The clinical importance of basioccipital developmental defects

Abstract

Dear Editor: We read with great interest the article by Tubbs et al. entitled “The human calvaria: a review of embryology, anatomy, pathology, and molecular development,” published in Child’s Nervous System (2012;28(1):23–31). In their paper, the authors provide an extensive review about the development, pathology, and molecular formation of the human calvaria. Although in their approach to the neurocranium the authors focus their attention on the human calvaria (also known as membranous cranium), this article also provides interesting information about the development of the cranial base (or chondrocranium). As highlighted by the authors, the embryological processes responsible for the formation of the final architecture of the skull starts with the chondrification of the mesenchyme of the spheno-occipital region during the seventh fetal week, further extending to the facial bones and lately spreading and expanding the neurocranial cavity to accommodate the enlarging forebrain. Although the presented knowledge about the embryological development of the calvaria seems to have clear clinical applications due to the importance of the several types of craniosynostosis in the neurosurgical pediatric practice, it is important to emphasize that the knowledge about the development of the skull base should not be regarded as secondary. In fact, although most of the skull base developmental defects usually represent incidental findings on imaging exams, they may constitute an unrecognized cause of recurrent meningitis, especially after paranasal sinus infections. With special clinical importance is the so-called “Canalis Basilaris Medianus” (CBM), a special type of basiocipital developmental defect. Although the reported incidence of such anomaly in old CT scans series was around 3.8% [3], a most recent report found a much higher incidence (around 20%) [6]. In early embryological development, the four sclerotomes of the first four somites (called hypoglossal somites) are incorporated into the skull base. At this location, these elements fuse and establish the basioccipital plate, which surrounds the notochord. The spinal and the pre-chordal part are responsible for the formation of the chondrocranium, also called the primordial cranium. The embryology and development of the sphenoid bone are complex and involve up to 15 separate enchondral and intramembranous ossification centers [5]. According to this mosaic-like formation, several interosseous sutures are established and may, in some cases, persist towards adulthood [11]. In fact, the absence of complete fusion of these embryological ossification centers may lead to the persistence of seven other types of anterior skull base foramina or pseudoforamina besides the “Canalis Basilaris Medianus’ [6]. Two theories have been proposed in the literature to explain the development of the CBM. The most accepted one suggests that the defect would be an osseous remnant of a basivertebral vein present only in the embryological phase of development [4, 8]. Another theory relates the origin of CBMs to notochordal development [2]. It is known that around the fifth week, the notochord becomes enclosed by the bodies of the upper cervical vertebrae. During regular embryological development T. A. Mattei (*) Neurosurgery Department, University of Illinois at Peoria, 828 NE Glen Oak Avenue, Apt. 302, Peoria, IL 61603-3285, USA e-mail: tobiasmattei@yahoo.com