A mystery illness said to have caused dozens of deaths in one Canadian province has turned out to be not such a mystery after all, with neurologists looking at 25 cases in detail.

In 2019, a neurological syndrome of unknown cause (NSUC) was first identified in east coast New Brunswick, which appeared to cause rapid-onset dementia and, in some cases, death.

Symptoms also included muscle atrophy, ataxia, myoclonus, hallucinations, behavioural changes, pyramidal signs and painful sensory sensations, with testing apparently ruling out other known disorders.

By April 2021, 48 people were identified as having symptoms of this mystery illness — labelled by the public health authority as NSUC — including six who had died, according to a report in JAMA Neurology.