OBJECT: The mesencephalic veins drain crucial brainstem areas. Due to the narrowness of the tentorial notch, these veins can become obstructed as a result of herniation or surgery, leading to hemorrhage and severe consequences for the patient. There is little in the literature about the mesencephalic veins. The aim of this study was to perform an exact analysis of their microanatomy.

METHODS: Fifty-two cadaveric hemispheres were examined under an operating microscope, and measurements were made with a digital caliper. The authors focused on the basal vein (BV), pontomesencephalic vein (PMV), peduncular vein (PV), lateral mesencephalic vein (LMV), and other smaller veins. The PMV was identified in 84.6% of specimens (mean diameter 0.54 mm); the PV, in 86.5% (mean diameter 0.86 mm); and the LMV, in 100% (mean diameter 1.07 mm). Four types of LMV were identified on the basis of the vein's course. Other smaller veins were also differentiated according to whether they drained mainly the cerebral peduncle, the lemniscal trigone, or the tectum. These veins and their junctions with other veins were depicted.

CONCLUSIONS: A thorough understanding of the microanatomy of the mesencephalic veins is crucial in brainstem surgery in order to avoid brain damage due to venous infarction and subsequent edema. Because knowledge of the course, variations, and outflow system of these veins could improve surgical outcome, they warrant special attention during surgery.