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| ==Overview==
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| '''Intracranial pressure''', ('''ICP'''), is the pressure in the [[cranium]] and thus in the [[brain]] tissue and [[cerebrospinal fluid]] (CSF); this pressure is exerted on the brain's intracranial [[blood]] circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF. Because the entire system is contained by bone and strong ligamentous connections, the pressures of the body, such as those caused by straining, exercise, and coughing, do not affect the brain or its environment. ICP is measured in millimeters of mercury ([[mmHg]]) and, at rest, is normally 7–15 mmHg for a [[Supine position|supine]] adult, and becomes negative (averaging −10 mmHg) in the vertical position.<ref name="Steiner">{{cite journal | author=Steiner LA, Andrews PJ | title=Monitoring the injured brain: ICP and CBF | journal=British Journal of Anaesthesia | volume=97 | issue=1 | year=2006 | pages=26–38 |url=http://bja.oxfordjournals.org/cgi/content/full/97/1/26 | pmid=16698860 | doi=10.1093/bja/ael110}}</ref> Changes in ICP are attributed to volume changes in one or more of the constituents contained in the cranium.
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| '''Intracranial hypertension''', commonly abbreviated '''IH''', is elevation of the pressure in the [[cranium]]. ICP is normally 0–10 mm Hg; at 20–25 mm Hg, the upper limit of normal, treatment to reduce ICP is needed.<ref name="Ghajar00">
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| {{
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| cite journal |author=Ghajar J |title=Traumatic brain injury |journal=Lancet |volume=356 |issue=9233 |pages=923–9 |year=2000 |month=September |pmid=11036909 |doi=10.1016/S0140-6736(00)02689-1
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| </ref>
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| ==The Monro-Kellie hypothesis==
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| The pressure-volume relationship between ICP, volume of CSF, blood, and brain tissue, and [[cerebral perfusion pressure]] (CPP) is known as the Monro-Kellie doctrine or the Monro-Kellie hypothesis.<ref>{{cite book | author=Monro A | title=Observations on the structure and function of the nervous system | location=Edinburgh | publisher=Creech & Johnson | year=1783}}</ref><ref>{{cite journal | author=Kelly G | title=Appearances observed in the dissection of two individuals; death from cold and congestion of the brain | journal=Trans Med Chir Sci Edinb | year=1824 | volume=1 | pages=84–169}}</ref><ref name=Mokri>{{cite journal |author=Mokri B |title=The Monro-Kellie hypothesis: applications in CSF volume depletion |journal=Neurology |volume=56 |issue=12 |pages=1746–8 |year=2001 |month=June |pmid=11425944 |doi= |url=http://www.neurology.org/cgi/content/full/56/12/1746}}</ref>
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| The Monro-Kellie hypothesis states that the cranial compartment is incompressible, and the volume inside the cranium is a fixed volume. The cranium and its constituents (blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another.<ref name=Mokri/>
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| The principal buffers for increased volumes include both CSF and, to a lesser extent, blood volume. These buffers respond to increases in volume of the remaining intracranial constituents. For example, an increase in lesion volume (e.g. epidural hematoma) will be compensated by the downward displacement of CSF and venous blood.<ref name=Mokri/> These compensatory mechanisms are able to maintain a normal ICP for any change in volume less than approximately 100–120 mL.
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| ==Increased ICP==
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| [[Image:Brain herniation MRI.jpg|left|thumb|Severely high ICP can cause the brain to herniate.]]
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| One of the most damaging aspects of [[brain trauma]] and other conditions, directly correlated with poor outcome, is an elevated intracranial pressure.<ref name="orlando">Orlando Regional Healthcare, Education and Development. 2004. [http://www.orlandoregional.org/pdf%20folder/overview%20adult%20brain%20injury.pdf "Overview of Adult Traumatic Brain Injuries."] Accessed January 16, 2008.</ref> ICP is very likely to cause severe harm if it rises too high.<ref name="Dawodu">Dawodu S. 2005. [http://www.emedicine.com/pmr/topic212.htm "Traumatic Brain Injury: Definition, Epidemiology, Pathophysiology"] Emedicine.com.Accessed January 4, 2007. </ref> Very high intracranial pressures are usually fatal if prolonged, but children can tolerate higher pressures for longer periods.<ref name="tolias">Tolias C and Sgouros S. 2006. [http://www.emedicine.com/med/topic3216.htm "Initial Evaluation and Management of CNS Injury."] Emedicine.com. Accessed January 4, 2007.</ref> An increase in pressure, most commonly due to head injury leading to [[intracranial hematoma]] or [[cerebral edema]] can crush brain tissue, shift brain structures, contribute to [[hydrocephalus]], cause the brain to [[brain herniation|herniate]], and restrict blood supply to the brain.<ref name="graham">Graham DI and Gennareli TA. Chapter 5, "Pathology of Brain Damage After Head Injury" In, Cooper P and Golfinos G. 2000. ''Head Injury'', 4th Ed. Morgan Hill, New York.</ref> It is a cause of reflex bradycardia. <ref name=firstaid> {{cite book |author=Deepak A. Rao; Le, Tao; Bhushan, Vikas |title=First Aid for the USMLE Step 1 2008 (First Aid for the Usmle Step 1) |publisher=McGraw-Hill Medical |location= |year=2007 |pages= |isbn=0-07-149868-0 |oclc= |doi= |accessdate=}} Page 254 </ref>
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| ===Pathophysiology===
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| The cranium and the [[vertebral body]], along with the relatively inelastic dura, form a rigid container, such that the increase in any of its contents—brain, blood, or CSF—will increase the ICP. In addition, any increase in one of the components must be at the expense of the other two; this relationship is known as the Monro-Kellie doctrine. Small increases in brain volume do not lead to immediate increase in ICP because of the ability of the CSF to be displaced into the spinal canal, as well as the slight ability to stretch the falx cerebri between the hemispheres and the tentorium between the hemispheres and the cerebellum. However, once the ICP has reached around 25 mmHg, small increases in brain volume can lead to marked elevations in ICP.
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| Traumatic brain injury is a devastating problem with both high mortality and high subsequent morbidity. Injury to the brain occurs both at the time of the initial trauma (the primary injury) and subsequently due to ongoing cerebral ischemia (the secondary injury). Cerebral edema, hypotension, and axonal hypoxic conditions are well recognized causes of this secondary injury. In the intensive care unit, raised intracranial pressure (intracranial hypertension) is seen frequently after a severe diffuse brain injury (one that occurs over a widespread area) and leads to cerebral ischemia by compromising cerebral perfusion.
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| [[Cerebral perfusion pressure]] (CPP), the pressure causing blood flow to the brain, is normally fairly constant due to autoregulation, but for abnormal [[mean arterial pressure]] (MAP) or abnormal ICP the cerebral perfusion pressure is calculated by subtracting the intracranial pressure from the mean arterial pressure: CPP = MAP − ICP <ref name="Steiner" />.<ref>{{cite journal | author=Duschek S, Schandry R | title=Reduced brain perfusion and cognitive performance due to constitutional hypotension | journal=Clinical Autonomic Research | volume=17 | issue=2 | year=2007 | pages=69–76 |pmc=1858602 | pmid=17106628 | doi=10.1007/s10286-006-0379-7}}</ref> One of the main dangers of increased ICP is that it can cause [[ischemia]] by decreasing CPP. Once the ICP approaches the level of the mean systemic pressure, it becomes more and more difficult to squeeze blood into the intracranial space. The body’s response to a decrease in CPP is to raise [[blood pressure]] and dilate [[blood vessel]]s in the brain. This results in increased cerebral blood volume, which increases ICP, lowering CPP further and causing a vicious cycle. This results in widespread reduction in cerebral flow and perfusion, eventually leading to ischemia and brain infarction. Increased blood pressure can also make [[intracranial hemorrhage]]s bleed faster, also increasing ICP.
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| Highly increased ICP, if caused by a one-sided space-occupying process (e.g., hematoma) can result in midline shift, a dangerous condition in which the brain moves toward one side as the result of massive swelling in a [[cerebral hemisphere]]. Midline shift can compress the [[ventricular system|ventricles]] and lead to buildup of CSF.<ref name="Downie">Downie A. 2001. [http://www.radiology.co.uk/srs-x/tutors/cttrauma/tutor.htm "Tutorial: CT in Head Trauma"] Accessed January 4, 2007.</ref> Prognosis is much worse in patients with midline shift than in those without it. Another dire consequence of increased ICP combined with a space-occupying process is [[brain herniation]] (usually uncal or cerebellar), in which the brain is squeezed past structures within the skull, severely compressing it. If brainstem compression is involved, it may lead to decreased respiratory drive and is potentially fatal. This herniation is often referred to as "coning".
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| Major causes of [[morbidity]] due to increased intracranial pressure are due to global brain [[infarction]] as well as decreased respiratory drive due to brain [[herniation]].
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| ===Intracranial hypertension===
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| Minimal increases in ICP due to compensatory mechanisms is known as stage 1 of intracranial hypertension. When the lesion volume continues to increase beyond the point of compensation, the ICP has no other resource, but to increase. Any change in volume greater than 100–120 mL would mean a drastic increase in ICP. This is stage 2 of intracranial hypertension. Characteristics of stage 2 of intracranial hypertension include compromise of neuronal oxygenation and systemic arteriolar vasoconstriction to increace MAP and CPP. Stage 3 intracranial hypertension is characterised by a sustained increased ICP, with dramatic changes in ICP with small changes in volume. In stage 3, as the ICP approaches the MAP, it becomes more and more difficult to squeeze blood into the intracranial space. The body’s response to a decrease in CPP is to raise blood pressure and dilate blood vessels in the brain. This results in increased cerebral blood volume, which increases ICP, lowering CPP further and causing a vicious cycle. This results in widespread reduction in cerebral flow and perfusion, eventually leading to ischemia and brain infarction. Neurologic changes seen in increased ICP are mostly due to hypoxia and hypercapnea and are as follows: decreased level of consciousness (LOC), [[Cheyne-Stokes respirations]], hyperventilation, sluggish dilated pupils and widened pulse pressure.
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| ===Causes===
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| Causes of increased intracranial pressure can be classified by the mechanism in which ICP is increased:
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| *'''mass effect''' such as brain tumor, infarction with oedema, contusions, subdural or epidural hematoma, or abscess all tend to deform the adjacent brain.
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| *'''generalized brain swelling''' can occur in ischemic-anoxia states, acute liver failure, hypertensive encephalopathy, pseudotumor cerebri, hypercarbia, and Reye hepatocerebral syndrome. These conditions tend to decrease the cerebral perfusion pressure but with minimal tissue shifts.
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| *'''increase in venous pressure''' can be due to venous sinus thrombosis, heart failure, or obstruction of superior mediastinal or jugular veins.
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| *'''obstruction to CSF flow and/or absorption''' can occur in hydrocephalus (blockage in ventricles or subarachnoid space at base of brain, e.g., by Arnold-Chiari malformation), extensive meningeal disease (e.g., infectious, carcinomatous, granulomatous, or hemorrhagic), or obstruction in cerebral convexities and superior sagittal sinus (decreased absorption).
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| {{main|hydrocephalus}}
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| *'''increased CSF production''' can occur in meningitis, subarachnoid hemorrhage, or choroid plexus tumor.
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| *Idiopathic or unknown cause ([[idiopathic intracranial hypertension]])
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| *[[Cerebral venous sinus thrombosis]]
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| *[[Acute liver failure]]<ref>{{cite journal |author=Polson J, Lee WM |title=AASLD position paper: the management of acute liver failure |journal=Hepatology |volume=41 |issue=5 |pages=1179–97 |year=2005 |pmid=15841455 |doi=10.1002/hep.20703}}</ref>
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| ===Signs and symptoms===
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| In general, symptoms and signs that suggest a rise in ICP including [[headache]], [[nausea]], [[vomiting]], ocular palsies, altered LOC, back pain and [[papilledema]]. If papilledema is protracted, it may lead to visual disturbances, optic atrophy, and eventually blindness.
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| In addition to the above, if mass effect is present with resulting displacement of brain tissue, additional signs may include [[pupillary dilatation]], abducens (CrN VI) palsies, and the [[Cushing's triad]]. Cushing's triad involves an increased [[systolic blood pressure]], a widened [[pulse pressure]], [[bradycardia]], and an abnormal respiratory pattern.<ref name="Sanders">Sanders MJ and McKenna K. 2001. ''Mosby’s Paramedic Textbook, 2nd revised Ed''. Chapter 22, "Head and Facial Trauma." Mosby.</ref> In children, a slow heart rate is especially suggestive of high ICP.
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| Irregular respirations occur when injury to parts of the brain interfere with the respiratory drive. [[Cheyne-Stokes respiration]], in which breathing is rapid for a period and then absent for a period, occurs because of injury to the [[cerebral hemisphere]]s or [[diencephalon]].<ref name="sgo">Singh J and Stock A. 2006. [http://www.emedicine.com/ped/topic929.htm "Head Trauma."] Emedicine.com. Accessed January 4, 2007.</ref> [[Hyperventilation]] can occur when the [[brain stem]] or [[tegmentum]] is damaged.<ref name="sgo"/>
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| As a rule, patients with normal blood pressure retain normal alertness with ICP of 25–40 mmHg (unless tissue shifts at the same time). Only when ICP exceeds 40–50 mmHg do CPP and cerebral perfusion decrease to a level that results in loss of consciousness. Any further elevations will lead to brain infarction and brain death.
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| In infants and small children, the effects of ICP differ because their cranial sutures have not closed. In infants, the [[fontanel]]s, or soft spots on the head where the skull bones have not yet fused, bulge when ICP gets too high.
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| ===Treatment===
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| The treatment for IH depends on the [[etiology]]. In addition to management of the underlying causes, major considerations in acute treatment of increased ICP relates to the management of stroke and cerebral trauma.
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| In patients who have high ICP it is particularly important to ensure adequate [[airway]], breathing, and [[oxygen]]ation. Inadequate blood oxygen levels ([[Hypoxia (medical)|hypoxia]]) or excessively high [[carbon dioxide]] levels ([[hypercapnia]]) cause cerebral blood vessels to dilate, increasing the flow of blood to the brain and causing the ICP to rise.<ref name="Su and Huh">Su F and Huh J. 2006. [http://www.emedicine.com/ped/topic3082.htm "Neurointensive Care for Traumatic Brain Injury in Children."] Emedicine.com. Accessed January 4, 2007.</ref> Inadequate oxygenation also forces brain cells to produce energy using [[Fermentation (biochemistry)|anaerobic metabolism]], which produces [[lactic acid]] and lowers [[pH]], also dilating blood vessels and exacerbating the problem.<ref name="orlando"/> Conversely, blood vessels constrict when carbon dioxide levels are below normal, so [[hyperventilating]] a patient with a [[ventilator]] or [[bag valve mask]] can temporarily reduce ICP. Hyperventilation used to be part of standard management of traumatic brain injuries but the constriction of blood vessels limits blood flow to the brain in a time when the brain may already be ischemic, and so is no longer widely used.<ref name="Shepherd">Shepherd S. 2004. [http://www.emedicine.com/med/topic2820.htm "Head Trauma."] Emedicine.com. Accessed January 4, 2007.</ref> Furthermore, the brain adjusts to the new level of carbon dioxide after 48 to 72 hours of hyperventilation, which could cause the vessels to rapidly dilate if carbon dioxide levels were returned to normal too quickly.<ref name="Shepherd"/> Hyperventilation is still used if ICP is resistant to other methods of control, or there are signs of [[brain herniation]] because the damage herniation can cause is so severe that it may be worthwhile to constrict blood vessels even if doing so reduces blood flow. ICP can also be lowered by raising the head of the bed, improving venous drainage. A side effect of this is that it could lower pressure of blood to the head, resulting in a reduced and possibly inadequate blood supply to the brain. Venous drainage may also be impeded by external factors such as hard collars to immobilise the neck in trauma patients, and this may also increase the ICP. Sandbags may be used to further limit neck movement.
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| In the hospital, blood pressure can be artificially raised in order to increase CPP, increase perfusion, oxygenate tissues, remove wastes and thereby lessen swelling.<ref name="Shepherd"/> Since [[hypertension]] is the body's way of forcing blood into the brain, medical professionals do not normally interfere with it when it is found in a head injured patient.<ref name="sgo"/> When it is necessary to decrease cerebral blood flow, MAP can be lowered using common [[antihypertensive agent]]s such as [[calcium channel blocker]]s.<ref name="orlando"/>
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| Struggling, restlessness, and seizures can increase [[metabolism|metabolic]] demands and oxygen consumption, as well as increasing blood pressure.<ref name="bech">Bechtel K. 2004. "Pediatric Controversies: Diagnosis and Management of Traumatic Brain Injuries." Trauma Report. Supplement to Emergency Medicine Reports, Pediatric Emergency Medicine Reports, ED Management, and Emergency Medicine Alert. Volume 5, Number 3. Thomsom American Health Consultants.</ref>.<ref name="Su and Huh"/> Analgesia and sedation (particularly in the pre-hospital, ER, and intensive care setting) are used to reduce agitation and metabolic needs of the brain, but these medications may cause low blood pressure and other side effects.<ref name="orlando"/>. Thus if full sedation alone is ineffective, patients may be [[paralysis|paralyzed]] with drugs such as [[atracurium]]. Paralysis allows the cerebral veins to drain more easily, but can mask signs of [[seizure]]s, and the drugs can have other harmful effects.<ref name="Su and Huh"/> Paralysing drugs are only introduced if patients are fully sedated (this is essentially the same as a [[general anaesthetic]])
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| Intracranial pressure can be measured continuously with intracranial transducers. A catheter can be surgically inserted into one of the brain's [[ventricular system|lateral ventricle]]s and can be used to drain CSF (cerebrospinal fluid) in order to decrease ICP's. This type of drain is known as an EVD (extraventricular drain).<ref name="orlando"/> In rare situations when only small amounts of CSF are to be drained to reduce ICP's, drainage of CSF via lumbar puncture can be used as a treatment.
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| [[craniotomy|Craniotomies]] are holes drilled in the skull to remove [[intracranial hematoma]]s or relieve pressure from parts of the brain.<ref name="orlando"/> As raised ICP's may be caused by the presence of a mass, removal of this via craniotomy will decrease raised ICP's.
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| A drastic treatment for increased ICP is [[decompressive craniectomy]], in which a part of the skull is removed and the [[dura mater]] is expanded to allow the brain to swell without crushing it or causing [[herniation]].<ref name="Shepherd"/> The section of bone removed, known as a bone flap, can be stored in the patient's abdomen and resited back to complete the skull once the acute cause of raised ICP's has resolved. Alternatively a synthetic material may be used to replace the removed bone section (see [[cranioplasty]])
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| A swollen optic nerve is a reliable sign that ICP exists.
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| ==Low ICP==
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| It is also possible for the intracranial pressure to drop below normal levels, though increased intracranial pressure is a far more common (and far more serious) sign. The symptoms for both conditions are often the same, leading many medical experts to believe that it is the change in pressure rather than the pressure itself causing the above symptoms.
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| ''Spontaneous'' intracranial hypotension may occur as a result of an occult leak of CSF into another body cavity. More commonly, decreased ICP is the result of lumbar puncture or other medical procedures involving the brain or spinal cord. Various medical imaging technologies exist to assist in identifying the cause of decreased ICP. Often, the syndrome is self-limiting, especially if it is the result of a medical procedure. If persistent intracranial hypotension is the result of a lumbar puncture, a "blood patch" may be applied to seal the site of CSF leakage. Various medical treatments have been proposed; only the intravenous administration of [[caffeine]] and [[theophylline]] has shown to be particularly useful.<ref name=Paldino>{{cite journal |author=Paldino M, Mogilner AY, Tenner MS |title=Intracranial hypotension syndrome: a comprehensive review |journal=Neurosurg Focus |volume=15 |issue=6 |pages=ECP2 |year=2003 |month=December |pmid=15305844 |doi= 10.3171/foc.2003.15.6.8|url=}}</ref>
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| ==References==
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| <references />
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| ==See also==
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| *[[Brain Trauma Foundation]]
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| *[[Traumatic brain injury]]
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| ==External links==
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| *Gruen P. 2002. [http://uscneurosurgery.com/infonet/glossary/m/monro%20kellie%20model.htm "Monro-Kellie Model" Neurosurgery Infonet. USC Neurosurgery]. Accessed January 4, 2007.
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| *National Guideline Clearinghouse. 2005. [http://www.guideline.gov/summary/summary.aspx?doc_id=3794&nbr=003020&string=intracranial+AND+pressure Guidelines for the management of severe traumatic brain injury.] Firstgov. Accessed January 4, 2007.
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| {{CNS diseases of the nervous system}}
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| {{Symptoms and signs}}
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| {{SIB}}
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| [[Category:Medical signs]]
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| [[Category:Neurotrauma]]
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| [[Category:Neurology]]
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| [[Category:Signs and symptoms]]
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| [[de:Hirndruck]]
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| [[fi:Kallonsisäinen paine]]
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| [[fr:Hypertension intra-crânienne]]
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| [[he:לחץ תוך-גולגולתי]]
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| [[ja:頭蓋内圧]]
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| [[pl:Wzmożone ciśnienie śródczaszkowe]]
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| [[pt:Pressão intracraniana]]
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| [[ro:Hipertensiunea intracraniana]]
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