Accessory intraventricular prominence of the occipital horn of the lateral ventricle

Object. Knowledge of normal variations in ventricular morphological features is important in endoscopic neurosurgery. Classically, two elevations are described on the medial wall of the occipital horn of the lateral ventricle: an upper bulb and a lower calcar avis. Nevertheless, a third, as yet unreported elevation may be present at the junction of the medial wall and the floor of the occipital horn. The authors report the frequency with which this third elevation was found in a series of cadaveric brains. Methods. The medial wall of the occipital horn of the lateral ventricle was studied in the three orthogonal planes in 45 formalin-fixed cadaveric hemispheres. The underlying structures responsible for the observed intraventricular prominences were exposed by microdissection. A third elevation was present, lying ventrorostral to the calcar avis, in seven (47%) of the 15 single hemispheres, and bilaterally in six (40%) of the 15 whole brains. After microdissection, a fiber bundle from the splenium of the corpus callosum was seen emerging in the occipital horn at the angle between the tail of the hippocampus and the bulb of the occipital horn. The most rostral fibers fanned out inferolaterally along the floor of the collateral trigone. The larger, posterior part protruded into the medial wall along the ventral border of the calcar avis as far as the tip of the occipital horn. Conclusions. Besides its importance as a variation of normal ventricular morphological features, the close relationship of this accessory intraventricular prominence to the tail of the hippocampus should be kept in mind when intervening neurosurgically so that damage to the underlying commissural fibers can be avoided.