Non-5q spinal muscular atrophies

Spinal muscular atrophies (SMAs) are hereditary disorders characterized by degeneration of motor neurons in the spinal cord and brainstem. The most common form of SMA is caused by decreased levels of the survival motor neuron (SMN) protein. This autosomal recessive condition results from mutations or homozygous deletions involving the SMN1 gene on chromosome 5q13. Non-5q SMAs are genetically heterogeneous, clinically diverse, and rare compared with 5q SMA.1 Various published classifications of non-5q SMAs have proved either inadequate or encyclopedic and confusing. Classification by distribution of weakness (distal, proximal, bulbar) and mode of inheritance is currently employed by most experts in this field.2,3 The classification scheme outlined in the table, although based on these premises, has its own limitations. It does not include all non-5q motor axonopathies or neuronopathies or SMA plus syndromes, and especially excludes conditions with uncertain nosology or those with no gene/locus information. The table is therefore offered as an aid to understanding non-5q SMAs by illustrating the …