Physiology of the Cerebrospinal Fluid and Intracranial Pressure

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CHAPTER 10 Physiology of the Cerebrospinal Fluid and Intracranial Pressure

The exquisite sensitivity of the central nervous system to physical and chemical injury has led to an ingenious series of protective and homeostatic systems that shroud, nurture, nourish, and maintain maximal function. These systems include rigid physical protection in the form of skull bones, hydraulic shock absorption in the form of cerebrospinal fluid (CSF), substrate supply and cellular homeostasis in the form of a rich vascular supply with continuous turnover of extracellular fluid, and protection from external noxious substances by the blood-brain barrier.

These highly developed protective systems can become detrimental, however, under certain pathologic conditions. In particular, encasement of brain tissue in a rigid structure (the skull) places an important constraint on the refined interplay between cerebral homeostatic processes, namely, volume regulation. Noncompressible matter such as fluid, when added to a rigid container, will generate pressure; in the case of the central nervous system, this results in the intracranial pressure (ICP). Under pathologic conditions, the intracranial volume and therefore ICP can rise, which presents a serious threat to the central nervous system. Furthermore, the effects of raised ICP can further contribute to the primary pathologic process.

A wide range of neurosurgical problems—including congenital lesions, neoplasms, metabolic and infectious syndromes, infarction, hemorrhage, and trauma—requires evaluation and treatment for elevated ICP. Effective clinical treatment of problems relating to raised ICP demands a detailed understanding of the physiologic mechanisms that generate and maintain normal ICP, the perturbations that lead to an elevated ICP, and the means by which elevated pressures can be reduced.

Historical Considerations

Although problems with brain swelling and the effects of removing pieces of skull were understood even in the times of Galen, Hippocrates, and the early Egyptian physicians, modern thinking about volume regulation inside the skull began with the writings of George Kellie and his mentor, Alexander Monro, who worked in Edinburgh at the turn of the 19th century. Monro,¹ in his seminal work on the brain and nervous system, wrote:

For, as the substance of the brain, like that of other solids of our body, is nearly incompressible, the quantity of blood within the head must be the same, or very nearly the same at all times, whether in health or disease, in life or after death, those cases only excepted in which water or other matter is effused, or secreted from the blood vessels; for in these a quantity of blood, equal in bulk to the effused matter, will be pressed out of the cranium.

Some years later, at a meeting of the Medico-Chirurgical Society of Edinburgh, George Kellie ² presented a report in which he advanced this idea and stated:

If these premises be true, it does not then appear very conceivable how any portion of the circulating fluid can ever be withdrawn from within the cranium, without its place being simultaneously occupied by some equivalent; or how anything new or exuberant can be intruded without an equivalent displacement.

These ideas later became recognized as the Monro-Kellie doctrine, which is formalized physiologically later. Subsequent advances in understanding ICP came with the ability to effectively monitor ICP under different circumstances. Quincke ³ first described lumbar puncture for the relief of “brain pressure” in 1911. However, it was not until the work of Guillaume and Janny⁴ in 1951 that ICP was continuously monitored. Lundberg⁵ later published results from a large series of patients in which he described several of the fundamental concepts used today in clinical ICP monitoring, including the Lundberg A, B, and C waves.

ICP monitoring today is considered commonplace in neurosurgical practice. ICP values are used as a measure of disease, as an indication of treatment response, and to monitor cerebral perfusion.

Normal Intracranial Pressure

The upper limit of normal ICP in adults and older children is usually given as 15 mm Hg, although the usual range is 5 to 10 mm Hg. Transient physiologic changes resulting from coughing or sneezing often produce pressures exceeding 30 to 50 mm Hg, but ICP returns rapidly to baseline levels.

ICP can be measured by low-volume displacement transducers to interface with CSF pathways in the intraventricular, intraparenchymal, subdural, or epidural spaces. The ICP waveform is normally pulsatile and can be divided into three major components (Fig. 10-1). The baseline or average level is commonly referred to as the ICP; rhythmic components superimposed on this level are associated with cardiac and respiratory activity. To completely describe ICP, one should specify the magnitude of the baseline or “steady-state” level and the amplitude and periodicity of the pulsatile components. Changes in these pulsatile components can be one of the earliest signs that the ICP is beginning to rise, as a reflection of the increased conductance of pressure waves through a “tightening” brain.

FIGURE 10-1 Normal intracranial pressure (ICP) waveform. The baseline pressure level is affected by rhythmic components caused by cardiorespiratory activity. Mean arterial blood pressure fluctuation with heart rate causes small-amplitude rapid pulsation, and respiration causes larger amplitude fluctuations of lower frequency. ICP is completely described only by information about both the baseline level and the pulsatile components.

Cardiac and respiratory activity creates pulsatile components by cyclic changes in cerebral blood volume. Left ventricular contraction contributes the cardiac component, which has a frequency similar to the peripheral arterial pulse. The exact vessels that transmit the peripheral pulse remain to be established. Early studies suggested that the choroid plexus and pial arteries were responsible,⁶ although more recent analysis has implicated the high-compliance venous blood vessels.⁷

The respiratory contribution to the ICP waveform arises as a result of fluctuations in arterial blood pressure and cerebral venous outflow during the respiratory cycle, generated by pressure changes in the thoracic and abdominal cavities. During inspiration, there is a fall in arterial blood pressure and an increase in pressure gradient from cerebral veins to central venous capacitance vessels. This gradient drives cerebral venous return, which is therefore increased on inspiration, with a concomitant drop in cerebral blood volume. Mechanical ventilation and intrathoracic disease may considerably alter the respiratory contribution to the ICP waveform.

If the ICP waveform is examined in more detail and at a higher chart speed, the waveform of highest frequency can be seen to consist of as many as five smaller peaks. Three of these are relatively constant (Fig. 10-2): the percussion wave (W1), the tidal wave (W2), and the dicrotic wave (W3).⁷^,⁸ The percussion wave is the most constant in amplitude and derives from pulsations in large intracranial arteries.⁹ The tidal wave has a more variable shape and is thought to arise from brain elastance. The tidal wave and the dicrotic wave are separated by the dicrotic notch, which corresponds to the dicrotic notch in the arterial pulse waveform.

FIGURE 10-2 Normal ICP waveform at rapid chart speed. Several small components can be seen, the most constant of which are the percussion wave (W1), the tidal wave (W2), and the dicrotic wave (W3).

Physical Principles

ICP is synonymous with CSF pressure and is defined as the pressure that must be exerted against a needle introduced into the CSF space to just prevent escape of fluid. ICP depends on several parameters that are features common to any hydrodynamic system: the internal (intracranial) fluid volume, the elastance of the system, the contribution from the atmosphere, and the orientation of the craniospinal axis relative to the gravitational vector.

Pressure (P) is defined as force (F) per unit area (A). Pressures are usually reported in units of either millimeters of water (mm H₂O) or millimeters of mercury (mm Hg). This convention arose because a fluid column will exert a pressure at its base proportional to the height of the column (h), the density of the fluid (ρ), and the gravitational constant (g). Because, for a given fluid, g and ρ are constant, pressures can be compared by relation to the height of a given fluid. ICP is variably expressed in either millimeters of mercury or millimeters of water. Dividing the pressure in millimeters of water by 13.6 yields the equivalent pressure in millimeters of mercury; this constant is based on the ratio of the density of water to mercury. The SI unit for pressure is the pascal (Pa), which is defined as 1 newton (N)/square meter; 1 kilopascal (kPa) is 7.5 mm Hg or 102 mm H₂O.

In actual fact, three different pressures contribute to ICP: atmospheric pressure, hydrostatic pressure, and filling pressure. Atmospheric pressure is the component resulting from transmitted atmospheric pressure to the brain, and therefore absolute ICP varies with altitude. This pressure is principally transmitted through the vasculature ¹⁰; however, ICP is typically reported relative to atmospheric pressure, and this component is ignored.

As with any column of fluid, the skull and spinal canal experience hydrostatic pressure caused by the weight of their contents. The contribution of hydrostatic pressure depends on the weight of fluid and tissue above the point of measurement, divided by the cross-sectional area at that level. For example, lumbar CSF pressure is greater in the sitting position compared with the lateral decubitus position ¹¹^,¹² as a result of the hydrostatic difference; increasing degrees of head-down tilting further increase the contribution of hydrostatic pressure.

The filling pressure of the system is determined by the volume of the intracranial contents and the elastance of the enclosing structures. The intracranial contents consist of blood, brain, CSF, and any pathologic masses. Elastance is a system parameter that is defined by the pressure change per unit of volume change,¹³ namely, the corresponding pressure change for any given volume increase in craniospinal contents (Fig. 10-3A). The relationship is not necessarily linear across all volumes and not necessarily constant under all physiologic conditions. Compliance is the inverse of elastance, and both measures are useful for understanding the physiology of ICP. Elastance arises as a combined result of both distention and displacement. In other words, as volume is added to the system, there are two principal routes for compensation, either expansion or loss of volume. In a physiologic sense, this can occur either by distention of the spinal dura mater or by displacement of CSF and blood. These concepts are expounded on subsequently in the discussion of non–steady-state dynamics. Atmospheric pressure, hydrostatic pressure, and filling pressure all contribute to the concept of steady-state dynamics.

FIGURE 10-3 The pressure-volume curve and the pressure-volume index describe the response of ICP to the addition of volume. A, The normal curve shows how compliance changes as greater volumes are added. The CSF system is in the phase of spatial compensation at point a compared with spatial decompensation at point b. B, Graphing log ICP against volume change allows calculation of the pressure-volume index, which is widely used as a description of intracranial compliance.

Steady-State Dynamics

In the absence of disease, baseline ICP and the amplitude of the pulsatile components of ICP remain constant despite a variety of transient perturbations. As alluded to previously, the skull should be considered a rigid container of noncompressible elements. ICP therefore depends on the relative constancy of total volume inside the skull, contributed to by CSF, blood, and brain tissue. The ability of the dural coverings to distend is very limited; therefore, any change in volume of one of the three intracranial components must occur at the expense of the other two if ICP is to be maintained. The following equation describes this relationship:

This relationship encapsulates the concept of the Monro-Kellie doctrine. The presence of an abnormal component, such as a tumor or hematoma (represented by V_OTHER in the equation), demands reciprocal changes in the volumes of brain, blood, or CSF to maintain ICP at physiologic levels. Even the physiologic state is far from static; it is rather a dynamic equilibrium. Constant changes induced by the heartbeat, systemic blood pressure, fluid status, and intrathoracic pressure invoke transient changes within the intracranial compartment that are considered part of the steady state.

General Physiology of the Cerebrospinal Fluid

In the adult, approximately 87% of the typically 1500-mL intracranial space is occupied by the brain, 9% by compartmental CSF (ventricles, cisterns, and subarachnoid space), and 4% by blood.¹⁴ The extracellular space is in direct contact with CSF and forms approximately 15% of the total brain volume.¹⁵ Total CSF is considered to include compartmental CSF and the extracellular space.

Cranial CSF volume as assessed in humans by Tanna and colleagues ¹⁶ was found to have a mean of 164.5 mL, with a range of 62.2 to 267 mL. Ventricular volume also varies considerably, as assessed with magnetic resonance imaging, from 7.49 mL to 70.5 mL, with a mean of 31.9 mL. Reasons for such variation are not clear; however, the amount of CSF in any organism reflects a dynamic balance between production and clearance.

CSF is principally produced by the choroid plexuses, which are invaginations of the pia mater into the ventricular cavities, specifically in the roofs of the third and fourth ventricles and the walls of the lateral ventricles. At these points, fronds of densely branching blood vessels are invested by pia mater and covered by specialized ependymal cells, the choroidal epithelium. The surfaces of cells in this structure are densely covered with villous processes to increase the surface area. A second site of CSF production is the ventricular ependyma, the proportional contribution of which arguably ranges from 50% to 100%.¹⁷^,¹⁸

For a considerable time, CSF was described as an ultrafiltrate of plasma, implying that hydrostatic pressure within blood vessels forced protein-free fluid through interendothelial spaces.¹⁹ However, close analysis of the composition of CSF (Table 10-1) shows multiple differences in composition at the ionic level, which is strongly against the idea of a simple filtration or dialysis process. CSF, in general, has a higher sodium, chloride, and magnesium concentration than one would expect in a plasma filtrate. The concentrations of potassium, calcium, urea, and glucose are lower. The overall osmolality is, however, similar. Current thinking therefore holds that a simple filtration process is modified by energy-dependent secretion and reabsorption processes.

TABLE 10-1 Concentrations of Solutes (mEq/kg H₂O) in Plasma and Lumbar Cerebrospinal Fluid in Humans

SUBSTANCE	PLASMA	CSF
Sodium (Na⁺)	150	147
Potassium (K⁺)	4.63	2.86
Magnesium (Mg²⁺)	1.61	2.23
Calcium (Ca²⁺)	4.7	2.28
Chloride (Cl⁻)	99	113
Bicarbonate (HCO₃⁻)	26.8	23.3
Amino acids	2.62	0.72
Osmolality	289.0	289.0
pH	7.397	7.30

Cl⁻, Na⁺, and K⁺ from Fremont-Smith F, Dailey ME, Merritt HH, et al. The equilibrium between cerebrospinal fluid and blood plasma: I. The composition of the human cerebrospinal fluid and blood plasma. Arch Neurol Psychiatry. 1931;25(6):1271-1289; Mg²⁺ and Ca²⁺ from Hunter G, Smith HV. Calcium and magnesium in human cerebrospinal fluid. Nature. 1960;186:161-162; HCO₃⁻ and pH from Bradley RD, Semple SJ. A comparison of certain acid-base characteristics of arterial blood, jugular venous blood and cerebrospinal fluid in man, and the effect on them of some acute and chronic acid-base disturbances. J Physiol. 1962;160:381-391; osmolality from Hendry EB. The osmotic pressure and chemical composition of human body fluids. Clin Chem. 1962;8:246-265.

Estimates of the rates of CSF production can be made experimentally by examining the clearance or turnover of injected substances,²⁰ by marker dilution techniques,²¹ or by ventriculocisternal perfusion.²² Estimates with these techniques have yielded values in the range of 0.35 to 0.37 mL/min for humans.²³^,²⁴ More recently, flow voids in magnetic resonance signal within the CSF system have been used to estimate CSF production rates. Feinberg and Mark²⁵ estimated the flow of CSF through the aqueduct in humans, which should in principle equate to the flow of CSF secretion in the lateral and third ventricles, as 0.48 mL/min. There are variations in absolute rates of CSF production, however, that clearly relate to the absolute weight of choroid plexus tissue in each subject. Furthermore, rates of CSF production follow a diurnal variation, with peak production rates in the late evening and early morning.²⁶

Because there is a constant production of CSF, there must be removal of CSF at the same rate. CSF circulates from choroid plexus, through the ventricles, to the cisterna magna, basal cisterns, and subarachnoid space. The principal site of physiologic CSF drainage is into the dural venous system, through the dural venous sinuses. Evaginations of the arachnoid membrane protrude into the lumen of the dural veins and form the arachnoid granulations or villi. This forms a valvular connection between the subarachnoid space and the dural sinus so that blood cannot reflux into CSF. A higher hydrostatic pressure in the subarachnoid space drives the bulk flow of fluid in the forward direction, therefore draining CSF volume. Studies of these structures have revealed that they can allow molecules up to several microns to pass, but only unidirectionally.²⁷ CSF reabsorption has been shown to cease at CSF pressures of less than 5 mm Hg.²³

Cerebrospinal Fluid Dynamics

The nonpulsatile volume of CSF at any point in time is dependent on a balance between the rate of formation, the rate of absorption, and the volume sink in the skull. The interactions between these parameters can be addressed by a mathematical model that analyzes the physiologic mechanisms of (1) CSF formation, (2) volume storage or compliance, and (3) fluid absorption (Fig. 10-4).²⁸^,²⁹

FIGURE 10-4 Equivalent electrical circuit showing the principal factors that govern ICP. CSF formation is represented as a current source (I formation), CSF storage is represented by a capacitative element (C). Resistance to CSF outflow is represented by a resistive element (R), and dural sinus pressure is P_d. This framework has allowed accurate description of CSF dynamics.

(From Marmarou A, Shulman K, Rosende RM. A nonlinear analysis of the cerebrospinal fluid system and intracranial pressure dynamics. J Neurosurg. 1978;48:332-344.)

The formation of CSF can be depicted as a pump that continuously introduces fluid into the CSF spaces of the neural axis. The rate of formation of fluid (I_f) is considered constant and independent of the pressure that has to be pumped against, in accordance with observations that the rate of CSF formation is affected minimally, if at all, by changes in the ICP. The fluid enters a compliant storage space, which can expand to accommodate the added volume, and proceeds through outflow pathways to be absorbed across the arachnoid villi into the dural venous sinuses. The compliance mechanism is represented by an element (C) that decreases its compressibility with increasing volume, much like the resistance offered by a rubber balloon at maximal inflation. Both the resistance of the CSF channels leading to the arachnoid villi and the resistance of the villi to fluid flow are combined into a single resistance element (R_o). This component represents the total resistance to the outflow of the CSF, which under normal conditions remains fixed and independent of ICP. The final element of this model is the dural sinus pressure (P_d). Fluid crossing the arachnoid villi must overcome this exit pressure.

From this conceptual framework, a theory has been developed to describe the interaction of the formation, storage, and absorptive elements in the steady state. First, both pressure and volume are in equilibrium, and there can be no net increase or decrease in the total volume of the CSF. The CSF formed must pass through the absorptive elements so that no net fluid is stored.

Because all fluid formed passes through the resistive element (R_o), a pressure gradient will be developed across the absorptive element that is equal to the product of fluid formation (I_f) and the fluid resistance (R_o). The greater the magnitude of flow or resistance, the greater the pressure gradient (I_f × R_o). As long as the system is in equilibrium, the pressure of the CSF spaces must be of sufficient magnitude to drive fluid through the arachnoid villi at the same rate it is being formed. This requires that the CSF pressure (ICP) be equal to the sum of the pressure gradient across the absorptive element (I_f × R_o) and the exit pressure. The equation

shows that the steady-state ICP is proportional to three parameters: (1) the rate of CSF formation, (2) the resistance to CSF absorption, and (3) the dural sinus pressure. When these parameters remain constant, ICP is unchanged, and the compliance element does not actively participate in ICP regulation.

An increase in CSF formation, outflow resistance, or venous pressure at the site of fluid absorption can alter this dynamic equilibrium and result in elevated ICP. Mathematical modeling has shown that the contribution of the product of CSF formation rate and outflow resistance (I_f × R_o) is approximately 10% of the total ICP.²⁹ The remainder is attributed to the magnitude of the dural sinus pressure (P_d). With this distribution, the outflow resistance would have to increase markedly to cause a significant rise in the ICP. However, much smaller elevations of sagittal sinus pressure (P_d) caused by venous sinus obstruction would be transmitted directly to the CSF system, thus raising resting ICP. From the equation, it is clear that changes in these elements can occur independently of each other.

If P_d rises, CSF absorption can remain constant, and a shift to a new ICP equilibrium can occur. This concept is supported by the work of Johnston,³⁰ who demonstrated normal CSF resistance in the presence of raised ICP induced by venous obstruction.⁴^,³⁰ In this case, there is no net change in the CSF volume and presumably no change in the compliance element.

Pulsation Models of Communicating Hydrocephalus: A New Concept

The pulsation of ICP has been the subject of study for many years. More recently, it has been considered to be an integral part of a new concept of hydrocephalus whereby the ICP pulse is more than a CSF reflection of the cardiac and respiratory pulsation. In the new concept, the flow of the arterial pulsations into the cranium is considered to be sequential, beginning from the large subarachnoid arteries at the skull base to the small arterioles in the parenchyma. Pulsations are dissipated into the subarachnoid CSF and into the choroidal arteries and ventricular CSF. These pathways are arranged in a series-parallel array of arteries and CSF spaces. The bulk flow of blood that remains after the pulsations have been filtered out continues through the capillary pathways and represents a windkessel mechanism.³¹ Most important, this model provides an alternative view to the “bulk flow theory” causing ventricular enlargement. Increased impedance to pulsations in the subarachnoid space increases pulsations in the blood flow to the choroidal arteries and the choroid plexus, thereby increasing the pulsations in the ventricular CSF. The ventricular pulsations exceed those in the subarachnoid space, and it is theorized that a transmantle pulse pressure gradient and subsequent ventricular expansion result. This theory is now being explored by mathematical models and experimental studies in the laboratory and in the clinical setting.³²^,³³ Future work will focus on demonstrating that this model accounts for the pathologic changes seen in communicating hydrocephalus.

Cerebral Blood Volume and Flow

The other dynamic component of the equation representing the Monro-Kellie doctrine is cerebral blood volume (CBV). CBV and cerebral blood flow (CBF) are essentially functions of the cerebral arteriovenous pressure difference and cerebrovascular resistance, which is largely determined by vessel caliber. Although, as described previously, there are rhythmic changes in blood volume as a result of the cardiorespiratory cycles, both CBV and CBF are maintained within narrow limits by the process of autoregulation. Vascular tone is influenced by both physical (force) and chemical (pH, PCO₂) signals, with the aim of maintaining CBF across a wide range of both arterial blood pressures (50 to 160 mm Hg) and levels of tissue metabolism (PaCO₂, 20 to 70). The mean normal adult CBF is 53 mL/100 g per minute.

Non–Steady-State Dynamics

The preceding processes maintain a constant ICP while there is no CSF accumulation or other change in intracranial volume. Unfortunately, many pathologic states, such as hematoma, tumor, hydrocephalus, and cerebral edema, are associated with changes in intracranial volume, which can, in turn, cause elevated ICP.

The most common clinical cause of raised ICP is traumatic brain injury, the pathology of which encompasses several possible V_OTHERS. Brain edema contributes extra volume to the intracranial contents in the form of water. Trauma may induce intracerebral collections of blood in extradural, subdural, subarachnoid, or intraparenchymal locations, which each contribute extra volume. Furthermore, trauma may induce changes in V_BLOOD as a result of disrupted autoregulation and hyperemia. The extent to which elevated CBV contributes to ICP after traumatic brain injury seems small, however, compared with edema.³⁴

Subarachnoid hemorrhage after rupture of an intracranial aneurysm differs from other intracranial hemorrhage. Bleeding with arterial pressure can potentially cause ICP to rise instantaneously, and as ICP approaches the mean arterial blood pressure, bleeding slows, but cerebral perfusion pressure is critically low.³⁵ The cause of this rise in ICP is a combination of intraparenchymal blood collection, increased CBV, and a strong vasomotor reaction to the ensuing injury.³⁶

Brain tumors provide a slowly increasing V_OTHER, which provides time for the organism to make compensatory changes. Therefore, ICP problems with tumors occur less commonly; however, a sudden change in intracranial volume (such as a hemorrhage into the body of a tumor) in a system that is already compensated and therefore dynamically stressed can have disastrous results. Also, prolonged periods of increased hydrostatic pressure (such as lying down) can create substantial increases in ICP in such patients. This phenomenon, coupled with the diurnal variation in CSF secretion, may underlie the early morning headaches and nausea experienced by these patients.

Other clinical conditions can result in an elevated ICP, including hydrocephalus, idiopathic intracranial hypertension, meningitis, and arteriovenous malformations. In several of these conditions, a direct addition of intracranial volume is not the primary cause. For example, in hydrocephalus, an impaired CSF drainage system causes CSF accumulation. The cause of elevated ICP in idiopathic intracranial hypertension is not known, although again, CSF production and drainage have been implicated as has elevated venous pressure.³⁷ Meningitis can influence ICP in several patterns, either by blocking CSF drainage pathways or by stimulating marked cerebral edema. Arteriovenous malformation, in contrast, represents a significant increase in V_BLOOD that can secondarily create V_OTHER if it is involved in a hemorrhagic event.

Brain edema is a specific pathologic process arising in response to a wide range of cerebral pathologic changes. It is defined as an increase in the brain tissue water content and therefore can be thought of as contributing to V_OTHER or V_BRAIN. At the turn of the 20th century, Reichardt ³⁸ reported differences between a dry swollen brain and a wet edematous brain. Klatzo³⁹ subsequently paved the way for all future discussions of edema by introducing the terms cytotoxic and vasogenic to indicate intracellular and extracellular water accumulation, respectively. The latter of these is traditionally associated with an open blood-brain barrier and fluid leakage. However, this division is a simplification for two reasons. First, for brain tissue water to rise, even under cytotoxic conditions, water has to enter the tissue from an external source, the most likely of which is blood vessels. Therefore, even cytotoxic edema has a “vasogenic” origin. It is the pathologic cause and the final site of edema accumulation that must distinguish these phenomena. Furthermore, studies in traumatic brain injury have suggested that the vasogenic component of injury may have been overemphasized⁴⁰^,⁴¹ and that the importance of a disrupted blood-brain barrier lies in the provision of a low-resistance pathway for movement of water to cells that are swelling cytotoxically.⁴² The predominance of traumatic cellular edema has been supported by studies in head-injured patients.⁴³ Cytotoxic and vasogenic edema may not, therefore, be separable entities, and a recently proposed classification of edema based on the terms intact barrier and open barrier may have merit.⁴⁴

Several possible alterations in intracranial volume are discussed in the preceding paragraphs. When such changes occur rapidly or exceed the ability for compensation by reciprocal volume reductions, ICP may begin to rise. Under these conditions, more descriptors of ICP are required than in the steady state.

Intracranial Pressure in Idiopathic “Normal-Pressure” Hydrocephalus

The term normal-pressure hydrocephalus was introduced in the thesis of Solomon Hakim in 1964 and the description later published in the landmark article by Adams and coworkers.⁴⁵ Together, they are credited with describing a specific syndrome associated with patients in whom ventricular enlargement occurred in the absence of elevated ICP and who presented with gait disturbance, dementia, and incontinence. Current guidelines for diagnosis and management of idiopathic normal-pressure hydrocephalus are now available.⁴⁶ According to these guidelines, the lumbar pressures for idiopathic normal-pressure hydrocephalus range from 60 to 240 mm H₂O. This range was arrived by consensus in a meeting of experts on the basis of their experience and what was reported in the available literature. Later, in a study of 151 patients diagnosed with idiopathic normal-pressure hydrocephalus, it was confirmed that the lumbar pressure measured in these patients varied over a wide range⁴⁷ (Fig. 10-5). Thus, the term normal-pressure hydrocephalus has been questioned as pressures varied considerably from the so-called normal pressure defined by Ekstedt.⁴⁸ Interestingly, the median pressure found by Marmarou⁴⁷ in patients with idiopathic normal-pressure hydrocephalus correlates well (9.0 mm Hg) with the value of the normal resting pressure by Ekstedt (10.0 mm Hg).

FIGURE 10-5 Distribution of lumbar pressure in patients with idiopathic “normal”-pressure hydrocephalus (NPH). Pressure varied over a wide range, calling normal-pressure hydrocephalus into question. The median pressure of 9.0 mm Hg was slightly lower than the average 10.0-mm Hg lumbar pressure found in patients without normal-pressure hydrocephalus.

Pressure-Volume Relationships

The relationship between intracranial volume and ICP is not linear. Volume-pressure relationships can be depicted by graphing the response of ICP to volume added into the neural axis (see Fig. 10-3). In the normal adult, Ryder and coworkers⁴⁹ showed that this relationship describes a hyperbolic curve. Along the flat portion of the curve, increases in volume affect ICP minimally because compensatory mechanisms can effectively maintain ICP in a normal range. This part of the curve is called the period of spatial compensation. As volume is added, the pressure changes per unit volume become increasingly large, and compliance lessens; this portion is called the period of spatial decompensation. Above 50 mm Hg, and as ICP approaches mean arterial pressure, the curve tends to flatten again; thus, the complete curve is not hyperbolic but rather sigmoid.

The reciprocal of the slope of this curve (ΔV/ΔP) represents the compliance of the system, which is maximal in the period of spatial compensation. The slope of the pressure-volume curve rises rapidly during spatial decompensation, and therefore compliance falls. Another method of expressing information about compliance is to plot ICP logarithmically against volume, which gives a straight line.⁵⁰ Its slope is the pressure-volume index (PVI), or the calculated volume in milliliters needed to raise ICP by a factor of 10 (Fig. 10-6A). In normal adults, PVI is 25 to 30 mL.⁵¹ When compliance is reduced by a pathologic process, PVI diminishes, and therefore small volume changes result in much greater pressure changes. Values less than 13 mL are considered clearly abnormal.⁵² PVI is age dependent, although normal infants have PVIs below 10 mL, and the adult PVI of 25 mL is reached at around 14 years of age.

FIGURE 10-6 Pressure-volume curves from a normal adult (A) and an adult with cerebral edema (B). In the pathologic state, the curve is steeper and the pressure-volume index (PVI) is greatly reduced as a result of V_OTHER. TBI, traumatic brain injury.

PVI can be measured clinically by infusion or withdrawal of small boluses of fluid in the CSF space, with concomitant measures of the pressure response. CSF outflow resistance can also be calculated from the rate decay of the pressure peak after a bolus infusion. A complete description of resistance measurement techniques is found in the report of Eklund and colleagues.⁵³ Methods of continuous PVI measurement have been devised with use of multiple time-averaged small-volume pulses.⁵⁴ Although these methods generate less pressure perturbation, they tend to underestimate compliance as measured with the conventional technique.

The compensatory abilities of brain, blood, and CSF at any given point on the pressure-volume curve are dependent on their respective volumes, their ease of egress from the skull, and the level of ICP at which the interactions are occurring, coupled with the rigidity of the skull. Although the brain occupies 80% of the intracranial space, this volume is effectively available for compensation only when increases in V_OTHER occur slowly. With more rapid changes, brain shifts and herniations are more likely to occur. Although blood and CSF occupy less of the intracranial space, their total volume can be reduced more rapidly.

These concepts become more complex, as do most models, when they are applied to clinical practice. Although short-term changes cause movement along a single pressure-volume curve, changing intracranial dynamics can create a new pressure-volume curve with time (Fig. 10-6B). Increases in CBV and cerebral edema are also likely to play a role in producing these dynamic pressure-volume interactions.⁵⁵ Thus, knowledge of the absolute pressure coupled with some expression of the slope of the ICP volume curve at any given time point provides a more complete description of the stability or instability of ICP.

Effects of Elevated Intracranial Pressure

Under non–steady-state conditions, failure of compensation mechanisms ultimately results in an elevated ICP, the pathologic consequences of which can be severe. First, continued perfusion of the brain relies on a cerebral arteriovenous pressure gradient. ICP is transmitted to the compliant cerebral veins, and therefore the cerebral perfusion pressure (CPP) is defined as the arterial inflow pressure minus ICP. If ICP increases, CPP falls; and if the lower limit of autoregulation is exhausted, CBF will begin to fall.

The autoregulatory reserve may be defined as the difference between the CPP at a given moment and the lower limit of autoregulation. Considering that the lower limit of autoregulation is within the range of 50 to 70 mm Hg, the autoregulatory reserve for a CPP of 90 would be 20 to 40 mm Hg. Thus, a CPP below the autoregulatory threshold exhausts the autoregulatory reserve.⁵⁶^–⁶⁰ When CPP decreases, the wall tension of reactive brain vessels decreases, thereby increasing the transmission of the arterial pulse wave to the intracranial contents.⁶¹ Similarly, when a reduction of CPP is caused by increased ICP, brain compliance decreases,²⁹^,⁶² which also serves to increase pulsatile transmission. Taken in concert, a decrease in CPP results in an increase of both blood pressure and ICP pulsatility. This relationship is highly predictive for fatal outcome⁶³ because as the ICP pulse amplitude levels off or starts to decrease, it implies that the cerebral vessels are no longer pressure reactive.

By use of these principles, it is clear that the correlation between spontaneous waves of blood pressure and ICP is dependent on the autoregulatory reserve. The correlation coefficient between changes in blood pressure and ICP is defined by Czosnyka ⁶¹ as the pressure reactivity index. An example of the use of the pressure reactivity index is illustrated in Figure 10-7. Examples of time-related changes of the pressure reactivity index are shown, in which the index increases from relatively low values (no association) to values approaching 1.0 (strong positive association). These values were calculated from a period of 1-hour terminal increase in ICP from 60 to 90 mm Hg in a head-injured patient who died (Fig. 10-7A). Figure 10-7B illustrates a transient change in pressure reactivity index during the period of an ICP plateau wave with rapid recovery when ICP returned to baseline. In summary, the pressure reactivity index provides a practical means of assessing the degree of autoregulation and is useful in elucidating the contribution of the cerebrovasculature to mechanisms causing ICP rise.

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