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But with essential tremor the shaking usually occurs when the person is doing an activity, like writing or lifting a spoon. It's often confused or even misdiagnosed as Parkinson's disease. "One sibling could have a hand tremor and the other sibling might have a voice and head tremor," says Lewitt. About half the people with essential tremor have a family member affected, but not always the same way. President John Adams had it so did his son, John Quincy Adams. Researchers do know there's a genetic component. The word "essential" refers to the fact that we essentially don't know what causes the tremor. "Essential tremor is a condition that has been known about for hundreds of years," Lewitt says. In fact, a small percentage of patients have such bad tremors that they're completely housebound, unable to manage the details of daily living. The shaking can interfere with just about everything - getting dressed, eating and drinking, using the telephone. "Essential tremor is a condition in which head, voice, hands or even all parts of the body are engaged in a tremor that can be as frequent as four to eight times per second, " says Peter Lewitt, a movement disorder specialist at Henry Ford hospital in Detroit. It is the most common of all movement disorders, affecting about 10 million people in this country. Terrified that her teenage daughter had Parkinson's disease, she contacted a neurologist, who diagnosed Shari Finsilver with essential tremor.