Approximately 20%-30% of SMA cases are type II (chronic infantile). Patients with SMA type II typically present between 7 and 18 months of age with general weakness and inability to sit or stand without assistance. Postural tremor of the fingers, musculoskeletal deformities, pseudohypertrophy of the gastrocnemius muscle, or respiratory failure may also be present.
Although the number of SMA types varies by classification system, the most frequently described are SMA types I through IV, of which types I through III (the juvenile forms) are the most common forms. The earliest age of symptom onset occurs in SMA type I, which is both the most common and most severe form of SMA and the type with the highest mortality rate. Whereas SMA type III is the mildest juvenile form, SMA type IV (adult onset) is the mildest and most benign form among all types.
SMA type I (acute infantile onset or Werdnig-Hoffmann disease): Symptom onset is usually at birth or by age 6 months. Hypotonia is present at birth in approximately 60% of patients with SMA type I. Most patients have severe muscle weakness and atrophy with bulbar dysfunction, resulting in respiratory difficulties and poor suckling, and 95% die by age 18 months. Approximately 60%-70% of SMA cases are type I.
SMA type II (chronic infantile onset or Dubowitz disease): As previously described, this is one of the common types of SMA. The average lifespan of patients with SMA type II ranges from age 2 years to the third decade of life, with death most commonly due to respiratory infection. This type accounts for 20%-30% of SMA cases.
SMA type III (chronic juvenile onset, Wohlfart-Kugelberg-Welander syndrome, Kugelberg Welander syndrome, mild SMA): Less severe than type I or II, SMA type III is characterized by slowly progressive muscle weakness that presents after 18 months of age. Most patients can walk but may have trouble climbing stairs or getting up off the floor owing to hip-extensor weakness. Although disease progression is slow and patients typically have a normal lifespan, patients with SMA type III often require a wheelchair with age. SMA types III and IV account for 10%-20% of SMA cases.
SMA type IV (adult onset): This is the mildest form of SMA. Although signs of muscle weakness may present as early as age 10 years, symptom onset typically occurs when patients are in their mid-30s. Patients with SMA type IV usually have a normal lifespan; disease progression is similar to SMA type III, with many patients eventually requiring a wheelchair.
In addition to the four main types of SMA, a wide range of SMA variants have been reported. These include SMA type 0 (a very severe, prenatal form), distal SMA, juvenile bulbar hereditary motor neuronopathy, and X-linked recessive bulbospinal muscular atrophy (Kennedy disease).
For more on SMA types, read here.
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Cite this: Stephen L. Nelson. Fast Five Quiz: Can You Diagnose the Different Types of Spinal Muscular Atrophy? - Medscape - Dec 06, 2018.