Scientific Background
Genetic spinal muscular atrophies (SMA) are caused by the loss of anterior horn cells in the spinal cord. The most common form of SMA is caused by deletions (rarely point mutations) of the SMN1 gene (e.g., SMA type 1 Werdnig-Hoffmann disease). In most cases of SMA, a search for deletions is performed, in individual cases a variant analysis is also performed.
Other rare forms, called "non-5q-SMA", have been identified by next generation sequencing. Clinically, this is a very heterogeneous group of disorders, some of which have other main symptoms (e.g., pontocerebellar hypoplasia). Essentially, they are characterized by loss of motor neurons in the spinal cord and lower brainstem and muscle weakness and atrophy. Differentiation is based on the age of manifestation, pattern of muscle involvement, anatomical location of the focus of manifestation, and inheritance pattern. Rough classification concerns a division into proximal and distal SMA (DSMA, corresponds to hereditary motor neuropathy).
References
Karakaya et al. 2018, Hum Mut 39:1284 / Teoh et al. 2017, Neural Plast 2017:6509493 / Rudnik-Schöneborn et Zerres 2017, medgen 29:21 / Peeters et al. 2014, Brain 137:2879