SMA is a neuromuscular disease passed on genetically to children by their parents.  You can not "catch" SMA by being around someone who has it.  It is a "Recessive" genetic disease, meaning that BOTH parents must carry a copy of the recessive SMA gene.  There is only a 25% chance each pregnancy of the child having SMA and a 75% chance each pregnancy that the child will be healthy.  One out of 40 people is a carrier of this recessive gene.  The brain is not affected, and they have been tested to have at least average to above average intelligence.  Please do not make the mistake of treating them as mentally impaired!!  Their bodies may not be perfect, but their minds are, so be sure to treat them that way!  SMA affects a child's muscular development, and the severity depends on what 'type' of SMA the child has.  There are four "Types" of SMA, Type 1, 2, 3 & 4.  The earlier  the symptoms are noticed, the more severe the type of SMA.  Type 1 is the most severe, affecting children while still in the womb or shortly after birth.  Type 4 is the least severe, affecting adults.

Type 1 children are diagnosed usually before 6 months of age, more often before 3 months of age.  Symptoms may even start in the womb.  Many mothers later recall the baby not moving as much the last month or so of pregnancy.  They are not able to hold up their heads, roll over, crawl, sit up without support, or walk.  All of their muscles are extremely weak, with the weakest muscles being the legs, upper arms, and neck.  Their chest may appear concave, or very skinny at the top, with a big belly.  Bell-shaped.  SMA affects all muscle systems as well including sucking, swallowing, digesting food, and excretion.  Constipation is a common problem as is being able to control excessive drooling (secretions), and getting proper nutrition and calories for proper weight gain.  A common cold can easily turn into pneumonia which is what usually takes the lives of these children, along with "respiratory failure" or when they no longer have the lung or chest muscles to be able to breathe on their own.  The major decision that must be made with Type I children is whether or not to put them on a ventilator or other breathing machine when they experience respiratory failure.  Current statistics show that the average lifespan of a child with SMA Type I, not put on permanent ventilation, is only 8 months of age, with 80% dying by the age of one, and the majority of the rest dying by age 2.  HOWEVER, these statistics are not a hard and fast rule.  Each child is affected so differently by SMA that they do not all follow the same path or progression.  Also, as more is learned about SMA, the lifespan of a Type 1 child can be lengthened depending on the severity of the symptoms for each particular child.  Last but not least, the line between each Type of SMA is not clearly defined, and it is common for a child to exhibit patterns of two types, thus confusing the issue of "life expectancy" for that child.  

Type I children most often have very little leg movement, very little upper arm movement. Many times their hands remain fisted and their hands/wrists are turned the "wrong" way.  The physical characteristics that often "gives them away" to having SMA is a bell shaped body, legs that stay in the "frog" position, moving the arms from the elbows down only, and the head tilted to the side because of lack of neck muscles. They often have bright, expressive faces and eyes.

Type 2 children are diagnosed before 2 years of age, usually more like 15 months.  These children are usually able to be in a sitting position without support, but often can not get there by themselves.  They can sometimes crawl with bracing and therapy, and on occasion may stand with braces.  Feeding and swallowing problems are not common in Type 2 children, though they are still possible.  They will usually never walk.  The lifespan of a Type 2 child varies so widely, there isn't one!  They could pass away at an early age or they could live well into adulthood.  As with all forms of SMA, weakness increases over time.

Type 3 children are diagnosed between 18 months of age and early adolescence.  In the beginning these children are able to stand and walk but usually have difficulty doing so.  They typically have a normal lifespan; however, as with all forms of SMA, weakness gets progressively worse and they usually will be wheelchair bound.

Type 4 SMA is an adult SMA, with symptoms beginning around age 35.   They also usually have a normal lifespan; though, as with all forms of SMA, weakness gets progressively worse.

(Information provided by www.smasupport.org.)