Spinal muscular atrophy is a genetic disease in which the motor neurons in the spinal cord degenerate. This causes muscle weakness, which is usually more severe in the legs than the arms, according to MedicineNet. There are several types of SMA.
SMA type 1 affects children before they are born. These children often move very little during pregnancy and in the few months after birth. Babies with SMA type 1 are very floppy and have trouble swallowing and feeding, according to MedicineNet. They usually die before their second birthday.
SMA type 2 becomes apparent after birth, usually when the child is aged between 3 and 15 months. Children experience respiratory problems and floppy limbs. These children can sit by themselves but are not able to stand or walk, as reported by MedicineNet.
SMA type 3 typically affects children between the ages of 2 and 17. These children can walk but have difficulty running or climbing stairs. There is also an adult form of SMA that usually comes on between the ages of 40 and 60 and has a life expectancy of roughly 5 years, as stated by MedicineNet.
Carriers of the genes for SMA have no symptoms, but they can have children who have the condition. For most types of SMA, children with the condition inherit one copy of the SMA gene from each of their parents. The chance of a pair of SMA gene carriers having a child with SMA is one quarter, according to MedicineNet.