Spinal muscular atrophies (SMAs) represent a rare group of inherited disorders that cause progressive degeneration of the anterior horn cells of the spinal cord. The exact cause of the degeneration is unknown. Loss of these cells results in a progressive lower motor neuron disease that has no sensory involvement and that is manifested as hypotonia, weakness, and progressive paralysis. Kugelberg Welander spinal muscular atrophy (also known as Wohlfart-Kugelberg-Welander syndrome or mild SMA) is a milder form of SMA, with symptoms typically presenting after age 18 months. [1, 2, 3]
SMAs were first described in the 1890s, by Guido Werdnig, a physician from the University of Vienna, in his lecture "On a Case of Muscular Dystrophy with Positive Spinal Cord Findings." Soon after, Professor Johann Hoffmann from Heidelberg University presented a paper describing a syndrome of progressive atrophy, weakness, and death during the early childhood period of siblings with genetically normal parents. Both physicians conducted autopsies on their patients and found severe atrophy of the ventral roots of the spinal cord. They also found histologic evidence of loss of motor neurons in the anterior horn cells of this region. Hoffmann called the syndrome spinale muskelatrophie (spinal muscular atrophy).
In the early 1960s, Byers and Banker classified SMA into categories based on the severity and age of onset of the symptoms, in an effort to predict prognosis. Their system, summarized below, became the basis for the most widely recognized system now used for the classification of SMA.
- Onset of symptoms before age 6 months
- Also known as infantile onset SMA, or Werdnig-Hoffmann disease
Type II 
- Onset of symptoms at age 6-18 months
- Also known as chronic SMA, juvenile SMA, or intermediate SMA
Type III 
- Onset of symptoms after age 18 months, usually in late childhood or adolescence
- Also known as Kugelberg Welander SMA, or mild SMA
Although Byers and Banker's classification system focuses on only the above 3 categories, many sources refer to a fourth type of SMA.
- This category is reserved for onset of symptoms during early adulthood.
- This disorder usually carries a much more favorable prognosis than do the other types of SMA.
This article focuses only on SMA types III and IV.
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Resource CenterSpinal Disorders
Spinal muscular atrophy (SMA) is caused by successive motor unit degeneration. Muscle atrophy, caused by a progressive loss of the anterior horn cells in the spinal cord, is universal. The motor nuclei in the lower brainstem, usually those of cranial nerves V-XII (V, VII, IX, XII), also may be involved. Various stages of degeneration can be observed histologically at these sites. As the nerve cells decrease in number, replacement gliosis, pyknosis, and secondary Wallerian degeneration in the roots and peripheral nerves are observed. These processes generally begin at the caudal end of the cord and typically are symmetrical. The lower limbs usually are affected sooner and more profoundly than are the upper limbs. This degeneration most often affects the proximal musculature before it impacts the distal. Note that, unlike in amyotrophic lateral sclerosis (ALS), no corticospinal tract involvement is seen in SMA.
Spinal muscular atrophy has an estimated incidence of 1 case per 15,000 live births. The genetic carrier prevalence is 1:80.
Spinal muscular atrophy has an estimated incidence of 1 case per 15,000-20,000 live births worldwide.
Spinal muscular atrophy (SMA) types III and IV, unlike types I and II, are consistent with survival well into adulthood.  Significant morbidity occurs from progressive weakness, and patients may frequently fall or may have difficulty with stairs. Most patients use wheelchair mobility by their fourth decade of life. Scoliosis and joint contractures are also extremely common. Morbidity associated with these conditions often can be minimized with spinal surgery, as well as with aggressive physical therapy. Respiratory failure in SMA types III and IV is not as common as in types I and II. Respiratory complaints usually can be managed medically, and mechanical ventilation seldom is necessary. [6, 7, 8]
Spinal muscular atrophy (SMA) affects all races equally.
Spinal muscular atrophy affects males and females at the same rate; however, disease progression is more severe in males.
The age of onset for spinal muscular atrophy is discussed above in the Background section.
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