Differentiating Flail Limb Syndrome From Amyotrophic Lateral Sclerosis

The aim of the study was to investigate differences between flail limb syndrome and amyotrophic lateral sclerosis.

A retrospective chart review identified 16 cases of amyotrophic lateral sclerosis and 16 of flail limb syndrome. Revised Amyotrophic Lateral Sclerosis Functional Rating Scale, compound muscle action potential amsplitudes, and rate of loss of vital capacity were compared.

Comparing amyotrophic lateral sclerosis and flail limb syndrome patients, rate of loss of vital capacity was 5.26% ± 0.33% versus 0.54% ± 0.06%, respectively (P < 0.05). No patient in the flail limb syndrome group had a rate of loss of vital capacity more than 0.65% per month. No patient in the amyotrophic lateral sclerosis group had a rate of loss of vital capacity less than 4.6% per month. The average ulnar nerve compound muscle action potential amplitudes were significantly lower in flail limb syndrome (P < 0.05). No significant difference was observed in the rate of Revised Amyotrophic Lateral Sclerosis Functional Rating Scale decline or average peroneal, tibial, and median nerve compound muscle action potential amplitudes.

In flail limb syndrome, an average monthly decrease in vital capacity exceeding 0.65% may suggest a spread of motor neuron loss to higher cervical anterior horn areas and raise the possibility of progression to amyotrophic lateral sclerosis. Larger prospective studies are needed to investigate the rate of VC decline in flail limb syndrome and limb-onset amyotrophic lateral sclerosis and to establish whether a cutoff score combining rate of loss of vital capacity and compound muscle action potential amplitude mainly of the ulnar nerve might predict progression of flail limb syndrome to amyotrophic lateral sclerosis, the knowledge of which can facilitate appropriate patient counseling.