Peripheral Venous Sampling for Ectopic Sources of ACTH
Patients with Cushing's syndrome with high ACTH in whom there is strong evidence for an ectopic source and no evidence, including inferior petrosal sampling, to support a pituitary source. Sampling should be preceded by fine cut CT scanning of the chest and abdomen.
(Discuss with radiology S.R.)
Allergy to contrast dye.
Ischaemic Heart Disease.
Consent (risks of bleeding from sheath sites, venous thrombosis).
Fast for at least 4 hours.
2 people to attend to assist sample processing.
30 red top Vacutainers.
30 EDTA (purple topped Vacutainers), labelled prior to study.
Syringes for sampling and flushing cannulae.
Arrangements to transfer for immediate centrifugation.
Rarely local bleeding.
Very rare adrenal infarction.
- One large catheter (14-16g) is placed peripherally to take a background venous level simultaneously with each selective site.
- Sites will include adrenal veins, high IVC, hepatic vein, azygos and hemiazygos veins, Rt atrium, Rt & Lt innominate and thymic vein, both jugular and both superior and middle thyroid veins.
- An arterial sample is needed as a difference between arterial and venous levels may be seen in a pulmonary source.
- If CT scanning or tumour markers have shown a possible tumour site then super selective samples will be taken from the venous drainage.
- Samples for ACTH should be stored in ice and spun within 15 minutes.
- ACTH is measured in all samples. Cortisol is measured in the adrenal, IVC and Rt atrium samples. Prolactin is measured in the high jugular and IVC samples.
Local and reported experience with this technique has been disappointing with difficulties arising due to the pulsatile secretion of ACTH and wide variations in venous drainage. Results can either be expressed as a ratio of the simultaneous background ACTH or considered positive when the selective sample exceeds the maximum level recorded in the background samples.
SENSITIVITY AND SPECIFICITY
The largest and most successful study was reported from Bart's (Drury etal). In this study high levels were found appropriately in 6 of 16 sources of ectopic ACTH, however in only 4 cases was this the major evidence for the source. Often the level found was only slightly higher than background and in 4 of the 5 patients whose source remains undiagnosed there were levels recorded higher than the background samples and these did not help localisation. CT and tumour markers were more helpful.
Drury P.L. et al., British Medical Journal 284: 9-12 (1982).