Endocrine Abstracts (2017) 50 P279 | DOI: 10.1530/endoabs.50.P279

Bilateral Inferior Petrosal Sinus Sampling (BIPSS) reliably differentiates pituitary from ectopic Cushing's syndrome, but does not predict pituitary tumour location, especially when lateralizing to the right

Louisa White1, Jake Tobin1, Olympia Koulouri1,2, Andrew Powlson1,2, Waiel Bashari1,2, Brendan Koo2 & Mark Gurnell1,2

1University of Cambridge, Cambridge, UK; 2Addenbrooke’s Hospital, Cambridge, UK.

Background: BIPSS is considered the gold-standard procedure for differentiating autonomous pituitary [Cushing’s Disease (CD)] and ectopic ACTH syndrome (EAS) sources of ACTH hypersecretion. A basal (i.e. unstimulated) pituitary:peripheral ACTH ratio >2, and/or a CRH stimulated pituitary:peripheral ACTH ratio >3, have been proposed as indicative of CD, with a range of sensitivities and specificities cited in the literature. Additionally, in CD a peak interpetrosal gradient of >1.4 has been reported to predict the side of the adenoma in approximately 2/3 of cases. We have re-examined the accuracy of these diagnostic thresholds in a cohort of patients undergoing BIPSS in our centre over the last 12 years, in whom confirmation of the underlying diagnosis was subsequently established.

Methods: 41 patients with biochemically proven ACTH-dependent Cushing’s syndrome, who had undergone BIPSS and had either subsequent histological confirmation of the ACTH source or significant improvement/cure of their Cushing’s were included in the study.

Results: Based on pre-CRH stimulation results, 90% of patients were deemed to have pituitary Cushing’s and 10% EAS. Post-CRH stimulation, 95% of patients reached the cut-off for CD. These yielded a sensitivity of 97% and a specificity of 100% for pre-CRH stimulation and 100% sensitivity and 67% specificity for post-CRH stimulation. The majority of BIPSS procedures lateralized to the right side (65%), but at surgery this was found to be correct in only half of cases. On the contrary, left lateralization had a much higher positive predictive value (80%). Overall, lateralization was accurate in 60% of cases.

Conclusion: Predominance of right sided lateralization on BIPSS is frequently (1:2) misleading. We propose that additional localizing strategies are pursued prior to pituitary surgery, such as detailed MRI sequences (e.g. SPGR MRI) or functional imaging modalities (e.g. 11C-Methionine PET). Whole gland exploration is particularly important for patients with inconclusive pre-operative localizing investigations.