During the course of a systematic study on the human
vertebrobasilar arterial system (De Caro et al. 1995), our
attention was drawn in one case to a strange morphology of
the ventrolateral aspect of one side of the medulla. While
initially our impression was that we were facing an actual
malformation, review of the structure of the brainstem as
described in most standard anatomical texts showed that it
could correspond to the circumolivary fasciculus (Williams
et al. 1995).
The subject was a 63-y-old man who had died from
rupture of an atherosclerotic aneurysm of the abdominal
aorta. The medical history was negative for neuropsychiatric
pathology. Brain examination showed severe atherosclerosis
of the cerebral arteries and hypoplasia of the initial segment
of the right posterior cerebral artery. A recent infarct was
found in the white matter of the right occipital lobe. After
removal of the vertebrobasilar arterial system, the right
pyramid showed a superficial groove extending from about
2 mm lateral to the foramen caecum obliquely downwards
to the right anterolateral sulcus of the medulla, at the level
of the superior margin of the inferior root of the hypoglossal
nerve (Fig. 1a). Due to this groove, the lateral part of the
pyramid formed a ridge that crossed the anterolateral sulcus
of the medulla passing between the 2 main roots of the
hypoglossal nerve and then continued, below the inferior
pole of the olive, over the lateral region of the medulla
ending at the level of the posterolateral sulcus (Fig. 1b). The
general appearance was that of a small cord forming a loop
around the anterior margin and the inferior pole of the
olive, with a descending part (8 mm long), a horizontal part
(6 mm long) and with a transverse diameter of 3 mm. After
formalin fixation, the brainstem was cut in 8 transverse
sections. At the level of the medulla the right pyramid
appeared larger than the contralateral and the small lateral
cord was separated from the main part of the pyramid by a
thin whitish transverse stria (Fig. 1c). Macroscopic examination
of the cerebellum showed nothing of relevance.