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KEY POINTS

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  • To gain an understanding of the mechanisms and anticipatory management of brain tissue displacement (herniation) and intracranial hypertension.

  • To understand available brain monitoring devices in measuring ICP and to appreciate their role in guiding early interventions to avoid secondary brain injury as hesitation to monitor intracranial pressure dynamics, and to aggressively pursue ICP management likely accounts for the vast majority of secondary brain injury in patients with reduced level of consciousness.

  • To foster an individualized patient approach in addressing abnormal ICP and flow dynamics within the practice of neurocritical care. Understanding the indications for brain monitoring via real-time parenchymal blood flow, oxygen tension, and chemistry surveillance, as well as mastering the current recommendations in aggressive management approaches toward elevated ICP such as induced hypothermia, suppression of abnormal electrical discharges, and early surgical decompression are necessary tools for the neurocritical care clinician.

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CONSIDERATION OF CEREBRAL PRESSURE AND FLOW DYNAMICS

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COMPARTMENTS AND MONRO-KELLIE DOCTRINE
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In adults, the cranial vault represents a closed, noncompliant structure. Two important exceptions exist in which intracranial compliance is increased. These are at the foramen magnum and craniectomy sites. Craniectomy refers to surgical bone removal to treat refractory intracranial hypertension or as a by-product of neurosurgical decompression for an alternate indication. This removal of bone leaves a palpable, soft, cranial defect covered only by dura, galea, and skin. The brain is distinguished from other organs by the unique challenge of monitoring brain function and intracranial dynamics in a structure enclosed by a bony vault. The noncompliant surrounding bone of the calvarium does not allow for significant volume change of the brain or adjustment of intracranial pressure (ICP) (Fig. 86-1A). As a result, the pressure within the fixed space of the calvarium must be carefully regulated by many mechanisms in order to be maintained within a physiologic range. Disruption of these mechanisms through trauma, space-occupying lesions, or edema leads to dysregulation of the delicate balance required to maintain normal pressure that results in significant neurologic and systemic dysfunction. For instance, the tentorial opening, which separates the supratentorial and infratentorial compartments, encloses, among other structures, the midbrain, posterior cerebral arteries, posterior communicating arteries, oculomotor, and sixth cranial nerves. These structures are frequently damaged during transtentorial herniation, leading to a chain of often irreversible, secondary injuries (Fig. 86-1B).

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FIGURE 86-1

A. Anatomical relationship of key intracranial structures. The two hemispheres within the supratentorial compartment are separated and stabilized by rigid dura duplications, known as the falx and the tentorium, respectively. These structures become clinically important in the setting of brain herniations; for example, as a late complication of subfalcine herniation the anterior cerebral artery (ACA) is compressed against the free edge of the falx, leading to ACA infarction. Whereas in lateral or descending transtentorial herniation, the posterior cerebral artery (PCA) is displaced inferiorly over the free edge of the tentorium, leading to herniation-induced occipital lobe ...

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