THE BASE OF THE SKULL

 

 

EMBRYOLOGY

The cranial base develops from four main components:

·        the chondrocranium (upper jaw) or neurocranium (forms the cranial base or basicranium)

·        the sensory capsules

·        the membranous bones (become the skull vault directly from the prechordal mesenchyme and parachordal mesenchyme (1,2)

·        and the viscerocranium.

A cartilaginous long bone has two primary growth centers that produce enchondral ossification in contrast with intramembranous ossification.

Link to Normal Cranial ossification

The base of the occipital bone is first formed by the parachordal cartilage and the bodies of three occipital sclerotomes. The hypophyseal cartilage and trabecula cranii fuse rostral to the occipital base plate to form both bodies of sphenoid and ethmoid. A number of other mesenchymal condensations arise on each side of the medial plate to form the lesser wing of the sphenoid rostrally and the greater wing of the sphenoid caudally. A third component, the periotic capsule, gives rise to the petrous and mastoid parts of the temporal bone. These components later fuse with the median plate and with each other, except for the openings through which the cranial nerves leave the skull (1,2).

The developmental pattern of the cranial base includes forward growth of each part of the anterior cranial fossa, increase of the middle cranial base angle and backward growth to increase the posterior cranial base space. During the fetal period there is widening of the middle cranial fossa while the anterior cranial base angle is almost constant.

 

 

ULTRASOUND

The cranial base structures give primary backbone not only to the brain but also to craniofacial tissue  (3,4). The most frequent anomalies that may involve the cranial base. Using our reference range tables we have followed two fetuses in which brachycephaly was found. Measurements of the anterior cranial angles were more than 2 SD above the mean in both fetuses. The diagnosis of craniosynostosis was confirmed after birth and one of the infants required neurosurgical intervention. Microcephaly was detected in two fetuses, the length of the sphenoid ridge and otic cartilage were less than 2 SD below the mean, however, measurements of the cranial base angles were within normal limits.

Different orofacial malformations, i.e. defects in closure of palate, prognathism, retrusion of jaws, etc., are closely related to abnormal development of the cranial base  (3-6). Otocephaly, in which a severe malformation of holoprosencephaly mainly arose from prechordal mesenchyme, showed hypoplasia of the ear in the lower anterior face  (7).

The anterior cranial base consists of ethmoid and sphenoid bones, and its development could directly affect the development of eye balls, nasal cavity and upper jaw   (8).

Several malformations are associated with abnormal development of the posterior cranial base, such as the constriction of the foramen magnum in thanatophoric dysplasia, occipital bone defects in anencephaly and posterior fossa compression in Chiari malformation (9-11).

 

The Skull Base

 

 

 

 

 

 

 

Conditions associated with dysplasia of the base of the skull (13)

Skeletal dysplasias

Various disorders

Achondrogenesis

Achondroplasia

Hypophosphatasia

Osteogenesis imperfecta

Thanatophoric dysplasia

Campomelic dysplasia

Mesomelic dysplasia

Microcephaly

Anencephaly

Neural tube defect

Chiari malformation

Mechanical compression

 

Craniosynostosis

Idiopathic

(scaphocephaly, brachycephaly, oxycephaly, plagiocephaly,

trigonocephaly, pachycephaly)

As part of other syndromes

Other associated conditions

Monogenic syndromes

 

Apert syndrome

Carpenter syndrome

Crouzon syndrome

Pfeiffer syndrome

Proteus syndrome

 

Chromosomal syndromes

 

Trisomy 18

Trisomy 21

Jacobsen syndrome (deletion 11q)

Wolf-Hirschorn syndrome (deletion 4p)

 

Teratogenically induced syndromes

 

Aminopterine syndrome

Fetal hydantoin syndrome

 

 
Hematological

 

Thalassemia

Sickle cell anemia

Polycythemia vera
Metabolic

 

Hyperthyroidism

Mucopolysaccharidosis (Hurler syndrome, Morquio syndrome)

Mucolipidosis III

Vitamin D deficiency/resistance

Idiopathic hypercalcemia

 

 

 

REFERENCES

 

  1. Larsen WJ. Human Embryology. Edinburgh: Churchill Livingstone, 1993: 311-40
  2. Sadler TW. Langman's Medical Embryology , 5th edn. Baltimore: Williams & Wilkins, 1985: 133-8
  3. Hoyte DA. The cranial base in normal and abnormal skull growth. Neurosurg Clin North Am 1991; 2: 515-37
  4. Madeline LA, Elster AD. Postnatal development of the central skull base: normal variants. Radiology 1995; 196: 757-63
  5. Kjaer I. Human prenatal palatal shelf elevation related to craniofacial skeletal maturation. Eur J Orthod 1992; 14: 26-30
  6. van den Eynde B, Kjaer I, Solow B, Graem N, Njaer TW, Mathiesen M. Cranial base angulation and prognathism related to cranial and general skeletal maturation in human fetuses. J Craniofac Genet Dev Biol 1992; 12: 22-32
  7. Cohen MM Jr, Sulik KK. Perspectives on holoprosencephaly. Part II. Central nervous system, craniofacial anatomy, syndrome commentary, diagnostic approach, and experimental studies. J Craniofac Genet Dev Biol 1992; 12: 196-244
  8. Kerr WJ, Adams CP. Cranial base and jaw relationship. Am J Phys Anthropol 1988; 77: 213-20
  9. Faye-Petersen OM, Knisely AS. Neural arch stenosis and spinal cord injury in thanatophoric dysplasia. Am J Dis Child 1991; 145: 87-9
  10. Muller F, O'Rahilly R. Development of anencephaly and its variants. Am J Anat 1991; 190: 193-218
  11. Frim DM, Jones D, Goumenerova L. Development of symptomatic Chiari malformation in a child with craniofacial dysmorphism. Pediatr Neurosurg 1990; 16: 228-31
  12. Degani, S., Leibovitz, Z., Shapiro, I., Gonen, R. & Ohel, G. Ultrasound evaluation of the fetal skull base throughout pregnancy. Ultrasound in Obstetrics & Gynecology 19 (5), 461-466.
  13. David DJ, Poswillo D, Simpson D. The craniosynostoses: causes, natural history, and management. New York: Springer-Verlag, 1982: 18-9