J Thorac Cardiovasc Surg 2001;121:968-970
© 2001 The American Association for Thoracic Surgery
Cardiopulmonary Support and Physiology |
Commentary
Hart G. W. Lidov, MD, PhD
Department of Pathology (Neuropathology), Children's Hospital, 300 Longwood Ave, Boston MA 02115
12/1/114100doi:10.1067/mtc.2001.112934
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Introduction
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In this study Rimpiläinen and colleagues have tested the hypothesis that an N-methyl-D-aspartate (NMDA) receptor blocking agent, memantine, might have a neuroprotective effect during deep hypothermic circulatory arrest (HCA). Their results, based on somewhat small experimental groups, suggest that this agent does not have the hoped for effect. My comments are focused on neurohistologic assessment and grading, which is a major instrument in this study and similar to procedures used by other groups with experimental deep HCA protocols.
Reliable and refined assessment and grading can only be as good as the initial histologic preparations, or at least this is a limiting factor. Benchmark studies in the 1960s and 1970s established that perfusion fixation followed by a period of in situ fixation before any mechanical manipulation of brain and spinal cord is markedly superior in terms of histologic results to the immersion fixation technique used in the present study.
1 Removal and handling before fixation, however gentle, produces artifacts, specifically vacuolation and dark neurons (so-called "blue-neuron" artifact). This happens even in neurosurgical biopsy specimens. A postmortem interval before fixation may also produce varying degrees of autolytic change depending on the precise conditions of time and temperature. In defense of immersion fixation, it should be acknowledged that this is essentially the method, whatever its limitations, routinely used in human autopsies, but this only underscores the handicap under which the clinical neuropathologist must function. Also, some studies, although not the present one, may combine histologic findings with techniques that are incompatible with fixation, and working with "suboptimal" material is unavoidable.
Awareness of some of the histologic vagaries indicates the pitfalls in constructing a histologic scoring system. For one thing, a satisfactory system must not mix effects that are probably artifact and genuine hypoxic-ischemic injury. In the present study it is possible that grade 1 lesions represent artifactual change—vacuolation and dark neurons that result from removal and bisection of unfixed brain and subsequent immersion fixation.
A grading scheme should probably consist of steps that are reliably distinguishable and are accepted or can be demonstrated to result from increasingly severe degrees of the same underlying pathologic process or insult. Presumably one does not arrive at the more severe stages without passing through the milder stages. A clue to the reliability of the grading scheme may be that the milder grades are present at the periphery of more severe lesions. It adds considerable complexity to have an element in this sequence that is an "independent" feature. Systems like this are used, for example, in some brain tumor grading schemes. In those cases, typically each feature is graded as present or absent and a "severity score" is the sum of the features present. Scoring this way raises its own questions: Are all features equally significant?
For hypoxic-ischemic injury several groups have adopted schemes that give progressively higher scores to more severe lesions. 2,3 The present authors' system varies somewhat from the usual practice. Their grades are as follows: 1 = edema; 2 = hemorrhages, more severe; and 3 = infarct, most severe. At the same time, Table III, in which given areas have scores greater than 3, seems to imply treating these as "features" rather than "grades." Some of the underlying assumptions are arguable or at least not clear.
Specifically, an unusual intermediate grade, "2 = hemorrhages," has been tucked in. Hemorrhages are not typically a part of hypoxic-ischemic injury. They may result from effects related to the bypass procedure, altered coagulation status, and, of course, there can be hemorrhage into infarcted tissue. However, this last should be subsumed under the infarct in terms of grading. It is possible that they are an acute phenomenon, but introducing a mix of acute (hours) and subacute (days) changes complicates interpretation. That an acute change should be present at 1 week is surprising.
My suspicion is that the histologic material supports only 2 grades, "no damage" and "definite hypoxic-ischemic injury," the authors' grades 0 and 3, respectively. Fortunately, it appears that restricting evaluation to this more coarse level would not alter the authors' conclusions. Within the sensitivity of the methods, no significant neuroprotective effect of memantine was discerned. Still, the authors previously found their evaluation sufficiently sensitive to recognize a positive neuroprotective effect of a different agent in a technically similar study. 4
It should be recognized that there are difficulties facing any investigator using neurohistologic assessment in large animal models. Many parameters are well studied in rodents and in paradigms where the aim is to produce hypoxic-ischemic injury, but these are not so well worked out in the intentionally milder conditions used in experimental surgical models of deep HCA or in dogs and piglets. Using numbers of large animals to work out baseline patterns for variables that may affect damage, such as time after injury and age of animals, is not practical. Manipulations such as placement of indwelling microperfusion catheters, as in the present study, or thermal probes further perturb intracranial pressure, cerebral blood flow, and the final histologic damage. And the experimenter is trapped in the "Catch-22" that damage that is survivable is only discernible by means of standard techniques after several days and will be relatively mild; damage sufficient to be readily identified at short intervals (such as hemorrhages in this study or acute vacuolation described in other studies 5) is likely to preclude survival and is hardly a realistic model with which to refine surgical procedures. The best neurohistology achievable can be some help with this.
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References
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Redmond JM, Gillinov AM, Zehr KJ, Blue ME, Troncoso JC, Reitz BA, et al. Glutamate excitotoxicity: a mechanism of neurologic injury associated with hypothermic circulatory arrest. J Thorac Cardiovasc Surg 1994;107:776-86.[Abstract/Free Full Text]
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Anttila V, Rimpilainen J, Pokela M, Kiviluoma K, Makiranta M, Jantti V, et al. Lamotrigine improves cerebral outcome after hypothermic circulatory arrest: a study in a chronic porcine model. J Thorac Cardiovasc Surg 2000;120:247-55.[Abstract/Free Full Text]
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Introduction
References