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Laurance Johnston, Ph.D.

Sponsor: Institute of Spinal Cord Injury, Iceland



Hypoxia is a state in which an adequate oxygen supply is lacking. One of the more common examples of a hypoxia condition is the high-altitude sickness suffered by recreational skiers and mountain climbers due to the rarified oxygen of higher elevations. Although hypoxia can cause a variety of problems, non-damaging intermittent hypoxia has actually been used by athletes to foster physiological adaptation similar to high-altitude training.

Evidence suggests that intermittent hypoxia has the potential to induce spinal plasticity (i.e., the ability of neurons to adapt in a function-restoring fashion), strengthening spared neuronal pathways after injury. Because in most injuries some neurons survive and still traverse the lesion site, this evidence has important implications for augmenting function after injury.

Physiologically, it is thought that intermittent hypoxia initiates the production of serotonin, a neurotransmitter which stimulates the synthesis of a key neuronal growth factor. In turn, this growth factor enhances the survival of existing neurons, and stimulates the development of new neurons and the synapses that relay signals to neighboring cells. This cascade of events helps to create function-restoring connections within the spinal cord.

Following up on studies demonstrating that intermittent hypoxia increases ankle strength after incomplete SCI, Dr. Heather B. Hayes and colleagues (USA) examined the impact of the therapy on walking ability in 19 subjects with incomplete SCI. Age ranged from 24 to 77, all but three were men, and the time lapsing since injury varied from three to 27 years. All subjects had either ASIA C or ASIA-D incomplete injuries (see appendix), ranging from the cervical C2 to the thoracic T12 level, and some walking ability.

Subjects were randomized into two treatment groups. In the first group, half the subjects received intermittent hypoxia through a breathing mask for five consecutive days. Each daily session consisted of fifteen 90-second hypoxic (i.e., short-burst, low-oxygen) episodes, each separated by a 60-second interval of normal oxygen. These subjects were compared with control subjects who breathed just normal oxygen through their masks, i.e., no hypoxic episodes. After a two-week interval, the groups were reversed, i.e., the controls now received intermittent hypoxia, and the initial hypoxia group normal oxygen.

In the second group, the experimental protocol was essentially the same, except daily treatment was followed by 30 minutes of overground walking therapy.

These experimental groups were evaluated by two assessments: 1) how fast one was able to walk 10 meters (i.e., speed), and 2) how far one could walk in six minutes (i.e., endurance). The results indicated that intermittent hypoxia treatment improved both walking speed and endurance. The impact was greatest when treatment was combined with overground walking therapy. For example, subjects who received hypoxia treatment combined with the walking program improved their endurance by 250% compared to controls. No adverse side effects were observed.