Hirayama Disease (Monomelic Amyotrophy): An Underreported Lower Motor Neuron Disease in the North American Adolescent Population
Monomelic amyotrophy, also known as Hirayama disease, is a lower motor neuron disease diagnosed overwhelmingly in adolescent males (>10:1 M:F, aged 15-25 years).1 The first cases were reported in 1959 in the Japanese population, and more recently, pediatric cases of Hirayama Disease have been found in North America.2
Afflicted patients typically have complaints of unilateral weakness in a single extremity of insidious onset occurring with equal frequency in the upper and lower extremities, often without pain or sensory changes. Upon clinical examination, marked weakness is isolated to one or two myotomes with gross unilateral muscle atrophy in the absence of any signs of upper motor neuron disease. In brachial monomelic motor atrophy (BMMA), asymmetric cervical MRI atrophy is evident in conjunction with symptoms in greater than 90% of cases, leading to a spine specialist referral.1 EMG often shows evidence of asymmetric abnormalities typical of a radiculopathy. Diagnostic criteria by Hirayama involved dynamic MRI evidence of worsening anterior spinal cord compression on neck flexion.3 It is believed that this compression results in unilateral ischemia to the anterior horn. Furthermore, this atrophy is rarely associated with osteophyte compression of the anterior spinal cord or with disc bulges.
Fortunately for this young population, the typical duration of symptoms is five years, and most commonly follows a benign, self-limiting course,1 allowing most patients to return to work or full daily activities. A cervical collar has been shown to alleviate symptoms and is the mainstay of nonsurgical management, however, many patients complain of some degree of persistent weakness as it is felt that the disease mechanism that results in a loss of lower motor neurons is not directly addressed. Proponents of a surgical option cite the associated improvement in EMG findings and unparalleled motor recovery not documented with nonsurgical management. However, given the benign course and spontaneous recovery associated with this disease and the rarity of this patient population, difficulties arise in designing a study that effectively illustrates the benefits of cervical decompression with superior clinical improvement compared directly to nonsurgical management. The risks of cervical fusion in a young patient, such as symptomatic adjacent segment disease, have to be weighed with the severity of motor symptoms prior to pursuing surgical options.
1. Nalini A, Gourie-Devi M, Thennarasu K, et al. Monomelic
amyotrophy: Clinical profile and natural history of 279
cases seen over 35 years (1976-2010). Amyotrophic
lateral sclerosis & frontotemporal degeneration 2014:1-9.
2. Hirayama K. [Juvenile muscular atrophy of unilateral
upper extremity (Hirayama disease)--half-century
progress and establishment since its discovery]. Brain
and nerve = Shinkei kenkyu no shinpo 2008;60:17-29.
3. Lu F, Wang H, Jiang J, et al. Efficacy of anterior cervical
decompression and fusion procedures for monomelic
amyotrophy treatment: a prospective randomized
controlled trial: clinical article. Journal of Neurosurgery.