Sutures and Fontanelles: Craniosynostosis
Craniosynostosis is a premature fusion of cranial sutures in infants that may lead to profound changes in craniofacial shape. These changes are a result of anatomic differences between the calvarial unit and skull base portion of the skull. Growth within the craniofacial skeleton is based on two key concepts: displacement and bone remodeling. Calvaria growth in the infant requires rapid and symmetrical displacement of each of the large bones (frontal, parietal, and occipital) of the skull along with osseous deposition along the sutures and within the bone matrix. Concomitant with these growth patterns is endocranial and ectocranial remodeling of the skull bones. Each of these patterns changes rapidly in infancy, continues into childhood, and in some cases continues even into adulthood. Following closely and symmetrically behind calvarial growth is skull base and facial growth. In the growth sequence, the anterior fossa completes its growth first, followed by the posterior and middle fossa. During this growth cycle, the skull base and face follow in form to the calvaria. When the three skull base fossae are examined individually, a unique growth pattern develops within each one. The anterior fossa relies on the growth at the sphenoid, ethmoid, and frontal bones primarily using the growth at the spheno-frontal and sphenoethmoidal sutures. Growth is rapid in this area up to about 7 years of age. The middle fossa continues its growth for an even longer period, into the teenage years, with the sphenopetrosal and petro-occipital synchondroses being most affected. The posterior fossa also continues an active growth pattern into childhood and the adolescent years. The intraoccipital synchondroses complete their growth in childhood, with the spheno-occipital synchondroses remaining active into adolescence. Concomitant to this skull base growth pattern is the growth rate of the facial skeleton. Growth within the face continues until well into the adolescent era, with a spurt that occurs during puberty. Critical angulation patterns of the maxilla are finalized in adolescence and closely follow the pattern of growth of the anterior fossa. As a result, if there is any form of premature fusion of any of the skull base sutures and synchondroses, these premature fusions can lead to significant alterations of the skull and facial alignment, resulting in a variety of different craniofacial anomalies. Head shape depends on which sutures are prematurely synostosed, the order in which they synostose, and the timing at which they synostose. Craniosynostosis may be of prenatal or perinatal onset or may occur later during infancy or childhood. The earlier synostosis occurs, the more dramatic the effect on subsequent cranial growth and development. The later synostosis occurs, the less the effect on cranial growth and development. Synostosed skulls with almost normal-shaped skulls have been observed. Therefore, skull and facial morphogenesis is a complex and multifactorial development, some of which we are only just beginning to understand.
The so-called single-suture synostoses (e.g., scaphocephaly) rarely involve the skull base and its concomitant sutures. There are two potential exceptions to this rule: plagiocephaly, which is due to premature closure of one of the coronal sutures, and trigonocephaly, where there is a premature closure of the me-topic suture.
Most craniosynostoses are congenital and manifest themselves predominantly during the period of intense cranial growth (birth through 3 years of age). Along with a small posterior fossa, cerebellar crowding occurs; Chiari malformation is a not uncommon finding associated with all these factors. Measurements of the foramen magnum typically show small aperture openings and other asymmetries. Angiographic studies have also shown significant alterations in the venous outflow patterns at the skull base, which can lead to intracranial venous hypertension. Venous hypertension is thought to be secondary to narrowing of the skull base foramina, particularly in the area of the jugular bulb. These skull base abnormalities are felt to be an important cause of hydrocephalus seen especially in syndromal craniosynostosis. Today, concepts about the pathophysiology of complex syndromic craniosynostoses have shifted from mechanical models involving tension forces of the dural structures to a molecular signalling disorder model. Mutations of the fibroblast growth factor receptors 1, 2, and 3, as well as of the TWIST and MSX2 genes, are commonly associated with the different phenotypes of this group of autosomal dominant (AD) malformations. Significant brain abnormalities have been reported in all syndromes. However, whether these abnormalities are secondary to the bone disease or primary (e.g., callosal agenesis) is still controversial. Recent evidence suggests that a white matter defect might be a primary disorder.
Craniosynostosis is initially manifested on radiographs by straight, narrow, sharp-edged sutures with marginal sclerosis. Later, the sutures become partially or entirely bridged by bone. Cranial growth is deviated in the direction of the prematurely fused suture. On three-dimensional (3D) CT, better appreciation of the small posterior fossa, cerebellar crowding, and sometimes associated Chiari malformation is possible. Also, narrowing of the skull base foramina, particularly in the area of the jugular bulb, which may give rise to intracranial hypertension, is better evaluated on CT. Additional structural intracranial abnormalities are easily shown on CT. For the surgeon, 3D CT is an excellent tool for preoperative and postoperative assessment in patients with complex craniosynostosis.
Secondary synostoses result from decreased ICP after shunting of hydrocephalus and only rarely result in microcephaly. This type of fusion does not produce craniosynostosis.
Skull deformity may also be the result of various metabolic diseases and skeletal disorders, such as the following:
Vitamin D deficiency rickets (healed)
Vitamin D–resistant forms of rickets
Hypothyroidism due to hormonal overdose