Spinal subdural hematomas (SSDH) are much less common than epidural hematomas; however, progression of symptoms due to compression tends to be faster. Spinal subdural hematomas are a rare entity, much more so than epidural hematomas. In a meta-analysis of over 600 spinal hematomas, only 4% were subdural. Symptoms are those of spinal cord compression or cauda equina syndrome, often presenting initially as back pain and/or radicular pain. Symptoms tend to develop more rapidly with subdural hematomas. Subdural hematomas occur within the dural sac; therefore, in contradistinction to epidural hematomas, the epidural fat is preserved and the dura is not displaced inward. The hematoma is bounded by the paired lateral denticulate ligaments and the dorsal septum, forming the inverted Mercedes-Benz sign on axial images. As such, it compresses the nerve roots but does not extend into the neural foramina or make direct contact with bone. Naturally, smaller collections will not expand the potential subdural space and will, therefore, not create the inverted Mercedes-Benz sign.
