Paralysis is caused by multiple conditions, which may be classified as those which affect the muscles themselves, and those which act indirectly, through the nerves that control muscle action. The most common external causes for paralysis include the occurrence of a stroke, head injury, spinal cord injury and multiple sclerosis. Paralysis is caused by multiple conditions, which may be classified as those which affect the muscles themselves, and those which act indirectly, through the nerves that control muscle action. The most common external causes for paralysis include the occurrence of a stroke, head injury, spinal cord injury and multiple sclerosis. The most important causes of paralysis are: Myopathy. Conditions which cause paralysis by directly inducing muscle weakness include: Congenital diseases, mostly genetic, that cause muscle impairment, such as muscular dystrophy. Inflammatory or autoimmune conditions of muscle inflammation, such as dermatomyositis and polymyositis.