HYPOXEMIC AND ISCHEMIC INJURY

Robert Kagan, M.D., F.C.A.P.

Medical Director of the MRI Scan Center, Medical Diagnostic Services and Diagnostic of America, and internationally know.

Anoxic-ischemic injury is a major cause of morbidity and children, as those who survive the initial events are frequently left with residual long-termed neurological sequela. As sophisticated cross-sectional imaging techniques have been developed to optimize the early visualization of the anoxic-ischemic injury, the imaging specialist has become more actively involved in the initial evaluation of this group of patients. Early manifestations vary in children of different ages; therefore, is important for the radiologist to recognize the spectrum of findings that can be seen.

The mechanisms underlying the development of brain damage from anoxia/ischemia are complex. Following a global insult, though initially the whole brain is subject to the insult, not all areas of the brain are equally injured. Depending on certain factors, such as the nature and duration of the insult and the level of maturity of the brain at the time of injury, different and distinct patterns of damage can develop. For instance, the relative maturity of the brain at the time of the insult to determine location of watershed infarction, and whether hemorrhage and subsequently gliosis will occur.

Hypoxemia is defined as insufficient blood oxygenation and ischemia as insufficient blood flow to a tissue bed. Hypoxemic-ischemic injury (H11) encompasses injuries resulting from either hypoxemia or ischemia. H11 is seen in the pediatric population secondary to accidental injuries, abuse, and hematologic abnormalities such as sickle cell disease; as a result of hypovolemia during surgery; and in association with injuries of the late prenatal and perinatal periods.

And although it is considered controversial, hyperbaric oxygen (HBO) may someday become routinely used to minimize the amount of damage to the brain following hypoxic-ischemic injury in childhood. The imaging plays a key role, not only to demonstrate the presence and location of the extent of brain injury. Brain imaging can show objective evidence of the efficacy of HBO. Documentation is essential for any new treatment modality in the field of medicine. With any new treatment there are always skeptics, and rightfully so. Irrespective of the value of any new treatment, without appropriate scientific documentation, the medical community will not accept it nor will insurance companies reimburse for it. This is the reality in today’s world. In regard to brain repair with particular reference to HBO, this documentation is mandatory and is now possible.

Functional imaging modalities include nuclear medicine, single photon emission computerized tomography (SPECT) and also functional magnetic resonance imaging methods such as diffusion – weighted imaging (D.W.I.). By modifying basic pulse sequences with MRI, they can be made to be very sensitive to the diffusion of water. Such "diffusion images" highlight areas of restricted water diffusion, such as those found on the inside of cells swollen with cytotoxic edema due to acute ischemia. This restricted motion of water leads to high signal on diffusion images allowing the rapid diagnosis of acute cerebral ischemia while it may still be reversible. This is one way to identify the "ischemic penumbra", which is the area of the brain at risk for extension of the initial brain injury. In addition to this functional information MRI gives the most detailed anatomic information, but it is very expensive.

Much of the data presented here today shows images produced by single photon emission computerized tomography currency (SPECT) which is found in many cardiologist offices and is frequently a part of the imaging department of most hospitals. The procedure involves an infinitesimal dose of an isotope, technetium-99m. About 40 minutes later, the patient lies still, but non-confined and a large cylinder moves 360-degrees around the head. It is necessary that the patient remain still. Under these circumstances, photons are emitted and images are produced by way of the computer. This is primarily done to demonstrate blood flow, but since the isotope crosses the blood brain barrier, it also measures metabolism. Sequential SPECT imaging gives excellent documentation of any brain therapeutic intervention and is used extensively for the evaluation of hyperbaric oxygenation.