Role of "major" and "minor" lambdoid arch sutures in posterior cranial fossa changes: mechanism of cerebellar tonsillar herniation in infants with multisutural craniosynostosis.

PURPOSE: This study aimed to explain the functional role of lambdoid arch sutures in the development of cerebellar tonsillar herniation. Posterior cranial fossa (PCF) changes were investigated in infants with premature synostosis of the major and minor sutures of the lambdoid arch without premature synostosis of the PCF synchondroses.
METHODS: Morphometric and volumetric PCF measurements were performed on preoperative high-resolution CT studies in 12 infants with multisutural craniosynostosis involving the lambdoid arch and compared with those of 12 age-matched healthy subjects.
RESULTS: All 12 patients had hypoplasia of PCF bone structures and normal volumes of the PCF and neural structures. PCF hypoplasia was related to exocciput length in infants with isolated involvement of major sutures, while it was related to posterior skull base hemifossae in infants with isolated involvement of minor lambdoid arch sutures. Foramen magnum AP diameter was reduced in babies with major suture involvement and tonsillar herniation, while foramen magnum AP and LL diameters were reduced in babies with minor suture involvement without tonsillar herniation. Right and left jugular foramen (JF) areas differed in all infants; however, the area of the smaller JF was significantly reduced only in infants with involvement of minor lambdoid arch sutures.
CONCLUSION: Hypoplasia of PCF bone structures due to sutural synostosis of the lambdoid arch is a required predisposing but not sufficient factor for the development of cerebellar tonsillar herniation through the foramen magnum. Normal PCF volume and foramen magnum anatomy may partly explain the development of cerebellar tonsil herniation in infants with lambdoid arch synostosis.