Anatomy of the junction of the inferior petrosal sinus and the internal jugular vein.

PURPOSE To evaluate the anatomy of the junction of the inferior petrosal sinus and the internal jugular vein. METHODS Using a previously described classification system, we prospectively classified venous anatomy bilaterally in 135 of 136 persons consecutively undergoing inferior petrosal sinus sampling. RESULTS Type IV anatomy, with no anastomosis between the inferior petrosal sinus and the internal jugular vein, was significantly less frequent in our series than in a previous series (1 versus 7%; P < .001). Venous anatomy did not differ significantly between the left and the right junctions or between men and women. Venous anatomy was symmetric in only 65% of subjects (86 of 133). We describe an uncommon variant anatomy, incomplete type IV, found in 4.5% of our subjects (six of 133), that may cause incorrect results of petrosal sinus sampling. CONCLUSION Bilateral sampling of pituitary venous effluent can be accomplished by the methods described, despite the presence of either incomplete or true type IV venous anatomy. Bilateral petrosal sinus sampling is anatomically possible in 99% of persons.