14 Chapter 1 Cerebrospinal fluid As the third component of the intracranial brain volume, we look into CSF. CSF accumulation can occur as the result of more secretion, increased resistance to CSF circulation or reduced resorption. Most evidence of alterations of CSF volume in sCS patients focus on obstruction of outflow of CSF and reduced resorption by elevated venous pressure in the sinus.43 Both causes can occur at the same time, where several pathogenetic mechanisms can play a role. Obstruction of CSF CSF produced in the choroid plexus flows from the intracranial compartment through the foramen magnum around the spinal cord and up to the granulations of Pacchioni back into the sagittal sinus. Potential obstructions of CSF in craniosynostosis patients could be a smaller foramen magnum, compromised CSF spaces of the posterior fossa, a small fourth ventricle and tonsillar herniation. 43-46 In Rijken et al. the smaller foramen magnum was related to premature closure of the anterior and posterior intraoccipital synchondroses.28 Crowding of the posterior fossa induces a reduction in volume of the CSF cisterns.47, 48 Because of the compromised fourth ventricle in CT scan studies some authors believe in a aqueduct stenosis, while others dispute this argument based on MRI studies.49 Tonsillar herniation, which is commonly observed in Crouzon-Pfeiffer patients, may affect the CSF flow at the level of the craniocervical junction and/or cisterna magna. 43 44 Whether tonsillar herniation develops as a result of elevated ICP or causes the ICP is an ongoing debate, but a higher cerebellar volume / posterior fossa volume ratio has been found to be a predisposing factor for the development of tonsillar herniation. 20, 29 At last, some studies have shown that premature closure of the lambdoid sutures is associated with development of tonsillar herniation.44, 50 The theory of mechanical CSF outflow obstruction remains challenged by the fact that posterior fossa decompressions could fail to sufficiently restore normal CSF circulation and, in several cases, tonsillar herniation arises without ventriculomegaly.45, 46 Reduced resorption of CSF Venous hypertension leads directly to ICH via increased cerebral blood volume and indirectly via venous hypertension-induced diminution of CSF absorption and an increase in brain interstitial fluid volume.14, 43 Normally the CSF pressure is slightly higher than the venous capillary bed pressure to maintain a gradient to drive the reabsorption of CSF. Increased venous pressure impairs this reabsorption of CSF.51 Impaired CSF absorption due to venous hypertension typically causes general dilatation of the inner and outer CSF spaces and progressive head enlargement if the skull is still capable of expanding. At the same time compensatory mechanisms as venous collaterals will develop or expand. In patients with craniosynostosis, the fused sutures do not allow
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