What are neuromuscular disorders?
Neuromuscular disorders represent a broad range of conditions that involve dysfunction of peripheral nerves, muscles or the communication between them. This most often results in muscle weakness, muscle atrophy (loss) and disturbance of sensation (like numbness and tingling). Neuromuscular disorders can cause other symptoms as well, depending on the type.
What are the different types of neuromuscular disorders?
There are several subgroups of neuromuscular disorders based on the area of dysfunction, with several types of conditions in each subgroup.
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A neuromuscular disorder represents any condition that affects the structure and/or function of any component of your neuromuscular system, including:
- Anterior horn cells.
- Nerve roots (including dorsal root ganglia).
- Plexuses (like the brachial plexus and lumbosacral plexus).
- Peripheral nerves.
- Neuromuscular junction.
- Muscle fibers.
Anterior horn cell conditions
Anterior horn cell diseases are often progressive, degenerative diseases (they get worse with time) of the motor neurons. Motor neurons are the cells in your brain and spinal cord that allow you to perform all movements, like walking, speaking and breathing. They relay commands from your brain to the muscles that carry out these functions.
Anterior horn cells specifically are motor neurons that are in the grey matter of your spinal cord. They’re essential for the movement of your skeletal muscles. The breakdown of these cells causes a lack of nerve supply (denervation) to your muscles, resulting in weakness.
Anterior horn cell diseases include:
- Amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease).
- Spinal muscular atrophy.
- Spinal-bulbar muscular atrophy (SBMA), or Kennedy’s disease.
- Polio.
Plexus disorders (plexopathies)
Soon after emerging from your spine at different spinal levels, the nerves may intermingle to form a complex network, or plexus. There’s a plexus in your upper limbs called the brachial plexus. And there’s another in your lower limbs called the lumbar or lumbosacral plexus.
These plexuses may be vulnerable to various types of injuries, like those from trauma, compression or even inflammation. This may cause pain, numbness and tingling, and weakness in the affected limb.
Nerve root conditions
Nerve roots are the short branches of a spinal nerve. Spinal nerves exit your spinal cord along the length of your spine between your vertebrae (spine bone segments). Each spinal nerve contains motor nerve root and sensory nerve root fibers. Motor nerve roots contain nerve fibers that carry commands from your brain and spinal cord through your peripheral nerves to your muscles.
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When surrounding tissues press on nerve roots, it can cause pain, numbness and tingling in different areas of your body. This is called radiculopathy. Compression of motor nerve roots causes muscle weakness.
There are different types of radiculopathies based on where the pinched nerve is along your spine:
- Neck (cervical radiculopathy).
- Middle to upper back (thoracic radiculopathy).
- Low back (lumbar or lumbosacral radiculopathy).
Peripheral nerve conditions
“Peripheral neuropathy” is an umbrella term for nerve diseases that affect any of the nerves outside of your brain and spinal cord (peripheral nerves). There are several types and several causes. You can’t move your muscles without the proper functioning of peripheral nerves, which is why these conditions fall under the even broader category of neuromuscular disorders.
Peripheral neuropathies may affect only parts of your body or most of your body. The most common form of peripheral neuropathy affects the parts of your limbs farthest away from your core, especially your feet and hands.
Examples of peripheral neuropathies include those related to:
- Diabetes.
- Alcohol use disorder.
- Malnutrition.
- Amyloidosis.
- Genetic or inherited causes like Charcot-Marie-Tooth disease.
- Autoimmune or inflammatory causes like chronic inflammatory demyelinating polyneuropathy (CIDP) and Guillain-Barré syndrome.
- Toxic exposures due to toxic chemicals, like cleaning fluids or pest killers.
Neuromuscular junction conditions
The neuromuscular junction (NMJ) is a connection (synapse) between the end of a motor nerve and a muscle. To communicate with the muscle, motor nerves release a molecule across the synapse called acetylcholine. Issues with this communication lead to certain neuromuscular junction disorders.
The main conditions that affect the NMJ include:
- Botulism.
- Lambert-Eaton myasthenic syndrome (LEMS).
- Myasthenia gravis.
Muscle conditions
Myopathies are conditions that directly affect your skeletal muscles (muscles that connect to your bones). These diseases attack muscle fibers, making your muscles weak. There are several types of myopathies. Some you’re born with (they’re genetic or inherited) and some you can develop later in life.
Myopathies don’t include direct muscle injuries, like a muscle tear, strain or bruise.
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Muscular dystrophy
Muscular dystrophy refers to a group of more than 30 genetic (inherited) conditions that affect the functioning of your muscles. Some forms include:
- Duchenne muscular dystrophy (DMD). This is the most common type.
- Becker muscular dystrophy (BMD).
- Myotonic dystrophy.
- Oculopharyngeal muscular dystrophy (OPMD).
Autoimmune and inflammatory myopathies
Autoimmune and inflammatory myopathies are conditions that happen due to issues with your immune system that cause problems with muscle function. Examples include:
- Dermatomyositis.
- Polymyositis.
- Immune-mediated necrotizing myopathy.
Other myopathies
Other myopathies include those related to:
- Electrolyte imbalances: Electrolyte imbalances, like hypercalcemia (high blood calcium) and hyperkalemia (high blood potassium), can cause muscle issues.
- Endocrine myopathies: These happen when hormones interfere with muscle function. This can happen from issues with your thyroid, parathyroid glands or adrenal glands, for example.
- Metabolic myopathies: Issues with genes that give instructions for enzymes that your muscles need cause metabolic myopathies. Examples include glycogen storage diseases and lipid storage disorders.
- Mitochondrial myopathies: These happen when there’s a defect in muscle mitochondria, which are the energy-producing part of cells.
- Periodic paralysis: Periodic paralysis is a group of inherited neurological disorders. They involve episodes in which your muscles become slack, weak and unable to contract.
- Toxic myopathies: This happens when a toxin or medication interferes with muscle structure or function.
What are the most common neuromuscular disorders?
Peripheral neuropathy is the most common type of neuromuscular disorder. This is mainly because it has several types and causes. Researchers estimate that more than 20 million people in the U.S. have some form of peripheral neuropathy.
But most people don’t think of peripheral neuropathies as neuromuscular disorders, as they have their own group. Some of the most well-known — but ultimately rare — neuromuscular diseases include ALS and muscular dystrophy.
About 30,000 people in the U.S. have ALS, with an estimated 5,000 new cases diagnosed each year. All the different types of muscular dystrophy combined affect about 16 to 25 per 100,000 people in the U.S.
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