Dr. Darryl De Vivo describes the disease progression for SMA Types I, II, III and IV.

Spinal muscular atrophy is viewed by us as a progressive neurodegenerative disease, but the measurement of progression varies depending on the severity of the clinical phenotype. So that in the Type I baby who becomes usually affected in early infancy, often around two or three months of age or such, we see that they drop off in their capacity to carry out critical functions like being strong enough to breathe for example. And so the progression under those circumstances is quite clear and obviously quite grave and such. And it puts the children at high risk of dying often within the first two years of life. In the Type II children and the Type III children, the progression is less obvious, more subtle over time. And we've debated repeatedly as to whether the disease is truly progressive as it relates to the loss of these motor neurons in the spinal cord or whether the children are not gaining sufficient strength as they get older to maintain their functions or to achieve new functions. But it's very clear from all the work that's been done by us and by others that the functional deterioration is evident in all three types of the disease. Children with spinal muscular atrophy are at risk for a number of complications and of course the more severe their clinical phenotype, the more severe and more likely are the complications. In the type I babies as we have discussed previously, respiratory failure is a very serious problem and you have to understand that breathing is dependent on your strength. So you have to lift your ribcage, drop your diaphragm down in order for the air to enter into the thoracic cage, and then it has to be expressed afterwards and such. So breathing is such a vital part of the motor functioning and if there's significant weakness then all these children are at great risk for respiratory failure. They may breathe adequately when they're healthy but if they have an occurring illness or the flu or pneumonia that can be extremely challenging and truly life threatening and such. The other major complication is stabilizing the musculoskeletal system. So again, strength is important to maintain proper alignment of our arms and our legs and our torso, spinal column, and so forth. So one of the big complications that these children have is a curvature of the spine or scoliosis and almost every one of these patients are at risk for developing significant scoliosis. As long as they remain ambulatory, the likelihood of developing serious scoliosis is much less, but when they lose the ability to ambulate or fail ever to develop the ability to ambulate, then they are at very great risk, almost 100% likelihood that they'll develop curvature of the spine. As that progresses, usually we have to consider surgical intervention in order to stabilize it or to correct the curvature of the spine.

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