[Hirayama Disease can be Caused by Loss of Attachment of the Cervical Posterior Dura to the Pedicle due to Immunological Abnormalities of the Dura and Posterior Ligaments: A New Hypothesis].

Hirayama disease is a rare and self-limiting cervical myelopathy related to neck flexion in adolescents, predominantly male, originally described by Hirayama in 1959 and until today reported worldwide. It characteristically presents with asymmetric distal upper extremity muscular weakness and atrophy without objective sensory disturbance or lower extremity involvement, which spontaneously halts within 3-5 years after onset with some sequelae. Neuroradiological and autopsy/neuropathological studies have shown that the condition is caused by chronic ischemic changes to the anterior horn cells of the lower cervical cord due to abnormal forward-shifting of the lower cervical posterior dura on neck flexion. Although the true pathomechanism of the dural changes is still unknown, various hypothetical theories have been formulated: "tight dural canal in flexion", "flexion myelopathy", "engorgement of the posterior venous plexus", and "disproportional growth between the vertebral column and the contents of the spinal canal during puberty" (Hirayama). However, rarity of the condition and asymmetric cord involvement cannot be explained with these theories alone. Combination of a "posterior epidural ligament [sparse at C6-C7] factor" proposed by Shinomiya , "loss of dorsal dural attachment from the pedicle even in neutral position in the MRI" pointed out by some neuroradilogists, and "pathological abnormalities of elastic and collagen fibers in the operatively-resected dura" reported by several spinal surgeons suggest that abnormal changes of the dura and posterior ligaments, as well as the existence of "myodural bridge" at C1-C2 described by some neuroanatomists, can cause the dural forward-shifting of the lower cervical cord. In addition to these mechanical factors, immunological abnormalities such as hyperIgEemia and upreguration of some cytokines and chemokines in serum/CSF of patients with Hirayama disease have been reported. Furthermore, immune cells comprise -17% of all cells in the dura in rats. In conclusion, although further immunological studies of the patient's cervical dura and surrounding structures are required, I propose, in addition to Hirayama's theory, a new hypothesis about the pathomechanism of Hirayama disease, "loss of dorsal dural attachment from the pedicle due to immunological abnormalities of the dura and posterior ligaments", which can resolve the remaining problems of the condition. (Received May 25, 2020; Accepted July 2, 2020; Published December 1, 2020).