Spinal muscular atrophies (SMAs) are a genetically and clinically heterogeneous group of rare debilitating disorders characterised by the degeneration of lower motor neurons (neuronal cells situated in the anterior horn of the spinal cord) and subsequent atrophy (wasting) of various muscle groups in the body.

[1] While some SMAs lead to early infant death, other diseases of this group permit normal adult life with only mild weakness.

Based on the type of muscles affected, spinal muscular atrophies can be divided into:[citation needed] When taking into account prevalence, spinal muscular atrophies are traditionally divided into:[citation needed] A more detailed classification is based on the gene associated with the condition (where identified) and is presented in table below.

In all forms of SMA (with an exception of X-linked spinal muscular atrophy type 1), only motor neurons, located at the anterior horn of spinal cord, are affected; sensory neurons, which are located at the posterior horn of spinal cord, are not affected.

By contrast, hereditary disorders that cause both weakness due to motor denervation along with sensory impairment due to sensory denervation are known as hereditary motor and sensory neuropathies (HMSN).