Cerebral Herniation Syndromes and Intracranial Hypertension

Matthew Koenig

• Publisher: Rutgers University Press
• Year: 2016
• ISBN:
  9780813579320
• Topic: Medicine
• Subtopic: Neurology

When the brain suffers an injury, the effects can be delayed and unpredictable. Cerebrospinal fluid can slowly build up, causing dangerously high levels of intracranial pressure (ICP), and the brain tissue can be displaced into adjacent compartments, resulting in cerebral herniation syndrome (CHS). Within the burgeoning field of neurocritical care, experts are just beginning to understand the nuanced, sometimes counterintuitive relationship between ICP and CHS.     Written by leading researchers who also have extensive first-hand clinical experience treating brain injury patients, Cerebral Herniation Syndromes and Intracranial Hypertension provides an up-to-date guide to this complex aspect of neurocritical care. Drawing from expertise gained working in high-volume medical centers, the book’s contributors reveal that there is no universal metric for gauging acceptable levels of intracranial pressure. Instead, they demonstrate the best practices for offering patients individualized care, based on their specific conditions and manifest symptoms.  
  Bringing together internationally-renowned neurocritical care experts from a variety of neurology, critical care, surgery, and neurosurgery disciplines, this volume takes a comprehensive look at a complicated issue. A concise, practical, and timely review, Cerebral Herniation Syndromes and Intracranial Hypertension offers vital information for all medical personnel concerned with improving neurocritical patient care.    

1

The Pathophysiology of Intracranial Hypertension and Cerebral Herniation Syndromes

BASICS OF INTRACRANIAL PRESSURE

Kevin Sheth

Margy McCullough

Much pathology of the brain involves a primary injury, such as trauma, infarction, or hemorrhage, as well as further damage in the days following an injury. During this time, the brain is susceptible to secondary insults that are frequently due to increases in intracranial pressure (ICP).

ICP is the pressure within the confines of the skull, which depends on a number of factors. ICP is normally 7 to 15 mmHg at rest for a healthy supine adult, measured at a level equal to that of the foramen of Monro; standing vertically, it typically falls below atmospheric pressure. It is lower in young children (usually 1–7 mmHg), is usually subatmospheric in newborns, and can be up to 18 mmHg in obese adults (1,2). At a steady state, pressure within the brain parenchyma and the intracranial extra-axial spaces is equal, largely due to free movement of the cerebrospinal fluid (CSF) (1,3). Changes in ICP are generally attributed to volume changes in one or more constituents of the cranium.

Under normal circumstances, ICP is maintained in a homeostatic range via intrinsic autoregulatory mechanisms, with occasional transient elevations associated with physiological events that increase central venous pressure and therefore ICP; these may include sneezing, coughing, and the Valsalva maneuver (4). Hip flexion (which decreases venous return), a change in head or neck position, external noxious stimuli, agitation, pain, and seizures can also increase ICP (3). Elevating the head generally leads to a fall in ICP, as CSF moves from cranial to spinal spaces.

ICP sustained at any pressure greater than 20 mmHg is considered pathologic. Based on population studies, ICP greater than 20 to 25 mmHg for a period of 5 minutes or longer poses a threat to adequate cerebral perfusion in adults, and small observational studies have suggested that keeping ICP lower than 20 to 25 mmHg is associated with better clinical outcomes (4–7). ICP in the range of 20 to 30 mmHg is considered moderately increased, whereas ICP that persistently exceeds 40 mmHg is severe and life threatening (1). An observational study reported that mean ICP peaks in patients with traumatic brain injury (TBI) between 2 and 5 days after the initial event (8).

CRANIAL CONTENTS

The cranial cavity, which the inflexible skull and dura protect, has a fixed volume of approximately 1400 to 1700 mL (3,9). Its major constituents include the brain, CSF, and intracranial blood. On average, the brain accounts for approximately 1200 mL of the volume (80% total cranial volume), and the blood and CSF each account for approximately 150 mL (10% total cranial volume each) (6).

Brain

The brain is composed of parenchymal tissue and water; water comprises slightly less than 80% of the brain, 75% to 80% of which is intracellular fluid and the remainder of which is interstitial (3,6). Brain tissue can be classified as either gray matter, also known as substantia grisea, or white matter, also called substantia alba. Gray matter contains most of the brain’s neuronal cell bodies, along with neuropil (dendrites and unmyelinated or myelinated axons), glial cells (astroglia and oligodendrocytes), and capillaries. The brain uses approximately 20% of the body’s oxygen, 95% of which goes to the gray matter; it is thus considered the more “active” of the two components. White matter, in comparison, does not contain neural cell bodies and primarily consists of myelinated axon tracts and glial cells.

Supportive septa, or dural reflections, divide the intracranial cavity and protect the brain from excessive movement. They include the falx cerebri, which divides the brain into two hemispheres, and the tentorium cerebelli, which divides the brain into anterior and posterior fossae. The brain parenchyma is largely incompressible and in the absence of pathology generally remains at a constant volume. It has a very small capacity for deformation in the presence of a mass lesion; any pressures exerting a force past that capacity are likely to cause movement of brain tissue into adjacent dural compartments in a process called herniation.

Cerebrospinal Fluid

CSF is the extracellular fluid in the ventricles and subarachnoid space that performs a number of major functions in the human nervous system. First, it provides physical support and buoyancy for the brain—CSF’s low specific gravity reduces the effective weight of the brain from 1.4 kg to 47 g, which reduces brain inertia and protects against deformation caused by acceleration or deceleration (10). Second, because CSF volume fluctuates reciprocally with changes in the intracranial blood volume, it helps to maintain a safe ICP. Third, because the brain has no lymphatic system, metabolic by-products are largely removed by the capillary circulation or directly by transfer through the CSF. CSF is also important in acid-base regulation and the control of respiration, and it regulates the chemical environment of the brain.

Resting ICP represents the equilibrium pressure at which CSF production and absorption are in balance (11). The average adult has between 90 and 150 mL of CSF within the subarachnoid and ventricular spaces; this volume is smaller in children (3). CSF is produced at approximately 20 mL/hr or a total of 500 mL/day and is in dynamic equilibrium with its resorption (5,6). Most CSF originates from the choroid plexuses, which are located in the floor of the lateral, third, and fourth ventricles; the meninges also produce a small amount of CSF (9). The production of CSF depends upon cerebral perfusion pressure (CPP, discussed in further detail later in this chapter). When CPP falls below 70 mmHg, CSF production falls as well due to reduced cerebral and choroid plexus blood flow. It moves from the lateral ventricles through the foramen of Monro to the third ventricle, via the aqueduct of Sylvius into the fourth ventricle, and then through the foramina of Magendie and Luschka into the subarachnoid space and basal cisterns (10,12).

A hydrostatic gradient passively reabsorbs CSF into the venous system primarily through the arachnoid villi of the dural sinuses, which act as one-way valves between the subarachnoid space and the superior sagittal sinus; some CSF also leaks out around the spinal nerve roots and through the walls of the capillaries of the central nervous system (CNS) and pia mater (3,12–14). The reabsorption process can be described with the following:

CSF drainage = (CSF pressure-sagittal sinus pressure)/outflow resistance

The outflow of CSF is normally of low resistance, so central venous pressure generally determines ICP in healthy patients (15). CSF pressure is highest in the lateral ventricles and decreases as it moves farther down the system (3,9). Of note, CSF production decreases and reabsorption increases to a slight degree with rising ICP (9).

Blood

The intracranial circulation of blood is about 1000 L/day and is determined primarily by cerebral blood flow (CBF) and cerebral vascular tone (3). Intracranial blood is separated into an arterial component and a venous component; venous blood needs to continually flow out of the cranial cavity in order to allow for continuous incoming arterial blood (16). CBF depends on a number of factors that can be categorized either as those affecting CPP or those affecting the radius of cerebral blood vessels. The Hagen-Poiseuille law, which describes the laminar flow of a uniformly viscous and incompressible fluid through a cylindrical tube with a constant circular cross section, can help explain the factors determining CBF:

CBF = (∆PπR4)/(8ηl)

Where ∆P is equal to CPP, R is the radius of the blood vessels, η is the viscosity of the blood, and l is the length of the blood vessels.

The brain is unique in that it produces energy almost entirely via oxidative metabolism—thus, adequate CBF to the brain must be maintained in order to both ensure the sufficient delivery of oxygen and substrates and the removal of the waste products of metabolism (17). CBF ranges from 20 mL/100 g/min in white matter to 70 mL/100 g/min in gray matter (which has higher metabolic needs and thus greater blood flow); in an adult brain weighing approximately 1400 g, this equals 700 mL/min, which is equal to approximately 15% of cardiac output (3). The brain accounts for only 2% of total body weight, so it clearly requires more oxygen than other organs; this oxygen requirement is known as the cerebral metabolic rate for oxygen, or CMRO2.

Cerebral perfusion pressure

CPP is often used as a measure of adequate blood flow to the brain and is determined by the pressure gradient between cerebral arteries and veins; it can be defined as CPP = MAP − ICP, where MAP is the mean arterial blood pressure and the ICP under normal circumstances is essentially the same as the venous pressure as it exits the skull. CPP is usually around 80 mmHg. As the ICP rises in situations of intracranial hypertension to a level close to that of the MAP, CBF and perfusion decrease significantly due to a decrease in CPP. In general, if CPP is less than or equal to 60 mmHg, there is impaired blood flow to the brain;...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• Contributing Authors
• 1. The Pathophysiology of Intracranial Hypertension and Cerebral Herniation Syndromes
• 2. Intracranial Pressure Monitoring and Waveforms
• 3. Controversies in Intracranial Pressure Monitoring
• 4. Cerebral Herniation Syndromes
• 5. Osmotic Agents for the Treatment of Intracranial Hypertension and Cerebral Edema
• 6. Metabolic Suppression and Induced Hypothermia for the Treatment of Intracranial Hypertension
• 7. The Surgical Management of Intracranial Hypertension and Cerebral Herniation Syndromes
• 8. The Multicompartment Management of Intracranial Hypertension
• 9. The Role of Intracranial Pressure in Multimodality Monitoring Strategies
• Index