Infantile SMA is caused by an altered gene that does not function they way it should. SMA is passed on by what is known as autosomal recessive inheritance. In our bodies, we all have 46 chromosomes, which form 23 pairs. 22 of these pairs are numbered (1 – 22), which are known as autosomes. The last pair of chromosomes, known as the sex chromosomes, determine the gender that individual. A female has two X chromosomes, whilst males have one X and one Y chromosome to make up the 23rd pair. In SMA, the altered gene is located on chromosome 5, an autosomal chromosome. As we all have two copies of chromosome 5 (one from our mother, and their other from our father), if we have one altered gene, the second copy acts as a back up and therefore we are not affected. For an individual to be affected with SMA, they need to have inherited 2 altered genes from their parents. This means, both parents must carry one copy of the altered gene which they both pass onto their child. If a couple already has a child with the disorder, each of their subsequent children has a 25 per cent (or 1 in 4) chance of inheriting the disorder too. Genetic counselling is available for these couples.

Infantile SMA is the most severe form. The symptoms include:

Children with Infantile SMA 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.

Generally speaking, the child’s survival depends on the age of onset of the condition. The earlier the symptoms appear, the shorter the expected life span. In some cases, the baby stops moving in the later stages of its mother’s pregnancy. Sometimes, the symptoms don’t appear for days, weeks or even months after birth. The onset can be sudden and dramatic, or very gradual. However, once the symptoms appear, the child quickly deteriorates.

DNA testing is available, however, that is generally the last part of the testing process. This is because there is so much information that is present on our DNA, it is too hard to test. By performing preliminary tests, the information that is gained from the results narrows down the section of DNA that needs to be investigated. Such preliminary tests include:

A simple blood test for an enzyme called creatine kinase (CK) is usually the first step in the diagnosis process. This enzyme leaks out of muscles that are deteriorating. It’s a nonspecific test, since CK levels are elevated in many neuromuscular diseases, but it’s often useful anyway. High blood CK levels aren’t harmful; they’re just an indicator of muscle damage.
 

An EMG and NCV are two tests that analyse how the muscles are nerves are functioning. EMG is an electrical recording of muscle activity that aids in the diagnosis of neuromuscular disease, while the NCV measures the ability of the nerve to conduct electrical signals. Both tests are usually performed at the same time using the same equipment.
 

A muscle biopsy removes a tiny section of muscle that is analysed at a miscroscopic level for signs of SMA. In children, a punch muscle biopsy may be used.

A child with SMA who must undergo surgery (for example, to correct scoliosis) needs to take special precautions. The surgical team, particularly the anesthesiologist, must thoroughly understand SMA.

Sometimes, especially in the early stages of SMA, the muscle cells that aren’t receiving nerve signals develop certain abnormalities as they try to "reach out" to nerves. These abnormalities can lead to dangerous reactions to muscle-relaxing drugs often used during surgery. Doctors can get around this problem if they’re aware of it, by using different drugs.