What is spinal muscular atrophy (SMA), and how does it differ from ALS?

SMA is a motor neuron disease characterized by the slow, progressive degeneration of motor neurons in the spinal cord (second motor neuron). This leads to progressive flaccid paralysis (paresis) and muscle wasting (myotrophy). Unlike ALS, in SMA the motor symptoms are limited to individual extremities (segmental SMA), and the spread of paralysis and muscle wasting from the initially affected region to adjacent muscle groups is significantly restricted. SMA also progresses much more slowly. SMA in adulthood may be associated with progressive motor deficits in the extremities (affecting walking, standing, and fine motor skills), while swallowing and respiratory function are not affected (or are affected very late in the course of the disease). This distinction applies to SMA in adulthood. If SMA occurs in childhood, motor deficits comparable to those seen in ALS may develop.