Paralysis Resource Guide - (Page 46) CONDITIONS SPINAL MUSCULAR ATROPHY Spinal muscular atrophy (SMA) refers to a group of inherited neuromuscular diseases that affect the nerve cells (motor neurons) and the control of voluntary muscles. SMA causes lower motor neurons in the base of the brain and the spinal cord to disintegrate, preventing them from delivering the necessary signals for normal muscle function. Involuntary muscles, such as those that control bladder and bowel function, are not affected in SMA. Hearing and vision are not affected, and intelligence is normal or above average. The three major childhood-onset forms of SMA are now usually called Type 1, Type 2 and Type 3. All three types are also known as autosomal recessive SMA—both parents must pass on the defective gene in order for their children to inherit the disease. All forms of SMA affect the skeletal muscles of the trunk and limbs. In general, those muscles closer to the center of the body are more affected than those farther away. SMA Type 1, the most severe form, mostly affects the neurons controlling the mouth and throat muscles and therefore involves more problems with chewing and swallowing. Respiratory muscles are involved to varying degrees in all forms of the disease. In SMA Type 1, the onset of the disease is noted within the first six months of the child’s life. Children with SMA Type 1 are unable to sit without support, and death usually occurs before age two. SMA Type 2 is an intermediate form of the disease. Onset is between 7 and 18 months. Children with SMA Type 2 usually learn to sit without support, but they don’t learn to stand or walk without aid. The child’s survival depends in large part on the degree of respiratory and swallowing difficulties. SMA Type 3 is a milder form of this condition. Onset occurs after the age of 18 months and most often between the ages of 5 and 15. Weakness of the muscles of chewing and swallowing is rare, and respiratory effects are generally not as severe as in the first two forms. These children may live into adulthood. Respiratory complications, if they occur, pose the most serious threat to life. At present, there is no known treatment that will stop or 46
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