"That is the big question. It makes no anatomical sense that patients with spinal cord injury should have sleep apnoea," says Professor David Berlowitz, the newly-created University of Melbourne Professor of Physiotherapy at Austin Health.
It's the research question that has plagued Prof Berlowitz for 22 years. "And I still don't know!" he says.
He is one step closer to solving the mystery, however, after the publishing the first research to find a physiological change that explains why so many people with SCI go on to develop obstructive sleep apnoea (OSA): evidence of anatomical changes to the reflex control of largest tongue muscle, the genioglossus, in people who have both a SCI and OSA.
He and his collaborators - many from the collaborative powerhouse of Austin Health, the Institute for Breathing and Sleep (IBAS) and the University if Melbourne, compared people with SCI who have OSA with able-bodied people who had the same degree of sleep apnoea. What they found was surprising:
"Milliseconds before a person takes a breath, the muscles of the upper airway stiffen, but in spinal cord injured patients with sleep apnoea, we found that the opposite occurred: their tongue muscle actually softens," says Prof Berlowitz.
The research has been published this week in the highly-regarded Journal of Physiology, with an accompanying editorial in recognition of its significance. One of Prof Berlowitz's students, Laura Gainche, is co-lead author.
Part of what makes the study so significant lies in the revelation that the OSA seen in so many spinal patients (somewhere between 60 and 70 per cent of all people with a SCI go on to develop OSA) has an entirely different cause to the OSA seen in able-bodied people.
"The normal treatment for sleep apnoea, CPAP [continuous positive airway pressure], is tolerated by 50 per cent of able-bodied people, but only 25 per cent of spinal cord injured people, and compared to able-bodied people, there's only tiny amount of air pressure required; it's very gentle CPAP," he says.
Prof Berlowitz says that this discovery will go on to inform other aspects of his long-term Sleep Health in Quadriplegia (SHiQ) research program, one of which involves testing the known treatments for OSA - such as CPAP - to see how effective they really are, and developing ideas for new treatments.
And of course, the solution to the bigger mystery still beckons: Why does the genioglossus muscle change in people who have a spinal cord injury and sleep apnoea?
Austin Health is building a new Centre of Excellence in Respiratory and Sleep Medicine, which will bring together the Department of Respiratory and Sleep Medicine at Austin Health and the Institute of Breathing and Sleep (IBAS), to create a world-class centre. To support research excellence like this, consider making a donation.