Have you ever noticed the shaky hands of a complete stranger and immediately made the diagnosis of Parkinson's disease? Or, have you noticed that your own hands are not as steady as they used to be, but have avoided telling your doctor for fear it may mean Parkinson's?

It may surprise you to learn that the most common cause of tremors is not Parkinson's disease -- a well-known disease associated with Michael J. Fox, Janet Reno and Muhammad Ali -- but something called Essential Tremor, or ET.

Like Parkinson's, ET is a neurological disorder that leads to tremors in the hands, head and voice, among other areas. Both are irreversible.

Whereas ET primarily causes tremors, Parkinson's is more likely to be accompanied by symptoms such as slowed movement, muscle stiffness, and walking problems that develop over many years.

Both ET and Parkinson's can begin at any age, but frequently develop later in life. Research suggests that about 5 percent of people over the age of 40 may have ET, compared with less than 1 percent who are believed known to have Parkinson's.

For the most people with ET, symptoms will slowly progress over decades, yet remain mild and have no impact on overall life. Parkinson's is a more rapidly progressive and disabling disorder that will require treatment in the majority of cases.

Distinguishing between ET and Parkinson's is relatively easy for an experienced physician. While ET causes shakiness or poor coordination when you are moving or outstretch your arms, Parkinson's tremors occur primarily when the body is at rest and you are not paying attention to the involved limb.

Despite the relative ease of distinguishing ET from Parkinson's, there are other disorders that share some of the symptoms, so they are best evaluated by a trained movement disorder neurologist.

Treatments to relieve symptoms of both ET and Parkinson's also vary widely and range from daily pills to brain surgery. Treatments should always be tailored to the individual based on their potential for benefit and side effects.

We don't yet understand what causes ET or Parkinson's, but researchers are working to identify the causes and develop better treatments. Neither disease has a cure, though medicines capable of slowing the progression may be on the horizon.

Dr. Fatta B. Nahab is an assistant professor of neurology at the University of Miami Miller School of Medicine.