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Intracranial Pressure
Intracranial Pressure J Hott, Jonathan Hott, PLC, Phoenix, AZ, USA HL Rekate, The Chiari Institute, Great Neck, NY, USA r 2014 Elsevier Inc. All rights reserved.
Approximations help to simplify the understanding of intracranial pressure (ICP). The normal intracranial contents consist of the brain parenchyma (80%), blood (10%), and cerebrospinal fluid (CSF, 10%). The modified Monro–Kellie hypothesis states that the sum of the intracranial volumes of the blood, brain, CSF, and other components (e.g., tumor, hematoma, and mass lesions) is a constant. An increase in one component must be offset by an equal decrease in another or else ICP will increase. These volumes are contained in the skull, which is an inelastic, completely closed container. Young children whose sutures have not yet fused are an exception; their cranium can expand to accommodate extra volume. The normal range of ICP is not absolute and varies with age. In adults and older children, the value is less than 10–15 mmHg. The range is 3–7 mmHg for young children and 1.5–6 mmHg for term infants. As measured in clinical practice, ICP is the product of atmospheric and hydrostatic pressures and the filling pressure of the neuraxis. Atmospheric and hydrostatic pressure are effectively constant. In contrast, the filling pressure is a dynamic state determined by the volume of the intracranial components (e.g., brain, blood, and CSF) and the elastance of the neuraxis. Elastance is defined as a change in pressure for a given change in volume. Thus, when elastance of the neuraxis is high, a large increase in pressure will be associated with an increase in the volume of any component of the neuraxis. The elastance of the craniospinal axis is low under normal conditions. Thus, normal compensatory mechanisms can maintain ICP within a nonpathological range.
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ICP is a dynamic state that reflects the volume of the craniospinal compartments. Contrary to what might be expected, ICP does not increase linearly. In the presence of an expanding mass lesion, ICP increases exponentially. The classic ICP elastance curve has three phases. First, as volume increases, there is little or no increase in pressure (low elastance). Second, a ‘break’ point on the elastance curve represents the loss of volume reserve within the craniospinal axis. Finally, the steep section of the curve indicates that elastance is high and the buffering capacity of the system has been exhausted. At this point, very small changes in intracranial volume produce very large changes in ICP.
See also: Hydrocephalus. Intracranial Pressure Monitoring. Myelomeningocele. Pseudotumor Cerebri (PTC). Shunts, Neurosurgical. Slit Ventricle Syndrome
Further Reading McComb JG (1983) Recent research into the nature of cerebrospinal fluid formation and absorption. Journal of Neurosurgery 59: 369–383. Rekate H and Olivero W (1990) Current concepts of CSF production and absorption. In: Scott RM (ed.) Hydrocephalus. Concepts in Neurosurgery, vol. 3. Baltimore, MD: Williams and Wilkins. Rekate HL (1989) Circuit diagram of the circulation of cerebrospinal fluid. In: Marlin A (ed.) Concepts in Pediatric Neurosurgery, pp. 46–56. Zurich: Karger. Rekate HL (2002) Biophysics of cerebrospinal fluid and shunts. Techniques in Neurosurgery 7: 186–196.
Encyclopedia of the Neurological Sciences, Volume 2
doi:10.1016/B978-0-12-385157-4.00757-0
Approximations help to simplify the understanding of intracranial pressure (ICP). The normal intracranial contents consist of the brain parenchyma (80%), blood (10%), and cerebrospinal fluid (CSF, 10%). The modified Monro–Kellie hypothesis states that the sum of the intracranial volumes of the blood, brain, CSF, and other components (e.g., tumor, hematoma, and mass lesions) is a constant. An increase in one component must be offset by an equal decrease in another or else ICP will increase. These volumes are contained in the skull, which is an inelastic, completely closed container. Young children whose sutures have not yet fused are an exception; their cranium can expand to accommodate extra volume. The normal range of ICP is not absolute and varies with age. In adults and older children, the value is less than 10–15 mmHg. The range is 3–7 mmHg for young children and 1.5–6 mmHg for term infants. As measured in clinical practice, ICP is the product of atmospheric and hydrostatic pressures and the filling pressure of the neuraxis. Atmospheric and hydrostatic pressure are effectively constant. In contrast, the filling pressure is a dynamic state determined by the volume of the intracranial components (e.g., brain, blood, and CSF) and the elastance of the neuraxis. Elastance is defined as a change in pressure for a given change in volume. Thus, when elastance of the neuraxis is high, a large increase in pressure will be associated with an increase in the volume of any component of the neuraxis. The elastance of the craniospinal axis is low under normal conditions. Thus, normal compensatory mechanisms can maintain ICP within a nonpathological range.
744
ICP is a dynamic state that reflects the volume of the craniospinal compartments. Contrary to what might be expected, ICP does not increase linearly. In the presence of an expanding mass lesion, ICP increases exponentially. The classic ICP elastance curve has three phases. First, as volume increases, there is little or no increase in pressure (low elastance). Second, a ‘break’ point on the elastance curve represents the loss of volume reserve within the craniospinal axis. Finally, the steep section of the curve indicates that elastance is high and the buffering capacity of the system has been exhausted. At this point, very small changes in intracranial volume produce very large changes in ICP.
See also: Hydrocephalus. Intracranial Pressure Monitoring. Myelomeningocele. Pseudotumor Cerebri (PTC). Shunts, Neurosurgical. Slit Ventricle Syndrome
Further Reading McComb JG (1983) Recent research into the nature of cerebrospinal fluid formation and absorption. Journal of Neurosurgery 59: 369–383. Rekate H and Olivero W (1990) Current concepts of CSF production and absorption. In: Scott RM (ed.) Hydrocephalus. Concepts in Neurosurgery, vol. 3. Baltimore, MD: Williams and Wilkins. Rekate HL (1989) Circuit diagram of the circulation of cerebrospinal fluid. In: Marlin A (ed.) Concepts in Pediatric Neurosurgery, pp. 46–56. Zurich: Karger. Rekate HL (2002) Biophysics of cerebrospinal fluid and shunts. Techniques in Neurosurgery 7: 186–196.
Encyclopedia of the Neurological Sciences, Volume 2
doi:10.1016/B978-0-12-385157-4.00757-0