Deep brain stimulation to treat Parkinson's makes some act irrationally

An implantable brain device meant to help Parkinson's disease patients better control their movements, has been found to have some worrying side-effects.

Parkinson's disease is a brain degenerative disorder that results from the deterioration of nerve cells in the brain that produce dopamine; it is a degenerative disorder of the central nervous system that affects the sufferer's motor skills and speech.

It's characteristics are muscle rigidity, tremor, difficulty walking and balancing, a slowing of physical movement and in extreme cases, a loss of physical movement.

Parkinson's disease is both chronic and progressive and while there is no known cause the symptoms are sometimes the result of toxicity, drugs, genetic mutation, head trauma, or other medical disorders.

The implantable brain device delivers 'Deep Brain Stimulation' (DBS), which initially offered significant benefits; however gradually some rather unsettling side effects began to appear.

It seems the DBS affected the ability of some patients to act rationally in certain situations.

The human brain is designed in such a way that when we are up against a tricky situation we stop and consider.

The DBS implant, while it stops the tremors experienced by Parkinson's patients, appears to take away this inhibition and makes people much more impulsive.

In a study involving 46 participants, 15 were Parkinson's patients who were taking dopamine drugs, 17 had received the DBS implant, while the other 14 were healthy elderly adults who did not have Parkinson's.

All were asked to tackle specialized computer games which tested their decision making behavior.

The first test used pairs of Japanese written characters and the participants were asked to select the 'correct' one in each pair.

The participants gauged their progress by observing the computer's response to each of their decisions - the computer would respond with either 'correct' or 'incorrect'.

The idea is that the more the game is played and the computer response watched, the more skilled the player becomes at spotting the correct characters.

When the symbols after a while were paired differently so that there might be pairs which were both correct, pairs which were both incorrect, and mixed pairs, the patients on dopamine and the healthy participants hesitated, while the DBS patients did not hesitate at all.

As soon as the implants were switched off the DBS patients also started to hesitate, which not only demonstrated that the implants were encouraging impulsive behavior, it also showed that the implants did not block learning (otherwise the patients would not have hesitated as they would not have learnt anything during the first round ).

Experts have known that Parkinson's medications can sometimes trigger compulsions, such as pathological gambling and the DBS device appears to stop the patients thinking rationally at times, with the result that decisions are made or things are done which are later regretted.

Experts say despite this undesirable side effect the benefits of the device nevertheless far outweigh the negatives.

They believe they are not a reason to avoid the therapy but rather that adjustments should be made to benefit the patient's quality of life.

The researchers say when the brain stimulation was turned off, the Parkinson's patients with the devices acted totally normally, regaining the ability to think appropriately under stressful situations.

The study is published in the journal Science.