Dominic Stone
Muscle atrophy is the loss of muscle tissue, which can be caused by a variety of factors, including disease, injury, inactivity, and aging. The three types of muscle atrophy are physiologic, pathologic, and neurogenic. Physiologic atrophy is caused by muscle disuse, which can be due to sedentary jobs, health problems, or aging. Pathologic atrophy is associated with aging, malnutrition, and diseases such as Cushing's disease. Neurogenic atrophy, the most severe form, occurs due to nerve injuries or diseases affecting the nerves that control muscles, such as amyotrophic lateral sclerosis (ALS). In addition, specific conditions like myositis, polio, and mitochondrial dysfunction can contribute to muscle atrophy. Intensive care unit (ICU) treatment and prolonged bed rest have also been linked to skeletal muscle atrophy. Treatments for muscle atrophy include physical therapy, exercise, surgery, and functional electrical stimulation (FES).
| Characteristics | Values |
|---|---|
| Type | Physiologic, Pathologic, Neurogenic |
| Causes | Inactivity, inadequate nutrition, aging, starvation, disease, injury, genetic disorders, metabolic disorders, nervous system disorders, neuromuscular diseases, inflammation, viral infection, autoimmune conditions, nerve damage |
| Diseases | Muscular dystrophy, myositis, polio, mitochondrial dysfunction, myasthenia gravis, spinal muscular atrophy, amyotrophic lateral sclerosis, carpal tunnel syndrome, multiple sclerosis, Duchenne muscular dystrophy, Becker-Kiener muscular dystrophy, limb-girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy, myotonic muscular dystrophy, facioscapulohumeral muscular dystrophy, congenital muscular dystrophy |
| Symptoms | Loss of movement, loss of strength, reduced muscle mass, muscle weakness, muscle pain, muscle stiffness, loss of endurance, impaired movement, involuntary muscle twitches, sensory disorders, loss of stamina, difficulty performing daily activities, slow walking, trouble climbing stairs, poor balance, falls, challenges with movement, delayed growth, hearing loss |
| Treatment | Exercise, physical therapy, surgery, functional electrical stimulation, ultrasound therapy, medication |
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What You'll Learn
Inactivity and immobility
Muscle atrophy due to inactivity can occur when a person remains immobile while recovering from an illness or injury. For example, limb muscle wasting is common in many critical illnesses, particularly sepsis, organ failure, hyperglycemia, and diseases associated with persistent systemic inflammation or oxidative stress. Additionally, prolonged bed rest in the absence of disease can also result in skeletal muscle atrophy.
Furthermore, muscle atrophy can be caused by neuromuscular diseases such as muscular dystrophy, which result in muscle weakness and loss of mass over time. These diseases are often genetic and affect the production of proteins necessary for healthy muscle formation. Examples of muscular dystrophy include Duchenne muscular dystrophy, Becker-Kiener muscular dystrophy, and limb-girdle muscular dystrophy.
In summary, inactivity and immobility are significant contributors to muscle atrophy, whether through physiologic atrophy, illness or injury-related immobility, sarcopenia, or neuromuscular diseases such as muscular dystrophy.
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Malnutrition and ageing
Muscle atrophy is the wasting or thinning of muscle mass. It can be caused by the disuse of muscles or neurogenic conditions. Physiologic atrophy, caused by insufficient muscle use, can be reversed with exercise and better nutrition. Malnutrition-related muscle atrophy may develop as a result of medical conditions that impair the body's ability to absorb nutrients. Cachexia, a complex metabolic condition that causes extreme weight loss and muscle atrophy, can develop as a symptom of another underlying condition, such as cancer, HIV, or multiple sclerosis (MS). People with cachexia may experience significant weight loss despite consuming a high number of calories.
As people age, their bodies produce fewer proteins essential for muscle growth, resulting in a condition called sarcopenia. Sarcopenia is a type of muscle atrophy that specifically affects older individuals. While everyone experiences some muscle loss with age, sarcopenia causes this loss to occur at a faster rate. The reduction in protein availability leads to shrinking muscle cells, contributing to the symptoms of sarcopenia, which include muscle weakness, loss of endurance, and difficulty performing basic daily activities.
Age-related hormonal changes also play a role in the development of sarcopenia. Lower levels of hormones such as testosterone and insulin-like growth factor (IGF-1) affect muscle fibers and contribute to muscle atrophy. Additionally, the reduction in nerve cells responsible for transmitting signals from the brain to initiate movement can further accelerate sarcopenia. The risk of sarcopenia increases significantly with age, with estimates ranging from 11% to 50% in individuals aged 80 and older.
Ageing, on the other hand, is a natural process that leads to a gradual loss of muscle mass and strength. This loss typically begins in our 30s, and we continue to lose muscle as we age. The rate of muscle loss can become more noticeable around age 60 and accelerate further after age 80. While sarcopenia is a common condition in older adults, it is not inevitable, and steps can be taken to slow down its progression.
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Muscular dystrophy
Duchenne muscular dystrophy (DMD) is the most prevalent form, predominantly affecting boys, although girls can exhibit milder versions of the disease. DMD patients may experience delayed motor milestones, frequent falls, and muscle pain and stiffness. The condition can progress to involve the heart and lungs, further complicating the disease.
Becker muscular dystrophy (BMD) is the second most common type, typically affecting boys, but girls can also experience milder symptoms. BMD symptoms usually appear between the ages of 5 and 60, but they often manifest during the teenage years.
Emery-Dreifuss muscular dystrophy (EDMD) primarily affects male children and young adults, causing muscle weakness in the shoulders, upper arms, and shins. It can also impact the heart, leading to potential cardiac complications.
Facioscapulohumeral muscular dystrophy (FSHD) commonly affects the muscles in the face, shoulders, and upper arms. It often affects one side of the body more than the other, causing symptoms such as difficulty closing the eyelids and mild hearing loss. FSHD symptoms typically appear before the age of 20.
Limb-girdle muscular dystrophy (LGMD) affects the muscles in the upper arms, upper legs, shoulders, and hips and can arise at any age. Oculopharyngeal muscular dystrophy (OPMD) weakens the muscles in the eyelids and throat, causing droopy eyelids and swallowing difficulties, with symptoms usually appearing in middle age.
Myotonic dystrophy, the most frequently diagnosed adult-onset form, affects both men and women, causing difficulty in muscle relaxation after use. It can impact the heart and lungs and lead to endocrine issues, such as thyroid disease and diabetes.
Congenital muscular dystrophies (CMD) are a group of muscular dystrophies present at or near birth, causing overall muscle weakness and potentially affecting joints, spine, breathing, intellectual development, and other systems.
Distal muscular dystrophy primarily impacts the muscles of the hands, feet, lower arms, and lower legs.
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Neurological damage
Neurogenic atrophy can be caused by various conditions and diseases, including Amyotrophic Lateral Sclerosis (ALS), which damages the motor nerve cells controlling the muscles. Other conditions include Guillain-Barre syndrome, carpal tunnel syndrome, spinal cord injuries, and multiple sclerosis. In addition, polio, an infectious disease that attacks the nervous system, can cause permanent paralysis and neurogenic muscle atrophy.
Mitochondrial dysfunction is another factor that can lead to neurogenic atrophy. Mitochondria are responsible for generating chemical energy for cells, and their dysfunction affects muscle tissue regulation. This can result in skeletal muscle atrophy as the muscles no longer receive signals from the nerves to contract.
Diseases and chronic conditions can also contribute to neurological damage and muscle atrophy. For example, muscular dystrophy is a group of progressive conditions that cause loss of muscle mass due to mutations in genes involved in protein production. Similarly, myositis refers to the inflammation of muscles, causing weakness and pain, which can develop after a viral infection or as an autoimmune condition.
Critically ill patients in intensive care units (ICU) often experience skeletal muscle atrophy due to prolonged bed rest and the severity of their illnesses. Additionally, patients treated with neuromuscular blockers are at an increased risk of muscle wasting. The factors contributing to ICU-associated muscle atrophy are complex and vary depending on hospitalization duration and the specific illness or injury.
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Mitochondrial dysfunction
Mitochondria are parts of cells that generate most of the chemical energy to power them. Mitochondrial dysfunction occurs when mitochondria don't work as well as they should due to another disease or condition. Mitochondrial dysfunction affects muscle tissue regulation, among many other processes, and can lead to skeletal muscle atrophy. Mitochondrial dysfunction has been recognised as an important sign of skeletal muscle atrophy, but its specific molecular mechanisms are unknown.
Mitochondrial diseases are a group of conditions that affect how mitochondria function in your cells. They can affect several organ systems in your body. Mitochondrial dysfunction can lead to a series of pathophysiological changes. Many conditions can lead to secondary mitochondrial dysfunction, including Alzheimer's disease, muscular dystrophy, type 1 diabetes, and multiple sclerosis (MS). If you have secondary mitochondrial dysfunction, you don't have a genetic mitochondrial disease, but you are still at risk of developing one if it runs in your biological family history.
Symptoms of mitochondrial diseases can be present at birth, but they can also arise at any age. They can range from mild to severe and could include poor growth, muscle weakness, muscle pain, vision and/or hearing loss, developmental delays or issues with cognitive development, diarrhoea or constipation, unexplained vomiting, acid reflux and/or swallowing difficulties, and respiratory (breathing) problems. A healthcare provider will diagnose a mitochondrial disease after a series of examinations and tests that may include a review of your medical and family history, a physical examination, a neurological examination, and a metabolic examination that includes blood and urine tests.
Mitochondrial dynamics and mitophagy are two different mitochondrial quality control mechanisms that are interrelated and mutually regulated. The former maintains the balance of the mitochondrial network, eliminates damaged or aged mitochondria, and enables cells to survive normally. Mitochondrial dysfunction characterised by mitochondrial dynamic imbalance and mitophagy disruption can lead to varying degrees of muscle atrophy, but the underlying mechanism of action is still unclear.
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Frequently asked questions
Muscle atrophy is caused by a lack of physical activity or immobilisation of joints, metabolic disorders, genetic disorders, nervous system disorders, inadequate nutrition, ageing, and diseases such as Cushing's disease, muscular dystrophy, myositis, polio, mitochondrial dysfunction, and myasthenia gravis.
As people age, their bodies produce fewer proteins that promote muscle growth, leading to a condition called sarcopenia, which is characterised by a decrease in muscle mass, strength, and function.
If a person does not use a particular muscle, the body will eventually break it down to conserve energy. This can occur if someone is recovering from an illness or injury, or if they have a sedentary lifestyle.
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