The venous drainage of the pituitary gland can be sampled in the inferior petrosal sinuses (Fig. 74-7). The finding of an ACTH gradient between petrosal sinus blood and peripheral blood in a patient with hypercortisolism localizes the site of ACTH production to the sella or parasellar region.26,26a Because the venous drainage of the pituitary gland tends to lateralize (the right half drains into the right inferior petrosal sinus and the left half drains into the left sinus), the site of an ACTH-producing microadenoma usually can be localized within the pituitary gland. The usefulness of the test is increased by sampling during the administration of CRH: the central/peripheral ACTH gradients and the right/left gradient are enhanced by this addition. The average gradient in Cushing disease is ˜15, with a lower limit of 1.7. The lower limit increases to 3 after CRH administration, which completely separates this population from those with nonpituitary causes of Cushing syndrome. Current data suggest that the test correctly identifies pituitary disease >95% of the time, which makes it the most powerful single test for differential diagnosis in Cushing syndrome.27 In previously untreated patients, the procedure correctly lateralizes the disease ˜85% of the time. Untoward effects are largely confined to the catheterization process. A few patients have had transient signs and symptoms of brainstem dysfunction or other neurological findings.27a These have not led to permanent disability. Other untoward effects include bleeding, bruising, and minor infections at the venipuncture site.