Spinal Muscular Atrophy (SMA) is a motor neuron disease. The motor neurons affect the voluntary muscles that are used for activities such as crawling, walking, head and neck control, and swallowing. It is a relatively common “rare disorder”: approximately 1 in 6000 babies born are affected, and about 1 in 40 people are genetic carriers.
Spinal Muscular Atrophy (SMA) refers to a group of inherited diseases of the motor nerves that cause muscle weakness and atrophy (wasting). The motor nerves arise from the spinal cord and control the muscles that are used for activities such as breathing, crawling, walking, head and neck control, and swallowing. SMA is a rare disorder occurring in approximately 8 out of every 100,000 live births, and affecting approximately 1 out of every 6,000 to 10,000 individuals worldwide.
SMA affects muscles throughout the body. In the most common types, weakness in the legs is generally greater than in the arms. Sometimes feeding, swallowing, and respiratory function (e.g., breathing, coughing, and clearing secretions) can be affected. When the muscles used for breathing and coughing are affected and weakened, this can lead to an increased risk for pneumonia and other respiratory infections, as well as breathing difficulty during sleep. The brain’s cognitive functions and the ability to feel objects and pain are not affected. People with SMA are generally grouped into one of four types (I, II, III, IV) based on their highest level of motor function or ability.
Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems.
With SMA type II, symptoms may not appear until age 6 months to 2 years.
Type III SMA is a milder disease that starts in childhood or adolescence and slowly gets worse.
Type IV is even milder, with weakness starting in adulthood.
Often, weakness is first felt in the shoulder and leg muscles. Weakness gets worse over time and eventually becomes severe.
Symptoms in an infant:
Breathing difficulty, leading to a lack of oxygen
Feeding difficulty (food may go into the windpipe instead of the stomach)
Floppy infant (poor muscle tone)
Lack of head control
Weakness that gets worse
Symptoms in a child:
Frequent, increasingly severe respiratory infections
Posture that gets worse
The first steps in diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with some simple tests to distinguish spinal muscular atrophy (SMA) from similar conditions (such as muscular dystrophy).
The doctor may order a blood test for an enzyme called creatine kinase (CK), an enzyme that leaks out of muscles that are deteriorating. This is a nonspecific test because CK levels are elevated in many neuromuscular diseases, but it’s often useful anyway. High blood CK levels aren’t harmful in and of themselves, but they do indicate that muscle damage has occurred.
The doctor probably will recommend genetic testing if SMA is suspected, because this is the least invasive and most accurate way to diagnose chromosome 5-related SMA (types 1-4). Genetic testing requires only a blood sample. However, it has implications for the whole family that must be considered (see Causes/Inheritance).
Genetic tests are available for chromosome 5-related SMA and for some of the other forms of SMA. See Athena Diagnostics, a Massachusetts company that offers genetic testing for many neuromuscular diseases, including SMA; and Gene Tests, a website supported by the National Center for Biotechnology Information and sponsored by the University of Washington-Seattle, that lists available genetic tests.
Reliability and specificity of genetic tests are improving, and the number of tests available is expanding rapidly as knowledge and technology improve. For more on getting a definitive genetic diagnosis, see The Genie’s Out of the Bottle: Genetic testing in the 21st century. Your MDA clinic team can guide you toward the right type of genetic testing for your situation.
In rare cases, doctors may order a muscle biopsy, which involves taking a small sample of muscle tissue, usually from the thigh, and looking at it under a microscope.
Other tests sometimes used to diagnose SMA include one that measures nerve conduction velocity — the speed with which signals travel along nerves — and one that measures the electrical activity in muscle, called an electromyogram, or EMG. Nerve conduction velocity tests involve sensations that feel like mild electric shocks, and EMGs require that short needles be inserted in the muscles.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications.
My son, Brayden was diagnosed with Spinal Miscular Atrophy on September 6, 2013! When he was 15 months we began to realize something was wrong! He never began to walk! He never even tried! We would stand him up with help and he just wouldn’t go! We underwent multiples of testing! Blood work after blood work! Everything came back fine! He became sick in march of 2013 and ended up in the hospital for dehydration and ended up leaving with a feeding tube! He was on it for about 2 weeks and showed improvement! But there was still something wrong! At 20 months, his pediatrician desided to send him to Siskins for developmental delay! I expressed some further concerns with this pt who thought it be best to see a neurologist who then recommended testing for SMA! The one test we never wanted to be positive! We were so crushed and in shock, but have somehow managed to get through it and come out stronger than we ever imagined! Brayden is a type 2! Although unable to walk, he is pretty strong! He is able to sit unsupported, able to crawl, and can cruise some along the couch, but is getting to where he can’t do that as much! He is healthy otherwise and we thank God each and everyday for our wonderful little boy!
Contributed by MOM Kristen Hobbs