Positive muscle phenomena--diagnosis, pathogenesis and associated disorders.

Positive muscle phenomena arise owing to various forms of spontaneous muscle hyperactivity originating in motor neurons or in the muscle itself. Although they are common in a wide range of neurological and non-neurological diseases, clinical and scientific data on these phenomena are limited, and their presence is sometimes overlooked. This gap in our knowledge hampers effective diagnosis and treatment. In this article, we review the clinical characteristics and approach to diagnosis of the various positive muscle phenomena, and discuss their genetic underpinnings and pathophysiological mechanisms. Finally, we outline an algorithm to discriminate between the different positive muscle phenomena.