Spinal muscular atrophy (SMA) is one of many disorders of muscles and the nerves that control them. These are called neuromuscular disorders. These disorders have similar symptoms but vary in which muscles are affected, how severe symptoms are, and when symptoms begin.
There is usually no cure for neuromuscular disorders. Treatment is focused on physical therapy, along with breathing and nutritional support. Here are some of the major categories and most common subtypes of neuromuscular disorders.
Amyotrophic lateral sclerosis (ALS)
Like spinal muscular atrophy, ALS is caused by motor neuron dysfunction. ALS is the most common type of motor neuron disease. It is usually not an inherited condition, unlike SMA. ALS is often called Lou Gehrig’s disease, after the baseball player who was diagnosed with the condition.1,2
Early symptoms of ALS include muscle stiffness, twitching, and weakness. Most people eventually lose the ability to walk, chew, speak, and breathe. The life expectancy is about 2 to 4 years after symptoms begin.1,2
Muscular dystrophy (MD)
Muscular dystrophies are diseases that cause weakness and muscle loss. People with MD have low levels of proteins needed to form healthy muscles. Each type affects different muscles, varies in severity, and progresses at a different speed. SMA is not muscular dystrophy. Some types of MD include:3-5
Duchenne muscular dystrophy (DMD)
Duchenne muscular dystrophy (DMD) is the most common form of childhood MD. It is often confused with type 3 SMA. Symptoms usually appear between 2 and 6 years old. It is caused by mutations in a gene that is used by muscle cells to make a protein called dystrophin. Since this gene is on the X chromosome, DMD affects mostly boys. Life expectancy is 15 to 45 years old.2,4,6
Becker muscular dystrophy (BMD)
Becker muscular dystrophy (BMD) is a milder form of DMD. It is often confused with type 4 SMA. Symptoms usually appear during childhood or early adulthood. Life expectancy is slightly below normal.2,4,6
Congenital muscular dystrophy (CMD)
Congenital muscular dystrophy (CMD) is a group of muscular dystrophies where symptoms appear at birth. Disease progresses slowly, and lifespan is shorter than normal. The symptoms and severity depend on the specific type of CMD. Severe types include Fukuyama CMD, Walker-Warburg disease, and muscle-eye-brain disease.4,6,7
Other types of muscular dystrophy
Other types of muscular dystrophy are caused by mutations in other genes that affect muscles. Some examples of these are:2,5,7
- Emery-Dreifuss muscular dystrophy (EDMD)
- Facioscapulohumeral muscular dystrophy (FSMD)
- Limb-girdle muscular dystrophy (LGMD)
- Myotonic dystrophy (DM)
- Oculopharyngeal muscular dystrophy (OPMD)
Myopathies
Myopathies are diseases where muscle fibers do not work properly. Symptoms of myopathies include muscle weakness, cramps, stiffness, and spasms. Common myopathies include:
Congenital myopathies
This is a broad category of muscle disorders that show symptoms at birth. Progression and severity vary between types. Some examples of congenital myopathies are nemaline myopathy, myotubular myopathy, and central core myopathy.2,6,8
Inflammatory myopathies
These myopathies happen when the immune system mistakenly attacks parts of the muscle. This can cause muscle weakness and pain. The main types of inflammatory myopathy are dermatomyositis, polymyositis, and inclusion-body myositis.9
Metabolic myopathies
These are caused by genetic defects in a certain enzyme needed for muscles to create energy. Symptoms vary depending on the type. Three main types are glycogen storage diseases, lipid storage diseases, and mitochondrial myopathy.10
Peripheral nerve diseases
Peripheral nerves are nerve cells outside of the brain and spinal cord. They send information from the brain and spinal cord to the rest of the body. Peripheral nerve diseases cause weakness in the hands and feet.11
The most common form of hereditary peripheral nerve disease is Charcot-Marie-Tooth (CMT) disease. Common symptoms of CMT are muscle weakness in arms and legs, loss of sensation, and difficulty walking. Symptoms usually appear in early childhood or early adulthood.12
Neuromuscular junction diseases
Neuromuscular junctions are the regions where motor neurons deliver signals to muscle fibers. These signals cause muscle contractions. Without these signals, muscles begin to waste away. Neuromuscular junction diseases result from defects in signal delivery by neurons or signal reception by muscles. The most common neuromuscular junction diseases include:13
Myasthenia gravis (MG)
Symptoms include weakness and fatigue of voluntary muscles, especially those in the eye, face, and throat. MG happens when your immune system mistakenly attacks proteins on muscle cells that receive signals.2,14
Congenital myasthenic syndromes (CMS)
This is a group of disorders of the neuromuscular junction. Symptom onset and severity vary greatly between people. The effects are similar to Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis, but the immune system is not involved.15
Other neuromuscular disorders
There are many other neuromuscular disorders. Some of these are caused by defects in “ion channels.” These are proteins that are important for muscles to contract and relax. Others are caused by defective mitochondria. These are cellular structures that produce energy for muscle cells.16
One example is cerebral palsy (CP). CP is a movement disorder caused by damage to the brain, usually before birth. Muscle stiffness and weakness, floppy limbs, and swallowing problems appear during infancy. Symptoms usually do not worsen over time, and life expectancy varies greatly.17