growth is a type of pancreatic neuroendocrine tumor (PNET) that develops from the insulin-producing cells. The defining characteristic of an insulinoma is its ability to autonomously and excessively secrete insulin into the bloodstream. This unregulated hormone production sets the stage for a variety of health issues, primarily related to dangerously low blood sugar levels.
Defining the Condition
Insulinomas arise specifically from the beta cells located within the Islets of Langerhans, which are clusters of cells scattered throughout the pancreas. These tumors violate the normal physiological control mechanisms that regulate insulin release based on the body’s glucose needs. The resulting high levels of insulin drive glucose out of the blood and into the cells, regardless of whether the body has sufficient fuel.
The vast majority of insulinomas (approximately 90%) are solitary and benign growths, meaning they are not cancerous. They are generally small, often less than two centimeters in diameter, which can make them difficult to locate using standard imaging. In rare instances, an insulinoma can be malignant, potentially spreading to organs like the liver or nearby lymph nodes.
The functional problem caused by the tumor is the constant, unregulated release of insulin, leading to hyperinsulinemic hypoglycemia.
Understanding the Effects
The persistent overproduction of insulin causes chronic hypoglycemia, or low blood sugar, which manifests through two main categories of symptoms: neuroglycopenic and adrenergic. Neuroglycopenic symptoms result from the brain being starved of glucose. These effects often worsen during periods of fasting or prolonged physical exertion and are temporarily relieved by consuming carbohydrates.
Neuroglycopenic symptoms can include:
- Confusion and difficulty concentrating
- Visual disturbances like blurred or double vision
- Changes in personality or mood
- Amnesia, uncharacteristic behavior, or seizures
- Loss of consciousness
Adrenergic symptoms are the body’s counter-regulatory response, involving the release of stress hormones like adrenaline to raise blood sugar. These physiological warning signs include profuse sweating, anxiety or nervousness, heart palpitations, a noticeable tremor, and intense hunger.
How Insulinomas Are Diagnosed
The diagnostic process begins by confirming inappropriate insulin secretion relative to low blood glucose. The gold standard for this is a supervised 72-hour fast conducted in a hospital setting. During this test, the patient is allowed only non-caloric fluids while blood samples are regularly drawn to measure glucose, insulin, and C-peptide levels.
C-peptide is a protein released when proinsulin forms insulin; measuring it confirms the excess insulin is produced internally by the tumor, ruling out factitious hypoglycemia. Diagnosis is confirmed when the patient develops hypoglycemia symptoms, has low plasma glucose, and exhibits inappropriately high levels of both insulin and C-peptide. This biochemical confirmation is a prerequisite before proceeding to imaging.
Once the functional diagnosis is made, the tumor must be localized within the pancreas to plan treatment. Imaging techniques are employed, including:
- Cross-sectional studies like computed tomography (CT) or magnetic resonance imaging (MRI) scans.
- Endoscopic ultrasound (EUS), which provides high-resolution images by placing a probe close to the pancreas.
- Specialized nuclear medicine scans, used in complex cases to pinpoint the tumor’s exact location before surgery.
Treatment Options
Surgical removal offers the definitive cure for the majority of patients with a benign insulinoma. The goal is to completely excise the tumor while preserving as much healthy pancreatic tissue as possible. Depending on the tumor’s location, the surgeon may perform an enucleation, which involves carefully scooping out the tumor while leaving the rest of the organ intact.
If the tumor is larger or located deep within the pancreatic head or tail, a partial pancreatectomy may be necessary. For the 90% of patients with a benign, localized tumor, surgical resection has a very high success rate, often immediately resolving the chronic hypoglycemia. Following successful surgery, patients typically no longer require medication to manage blood sugar.
For patients who are not suitable surgical candidates or who have malignant or metastatic disease, medical management is the focus. Medications control hypoglycemia symptoms by either inhibiting insulin release or counteracting its effects. Diazoxide is frequently used to suppress insulin secretion directly from the beta cells, helping to maintain higher blood glucose levels.
Another class of medications, such as somatostatin analogs, may be administered to inhibit the release of hormones, including insulin, from the tumor cells. These medical treatments are palliative; they manage symptoms and improve quality of life, but they do not eliminate the tumor itself.