Demyelination and Deoxygenation?
Abstract & Commentary
By Joseph E. Safdieh, MD
Assistant Professor of Neurology, Weill Cornell Medical College
Dr. Safdieh reports no financial relationships relevant to this field of study.
Synopsis: In an animal model, hypoxia may play a significant role in causing tissue damage and neurologic deficits in demyelinating disorders, and early oxygen therapy may improve function.
Source: Davies AL, et al. Neurological deficits caused by tissue hypoxia in neuroinflammatory disease. Ann Neurol 2013;74:815-825.
Multiple sclerosis is a neurologic disorder that causes multifocal episodes of central nervous system demyelination as well as delayed neurodegeneration with many possible neurologic symptoms and signs. However, various previous studies have concluded that inflammation alone may be sufficient to cause clinical deficits without demyelination. The authors of this study attempt to prove that tissue hypoxia plays a significant role in the development of neurologic deficits in an animal model of demyelination.
The authors induced experimental autoimmune encephalomyelitis (EAE) in rats by immunizing them with recombinant myelin oligodendrocyte glycoprotein. The rats developed typical EAE with an early asymptomatic phase, the first peak of symptoms, remission, and relapse. Control and experimental animals were studied. Animals were grouped by disease phase and were injected with pimonidazole 4 hours prior to terminal anesthesia. Pimonidazole readily enters CNS tissues and is converted to a form that permanently binds hypoxic tissue but does not bind to tissues with normal oxygenation. A cohort of the animals underwent in vivo oxygen probe measurement as well to detect spinal cord tissue oxygenation at various phases of EAE.
The authors looked at spinal cord tissue in EAE and control animals. Animals with EAE demonstrated severe inflammation of the spinal cord in association with the development of ascending weakness. Activated microglia were detected primarily in the white matter and meninges of the EAE animals, but also noted in the gray matter, and their presence correlated with the degree of neurologic deficit. Significant pimonidazole labeling was detected in the lumbar spinal cord of EAE animals, and the intensity of labeling correlated with severity of neurologic deficit, and returned to normal during remission. Pimonidazole labeling was present in both the gray matter and white matter of EAE animals, and was not detected in control animals. Of note, no demyelination was noted in the initial attack of EAE by luxol fast blue staining. From these results, the authors concluded that the inflamed tissue in EAE spinal cord, but not controls, was hypoxic, based on pimonidazole labeling.
To further study this phenomenon, the authors then performed in vivo spinal cord oxygen probe measurements in anesthetized animals in the EAE and control groups. They determined that controls had normal spinal cord oxygenation whereas EAE animals had significant spinal cord hypoxia, which normalized during remission and became hypoxic again during relapse. Additionally, spinal cord vessel size differed significantly between the EAE and control groups, with larger diameter vessels in the EAE groups. This would be consistent with vascular compensation for tissue hypoxia. Vessel size was greatest during the relapse phase in EAE animals.
The authors then performed a study treating the animals with normobaric oxygen therapy (95% oxygen) vs room air acutely at onset of first neurologic deficit. These animals were also terminally anesthetized after pimonidazole injection. Notably, labeling for pimonidazole was absent in the oxygen-treated animals suggesting normal spinal cord oxygenation, and this was further demonstrated in vivo with spinal cord oxygen probe measurements. Another finding was that acute oxygen administration improved the expression of neurologic deficits in EAE animals compared to those administered room air. Continuous oxygen therapy for the next 7 days led to persistence of functional improvement.
This study is important for several reasons. First, it demonstrates that inflammatory changes in EAE are associated with tissue hypoxia in the absence of demyelination. Second, it demonstrates that acute oxygen therapy at the onset of deficits mitigates these tissue changes and leads to functional improvement. These findings are provocative and should lead to further study of the role of tissue hypoxia in multiple sclerosis. Perhaps a study of acute oxygen therapy in patients with relapses of multiple sclerosis might be undertaken based on these findings. That said, caution should be used before applying the results of animal studies of EAE to humans with multiple sclerosis without further clinical investigation in a placebo-controlled trial.