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.
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