Raybaud Charles, Jallo George I
Neuroradiology, Hospital for Sick Children, University of Toronto, Toronto, ON, Canada.
Division of Pediatric Neurosurgery, Johns Hopkins Hospital, Baltimore, MD, United States.
Handb Clin Neurol. 2018;155:25-48. doi: 10.1016/B978-0-444-64189-2.00002-0.
The metamerically associated normal hindbrain and normal posterior fossa are programmed to grow together in such a way that the tonsils are located above the foramen magnum and surrounded by the cerebrospinal fluid (CSF) of the cisterna magna. This allows the pulsating CSF to move freely up and down across the craniovertebral junction (CVJ). A developmental mismatch between the rates of growth of the neural tissue and of the bony posterior fossa may result in the cerebellar tonsils being dislocated across the foramen magnum. The cause of this may be, rarely, an overgrowth of the cerebellum. More commonly, it is due to an insufficient development of the posterior fossa, possibly associated with a malformation of the craniocervical joint. When it is not due to a remediable cause, such a herniation is called a Chiari 1 deformity. This definition is anatomic (descent of the tonsils below the plane of the foramen magnum) and not clinical: many patients with the deformity are and will remain asymptomatic. Most authors consider that a descent of 5 mm or more is clinically significant but other factors, such as the diameter of the foramen magnum and the degree of tapering of the upper cervical "funnel," are likely to be as important. Morphologic markers of severity on magnetic resonance imaging are, beside the degree of descent, the peg-like deformity of the tonsils, the obstruction of the surrounding CSF spaces (at the craniocervical junction and in the whole posterior fossa), a compression of the cord, an abnormal signal of the cord, and a syringomyelia, typically cervicothoracic. The syringomyelia is assumed to be explained by the "Venturi effect" that is associated with the increased velocity of the CSF across the restricted CSF spaces. Radiologically, the etiopathogenic assessment should address the size and morphology of the posterior fossa, and the functional status of the craniocervical flexion joint. The posterior fossa is best evaluated on sagittal cuts by the posterior fossa pentagon proportionality associated with the line of Chamberlain, and on coronal cuts, by showing a possible shallowness of the posterior fossa. The functional status of the craniocervical joint is altered in case of a proatlantal hypoplasia, as this condition results in a cranial shift of the joint that brings the tip of the dens and of the flexion axis in front of the medulla, that is, in a situation of osteoneural conflict. Less commonly, similar conflicts may also occur when an abnormal craniocervical segmentation results in an instability of the joint.
节段性相关的正常后脑和正常后颅窝按程序共同生长，使得扁桃体位于枕骨大孔上方，并被枕大池的脑脊液（CSF）所包围。这使得搏动的脑脊液能够在颅颈交界（CVJ）处自由上下移动。神经组织和骨性后颅窝生长速率之间的发育不匹配可能导致小脑扁桃体通过枕骨大孔发生移位。其原因可能很少是小脑过度生长。更常见的是，这是由于后颅窝发育不足，可能与颅颈关节畸形有关。当不是由可补救的原因引起时，这种疝称为Chiari 1畸形。这个定义是解剖学上的（扁桃体下降至枕骨大孔平面以下）而非临床的：许多有这种畸形的患者现在无症状，将来也仍会无症状。大多数作者认为下降5毫米或更多在临床上具有重要意义，但其他因素，如枕骨大孔的直径和上颈椎“漏斗”的变窄程度，可能同样重要。磁共振成像上严重程度的形态学标志物，除了下降程度外，还有扁桃体的钉状畸形、周围脑脊液间隙（在颅颈交界和整个后颅窝）的阻塞、脊髓受压、脊髓异常信号以及通常为颈胸段的脊髓空洞症。脊髓空洞症被认为是由与脑脊液在受限脑脊液间隙中流速增加相关的“文丘里效应”所解释。在放射学上，病因评估应涉及后颅窝的大小和形态以及颅颈屈曲关节的功能状态。后颅窝在矢状切面上最好通过与张伯伦线相关的后颅窝五边形比例来评估，在冠状切面上，通过显示后颅窝可能变浅来评估。在寰椎发育不全的情况下，颅颈关节的功能状态会改变，因为这种情况会导致关节的颅骨移位，使齿状突尖端和屈曲轴位于延髓前方，即处于骨神经冲突的状态。较少见的是，当异常的颅颈节段化导致关节不稳定时，也可能发生类似的冲突。
