gms | German Medical Science

ESBS 2005: Skull Base Surgery: An Interdisciplinary Challenge
7th Congress of the European Skull Base Society held in association with
the 13th Congress of the German Society of Skull Base Surgery

18. - 21.05.2005, Fulda, Germany

Embryology of skull base malformations

Meeting Contribution

Search Medline for

  • A. Prescher - Institute of Anatomy, Technical University of Aachen, Aachen, Germany

ESBS 2005: Skull Base Surgery: An Interdisciplinary Challenge. 7th Congress of the European Skull Base Society held in association with the 13th Congress of the German Society of Skull Base Surgery. Fulda, 18.-21.05.2005. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc05esbs01

doi: 10.3205/05esbs01, urn:nbn:de:0183-05esbs019

Published: January 27, 2009

© 2009 Prescher.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.en). You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.


Outline

Text

Several categories of malformations can be observed at the skull base. Often serious malformations are combined with syndromes, such as Apert-syndrome or Crouzon-syndrome. This contribution will not deal with these obvious pathological occurrences, but will focus on minor defects responsible for celes, which can cause recurrent bacterial meningoencephalitis. In some cases such a recurrent bacterial meningitis can be the first symptom of an occult malformation of the skull base [12].

In early development the 4 sclerotomes of the first four somites, called hypoglossal somites, are incorporated into the skull base [8]. There these elements fuse and establish the basioccipital plate. This part of the skull base is also termed as the spinal part of the skull, because it surrounds the notochord. Consequently the spinal part ends at the anterior border of the notochord. In front of this posterior section the prechordal part is located. Centrally this structure is perforated by the anterior basicranial foramen allowing the adenohypophysis to enter the cranial cavity. The spinal and the prechordal part are responsible for the formation of the chondrocranium, also called the primordial cranium. This primordial cranium will be completed to the definitive cranium by desmally developing parts, summarized as desmocranium. At several locations these two parts have to fuse together harmonically. These areas can be the scene of maldevelopments. For example the region of the olfactory groove is such a developmentally complicated area. In this part the chondrally developed ethmoidal complex must be fitted into the ethmoidal incisure of the frontal bone (Figure 1 a [Fig. 1]). At first osseous sutures are marking the fusion zone. Later these zones completely ossify. If the different parts do not fuse together regularly, clefts can appear which persist during life, as shown in a specimen belonging to a 86 year old male (Figure 1 b [Fig. 1]). In this context the always visible foramen caecum at the anterior border of the ethmoidal plate is also an interesting structure. Regularly it is bordered at its anterior margin by the frontal bone and at its posterior circumference by the ethmoid bone [5], so that it is also configurated by the above described insertion of the ethmoidal plate into the frontal bone. Many anatomical textbooks describe an emissarial vein as content, especially in childhood. In contrast to this Kaplan et al. [4] in 1973 were not able to demonstrate even one vein in 200 specimens. They only found collagenous tissue. Therefore the caecal foramen can be seen as a physiologically appearing defect in the borderland between the ethmoid complex and the frontal bone. Holl (1893) [3] mentions, that it may be important for the development of anterior celes, especially if the important nasal process of the frontal bone is not established.

Another critical point of the development is the composition of skull base structures by several subunits. Typical examples are the sphenoidal and the occipital bone. In early development the sphenoid bone is built up by independent cartilaginous precursors: presphenoid (unpaired), basisphenoid (unpaired), alisphenoid (paired), orbitosphenoid (paired), and the lateral parts of the pterygoid process (paired). According to this mosaic-like formation several interosseous sutures are established, which can persist towards adulthood. A characteristic observation is the very rare intersphenoidal fissure between the presphenoidal and the basisphenoidal part. Furthermore in the contact area between the presphenoid, the basisphenoid, and the alisphenoid an ossification defect is formed called Canalis craniopharyngeus lateralis Sternberg according to Maximilian Sternberg in 1888 [14]. At first this defect is formed like an irregular foramen, because the incongruent surrounding osseous parts do not fit together. In a second step a cleft results. During further development the cleft becomes rounded like a foramen and at last even this structure diminishes. In some adult cases the small canal can persist and is termed as “Canalis carniopharyngeus lateralis Sternberg”. This canal can be observed in about 4% of adult skulls and leads into the sphenoid sinus or into the roof of the pharynx. In rare cases the whole cleft can persist as a greater defect, where celes may occur. A typical case was described by Schick et al. [11].

In the region of the sphenoid bone the former anterior basicranial opening is an also important structure. During development this opening gives way to the adenohypophysis originating in Rathke´s pouch. In further development the adenohypohysis has to enter the cranial cavity and uses this basicranial foramen. In some cases the anterior basicranial foramen can also persist as a fine canal termed as median craniopharyngeal canal (Landzert). The persisting craniopharyngeal canal will be observed in about 1% of the cases. If a large defect persists, the sphenopharyngeal cele may originate at this place.

In the dorsal region of the skull the occipital bone must also be considered. The basal part of this bone is formed as a part of the chondrocranium, whereas the cranial part is developed desmally. The border between these two parts is marked in foetal skulls by a typical suture: the transverse occipital suture or sutura mendosa. If this suture persists completely towards adulthood, an 'Inca bone' can be seen. Melting defects between the desmally formed cranial part and the chondrally formed basal constituent of the occipital bone may form atypical openings in the squama of the occipital bone. Often these defects are localised in the midline, where the occipital celes may occur.

The classification of celes according to Lang [5] includes the fronthoethmoidal cele, the transethmoidal cele, the sphenoethmoidal cele, the sphenopharyngeal cele, the sphenoorbital cele, and the occipital cele. It becomes obvious, that the main types of celes are indeed often located at the developmentally explainable weak areas of the skull base. Furthermore abnormal canals in the skull base, which may sometimes occur as anatomical variations, can be the origin for cerebrospinal fluid leakage. A typical example was presented by Schick et al. [13]. In the described case a transclival osseous defect was present and therefore a basilar clefting or a median basilar canal was discussed. The basilar clefting (Sauser`s fissure) is a very rare occurrence which belongs to the great entity of manifestations of the occipital vertebra [10]. The median basilar canal and its related different expressions is explained in literature by two main theories: some authors [2], [9] suggest a vascular origin due to emissary veins, whereas the other side (e.g. [7]), explains the canal as location of notochordal remnants. Neiß [7] created in this sense the term “ Canalis chordae”. Concerning the vascular explanation the median basilar canal is interpreted as the canal for the former basivertebral veins [2], [6].

According to these embryological facts it can be summarised, that persisting defects of the skull base are often very small ones. Therefore, a thorough and sensitive radiological investigation is absolutely necessary for detection [1]. An exact knowledge of the developmental background will provide hints where defects must be searched.

It becomes obvious, that Karl Ernst von Baer, the discoverer of the human egg, was absolutely right, when he claimed in 1828 that developmental science is absolutely important for the explanation of living beings and their variations. Especially it must be said, that malformations and teratogenetic occurrences are not arbitrary appearances but consequent followings of a disturbed regular development.


References

1.
Carrol ED, Latif AH, Misbah SA, Flood TJ, Abinun M, Clark JE, Pugh RE, Cant AJ. Recurrent bacterial meningitis: the need for sensitive imaging. BMJ. 2001;323:501-3.
2.
Gruber W. Über den anomalen Canalis basilaris medianus des Os occipitale beim Menschen. Mém Acad Imp Sci St Petersburg sér VII. 1880; tome 27, no 9.
3.
Holl M. Über das Foramen caecum des Schädels. Anzeiger der Kaiserl. Akad. d. Wissensch. Math.-Naturw. Kl. 1893; Nr. 24: 250-1.
4.
Kaplan HA, Browder A, Browder J. Nasal venous drainage and the foramen caecum. Laryngoscope. 1973;83:327-9.
5.
Lang J. Praktische Anatomie. In: Lang J, Wachsmuth W (eds), Bd. I/1, Teil A. Berlin New York Heidelberg: Springer; 1985.
6.
Lang J. Praktische Anatomie. In: Lang J, Wachsmuth W (eds). Bd. I/1, Teil B. Berlin New York Heidelberg: Springer; 1979.
7.
Neiß A. Nachweis des Canalis chordae (Canalis basilaris medianus) im Röntgenbild. Röfo. 1956;84:206-8.
8.
O`Rahilly R, Müller F. The early development of the hypoglossal nerve and occipital somites in staged human embryos. Am J Anat. 1984;169:237-57.
9.
Oehmke HJ. Die Bedeutung des Canalis basilaris und seine Darstellung im Röntgenbild. Gegenbaurs Morphol Jahrb. 1963;104:459-75.
10.
Prescher A. The craniocervical junction in man, the osseous variations, their significance and differential diagnosis. Ann Anat. 1997;179:1-19.
11.
Schick B, Brors D, Prescher A. Sternberg`s canal - cause of congenital sphenoidal meningocele. Eur Arch Otorhinolaryngol. 2000;257:430-2.
12.
Schick B, Draf W, Kahle G, Weber R, Wallenfang T. Occult malformations of the skull base. Arch Otolaryngol Head Neck Surg. 1997;123:77-80.
13.
Schick B, Prescher A, Hofmann E, Steigerwald C, Draf W. Two occult skull base malformations causing recurrent meningitis in a child: a case report. Eur Arch Otorhinolaryngol. 2003;260:518-21.
14.
Sternberg M. Ein bisher noch nicht beschriebener Kanal im Keilbein des Menschen. Anat Anz. 1888;3:784-5.