Types
SMA manifests over a wide range of severity affecting infants through adults. The disease spectrum is variously divided into 3–5 types, in accordance either with the age of onset of symptoms or with the highest attained milestone of motor development.
The most commonly used classification is as follows:
| Type | Eponym | Usual age of onset | Characteristics | OMIM |
|---|---|---|---|---|
| I: Infantile | Werdnig–Hoffmann disease | 0–6 months | The severe form manifests in the first months of life, usually with a quick and unexpected onset ("floppy baby syndrome"). Rapid motor neuron death causes inefficiency of the major bodily organs - especially of the respiratory system - and pneumonia-induced respiratory failure is the most frequent cause of death. Babies diagnosed with SMA type I do not generally live past two years of age, with death occurring as early as within weeks in the most severe cases (sometimes termed SMA type 0). With proper respiratory support, those with milder SMA type I phenotypes, which account for around 10% of cases, are known to live into adolescence and adulthood. | 253300 |
| II: Intermediate | Dubowitz disease | 6–18 months | The intermediate form affects children who are never able to stand and walk but who are able to maintain a sitting position at least some time in their life. The onset of weakness is usually noticed some time between 6 and 18 months. The progress is known to vary greatly, some patients gradually grow weaker over time while others through careful maintenance avoid any progression. Body muscles are weakened, and the respiratory system is a major concern. Life expectancy is somewhat reduced but most SMA II patients live well into adulthood. | 253550 |
| III: Juvenile | Kugelberg–Welander disease | >18 months | The juvenile form usually manifests after 18 months of age and describes patients who are able to walk without support at some time, although many later lose this ability. Respiratory involvement is less noticeable, and life expectancy is normal or near normal. | 253400 |
| IV: Adult-onset | Adulthood | The adult-onset form (sometimes classified as a late-onset SMA type III) usually manifests after the third decade of life with gradual weakening of muscles – mainly affects proximal muscles of the extremities – frequently rendering the patient wheelchair-bound. Other complications are rare, and life expectancy is unaffected. | 271150 |
The most severe form of SMA type I is sometimes termed SMA type 0 (or severe infantile SMA) and is diagnosed in babies that are born so weak that they can survive only a few weeks even with intensive respiratory support. SMA type 0 should not be confused with SMARD1 which may have very similar symptoms and course but has a different genetic cause than SMA.
Development milestone attainment is commonly measured using a specially modified Hammersmith Functional Motor Scale.
The eponymous label Werdnig-Hoffmann disease (often misspelled with a single "n") refers to the earliest clinical descriptions of childhood SMA by Johann Hoffmann and Guido Werdnig. The eponymous term Kugelberg-Welander disease after Erik Klas Hendrik Kugelberg (1913-1983) and Lisa Welander (1909-2001), who distinguished SMA from muscular dystrophy. Rarely used Dubowitz disease (not to be confused with Dubowitz syndrome) is named after Victor Dubowitz, an English neurologist who authored several studies on the intermediate SMA phenotype.
Read more about this topic: Spinal Muscular Atrophy
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