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The pathophysiology of brain edema and elevated intracranial pressure

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The pathophysiology of brain edema and elevated intracranial pressure Powered By Docstoc
					           The pathophysiology of brain edema
            and elevated intracranial pressure
                                                     ANTHONY MARMAROU, PHD



                                                                    s TRAUMATIC BRAIN EDEMA—VASOGENIC


T
          he contribution of brain edema to brain
          swelling in cases of traumatic injury,                      OR CELLULAR?
          ischemia, and tumor remains a critical                    By definition, edema is an abnormal accumulation
          problem for which there is currently no                   of fluid within the brain parenchyma; it is subdivid-
effective clinical treatment. It is well documented                 ed into vasogenic and cytotoxic types. Vasogenic
that in head injury, swelling leads to an elevation in              edema is defined as fluid originating from blood ves-
intracranial pressure (ICP), which is a frequent                    sels and accumulating around cells. Cytotoxic
cause of death, and to very poor prognosis in sur-                  edema is defined as fluid accumulating within cells
vivors. This swelling process has been classified into              as a result of cell injury. The most common cytotox-
four distinct degrees of severity based on studies of               ic edema occurs in cerebral ischemia. Neurotoxic
the Traumatic Coma Data Bank. Of great impor-                       edema is a subtype of cytotoxic edema caused by
tance is the fact that the degree of swelling assessed              high levels of excitatory amino acids. Heretofore,
on the first CT scan, obtained soon after injury, is                the edema specific to traumatic brain injury has gen-
highly correlated with outcome (P < .0002).                         erally been considered to be of “vasogenic” origin,
                                                                    secondary to traumatic opening of the blood-brain
s BRAIN SWELLING—EDEMA OR VASCULAR                                  barrier. However, all three forms of edema can coex-
  ENGORGEMENT?                                                      ist, and their relative contributions to brain swelling
Our experimental and clinical studies provide                       and elevated ICP have not been identified. This is a
strong evidence that edema is primarily responsible                 critical problem, as effective treatment will clearly
for the swelling process. For the past several decades,             depend on the type of swelling.
it has been generally accepted that the swelling                        Our own studies in this area are in sharp contrast
process accompanying traumatic brain injury is                      to the general belief that traumatic brain injury
mainly due to vascular engorgement, with blood                      results in a predominantly extracellular edema sec-
volume providing the increase in brain bulk and                     ondary to blood-brain barrier opening. Although a
subsequent rise in ICP. Edema was thought to play a                 vasogenic component may be present, we strongly
minor role. However, our recent findings indicate                   suspect that the type of swelling in traumatic brain
that edema, not vascular engorgement, is responsi-                  injury with or without associated mass lesion is pre-
ble for brain swelling and that blood volume is actu-               dominantly a cellular edema. A lack of barrier open-
ally reduced following traumatic brain injury. Thus,                ing in the presence of continued swelling has been
it is important to shift our attention to brain edema               noted in our clinical studies of head-injured patients
and to understand the pathophysiologic mecha-                       in whom magnetic resonance “water maps” were
nisms responsible for water movement into brain.                    obtained with gadolinium challenge. Experimentally,
                                                                    we have strong evidence that the type of swelling in
                                                                    diffuse injury is predominantly cellular.
From the Department of Neurosurgery, Virginia Common-
wealth University Medical Center, Richmond, Va.                     s IONIC DYSFUNCTION IN BRAIN INJURY
Address: Anthony Marmarou, PhD, Department of Neuro-
surgery, Virginia Commonwealth University Medical Center,           It is well documented that ionic dysfunction occurs
1001 East Broad Street, Suite 235, Richmond, VA 23219; e-           with traumatic brain injury and that extracellular K+
mail: amarmaro@vcu.edu.                                             is transiently increased as a result of the depolariza-


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tion synchronous with mechanical insult. This loss              lular swelling and cytotoxic edema, which we have
of ionic homeostasis should be accompanied by a                 shown to be the primary contributor to raised ICP.
concomitant movement of sodium. The seminal                     In traumatic brain injury, the initiating factors,
studies by Betz et al and Gotoh et al measured uni-             which result in the movement of ions, may differ
directional movement of sodium into brain follow-               from those primarily responsible in ischemia. For
ing an ischemic injury, and work by other investiga-            example, ATP reduction may not be due to
tors has demonstrated a clear relationship between              decreased cerebral blood flow since blood flow in
tissue water content and sodium accumulation. As                traumatic brain injury persists and delivery of sub-
we have demonstrated a predominantly cellular                   strate is maintained.
swelling, the extension of our work to the study of
ionic movement is fundamental to a deeper under-                s LABORATORY MODELS OF TRAUMATIC BRAIN
standing of the formation of traumatic brain edema.               INJURY WITH BRAIN SWELLING
   Traumatic brain injury triggers a cascade of
events, including mechanical deformation, neuro-                The study of traumatically induced swelling in the
transmitter release, mitochondrial dysfunction, and             laboratory has been difficult, in part because of a
membrane depolarization, that leads to alterations              lack of models that produce marked, rapid swelling
in normal ionic gradients. Excitatory amino acids               and, most important, a steady rise in ICP. Fluid per-
released via mechanical deformation and mem-                    cussion, or direct dural impact, results in a sudden
brane depolarization activate ligand-gated ion                  rise in blood pressure that is sufficient to breach the
channels, which allow ions to move down their                   blood-brain barrier and is not suitable for the study
electrochemical gradients. In addition, membrane                of edema produced by closed head injury. Moreover,
depolarization resulting from ionic flux and trauma             ICP increases only transiently and declines over
triggers voltage-sensitive ion channels, providing              time. Similarly, the classic model of subdural
further routes for ionic movement. These ionic dis-             hematoma, which has been used by many investiga-
turbances are identified by an increase in extracel-            tors, also produces only a transient rise in ICP fol-
lular potassium ([K+]ecs) with a concomitant                    lowed by a gradual recovery toward baseline.
decrease in extracellular sodium ([Na+]ecs), calcium,              For diffuse injury, we solved this problem with
and chloride.                                                   our development of a rat impact-acceleration model
   Restoration of ionic homeostasis is accomplished             that develops marked swelling, a profound diffuse
via cotransport and countertransport processes such             axonal injury, and a steadily rising ICP when sec-
as the Na+-K+ ATPase, Na+/K+/2Cl– cotransporter,                ondary insults are superimposed upon the mechani-
Na+-H+ transporter, and Na+-Ca2+ exchanger.                     cal trauma. For mass lesions, we found that super-
However, if the injury is severe, or if secondary               imposing a controlled subdural hemorrhage follow-
insults occur, disruption of ionic homeostasis persists         ing impact-acceleration injury resulted in similar
as the cotransport and countertransport processes are           effects. This combination, which mimics the clini-
impaired and become incapable of returning ion                  cal scenario better than a subdural hemorrhage
concentrations to their normal levels. Moreover, in             alone, results in a remarkable, steady increase in
the absence of adequate levels of ATP resulting from            ICP to greater than 50 mm Hg .
either an ischemic reduction in cerebral blood flow
                                                                s MAGNETIC RESONANCE TECHNOLOGY:
or insufficient production of ATP due to mitochon-
                                                                  DIFFUSION-WEIGHTED IMAGING TECHNIQUES
drial dysfunction, energy-dependent ion pumps and
                                                                  AND PROTON SPECTROSCOPY
cotransport and countertransport processes are inef-
ficient in counteracting the normal dissipative flux            It is not possible to differentiate between vasogenic
of ions down their electrochemical gradients.                   (extracellular) and cytotoxic (cellular) edema by
   We hypothesize that the net balance of ionic                 conventional tissue water measurement. In vivo dif-
movement that accompanies brain injury results in               fusion-weighted imaging is a magnetic resonance
the movement of cations out of the extracellular                technique that, by employing strong magnetic field
space into cells. The movement of sodium and cal-               gradients, is sensitive to the random molecular
cium is passively followed by chloride to maintain              translation of water protons. Maps of the apparent
electroneutrality, and is followed isosmotically by             diffusion coefficient (ADC) can be derived from a
water. If sustained, ionic disturbances result in cel-          series of diffusion-weighted images obtained with


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different magnetic fields. Recent findings in experi-               ment, such calcium loading leads to mitochondrial
mental ischemia models suggest that ADC values                      permeability transition, which is linked to overt
can provide earlier and more specific information                   mitochondrial failure. The accuracy of this assump-
about tissue damage and characteristics of edema.                   tion is supported by several studies. More impor-
Several studies have adopted this concept in distin-                tantly, studies by Povlishock et al have shown that
guishing the type of edema in ischemia and trau-                    cyclosporin A, which blocks mitochondrial perme-
matic brain injury. The application of such new                     ability transition, translates into significant neuro-
imaging techniques can quantify the temporal                        protection.
changes and document the type of edema that is                         N-acetylaspartate (NAA) was discovered in 1956
occurring during both the acute and the late stages                 by Tallan, More, and Stain and is represented by the
of edema development.                                               largest peak in a proton spectrum (1H MRS). It is
                                                                    synthesized in mitochondria from L-aspartate and
s N-ACETYLASPARTATE ASSESSED BY PROTON                              acetyl-CoA in a reaction catalyzed by an N-acetyl-
  SPECTROSCOPY AND MITOCHONDRIAL DYSFUNCTION                        transferase. NAA has been found histochemically to
                                                                    be a constituent of neurons and axons, with lesser
We hypothesize that mitochondrial dysfunction                       amounts in glial cells. Large numbers of studies show
prevents the restoration of ionic and cell volume                   NAA absent or reduced in brain tumor (glioma),
homeostasis. Hovda’s group has clearly implicated                   ischemia, degenerative disease, and inborn errors of
ionic shifts as the major mechanisms for cellular and               metabolism, and it is accepted fact that NAA levels
organelle swelling. Moreover, Muizelaar has recent-                 correspond to tissue damage. Studies of NAA in
ly demonstrated in rat brain suspensions that mito-                 severe brain injury are relatively few, and the tempo-
chondrial function was profoundly reduced after                     ral course of NAA changes in trauma in association
traumatic brain injury and that this was largely cal-               with ADC values has not been studied. Because
cium-dependent. Povlishock has posited that the                     NAA is synthesized by the mitochondria, it is rea-
shear and tensile forces of injury induce mechanical                sonable to posit that reduced tissue NAA will be
poration of cell membranes that can cause an “exci-                 associated with regions of low ADC, low ATP, ionic
tatory amino acid storm” that leads to intracellular                dysfunction, and brain edema. This report describes
calcium influx with subsequent mitochondrial load-                  the most recent information available on NAA
ing. It is envisioned that in a permissive environ-                 reduction in human head injury.




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