Atonia

Atonia is the state of diminished or absent muscle tone in a part of the body. In medicine, it most often refers to hypotonia of skeletal muscles, characterized by reduced resistance to passive movement and flaccidity. It is distinct from paralysis, which implies an inability to move, and from ataxia, which involves impaired coordination.

In neonates and infants, hypotonia is described as a "floppy" state and may be central (originating in the brain or spinal cord) or peripheral (affecting nerves or muscles). Causes include genetic or congenital disorders, metabolic diseases, infections, endocrine problems such as hypothyroidism, birth injuries, or prenatal factors.

A related but separate concept is uterine atony, the loss of uterine muscle tone after delivery, which is a leading cause of postpartum hemorrhage. Risk factors include overdistended uterus, prolonged or vigorous labor, multiparity, magnesium sulfate or other sedatives, and infection.

Diagnosis relies on the clinical assessment of tone, reflexes, and motor response, with neurological examination guiding further testing. Investigations may include metabolic panels, genetic testing, imaging, or electrophysiological studies to identify underlying causes.

Treatment targets the underlying cause and may include supportive care, physical therapy to improve tone in infants, and specific therapies for metabolic or endocrine disorders. In uterine atony, urgent management with uterotonic medications, fluids, blood products, and sometimes surgical interventions is required.

Prognosis varies with cause and timeliness of treatment; some cases resolve, while others reflect chronic neuromuscular or developmental conditions. See also hypotonia, floppy infant syndrome, postpartum hemorrhage.