ACTH increment post total bilateral adrenalectomy for Cushing's disease: a consistent biosignature for predicting Nelson's syndrome.

Nelson's syndrome (NS) is regarded as an aggressive complication of total bilateral adrenalectomy (TBA) for Cushing's disease (CD). This challenge may be addressed by using clinical criteria to guide frequency of neuroimaging to enable timely management of NS and also avoid unnecessary frequent imaging. All patients (n = 43) with CD subjected to TBA over 35 years at a tertiary care centre were included. NS was defined as a newly appearing or expanding (> 2 mm) pituitary adenoma with or without ACTH levels exceeding 500 pg/ml. Pre-and post-TBA parameters like clinical symptomatology, cortisol, ACTH and radiology were analysed for the prediction of NS. NS developed in 39.5% (n = 17) patients with a median follow-up of 7 years. Half of them had new appearance, while rest had an expansion of pre-existing pituitary tumour. Majority (90%) had ACTH above 500 pg/ml. On Cox proportional hazards analysis, frequent discriminatory features of protein catabolism (≥ 4) (HR 1.15, CI 0.18, 7.06), proximal myopathy (HR 8.82, CI 1.12, 69.58) and annual ACTH increment of 113 pg/ml (HR 12.56, CI 1.88, 88.76) predicted NS. First post-operative year ACTH indices predicting NS included ACTH rise of 116 pg/ml and absolute ACTH of 142 pg/ml (sensitivity, specificity exceeding 90%). Annual ACTH increment exceeding 113 pg/ml, ≥ 4 discriminatory features and uncontrolled hypertension had the best overall prediction. Patients who developed NS had higher rebound rise of ACTH following TBA and a more severe disease phenotype at baseline. Consistent ACTH increment can be used as a marker for predicting the development of NS.