For people with Parkinson’s disease, deep brain stimulation can calm the tremors and tame the unwanted movements that come with the progressive neurodegenerative disease. Electrodes implanted in the brain, controlled by a device placed under the skin of the chest and equipped with an on-off switch, can ease troubling symptoms and synchronize complex motions.

A new paper reports a worrying development: Nine proficient swimmers lost their ability to swim after DBS surgery for Parkinson’s, even though the implant improved other movements — walking, for one — that require coordination of the limbs. Why swimming mastery disappeared remains a mystery, but the doctors who described the cases Wednesday in Neurology wanted to sound an alarm right away.

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