The Flail Limb Syndrome

The flail limb syndrome is primarily a lower motor neuron disorder that initially affects proximal arm muscles (flail arm syndrome-FAS) or distal leg muscles (flail leg syndrome-FLS). Both were recognized early on (1886 for FAS and 1918 for FLS) as somewhat distinct from classic amyotrophic lateral sclerosis (ALS). Descriptions in the literature are case series with limited information on electrophysiologic features (central and peripheral), cognitive involvement, and genetic mutations. What follows is a compilation of these features. The flail limb syndromes are rare, representing ~7%-8% of ALS. They have a higher ratio of males to females compared to classic ALS. Both are defined by predominant focal arm or leg weakness for ~2 years before progression to other regions, although there can be early and mild clinical or electrophysiologic evidence for denervation and reinnervation in other regions during the initial period. Ultimately, there is progression to respiratory failure, but at a slower rate compared to classic ALS. Upper motor neuron clinical signs are variable, but transcortical magnetic stimulation paradigms and magnetic resonance imaging tractography support upper motor neuron loss. Tests of the split hand pattern show it is rare compared to ALS. Dementia is also rare. Genetic testing supports a spectrum of ALS-related gene mutations but at a lower frequency than with classic ALS, and no gene mutation is predominant. Diagnosis requires ~2 years of regional stability to predict the better prognosis for the flail limb syndromes.