Sagittal Craniosynostosis

GENESIS

Early descent of the fetal head into the maternal pelvis (as early as 4–6 weeks before delivery) with fetal head entrapment that results in biparietal constraint is considered the most common cause of sagittal craniostenosis. This mode of genesis is shown in Fig. 29.2 . Synostosis limits lateral cranial expansion, resulting in dolichocephaly with progressive frontal and/or occipital prominence ( Fig. 30.1 ), and ridging is usually palpable along the posterior portion of the sagittal suture ( Fig. 30.2 ). The platypelloid pelvis, which is relatively small in the anteroposterior dimension compared with its lateral dimensions, would hypothetically constitute the greatest risk for this type of entrapment head constraint. Prolongation of gestation in pregnant mice via placement of a cervical clip resulted in fetal crowding and craniosynostosis. Further support for the importance of biomechanical forces in sagittal craniosynostosis derives from a study of sutural histology in completely synostotic and nonsynostotic sagittal sutures that were removed at the time of surgery. There is a definite qualitative difference in the trabecular pattern for both the vector and direction of suture fusion initiation for the partially and completely synostotic suture compared with the open portion of the same suture ( Fig. 30.3 ). Within the synostotic suture, the trabecular pattern is more disorganized and less polarized than it is within the open suture, where trabeculae are oriented parallel to the direction of biomechanical growth–stretch forces. Bony ridging is most prominent in the posterior half of the synostotic sagittal suture (in the plane of the widest cranial diameter), consistent with this being the initiation site of synostosis and the site of the most longstanding sutural fusion.

Sagittal craniosynostosis is the most common type of synostosis, accounting for 50–60% of cases and occurring in 1.9 per 10,000 births, with a 3.5:1 male:female gender ratio. Only 6% of cases are familial, with 72% of cases sporadic and no paternal or maternal age effects noted. Twinning occurred in 4.8% of 366 cases, with only one monozygotic twin pair being concordant, and intrauterine constraint was considered a likely cause in many cases. The predilection for occurrence in males has been attributed to the more rapid rate of head growth in the male fetus during the last trimester of gestation. The larger head is more likely to become seriously constrained, and the associated twinning also contributes toward fetal head constraint.