The clinical and operative findings are reviewed in 47 patients with intractable hemifacial spasm. The syndrome was classical in its features in 45 patients and atypical in two. Mechanical compression distortion of the root exit zone of the facial nerve was noted in all 47 patients. In 46 the abnormality was vascular cross-compression, usually by an arterial loop. In one patient, a small cholesteatoma was discovered and removed. Morbidity and postoperative results are discussed.

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Two patients developed subdural tension pneumocephalus after undergoing posterior fossa surgery performed in the sitting position. The mechanism for entry of air into the intracranial compartment is analogous to the entry of air into an inverted soda-pop bottle. As the fluid pours out, air bubbles to the top of the container. We have thus referred to this as the "inverted pop-bottle syndrome." Computerized tomography provided prompt diagnosis and confirmed brain displacement. Twist-drill aspiration of the air resulted in improvement in both patients, although one patient subsequently died from an intracerebellar hemorrhage. Tension pneumocephalus appears to be another potential complication of posterior fossa surgery in the sitting position. This condition is easily diagnosed and treated, and should be considered whenever a patient fails to recover as expected following posterior fossa surgery.

SUMMARY The autoregulatory capability of regional areas of the brain and spinal cord was demonstrated in 18 rats anesthetized with a continuous infusion of intravenous pentothal. Blood flow was measured by the injection of radioactive microspheres (Co 57 , Sn 113 , Ru 103 , Sc 4 *). Blood flow measurements were made at varying levels of mean arterial pressure (MAP) which was altered by neosynephrine to raise MAP or trimethaphan to lower MAP. Autoregulation of the spinal cord mirrored that of the brain, with an autoregulatory range of 60 to 120 mm Hg for both tissues. Within this range, cerebral blood flow (CBF) was 59.2 ± 3.2 ml/100 g/min (SEM) and spinal cord blood flow (SCBF) was 61.1 ± 3.6. There was no significant difference in CBF and SCBF in the autoregulatory range. Autoregulation was also demonstrated regionally in the left cortex, right cortex, brainstem, thalamus, cerebellum, hippocampus and cervical, thoracic and lumbar cord. This data provides a coherent reference point hi establishing autoregulatory curves under barbiturate anesthesia. Further investigation of the effects of other anesthetic agents on autoregulation of the spinal cord is needed. It is possible that intraspinal cord compliance, like intracranial compliance, might be adversely affected by the effects of anesthetics on autoregulation. Stroke Vol 17, No 6, 1986 HEMODYNAMIC AUTOREGULATION, a mechanism intrinsic to the cerebrovascular system, and also present in many other tissues, maintains tissue blood flow within a narrow range despite changes in perfusion pressure. It has been firmly established as the primary blood flow regulatory mechanism in the brain and constitutes a physiological adjustment that is of major importance in maintenance of a homeostatic internal environment of the brain. The first observation of cerebral autoregulation was made by Fog in 1934. 1 -2 He observed the responses of pial vessels of cats through a cranial window under conditions of varying arterial blood pressure and noted dilation with a fall in blood pressure and constriction with a rise in blood pressure. Lassen, in his review in 1959, established the concept of cerebral autoregulation based on actual measurements of blood flow during blood pressure alterations. 3 Since that time, cerebral autoregulation has been extensively investigated and it has been found that under normotensive conditions mean arterial blood pressure can be varied from a lower limit of 60 mm Hg to an upper limit of 130 mm Hg without any measurable change in blood flow.4 Below these limits, cerebral blood flow markedly falls, and above these limits, forced dilation of arterioles occurs which is associated with disruption of the blood brain barrier and edema formation. Received July 17, 1985; revision #1 accepted January 29, 1986. vascular dynamics. However, a comparison between the autoregulatory capabilities of these two tissues has not been demonstrated. The purpose of this study was to simultaneously measure spinal cord and cerebral blood flow under conditions of varying...

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