Jerky movements and muscle spasms?

Brain injury: In a very young child or infant, most likely due to cerebral palsy or syringomyelia; seen first in a teenager or young adult, head or spinal cord injury leading to spasticity. In mid life multiple sclerosis. In later mid life stroke or brain tumor will do this. Then there are other more unusual illnesses such as motor neuron disease.

Question is unclear: In general jerky movements indicate an injury in the nervous system. If they are occurring with spasms they may be due to a stable brain injury from birth or infancy, or to an evolving injury to the spinal cord or brain. Certain muscle diseases (genetic) will lead to poor contol and spasm. Some are temperature sensitive.