Spinal Muscular Atrophy (SMA) is a genetic condition in which the muscles throughout the body are weakened because the cells in the spinal cord and motor nerves do not work properly. Approximately 10-16 out of every 100,000 children are born with this condition. SMA is characterized by muscle weakness and decreased muscle tone. This weakness affects legs more than arms and proximal muscles (those closest to the body like hips and shoulders) more than distal muscles (those farthest from the body like fingers and toes). This weakness tends to be equal between the right and left sides of the body. In addition to the muscles we have in our arms and our legs, we also have muscles in our chest which help us with breathing. In many cases of SMA these muscles are also affected, leading to some breathing problems.
The severity of this condition varies greatly between individuals, so the disease is broken up into three main groups. These are referred to as SMA type I, SMA type II, and SMA type III. The type of SMA a child has is determined by the highest motor function a child achieves.
SMA Type I
Also known as Werdnig-Hoffmann Disease, is the most severe form of SMA and defined as children who are not able to sit independently. Children with SMA type I usually present to the doctors before 6 months of age because of their severe low muscle tone, delayed gross motor skills, breathing difficulties, lack of reflexes, and spontaneous tongue movements (referred to as tongue fasciculations). Due to respiratory problems, children with SMA type I often have a significantly shortened life span.
SMA Type II
Is an intermediate form of SMA and is defined as children who can sit unassisted but cannot walk or stand. Children with this form of SMA usually present to the doctors before 18 months of age because of low muscle tone and delayed gross motor skills. Individuals with SMA type II can also have the typical tongue movements and/or a minor tremor of both hands. SMA type II can cause varying degrees of breathing difficulties. In many cases of SMA type II children and adults also experience a shortened lifespan.
SMA type III
Also known as Kugelberg-Welander Disease, is the least severe form of SMA and is defined as individuals who are able to stand and walk unassisted. Usually individuals with this form of SMA have limited, if any, breathing difficulties, and may show little or no decline. This form of SMA often resembles muscular dystrophy but can be identified by specialized testing.
While these three forms are considered one disease with one genetic cause, there is great variability in the severity of the condition and the challenges a child or individual faces.