Ask any mountaineer or miner and they'll tell you that time in a low-oxygen environment can be a dangerous business – it can cause altitude sickness and other health problems. But for people with spinal injuries, short periods of hypoxia may be a promising therapy for regaining mobility.
About 59 per cent of people who experience trauma to the spinal cord suffer incomplete injuries, meaning that some neural pathways between their brain and the neurons that extend to their limbs remain intact.
"Strengthening those weak, lingering connections is key to regaining movement," says engineer Randy Trumbower, at Emory University in Atlanta, Georgia. But, often, those networks cannot be fully activated through physical therapy alone.
Keen on finding a better way to awaken those dormant connections, he teamed up with Gordon Mitchell at the University of Wisconsin in Madison, and his colleagues, who were studying the effects of sleep apnoea – or interrupted breathing – in mice.
Base camp to summit
Mitchell had found that periodic exposure to low oxygen levels triggers the release of the brain chemical serotonin. This in turn prompts the production of a growth factor that coaxes new connections to form between neurons present in the brainstem and spinal cord that regulate breathing.
Trumbower wondered whether there was a way to control the release of this neural growth factor to rejuvenate an injured spine. The idea led his team to run the first test looking into the benefit of controlled hypoxia on people with neurological conditions.
The team recruited 19 people with a wide variety of spinal cord injuries, all of whom had difficulty moving around for long periods of time without assistance – from a walkler for example – but could walk at least one step unassisted. Half of the subjects wore a breathing mask for 40 minutes a day for five days, alternating between inhaling 9 per cent oxygen for 90 seconds and normal air with 21 per cent oxygen for 60 seconds.
"If you were to convert 9 per cent oxygen to altitude, you can think of yourself being at about 26,000 feet – at Mount McKinley but not quite Mount Everest," says Trumbower. He adds that participants do not experience discomfort because they are not exposed to the hypoxic air for long. "You're continuously alternating between the summit and base camp," he says.
Faster and further
At the end of the five days, the participants who received low-oxygen treatment walked a 10-metre course an average of 3.8 seconds faster than those who had inhaled air with normal oxygen levels.
The team also found that subjects who attempted to walk for 30 minutes after each of their daily treatments not only walked faster at the end of the regimen, but also walked farther – averaging an extra 100 metres during a 6 minute walk, compared with those who walked for 30 minutes each day after receiving normal oxygen.
"It may seem a little counterintuitive, but these findings make very good sense," says neurosurgeon Michael Fehlings at the University of Toronto, Canada. "Because hypoxia acts as a stress to the nervous system, it can elicit a beneficial response if controlled correctly."
He says he is curious to see if the treatment will provide benefit for people who have more severe spinal cord trauma and cannot walk unassisted.
Although the therapeutic effects from this trial did not last for more than two weeks, Trumbower is hopeful that longer treatment regimens will produce lasting effects.
"Our hope is that this therapy can be used for months, not just days, and enhance the improvements of physical therapy."
Journal reference: Neurology, in press
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