When do essential tremor patients develop head tremor? influences of age and duration and evidence of a biological clock

Background: Essential tremor (ET) is a chronic, progressive neurological disease. Head (neck) tremor may eventually develop in as many as 30-60% of patients, yet it is unclear why. Is its appearance merely a function of advancing disease duration? Alternatively, is patient age a primary factor? The latter would argue for the presence of a biological clock that is important for the expression of this clinical feature of ET. Methods: A total of 363 ET patients were enrolled in a cross-sectional, clinical-epidemiological study. Each ET patient underwent a 20-min videotaped neurological examination which included an assessment of the presence/absence of head tremor. Results: Head tremor was present on examination in 140 (38.6%) patients. Young patients, even with longer-duration tremor, rarely had head tremor: 2/27 (7.4%) patients <40 years old with tremor duration ≥10 years had head tremor versus 121/283 (42.8%) older patients (>60 years old) with tremor duration ≥10 years (p < 0.001). In a multivariate logistic regression analysis, while head tremor was associated with age (p < 0.001) it was not independently associated with tremor duration (p = 0.26); interestingly, it was associated with gender in that model (p < 0.001). With the exception of 1 patient, head tremor did not begin before the age of 36. Conclusions: Data suggest that the appearance of head tremor in ET depends on a biological factor that is intrinsic to the patient (i.e. age) and is not a clear consequence of advancing disease duration.