Progressive brachial diplegia unmasking flail arm syndrome, a regional variant of amyotrophic lateral sclerosis: a case report

Background

Flail arm syndrome (FAS) is an uncommon variant of amyotrophic lateral sclerosis (ALS) characterized by progressive, predominantly lower motor neuron weakness confined mainly to the upper limbs. Because structural cervical disease is common in older adults, its diagnosis may be delayed or misattributed to compressive or peripheral nerve disorders. Case Presentation: A 69-year-old man developed progressive asymmetric upper limb weakness over two years, evolving into bilateral brachial diplegia with preserved lower limb function. Neurological examination showed marked upper limb atrophy, fasciculations, and extensor plantar responses, without sensory deficits. MRI showed no significant cervical compression, while electromyography revealed multisite denervation, supporting the diagnosis of flail arm syndrome as a phenotype of ALS. Multiple alternative diagnoses, including cervical myelopathy, multifocal motor neuropathy, and myopathy, were systematically excluded.