The cause of most cases is unclear as of 2018.[4] More than 90% of recent cases have followed a mild viral infection such as from enteroviruses.[4] While polio can cause AFM, since 2014, it has not been involved in cases in the United States.[3][6] The underlying mechanism involves damage to the spinal cord's grey matter.[3] Diagnosis may be supported by medical imaging of the spine, nerve conduction studies, and cerebrospinal fluid testing.[3][4]