Very severe spinal muscular atrophy (SMA type 0): an expanding clinical phenotype.

The classical form of severe spinal muscular atrophy (SMA type 1; Werdnig-Hoffmann disease) has a very consistent clinical phenotype that is well recognized by paediatricians. It usually presents at birth or within the first few months of life. There is general hypotonia, with axial and limb weakness; the legs are affected more than the arms and proximal muscles more than distal, leaving residual spontaneous activity in the feet and in the forearms and hands. Facial muscles are spared so that the infant usually has a bright normal expression. The intercostal muscles are always affected, whereas the diaphragm is spared, allowing adequate spontaneous respiratory activity until the infants are precipitated into respiratory failure by an incidental respiratory infection, or aspiration. With rare exception they die by 2 years of age with a median around 7 months and with about 80% of the children dying by the time they are 1 year old. There is a consistent homozygous deletion in exons 7 and 8 of the telomeric copy of the survival motor neuron (SMN) gene. In the current issue of the journal, MacLeod and her colleagues have documented five cases of more severe spinal muscular atrophy, with a history of diminished fetal movements in utero and presenting at birth with asphyxia and severe weakness.