A rare inherited progressive condition where the muscles of the arms, legs and face become increasingly weak and stiff due to damage to nerve cells that control muscle movement. The legs are affected first and then the arms and face - the symptoms ascend up the body. This condition involves mutations in the same gene and overlapping symptoms with juvenile primary lateral sclerosis but the difference is that primary lateral sclerosis only involves degeneration of the upper motor neurons whereas infantile-onset spastic paralysis is more severe and involves degeneration of upper and lower motor neurons. See also Hereditary spastic paralysis, infantile onset ascending