Displacement of one or both cerebellar tonsils through the foramen magnum ( Fig, 3) is a common occurrence in patients with posterior fossa mass lesions and usually precedes upward cerebellar herniation. Although the condition may be tolerated for considerable periods of time without noticeable symptoms, neck tilt or nucal rigidity is often evidence on physical examination. Such findings must be carefully distinguished from neck stiffness resulting from meningitis or subarachnoid hemorrhage. Lumbar puncture in a patient with transforaminal tonsillar herniation can result in a sudden reduction in pressure within the spinal canal and further descent of the cerebellar tonsils through the foraminal notch. In questionable cases, CT or magnetic resonance imaging should be performed to exclude a posterior fossa mass.
Fig. 3 Diagram of upward (transtentorial) and downward (transforaminal) cerebellar herniation secondary to infratentorial mass lesion. (Reproduced with permission from Mi.lh.orat, .119,7,8,),)
A constant hazard of transforaminal cerebellar herniation is sudden respiratory arrest. This can occur at any time, occasionally in response to otherwise innocuous stimuli such as a cough or sneeze. In rapidly developing herniations, the cerebellar tonsils as well as the medulla and the upper cervical spinal cord can undergo ischemic infarction as a consequence of compression of one or both posterior inferior cerebellar arteries against the rim of the foramen magnum ( B§.lL§n.d...lM.CC.0.rm.iCk 1.9.72; Plum .and .Posner ..1992).
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