Last time I introduced the topic of the ischemic penumbra, which is the zone of living brain tissue between normal brain and the central dead zone of a stroke. The brain tissue in the penumbra is receiving enough blood to remain alive, but not enough to continue to function.
Much of the penumbra seems quite fragile. We know, for example, that a window of only a few hours exists between the onset of a stroke and the ability of clot busting drugs such as Tissue Plasminogen Activator (TPA) to help. In other words, if a patient receives TPA soon enough, blood flow resumes and much of the penumbra will survive. After a few hours, however, even though TPA may still restore blood flow, much of the penumbra has already died.
The late Dr. Richard Neubauer believed that some of the ischemic penumbra survived long term. He based his belief, in part, on clinical observations that patients occasionally recovered substantial function following carotid bypass surgery long after a stroke. As far as I am aware, Dr. Neubauer was the first person who tried to revive the stroke penumbra with hyperbaric oxygen, and he first published his ideas in a widely read journal in 1990. Although he wasn’t the first to call the surviving brain cells in the penumbra “idling neurons,” he certainly popularized the term.
The penumbra theory of idling neurons forms the rationale for hyperbaric oxygen therapy of stroke patients. These neurons are alive but don’t get enough oxygen to develop electrical activity. Like all tissue suffering oxygen starvation, repeated treatment with hyperbaric oxygen stimulates new blood vessels to grow and partially restore the oxygen supply. Once the oxygen level reaches a threshold, the idling neurons resume electrical activity and begin to function. Because of brain-tissue plasticity, these brain cells can even learn some of the function of adjacent neurons that have died.
The penumbra theory is also the rationale for hyperbaric oxygen treatment of traumatic brain injury and cerebral palsy. Like stroke, both these disorders are characterized by central dead zones surrounded by zones of idling neurons.