Scientists are uncertain about the cause of primary orthostatic tremor and suspect that it may be a rare form of essential tremor, according to WebMD. While scientists also have limited understanding of essential tremor, research suggests this common condition occurs when the thalamus triggers abnormal electrical signals in the brain.
Primary orthostatic tremor is a neurological disorder that causes rapid trembling in both legs when standing, the National Organization for Rare Disorders notes. Tremors force muscles in the legs to contract involuntarily, causing the individual to feel unsteady or in danger of falling; however, the feeling usually stops or becomes less intense when the person lies down, sits or walks. The tremors can occur at such a rapid pace that the condition may not be outwardly visible to others.
Primary orthostatic tremor has only been defined since 1984, and some factions of the medical community believe the condition is an extreme physiological reaction to standing, NORD states. While the condition is not always perceptible by sight, physicians can detect the rhythmic shaking of the legs during a physical examination or use a stethoscope to listen to vibrations of the muscles. The tremors and the resultant sense of imbalance can become increasingly severe, sometimes causing an individual soreness, weakness or stiffness in the legs. While orthostatic tremor does not lead to other conditions, it can interfere with daily life, especially for individuals who experience shaking immediately upon standing.