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Few distinct motor neurone disease (MND) variants—for example, familial amyotrophic lateral sclerosis (fALS) and spinal muscular atrophy (SMA)—are caused by definitive gene mutations. Within the phenotypic spectrum of the superoxide dismutase gene (SOD1)-associated fALS variants, cases showing exclusive involvement of the lower motor neurone (LMN) are known. Furthermore, the SMA group contains heterogeneous diseases that are mainly characterised by isolated LMN degeneration. From a clinical point of view, the distinction between ALS with exclusive or mainly LMN affection and a pure SMA in adult cases is often demanding. Respiratory failure regularly occurs during the course of both entities, and diaphragmatic denervation is present in some cases. Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a specific and rare variant of infantile SMA caused by homozygous mutations in the IGHMBP2 gene. Disease onset is usually within the first year of life, but juvenile SMARD1 starts in early childhood. The phenotypic hallmark is the early …
Competing interests: None.