Types of Neuromuscular Disorders

Neuromuscular disorders are conditions that affect the nerves that send electrical signals to muscles to control movement. When the nerves are damaged, communication between the nerves and muscles becomes disrupted. This results in significant muscle weakness, wasting, and loss of function.

Nerves communicate with muscles through the release of neurotransmitters at the neuromuscular junction, the space between a nerve cell and a muscle fiber. Neuromuscular disorders can damage the nerve itself or the neuromuscular junction, where the signal is transmitted from a nerve to a muscle.

Symptoms and Diagnosis

Most neuromuscular disorders begin by affecting large skeletal muscles, such as those in the arms and legs, but can progress to affect the smaller muscles of the eyes, throat, and chest, which can lead to other problems.

Symptoms of neuromuscular disorders include:

• Muscle weakness
• Muscle wasting (atrophy)
• Muscle twitches, cramps, or spasms
• Muscle pain
• Numbness and tingling
• Decreased coordination
• Droopy eyelids and double vision due to eye muscle weakness
• Difficulty swallowing due to weakness of the pharynx
• Difficulty breathing due to weakness of the diaphragm
• Poor balance

A diagnosis of a neuromuscular disorder begins with a physical exam from a physician. Your doctor will ask you about your medical history, family history, and symptoms. They will also examine your muscle strength, muscle tone, and reflexes.

Diagnostic tests may be ordered to help determine a diagnosis, including:

• Blood work to assess levels of enzymes and inflammatory markers
• Genetic testing to assess for genetic risk
• Magnetic resonance imaging (MRI) of your brain and spinal cord to assess for damage
• Electromyography (EMG) testing to assess the electrical activity of your muscles
• Nerve conduction tests to assess how signals travel from your nerves to your muscles
• Muscle biopsies to examine the quality of your muscle tissue
• Lumbar puncture, also called a spinal tap, to assess the quality of your cerebrospinal fluid within your spinal canal

Muscular Dystrophies

Muscular dystrophies are a group of genetic diseases characterized by a gradual loss of motor function, muscle weakness and wasting away, gait problems, progressive respiratory failure, and cardiomyopathy.

There are nine different types of muscular dystrophy, all caused by genetic mutations, but the most common forms are Duchenne muscular dystrophy and Becker muscular dystrophy.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy usually begins in boys between 2 and 6 years of age, and is characterized by general muscle weakness and atrophy affecting the arms and legs. The condition progresses to all muscles, including the diaphragm, which controls breathing.

Becker Muscular Dystrophy

Becker muscular dystrophy usually occurs later, during adolescence to early adulthood, and progresses more slowly than Duchenne muscular dystrophy. Muscle weakness and atrophy are the characteristic symptoms.

Myopathies

Myopathies, meaning diseases of muscles, are classified into the following categories:

• Congenital: Occurring from birth from inherited genes and affecting all voluntary muscles of the body, including those involved in swallowing and breathing
• Distal: Occurring from inherited genes, with onset in childhood or early adulthood, and affecting the lower arms and legs
• Endocrine: Occurring due to deficient levels of thyroid hormone due to an underactive or overactive thyroid gland
• Inflammatory: Occurring because of an autoimmune response that attacks the muscles
• Metabolic: Occurring from a genetic mutation that disrupts metabolic processes within the body, resulting in widespread muscle weakness

Unlike other neuromuscular conditions, congenital and endocrine myopathies are usually not progressive and symptoms do not worsen over time. On the other hand, distal myopathies are slowly progressing but not considered life-threatening. Inflammatory and metabolic myopathies can vary in severity based on the age of onset.

Motor Neuron Diseases

Motor neuron diseases damage motor neurons, which are specific nerve cells that control contraction of muscle fibers. With motor neuron diseases, the muscles become weak and lose function over time from a lack of electrical signaling from the nerves to the muscles.

The most common forms of motor neuron disease are amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig’s disease, is a genetic disorder that results from hardening of the spinal cord. It causes damage to the nerves that control muscles and voluntary movement. ALS can affect people of any age, although middle-aged adults in their 50s are most commonly diagnosed with the condition.

Symptoms of ALS include muscle weakness, atrophy, stiffening, spasticity, muscle twitches, and cramping. Approximately 80% of cases begin with muscle weakness or spasticity in one arm or leg.

As ALS progresses, more muscles become affected, causing widespread muscle weakness and paralysis. Typical life expectancy after age of diagnosis is typically three to five years, although 10% to 20% of patients with ALS survive for 10 years or more.

Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a genetic disorder caused by a genetic mutation that causes damage to a motor neuron protein crucial to normal functioning of motor neurons. There are several different forms of SMA, with ages of onset that vary between childhood, adolescence, and adulthood.

SMA most commonly causes weakness of the muscles closest to the body like the back, shoulders, hips, and thighs. The lower body is usually more affected than the upper body. Patients with SMA also have diminished deep tendon reflexes, and often develop scoliosis, abnormal spinal curvature, from weakness of the spinal muscles and breathing difficulties if the diaphragm is affected.

Infants diagnosed with type 0 SMA present with severe muscle weakness and heart failure, while infants with type 1 SMA have difficulty breathing and swallowing and survive for only a few years into childhood.

Infants diagnosed with type 2 SMA never gain the ability to stand or walk independently, but survive into young adulthood. Older children and teens diagnosed with type 3 SMA who have learned to stand and walk slowly lose their ability to do so.

Type 4 SMA is usually diagnosed during the late teens or adulthood, and those affected are able to maintain their ability to walk but have muscle weakness. Patients with type 3 and type 4 SMA have a better life expectancy if the respiratory muscles are not affected, with those with type 4 SMA typically having normal life expectancies.

Ion Channel Diseases

Ion channel diseases are a group of disorders that affect the ability of muscles to contract due to altered levels of potassium ions in the blood. This often results in periodic paralysis or temporary loss of the ability to contract muscles. This happens because of inherited genetic mutations that cause defective sodium-potassium channels within muscle cells.

Periodic paralysis caused by too much potassium affects men and women equally, with onset in childhood and decreased frequency of attacks after middle age. Periodic paralysis caused by too little potassium also affects men and women equally, with onset in childhood or adulthood. However, it slowly progresses to permanent leg weakness often after the age of 50.

Periodic paralysis can also occur from Andersen-Tawil syndrome, an inherited disorder that affects the gene controlling sodium-potassium channels. It decreases the ability of muscles to contract, including the heart muscles. That's why Andersen-Tawil syndrome is more serious than the other forms of periodic paralysis. It can result in the development of cardiac rhythm irregularities. Andersen-Tawil syndrome occurs during childhood or adolescence, and can slowly progress to permanent muscle weakness over time.

Mitochondrial Diseases

Mitochondria are in nearly every cell of the body, and are responsible for processing oxygen and converting substances from the foods we eat into energy. Mitochondria produce 90% of the energy our bodies need to function.

Mitochondrial diseases occur when mitochondria fail to produce enough energy for the body to function properly.

Mitochondrial Myopathies

Mitochondrial myopathies are caused by mitochondrial diseases that damage the mitochondria, resulting in damage to the nerve and muscles cells, which have high energy demands. Symptoms of mitochondrial myopathies include muscular and neurological complications, such as muscle weakness, fatigue, exercise intolerance, poor balance and coordination, seizures, heart problems, vision problems, hearing loss, and developmental delays.

Friedreich's Ataxia

Friedreich's ataxia is another condition caused by mitochondrial disease, and results in muscle weakness and ataxia, or a loss of balance and coordination. Friedreich's ataxia affects the spinal cord, peripheral nerves that control muscles, and the cerebellum, the portion of the brain that aids in balance and coordination of movements.

Friedreich's ataxia is a genetic condition caused by a gene mutation, and a diagnosis is usually made between 10 and 15 years of age. Symptoms of Friedreich's ataxia progress slowly, and many people with the condition live active and fulfilling lives.

Neuromuscular Junction Diseases

Neuromuscular junction diseases affect the neuromuscular junction, the gap between a nerve cell and a muscle fiber where the neurotransmitter acetylcholine is released from the nerve to elicit contraction of the muscle fiber.

Myasthenia Gravis

Myasthenia gravis is an autoimmune disease that causes inflammation throughout the body. With myasthenia gravis, the body produces antibodies that attack the receptors for acetylcholine, reducing the body’s ability to contract muscles. This leads to weakness, atrophy, and fatigue.

Muscle weakness occurs in the arms, legs, face, and eyes. It can cause double vision and droopy eyelids. Myasthenia gravis can occur at any age and the cause is unknown, although damage to the thymus gland or infection from bacteria or viruses may trigger an autoimmune reaction.

Life expectancy is usually not affected, and many people with myasthenia gravis live active lives.

Peripheral Nerve Diseases

Peripheral nerve diseases affect the peripheral nerves that exit from the spinal cord and control the muscles of the arms and legs.

Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease is a class of peripheral nerve disorders that cause muscle weakness and atrophy as well as loss of sensation, most commonly in the legs and feet. However, the hands and arms are occasionally affected. Other symptoms of Charcot-Marie-Tooth disease include joint contractures, poor balance and coordination from muscle weakness, loss of fine motor movements if the hands are affected, and scoliosis due to weakness of the spinal muscles.

Charcot-Marie-Tooth disease is a genetic disorder caused by a gene mutation that damages myelin, an insulating sheath that surrounds all nerves and aids in the conduction of electrical signals. Progression of Charcot-Marie-Tooth disease is gradual, with an average age of onset in adolescence or early adulthood, and life expectancy is often not affected.

Treatment

At this time, there is no cure for neuromuscular disorders. However, there are treatment options that can help improve symptoms, halt disease progression, and enhance quality of life, including:

• Medications to suppress the immune system and decrease symptoms of autoimmune conditions
• Pain management
• Physical and occupational therapy to maintain muscle strength, range of motion, joint mobility, and overall level of functioning
• Braces and splints to decrease and prevent muscle contractures and maintain functional range of motion
• Assistive devices such as canes, crutches, walkers, and wheelchairs to help with overall mobility
• Apheresis, a process that filters out antibodies from the blood
• Clinical trials for new medications and treatment techniques

Summary

Neuromuscular disorders are usually genetic or a result of an autoimmune disease. Even for those that cannot be cured, there are treatment options available to help manage symptoms and delay disease progressions. The earlier you get treated, the better. Therefore, if you start noticing problems with movement and coordination, it's best to call your doctor and get evaluated.

A Word From Verywell

While there is no cure for neuromuscular disorders, research is ongoing and treatments have come a long way for many of these conditions to provide an excellent quality of life for those with these disorders. Medications to halt symptom progression and physical therapy to maintain mobility and muscle strength can help people with neuromuscular disorders effectively manage their conditions.

It is important to start treatment early to manage symptoms and prevent progression of your neuromuscular disorder. This can help you maintain maximum muscle strength and mobility to complete everyday tasks, postures, and movements like sitting, standing, getting in and out of bed, in and out of a chair, and walking with as much independence as possible.