Despite the effect on swimming, he said all nine patients preferred to keep their stimulators on, because they provided so much relief from symptoms.

Australian doctors had reported a similar case in 2015, in just one patient. With the device turned on, he could not even float. Trying to swim, he could not synchronize his arms and legs, and turned his whole trunk in an awkward, useless way. With the device off, he easily swam laps.

The Australian doctors said that three other patients with stimulators for Parkinson’s had drowned, and that they did not know if the deaths were related to the device.

Dr. Michael S. Okun, the medical director of the Parkinson’s Foundation, said the new report from Switzerland raised questions that needed more rigorous study. In any case, he said, all Parkinson’s patients, whether they have stimulators or not, should be warned not to swim alone, because the disease itself can make them suddenly unable to move and put them at risk for drowning.

Deep brain stimulators, first introduced more than 20 years ago, emit electrical signals that interfere with the abnormal electrical impulses in the brain that cause Parkinson’s symptoms. Sometimes described as pacemakers for the brain, the devices can provide great relief from tremors, stiffness and difficulty moving.

How the devices could interfere with swimming is not known. Dr. Baumann and his colleagues suggested that in some patients the signals may somehow affect a brain region that is crucial for coordinating limb movement. He said other complicated, learned skills might also be affected: Some patients said they could no longer ski after receiving stimulators, and one said he could not play golf anymore.

The earlier Australian report prompted Medtronic to post an “urgent field safety notice” in 2016, warning that a side effect of brain stimulators could be a loss of coordination that might leave a patient unable to swim.