More than 70 percent of people with quadriplegia also have obstructive sleep apnea (OSA), a disorder in which the upper airway narrows and closes repeatedly while people are asleep. A new study in The Journal of Physiology sought to understand why.

Researchers wanted to know how throat muscles are impacted by quadriplegia. In sleep labs in Sydney and Melbourne, Australia, they tested volunteers who had sleep apnea. Some of those volunteers also had quadriplegia.

A breathing mask was used to simulate the narrowing of the airway that is characteristic of sleep apnea. Electrodes inserted into the genioglossus, the largest muscle that surrounds the upper airway, measured reflex responses to the airway narrowing.

Researchers expected that those reflexive responses would be similar for volunteers with and without quadriplegia, since those researchers thought quadriplegia didn’t affect a patient’s airway.

Instead, study results showed some people with quadriplegia, “Key protective throat reflexes were absent, or indeed led to the switching off (instead of activation) of the genioglossus muscle. These findings help to explain why OSA is so common in those with tetraplegia and sleep apnea.”

The study was done on volunteers who were awake at the time, so researchers can’t say whether or not those reflexive responses would be the same when volunteers were sleeping.

“Regardless,” researchers said in a study announcement, “observations provide valuable insight into upper airway reflex responses, and pave the way for future physiological sleep reflex studies looking to develop effective and tolerable targeted therapies for those with [quadriplegia] and sleep apnea.”