After a low-energy fall, an 83-year-old man presented with bilateral weakness of the upper arms without loss of sensation associated with a rigid cervical spine (ankylosing spinal disorder, ASD). Because of an atypical presentation during history, examination and initial imaging, a late diagnosis of a transdiscal C4-C5 fracture was made by dynamic radiographs. Anterior cervical discectomy and fusion were performed with delay. Strength improved from grade C to D (American Spinal Injury Association classification) after surgery. To our knowledge, this is the first description of a bilateral, isolated upper limb C5 paralysis without any loss of sensation caused by a transdiscal C4-C5 fracture. A high clinical and diagnostic index of suspicion is mandatory to make the diagnosis. We present three clinical 'Awareness Criteria' (1: recognition of ASD; 2: high index of fracture suspicion; 3: necessary imaging) helping clinicians to safely and promptly diagnose occult spinal fractures in ASD.