The weakness can progress to the opposite limb, although whether this progression is typical or rare is under discussion. Cases of patient improvement and deterioration have both been described, but are atypical. Initially MMA can be confused for slowly progressing case of other neurological diseases such as amyotrophic lateral sclerosis (ALS); initial symptoms can be similar, but their causes are apparently different, and their outcomes markedly so. Diagnostic tools have improved since first described, and a few therapies are being introduced. But sometimes several years of observation are needed before a definitive diagnosis can be made.Use of a cervical collar may afford relief, and some researchers advocate its therapeutic use. There is also a slowly progressive variant of MMA known as OSullivan-McLeod syndrome, which only affects the small muscles of the hand and forearm and has a slowly progressive course. Epidemiology
MMA is described most frequently in Asia, with studies of a few hundred individuals emerging from Japan, China and India; it is much less commonly seen in North America and Europe. The disease (disorder) was first described by Keizo Hirayama in 1959 as "juvenile muscular atrophy of unilateral upper extremity". In 1984 Mandavilli Gourie-Devi (et al.) introduced the term "monomelic amyotrophy". The disease primarily (but not exclusively) affects young (15- to 25-year-old) males. As of 2014 there had been less than 1500 described cases, starting with the patients in Hirayamas 1959 study. The condition is disproportionately high in Asia but no conclusive reason has been found for this.
To date the largest studies recorded are Japan (333 cases), India (279 cases), and China (179 cases). Etymology
Both the names for the disorder and its possible causes have been evolving since first reported. Because this condition has been found almost exclusively in healthy young adults and stabilizes after a few years, a span of 23 years elapsed between the time Hirayama first described the condition, and the first death (from cancer), and autopsy. Over the several decades since first reported, a majority of researchers now describe this as a biomechanical condition linked to adolescent growth spurt. However, this is not yet a universal conclusion and there are anomalous reports that suggest an incomplete understanding of the condition. See also
Lower motor neuron lesion

Notes
References
External links

monomelic_amyotrophy at NINDS