Diagnostic difficulties in long-standing insulinoma with near-normal plasma insulin levels

Insulinoma tumors are often difficult to detect, as the symptoms largely precede occurrence of a visualized tumor. We report the case of an insulinoma patient with long delayed diagnosis and marked adaptation to extreme hypoglycemia. The patient with a 7-yr history of seizures was found to have plasma glucose concentration during a starvation test as low as 16 mg/dl, with no clinically significant symptoms and concomitant normal plasma insulin levels: 10-30 microIU/ml. All attempts to localize a tumor with repeated abdominal ultrasound examinations or computed tomography scanning were unsuccessful. The patient did not tolerate the introduced oral treatment with diazoxide. Once it had become technically available, endoscopic ultrasonography of the pancreas was performed. It revealed a 10 mm tumor in the pancreatic head. The tumor was subsequently removed surgically. During the operation plasma insulin concentration rose almost 15-fold, which confirmed the insulin-secreting character of the growth. Microscopic examination revealed benign insulinoma, with partially trabecular structure. One month after the operation the patient had normal plasma glucose values of 60-120 mg/dl, but she constantly complained of excessive thirst, which occurred soon after the operation and slowly subsided in the following weeks. In conclusion, the present report demonstrates that insulinoma should be considered and searched for in every case of hypoglycemia associated with normal insulin levels. It also confirms the essential role of endoscopic ultrasonography in the diagnosis of insulin-secreting tumors.