Essential Tremor

Epidemiology

ET was first described in 1817 by James Parkinson, who differentiated between parkinsonism and what was later defined as essential tremor. In 1887, Dana compiled the first organized account of ET. Critchley reported on a large study of the condition in 1949.

ET affects all ethnic and geographic populations, with the possible exception of certain isolated communities in New Guinea. The frequency of ET appears to be independent of gender and increases with age. Although the disorder may initially become apparent during childhood or adolescence, onset is most common during adulthood, with a mean of about age 45.

ET is generally considered one of the most common neurologic movement disorders. As mentioned previously, current estimates suggest that ET may be 10 to 20 times more prevalent than PD. Although approximately 5 million individuals in the United States have been diagnosed with ET, some investigators indicate that as many as 10 million people in the US may have the condition. In other words, despite causing significant disability and handicap, the disorder often remains unrecognized. For example, some patients may fail to obtain medical attention if the tremor is mild, not recognizing that the likelihood of functional disability and resultant handicap may increase with advancing age. Functional disability associated with ET may affect handwriting; speech; the performance of fine manipulations of the hands; and various activities of daily living (ADLs), including the capacity to feed oneself, drink fluids from a glass or cup, or dress. Functional disability may also affect the ability to perform certain on-the-job tasks, compelling some patients to change jobs or retire, and may result in embarrassment, causing withdrawal from social activities. In addition, some ET patients may fail to seek medical attention if they are unaware of the availability of effective treatments; improperly attribute tremor to advancing age; or fear that the impact of tremor on their QOL will not be taken seriously.

Estimates of the prevalence of ET are extremely variable, ranging from 0.08 to 220 cases per 1,000 individuals, i.e., a 2,750-fold difference. Among 4 studies that provided age-stratified data, the prevalence of ET in persons over the age of 60 years was 13.0 to 50.5 cases per 1,000. The substantial variations in prevalence estimates may be due to inherent difficulties in studying ET as well as other factors, including...

• Variations in study designs
• Significant differences in study populations
• Lack of a consensus definition of ET
• Use of different screening instruments
• Difficulties in differentiating between mild ET and enhanced physiologic tremor
• Difficulties in classification when tremor is coexistent with other neurologic conditions
• Tendency of hospital- and clinic-based (as opposed to community-based) studies to underestimate the total number of people with ET*

*The ascertainment of patients through retrospective use of medical records may include only a fragment of those affected by ET, disproportionately representing those with a positive family history or marked disease. Most investigators agree that prevalence may be most accurately represented by community-based studies in highly defined geographic regions with subjects who are determined through personal screening, such as through surveys or questionnaires, and follow-up examinations.

To more clearly ascertain the frequency and range of expression of ET in the general population, additional prospective, community-based investigations are required, utilizing uniform diagnostic criteria. However, validating the accuracy of diagnostic criteria for ET will only be possible upon identification of the precise genetic marker.