Bilateral inferior petrosal sinus sampling (BIPSS) is the gold standard investigation in Cushings disease for identifying the pituitary as the ACTH source. This technique aims to demonstrate a gradient of central:peripheral ACTH levels of >2:1 in such patients, or >3:1 after CRH stimulation. In patients without significant pituitary MRI abnormalities this facilitates neurosurgical exploration.
The test is limited by difficulties in achieving adequate sinus cannulation in some patients. Sinus to peripheral prolactin ratio is being used in some centres to confirm cannulation and allow for result adjustment, with ratio >1.8 indicating success. The prolactin-adjusted ACTH ratio may also guide adenoma lateralisation.
In our tertiary referral centre, between 2010 and 2015, 15 patients with confirmed ACTH-dependent Cushings underwent BIPSS with the addition of prolactin measurements from 2013. We aimed to demonstrate whether this modification improved the diagnostic yield by careful review of electronic case records of all fifteen patients.
Overall, 13 patients had definite pituitary Cushings disease; 12 subsequently confirmed on hypophysectomy and one with empty sella syndrome who underwent adrenalectomies. Two of 15, despite clinical suspicion, had negative results. The first predated prolactin adjustment, however the other had a low prolactin ratio indicating possible procedural failure and a second BIPSS is planned. Curiously, of the five patients who underwent BIPPS with prolactin measurement, only one met criteria for adequate bilateral cannulation. However, in three of the other four there was a clear central:peripheral ACTH gradient despite a central:peripheral prolactin ratio of <1.8.
BIPSS is a reliable test to confirm Cushings disease; we report an 87% success rate over the past 5 years. However, our data sheds some doubt on the reliability of using prolactin to confirm successful BIPSS, as we cannot easily explain a clear central ACTH excess with an apparently non-diagnostic test. This merits further study in larger series.