In a retrospective case note study of patients referred to a specialist clinic for motor neuron disorders we identified a subgroup of patients with severe wasting and weakness of the arms without significant functional involvement of other regions. This “flail arm syndrome” was identified in 39/395 (10%) of the subjects studied. Their clinical characteristics were compared with the amyotrophic lateral sclerosis (ALS) group as a whole. The age of onset was similar between the two groups but the male:female ratio was 9:1 in the flail arm group, compared with 1.5:1 in the ALS group (p=0.0015). Although there was a trend towards improved survival in the flail arm group (median survival 57 (95%CI 45–69) months) compared with the ALS group (39 (95%CI 35–43) months), this did not reach significance (p=0.204) and was not an independent prognostic factor. As many patients with flail arm syndrome develop upper motor neuron signs in the lower limbs this syndrome probably represents a variant of ALS. It seems likely that unknown factors linked to male sex predispose a proportion of patients to develop the flail arm phenotype.

Historically, physicians have categorised illnesses according to their clinical characteristics. As a result the adult sporadic motor neuron disease syndromes have been described according to the site of lesion onset. Most patients present with both upper and lower motor neuron signs in the limbs (classic motor neuron disease or ALS). Rarer forms of predominantly upper and lower motor neuron syndromes are also recognised (primary lateral sclerosis and progressive muscular atrophy respectively). We have noted that a subgroup of patients with classic ALS present with progressive wasting and weakness of the arms, with little or no functional impairment of the bulbar muscles …