Absence of Intercavernous Venous Mixing: Evidence Supporting Lateralization of Pituitary Microadenomas by Venous Sampling*

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Abstract
Preoperative lateralization of endocrine-active pituitary tumors may be possible by bilateral inferior petrosal sinus (IPS) sampling for hormone measurement. The reliability of this technique depends on unilateral drainage of blood from the adenoma within the pituitary gland, in its course from the gland to the ipsilateral cavernous sinus, and from the cavernous sinus to the junction of the IPS and the internal jugular vein. Anatomical studies demonstrate unilateral drainage of each hemihypophysis into the ipsilateral cavernous sinus. The degree of mixing of blood between the cavernous sinuses, via the intercavernous sinus, and between the IPSs, via the basilar plexus, is unknown. We determined the extent of mixing of blood flowing from the superior orbital vein to the junction of the IPS and the internal jugular vein in four rhesus monkeys. After catheterization of a superior orbital vein and both IPSs, 99Tc-colloidal sulfur was infused into the superior orbital vein while blood samples were taken simultaneously from both IPSs and a peripheral vein at 3-min intervals. Mean relative radioactivity was 100 ± 14% (±SEM) in the ipsilateral IPS, 8 ± 2% in the contralateral IPS, and 3 ± 1% in a peripheral vein. Isotope reaching the ipsilateral IPS before recirculation from the periphery was 10- to 313-fold more concentrated than in the contralateral IPS. These findings suggest that mixture of blood between the cavernous sinuses and between the IPSs is insignificant and support the capability of preoperative lateralization of pituitary microadenomas by bilateral and simultaneous IPS sampling.