Per Borghammer’s “aha” moment came nearly 20 years ago. The neuroscientist was reading a paper from researchers who were examining whether REM sleep behaviour disorder (RBD), a condition that causes people to act out their dreams and is often found in people who later develop Parkinson’s disease, could be an early form of the neurological condition.

Rather than starting with the brain, however, the team instead looked for nerve cell loss in the heart. Though Parkinson’s is historically associated with nerve cell depletion in the brain, it also affects neurons in the heart that manage autonomic functions such as heart rate and blood pressure. And, says Borghammer, “In all of these patients, the heart is invisible; it is gone.”

Not literally, of course. But in these people, the neurons that produce the neurotransmitter norepinephrine, which helps control heart rate, were so depleted that their hearts didn’t show up on scans using radioactive tracers. This kind of neuron loss is associated with Parkinson’s, but at the time, none of the people had been diagnosed with the disease and their brain scans seemed normal.

What struck Borghammer was that Parkinson’s didn’t seem to follow the same trajectory in everyone it affected: RBD strongly predicts Parkinson’s, but not everyone with Parkinson’s experiences RBD.

“I realised that Parkinson’s must be at least two types,” says Borghammer – when neuron loss starts outside the brain, eventually working its way in, and when neuron loss is largely restricted to the brain from the beginning. By 2019, Borghammer,…