Eliciting an accurate diagnosis of Cushing's disease before transphenoidal hypophysectomy is essential to avoid unnecessary surgery. Localisation of the tumour increases the chance of operative success.
In the right hands, Inferior Petrosal Sinus Sampling (IPSS) can provide invaluable information in distinguishing ectopic ACTH from pituitary Cushing's and localising the tumour in patients who are known to have Cushing's syndrome. Due to the invasive nature of the procedure and 2% complication rate, IPSS has not been carried out in normal volunteers. As a result, it cannot be used to make the diagnosis of Cushing's syndrome.
Between 1991 and 2002, 36 patients underwent IPSS; all these procedures were performed by the same radiologist. Two patients were not included due to unsuccessful cannulation of the pituitary sinuses. Thirty-four patient notes were analysed. Twenty-eight patients had a pituitary tumour and six patients were later found to in fact have a normal pituitary. Three of these had a positive IPSS but did not have a pituitary tumour. Two of these patients had persistence of their symptoms after hypophysectomy but did not have Cushing's disease, and one, bilateral adrenal hyperplasia. Three had a negative IPSS supported by the final diagnosis of adrenal adenoma or ectopic ACTH. Our results show that a negative result on IPSS is a strong indicator of a tumour not being present (Sensitivity = 100%, NPV=100%). A positive result does not indicate that a tumour is present in the pituitary for certain (Specificity = 50%, PPV = 90%).
IPSS should only be used in patients with definite Cushing's syndrome. It is excellent in distinguishing pituitary Cushing's from ectopic ACTH or adrenal adenoma, but it should not be used to 'make' the diagnosis of Cushing's syndrome in patients who have borderline biochemical evidence of Cushing's as it may give false positive results.