Endocrinology · Pancreatic Neuroendocrine Tumors
The facts most likely to be tested
Insulinoma presents with Whipple's triad: symptomatic hypoglycemia, blood glucose < 50 mg/dL, and resolution of symptoms with glucose administration.
Patients frequently exhibit neuroglycopenic symptoms such as confusion, visual disturbances, seizures, and focal neurologic deficits.
Diagnostic confirmation requires a 72-hour supervised fast demonstrating inappropriately elevated insulin, C-peptide, and proinsulin levels in the setting of hypoglycemia.
Elevated C-peptide levels distinguish insulinoma from exogenous insulin abuse, where C-peptide levels are suppressed.
Insulinoma is the most common pancreatic neuroendocrine tumor and is associated with MEN type 1 syndrome (MEN1).
Initial localization of the tumor is best achieved via endoscopic ultrasound (EUS) or CT/MRI of the abdomen.
The definitive treatment for a localized insulinoma is surgical resection (enucleation or distal pancreatectomy).
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A 42-year-old woman presents to the emergency department after a witnessed seizure at home. She reports several months of intermittent dizziness, palpitations, and confusion that typically occur in the morning before breakfast and resolve after eating a snack. Physical examination is unremarkable, but a point-of-care glucose is 42 mg/dL. Laboratory studies reveal an elevated serum insulin level, elevated C-peptide, and elevated proinsulin in the presence of hypoglycemia.
What is the most likely diagnosis?
Insulinoma
The patient exhibits Whipple's triad and biochemical evidence of endogenous hyperinsulinism (elevated C-peptide), which is diagnostic for an insulinoma.
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Etiology / Epidemiology
Rare neuroendocrine tumor of pancreatic beta cells. Often associated with MEN type 1 syndrome.
Clinical Manifestations
Presents with Whipple's triad: hypoglycemic symptoms, low plasma glucose, and symptom relief with glucose administration.
Diagnosis
Gold standard is the 72-hour supervised fast. Diagnostic if glucose < 45 mg/dL with inappropriately high insulin.
Treatment
Definitive treatment is surgical resection. Use diazoxide for medical management of unresectable disease.
Prognosis
90% are benign solitary adenomas. Malignancy is rare but requires aggressive staging.
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Epidemiology & Etiology
Insulinomas are the most common functional pancreatic neuroendocrine tumors. While usually sporadic, they are a classic component of MEN type 1 (Wermer syndrome). Patients typically present between ages 30-60.
Pertinent Anatomy
Tumors arise from the islets of Langerhans within the pancreas. They are distributed equally throughout the head, body, and tail of the organ. Precise preoperative localization is mandatory for successful resection.
Pathophysiology
Autonomous, unregulated secretion of insulin and C-peptide occurs despite profound hypoglycemia. This hyperinsulinemia suppresses hepatic gluconeogenesis and glycogenolysis. The resulting neuroglycopenia and sympathetic discharge drive the clinical syndrome.
Clinical Manifestations
Patients exhibit Whipple's triad: documented hypoglycemia, neuroglycopenic symptoms (confusion, visual changes, seizures), and resolution upon glucose intake. Seizures and coma are critical red flags. Symptoms often occur during fasting or exercise.
Diagnosis
The 72-hour supervised fast is the gold standard. Diagnostic criteria include glucose < 45 mg/dL, insulin ≥ 3 µU/mL, and C-peptide ≥ 0.6 ng/mL. Elevated proinsulin levels are also highly sensitive.
Treatment
Surgical enucleation or partial pancreatectomy is the curative standard. For patients who are poor surgical candidates, diazoxide inhibits insulin release. Thiazide diuretics are often required to manage the fluid retention caused by diazoxide.
Prognosis
Most tumors are benign adenomas with excellent long-term survival post-resection. Metastatic disease occurs in 5-10% of cases, typically to the liver or regional lymph nodes. Post-operative monitoring for recurrent hypoglycemia is essential.
Differential Diagnosis
Factitious hypoglycemia: elevated insulin with suppressed C-peptide
Sulfonylurea ingestion: elevated insulin and C-peptide with positive drug screen
Non-islet cell tumor hypoglycemia: low insulin and low C-peptide
Reactive hypoglycemia: symptoms occur post-prandially, not during fasting
Addison's disease: hypoglycemia associated with hyperpigmentation and hypotension