Benign Essential Tremor is a neurologic movement disorder characterized by involuntary fine rhythmic tremor of a body part or parts, primarily the hands and arms (upper limbs). In many affected individuals, upper limb tremor may occur as an isolated finding. However, in others, tremor may gradually involve other anatomic regions, such as the head, voice, tongue, or roof of the mouth (palate), leading to difficulties articulating speech (dysarthria). Less commonly, tremor may affect muscles of the trunk or legs.
In individuals with the condition, tremor tends to occur while voluntarily maintaining a fixed posture against gravity ("postural tremor") or while performing certain goal-directed movements ("kinetic intention tremor"). Although tremor is typically absent with rest--i.e., when the affected muscle is not voluntary activated--some individuals with advanced disease may develop resting tremors.
Although symptom onset may occur during childhood or adolescence, the condition most commonly becomes apparent during adulthood, at an average age of 45 years. Benign Essential Tremor is generally considered a slowly progressive disorder. Disease progression is characterized by an increase in tremor amplitude, causing difficulties in performing fine motor skills and varying degrees of functional disability. For example, hand tremor may gradually cause difficulties with manipulating small objects, drinking fluids from a glass, eating, writing, or dressing. (As mentioned above, in some affected individuals, disease progression may also include extension of tremor to other muscle groups.)
Benign Essential Tremor may appear to occur randomly for unknown reasons (sporadically) or be transmitted as an autosomal dominant trait. Researchers suggest that changes (mutations) of different genes may be responsible for the disorder (genetic heterogeneity). For example, during genetic analysis of several affected families (kindreds), investigators located a gene for the disorder, known as "FET1," on the long arm (q) of chromosome 3 (3q13). In another kindred, the disorder was determined to result from mutations of a gene, designated "ETM2," on the short arm (p) of chromosome 2 (2p22-p25).