Hirayama disease: From flexion myelopathy to an instability-centric disorder

Hirayama disease has long been regarded as a juvenile, self-limiting cervical flexion myelopathy caused by disproportionate growth between the spinal canal, its meningeal coverings, and its contents.[1] Disabling wasting, deformity, and weakness of hands in young men are hallmarks of the disease. Although leg weakness or stiffness is usually associated, these are not the primary clinical symptoms. Imaging features are characteristic wherein there is abnormal lower cervical spinal kyphotic curvature on flexion of the neck, and a well-defined extradural mass emerges in the lower cervical spine predominantly posterior to the spinal cord on flexion of the neck and disappears on neck extension. The classical emphasis on dynamic cord compression presumably by abnormally “tight” dura over the anteriorly located osteophytic bars, demonstrated on flexion magnetic resonance imaging, has shaped both diagnostic criteria and treatment strategies. However, this compressive paradigm does not satisfactorily explain the selective anterior horn cell involvement, asymmetry of weakness, or the characteristic evolution and eventual arrest of the disease. A conceptual shift emerged when Hirayama disease was examined through the lens of cervical instability, as proposed by Goel et al.[2] In this framework, the fundamental pathology is not dural tightness or transient compression, but subtle, longstanding, and progressive instability of multiple segments or of the entire cervical spine that more often includes the atlantoaxial articulation. As has been speculated earlier, more than neural compression or deformation, it is instability that is the cause of symptoms. Although it is difficult to conclude as to how instability causes symptoms, it is possible that repeated abnormal micro-movements during neck activity result in chronic, asymmetric injury to the spinal cord in general and anterior horn cells in particular, producing the distinctive distal upper-limb amyotrophy. From this perspective, the radiological hallmarks seen on flexion magnetic resonance imaging – forward migration of the posterior dura, expansion of the posterior epidural space, and venous plexus engorgement – are better understood as secondary, naturally protective or adaptive phenomena, reflecting abnormal motion rather than constituting the primary disease mechanism.[2-4] The absence of overt compression or demonstrable instability on dynamic imaging or in the neutral position does not negate this diagnosis; rather, it underscores the concept of occult, vertical, or central instability, a principle that has been repeatedly emphasized in our reports on degenerative and nondegenerative spinal disorders.[5] The natural history of Hirayama disease further supports this interpretation. The initial progressive phase corresponds to a period of active instability, whereas the subsequent spontaneous arrest reflects biological or mechanical stabilization with skeletal and muscular maturity. Thus, the so-called “self-limiting” nature of the disease is not the resolution of pathology, but accommodation to instability and the subsequent natural response. This reconceptualization has direct therapeutic implications. Traditional management strategies – prolonged cervical collar use, multisegmental bone decompression, or procedures aimed at dural expansion – primarily restrict motion or address secondary changes. In contrast, posterior cervical stabilization, as advocated by Goel et al., targets the primary pathology by eliminating abnormal movement at the affected segments.[2,6] Clinical improvement following fixation, even in the absence of decompression, signifies the role of muscle weakness-related spinal instability and challenges the notion that neuronal loss in Hirayama disease is irreversible. Viewed in this context, Hirayama disease becomes part of a broader spectrum of spinal disorders in which instability precedes compression and stabilization precedes recovery. This philosophy aligns with conditions such as cervical spondylotic myelopathy, Chiari malformation, and craniovertebral junction anomalies, where fixation alone can result in dramatic neurological improvement.[3-5] By shifting the focus from flexion-induced compression to segmental instability, the concept transforms Hirayama disease from a condition managed expectantly into one that is mechanically defined and surgically addressable. As this perspective gains wider acceptance, it has the potential to redefine treatment algorithms and improve long-term functional outcomes in affected patients.