Function changes in spinal muscular atrophy II and III. The DCN/SMA Group.

The course of spinal muscular atrophy (SMA) is not well established except for those patients whose age of onset is before 6 months and who achieve only "sit with support" as their maximum function (Werdnig-Hoffmann disease or SMA I). This study shows that there is another group of SMA patients whose age of onset and maximum function achieved can be used as prognostic guides. Fifty percent of SMA patients who could walk without assistance and whose onset was prior to age 2 years lost the ability to walk independently by age 12. Fifty percent of SMA patients who walked and whose onset was between 2 and 6 years of age lost walking ability by age 44 years. Fifty percent of SMA patients who could walk with assistance as their best function ever achieved lost this ability by age 7 years, unrelated to age of onset; none could walk with assistance after age 14 years. Seventy-five percent of SMA patients who developed the ability to sit independently as their best function were still sitting after age 7 years independent of age of onset; 50% of this group could sit independently after age 14 years. Eighty-five percent of SMA patients who could walk could not negotiate stairs without holding onto a rail. They could raise their hands above the head; however, as they lost walking ability, they lost this function as well. Only one SMA patient whose maximum function was sitting independently could get to the sitting position on his own. Only two of these patients could hold their hands above their heads. All patients with SMA lose function over time. This function loss occurs slowly and is related primarily to maximum function achieved; knowledge of age of onset provides helpful information, especially for predicting the loss of independent walking.