Stimulating either of a pair of crucial nerves that carry messages from the brain to several major organs could be an effective way to treat people with severe depression.

An international team of researchers conducted a clinical trial on 493 adults whose major depression hadn't previously responded to treatment. Participants were fitted with a device capable of stimulating one of their vagus nerves, which was then activated in half the group to transmit signals to areas of their brain that regulate moods.

Assessments were conducted for roughly ten months, after which participants who received the stimulation treatment showed improvements in their depressive symptoms, in their quality of life, and in their ability to carry out the daily tasks of everyday life.

"On average, each patient had already tried 13 treatments that failed to help them before they enrolled in the trial, and they had spent more than half of their lives sick with depression," says Charles Conway, a professor of psychiatry at Washington University in St Louis.

"But despite that super-high level of sustained illness, we still see statistically significant, measurable improvements in depressive symptoms, quality of life and functional outcomes."

While VNS has shown promise for treating depression before, evidence of its success hasn't always been clear cut, leaving treatment difficult to afford for many in the US on insurance cover.

There are some reasons to be cautious. Based on the study's primary measure of depressive symptoms, the Montgomery-Åsberg Depression Rating Scale (MADRS), there were no differences between the treatment and non-treatment groups.

Very few participants from either group reported a full remission of their depression.

Even with those caveats in mind though, the study's results are promising, particularly when it comes to helping people escape the mental paralysis that can come along with major depression.

"What's really important here is that patients themselves were reporting that their lives were improving," says Conway.

"They're saying they are seeing meaningful improvements in their ability to function and live their lives."

Up to 30 per cent of those with severe depression fail to benefit from standard antidepressants, with their mental health problems compounded by an increased risk of suicide, hospitalization, and disability.

The same clinical trial is being used to see if VNS can make a difference for people with bipolar too. In addition, the researchers want to continue the trial for another four years, and see if the benefits are particularly noticeable in certain groups of people.

"The nice thing about vagus nerve stimulation, we know from other studies, is that when the patient responds, the effects usually stick," says Conway.

The research has been published in two papers in Brain Stimulation, here and here.