Hirayama Disease

Introduction

Hirayama disease is a rare, slowly progressive, and self-limiting cervical myelopathy related to neck flexion in young adults. It is characterized by a pure focal amyotrophy in the C7-T1 innervated musculature. It presents classically in an adolescent male with progressive asymmetric upper extremity muscular weakness and atrophy. During neck flexion, forward displacement of the posterior dural sac is thought to result in minimal but additive repetitive damage to the lower cervical spinal cord secondary to cord compression and/or venous congestion ( Fig. 35.1 ). The only reported autopsy case demonstrated asymmetric ventral spinal cord flattening and necrotic changes of anterior horn cells centered in the lower cervical spinal cord. Importantly, it markedly differs from other motor neuron diseases by its self-limiting course, which spontaneously halts after a period of 5 years in 90% of patients.

Overview

Historically, the cervical flexion–induced myelopathy of Hirayama disease has been mainly identified and reported in Asian countries. Perhaps due to greater awareness, the rare disease has recently been reported in a wider ethnic cohort.

Although quite rare, Hirayama disease classically presents in an adolescent male patient with asymmetric upper extremity weakness and atrophy. There is a strong male predominance (7 : 1), and onset of symptoms generally occurs between adolescence and young adulthood (early third decade of life). Symptoms generally consist of asymmetric muscular weakness and atrophy in the C7-T1 myotomal distribution with relative sparing of the brachioradialis muscle. There is generally no sensory loss, and the lower extremities are spared. Although approximately 90% of Hirayama cases present with a history of unilateral onset, a minority of cases present with symmetrical upper extremity involvement.

The natural history of Hirayama disease is curious and differs from other more ominous motor neuron diseases. Initially, the classically asymmetric upper extremity weakness and atrophy progresses over the course of a few years but then halts within 5 years in 90% of patients. Treatment is dependent on early recognition and consists of conservative treatment with cervical collar placement to avoid neck flexion for a number of years. Decompressive surgery is controversial and only advocated for patients with persistent deterioration despite treatment.

The cervical flexion–induced myelopathy in Hirayama is associated with dynamic alterations along the lower cervical cord, eventually resulting in ischemia, infarction, and gliosis of anterior horn cells. The pathognomonic dynamic magnetic resonance imaging (MRI) findings of Hirayama disease are impressive. On 30 to 40 degrees of neck flexion, anterior shifting of the posterior cervicothoracic dura is seen, with sometimes dramatic venous engorgement in the expanded epidural space. The anterior shifting of the dura and venous engorgement obliterate the subarachnoid space and compress the lower cervical spinal cord against the vertebral bodies ( Fig. 35.2 ).

Hirayama patients have been shown to have increased range of cervical motion on radiographs, with hypermobility possibly representing an etiological or contributing factor to pathogenesis. Although the exact pathogenesis of Hirayama disease is yet unknown, the principal theory is that the growing adolescent or young adult patient may have an imbalanced growth between the cervical dura and the remainder of the cervical spine. The patient with a cervical dura that has lagged in growth as compared with other cervical elements is therefore thought to be unable to fully compensate for dynamic increases in posterior length during flexion. During flexion, the relatively short dura becomes taught and shifts forward. The transient anterior displacement of the posterior dura obliterates the subarachnoid space and creates a large posterior epidural space filled by engorged epidural venous plexus. Case reports with epidural venography have confirmed that posterior enhancement represents an engorged epidural venous plexus. In conjunction, the anteriorly displaced dura and engorged venous plexus transiently compress the anterior spinal cord against the lower cervical vertebral bodies. Because angiographic case reports have shown that arterial spinal flow is not obstructed, the mechanism for anterior horn spinal cord damage is thought to result from transient but repetitive mechanical compression and/or venous congestion. Lastly, the importance of posterior epidural ligaments has been entertained. Both fine elastic ligaments as well as larger ligaments exist normally in the dorsal epidural space. Because these ligaments are likely to resist against separation of the posterior dura from the ligamentum flavum, their paucity could prove to be an etiological factor in the disease.