Prenatal diagnosis in known affected families is also under investigation. There are three main types of SMA occurring in childhood, conventionally divided by age of onset. These will be described in their most typical presentation, although it is important to realize that clinical features and progression overlap. Acute Infantile SMA, or Werdnig-Hoffman disease, is the most severe form. Symptoms may be present in utero with decreased fetal movement.
Affected infants are hypotonic and areflexic.
Proximal muscles are affected more than distal. Tongue fasciculations and atrophy may be seen. Extraocular movements generally remain intact. Development other than gross motor is normal in these children, which may give the family a false sense of improvement. Death may occur by the age of 6 months secondary to loss of gag reflex with resultant aspiration and pneumonia.