Sudden weakness of the left arm may be related to the following causes: First, it may be related to trauma. If there is an injury to the left arm, muscle strain, joint dislocation and other diseases, the phenomenon of weakness of the left arm may occur. Second, sudden weakness of the left arm may be related to cerebrovascular accidents and other diseases, and this condition can be clearly diagnosed through CT and MRI of the brain. Third, sudden weakness of the left arm may be related to cervical spondylosis or neck injury. Neck injury or cervical spondylosis may compress the nerves of the upper limbs, which often results in sudden weakness, numbness and inability to use the left arm.