Cushing's syndrome: inferior petrosal sinus venography (PA and lateral) to confirm catheter position prior to venous sampling.
Percutaneous catheterisation of one inferior petrosal sinus, or if possible both simultaneously, is useful in the investigation of ACTH- dependent Cushing's syndrome. The finding of a 2:1 gradient in ACTH levels in venous samples from the inferior petrosal sinus(es) compared with peripheral blood suggests that the syndrome is likely to be of pituitary origin but is only seen in 50 per cent of cases. However, if 1 00~g corticotrophin releasing hormone, CRH41, is given intravenously, a rise in ACTH levels in the petrosal sinuses of over twice the basal value establishes a diagnosis of pituitary dependent Cushing's disease in 80 per cent of cases and may lateralise a microadenoma to one side of the fossa. Venous sampling from other sites may help to locate an ectopic source of ACTH production (see chapter 8).
