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An insulinoma is a tumour of the pancreas derived from the beta cells which while retaining the ability to synthesize and secrete insulin is autonomous of the normal feedback mechanisms. Patients present with symptomatic hypoglycemia which is ameliorated by feeding. The diagnosis of an insulinoma is usually made biochemically with low blood sugar, elevated insulin, pro-insulin and C-peptide levels and confirmed by medical imaging or angiography. The definitive treatment is surgery.
Insulinomas are rare neuroendocrine tumours with an incidence estimated at 1 to 4 new cases per million persons per year. Insulinoma is one of the most common types of tumour arising from the islets of Langerhans cells (pancreatic endocrine tumours). Estimates of malignancy (metastases) range from 5% to 30%. Over 99% of insulinomas originate in the pancreas, with rare cases from ectopic pancreatic tissue. About 5% of cases are associated with tumours of the parathyroid glands and the pituitary (Multiple endocrine neoplasia type 1) and are more likely to be multiple and malignant. Most insulinomas are small, less than 2 cm.
Patients with insulinomas usually develop neuroglycopenic symptoms. These include recurrent headache, lethargy, diplopia, and blurred vision, particularly with exercise or fasting. Severe hypoglycemia may result in seizures, coma, and permanent neurological damage. Symptoms resulting from the catecholaminergic response to hypoglycemia (i.e. tremulousness, palpitations, tachycardia, sweating, hunger, anxiety, nausea) are not as common. Sudden weight gain (the patient can become massively obese) is sometimes seen.
The diagnosis of insulinoma is suspected in a patient with symptomatic fasting hypoglycemia. The conditions of Whipple’s triad need to be met for the diagnosis of “true hypoglycemia” to be made:
The following blood tests are needed to diagnose insulinoma:
If available, a proinsulin level might be useful as well. Other blood tests may help rule out other conditions which can cause hypoglycemia.
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