Diseases of the muscular system range from inherited conditions that progressively destroy muscle tissue to autoimmune attacks that cause inflammation and weakness. They can be grouped into several major categories: muscular dystrophies, inflammatory myopathies, neuromuscular junction disorders, metabolic myopathies, and conditions where nerve damage leads to muscle wasting. Each has distinct causes, symptoms, and outcomes, but they share a common thread: they interfere with the muscle’s ability to contract, repair itself, or receive signals from the nervous system.
Muscular Dystrophies
Muscular dystrophies are a group of genetic diseases in which mutations cause the body to produce defective or missing proteins that muscles need to stay intact. The most well-known is Duchenne muscular dystrophy (DMD), which affects roughly 1 in every 5,000 males between ages 5 and 9. DMD is caused by mutations that completely eliminate a critical structural protein in muscle fibers. Without it, muscle cells break down with normal use and are gradually replaced by scar tissue and fat. Boys with DMD typically begin showing weakness in their hips and thighs in early childhood and often need a wheelchair by their early teens.
Becker muscular dystrophy involves mutations in the same gene but produces a partially functional version of that protein. It progresses more slowly, and symptoms may not appear until the teenage years or later. DMD is about three times more prevalent than Becker. A third common form, myotonic dystrophy, is the most frequently diagnosed muscular dystrophy in adults. It causes prolonged muscle stiffness after contraction (you grip something and can’t easily let go) along with gradual weakness, and it can also affect the heart, eyes, and hormonal systems.
In 2023, the FDA approved the first gene therapy for DMD, a treatment designed for children ages 4 to 5 that delivers a shortened but functional version of the missing protein gene directly into muscle cells. It was granted accelerated approval, meaning its long-term benefits are still being confirmed in ongoing trials.
Inflammatory Myopathies
Inflammatory myopathies are autoimmune conditions where the body’s immune system mistakenly attacks its own muscle tissue. The three main types are dermatomyositis, polymyositis, and inclusion body myositis. All three cause progressive muscle weakness, but they differ in who they affect, what the immune system targets, and how they present.
Dermatomyositis stands out because it produces visible skin changes, often before any muscle weakness appears. A distinctive purplish rash around the eyelids and raised, scaly patches over the knuckles can precede weakness by months or even up to two years. The underlying problem is an immune attack on the tiny blood vessels supplying muscle tissue. The body deposits immune complexes on capillaries, which damages blood flow and causes muscle fibers to die from the outside in.
Polymyositis has no skin involvement. Instead, specialized immune cells directly invade individual muscle fibers. The weakness develops over weeks to months, typically affecting the hips and shoulders first. People notice trouble climbing stairs, getting out of a chair, or lifting their arms above their heads. Muscle pain and tenderness are less common in polymyositis than in dermatomyositis.
Inclusion body myositis is the most common inflammatory muscle disease in people over 50. Unlike the other two, it often affects the forearms and thigh muscles, causing grip weakness and frequent falls. It responds poorly to the immune-suppressing treatments that help dermatomyositis and polymyositis.
Neuromuscular Junction Disorders
Your muscles contract when a nerve signal triggers the release of a chemical messenger at the point where nerve meets muscle. Diseases that disrupt this junction cause weakness not because the muscle itself is damaged, but because the signal never arrives properly.
Myasthenia gravis is the most common of these disorders. The immune system produces antibodies that block or destroy the receptors on muscle cells that receive nerve signals. This makes muscles weaken with repeated use and partially recover after rest. Drooping eyelids and double vision are often the first symptoms because the small muscles controlling eye movement fatigue easily. In more severe cases, weakness spreads to the limbs, throat, and breathing muscles. Myasthenia gravis does not destroy muscle tissue the way dystrophies do, so treatment that suppresses the immune response or boosts signal transmission can significantly improve function.
Metabolic and Mitochondrial Myopathies
Muscles need a constant supply of energy to contract. Metabolic myopathies are genetic conditions that break one of the biochemical pathways muscles use to produce that energy. The result is exercise intolerance, cramping, and sometimes episodes where muscle tissue breaks down rapidly after exertion.
One well-known example, McArdle disease, prevents muscles from breaking down their stored sugar (glycogen) for fuel. People with McArdle disease experience pain and stiffness within minutes of starting exercise, but often feel better if they rest briefly and then continue, as the body switches to alternative fuel sources.
Mitochondrial myopathies involve defects in the mitochondria, the structures inside cells responsible for converting nutrients into usable energy in the form of ATP. Nearly all mutations linked to mitochondrial disease reduce the efficiency of the energy-production chain inside these organelles. Because mitochondrial DNA is small (about 16,500 base pairs, compared to billions in nuclear DNA), it is especially vulnerable to mutations. Symptoms vary widely depending on which tissues are most affected, but muscle weakness, exercise intolerance, and a buildup of lactic acid during activity are hallmarks. Some mitochondrial disorders also affect the brain, heart, and other organs that have high energy demands.
Motor Neuron Diseases and Muscle Wasting
Motor neuron diseases destroy the nerve cells that control voluntary movement. While the primary problem is in the nervous system, the downstream effect on muscles is devastating. Amyotrophic lateral sclerosis (ALS) is the most recognized example. As motor neurons in the brain and spinal cord degenerate, the muscles they control stop receiving signals, shrink, and eventually fail. Visible twitching (fasciculations) and progressive atrophy are characteristic signs. ALS typically leads to respiratory failure as the muscles that control breathing weaken.
Spinal muscular atrophy (SMA) is another motor neuron disease, but it is genetic rather than acquired. It primarily affects infants and children, causing weakness that begins in the muscles closest to the trunk and can range from severe, life-threatening weakness in infancy to milder forms that allow independent walking into adulthood.
Common Symptoms Across Muscle Diseases
Despite their different causes, most muscle diseases share a recognizable pattern. The most common signs are weakness, stiffness, cramps, and spasms. Weakness is almost always “proximal,” meaning it starts in the muscles of the hips, thighs, and shoulders before spreading outward. This is why difficulty climbing stairs, rising from a chair, and reaching overhead are such consistent early complaints. Unlike nerve damage, muscle diseases typically do not cause numbness or tingling.
Some conditions caused by hormonal imbalances also damage muscles. An overactive thyroid, for example, can cause muscle weakness alongside eye bulging and an enlarged thyroid gland. Certain medications, particularly cholesterol-lowering statins and corticosteroids used long-term, can also trigger muscle damage that mimics primary muscle disease.
How Muscle Diseases Are Diagnosed
Diagnosis usually starts with blood tests that measure enzymes released when muscle fibers are damaged. From there, two key tests help narrow things down. Electromyography (EMG) involves inserting a thin needle into a muscle to record its electrical activity. It is minimally invasive and helps distinguish whether weakness is coming from the muscle itself or from the nerves supplying it. EMG is highly accurate for detecting nerve-related problems (100% sensitivity in one study of 123 patients), but less reliable for identifying muscle diseases on its own.
Muscle biopsy, where a small sample of tissue is removed and examined under a microscope, remains the most definitive test for many myopathies. In the same study, biopsy correctly identified the diagnosis in about 89% of cases and was 97% specific for detecting muscle disease. The typical diagnostic path starts with EMG and moves to biopsy when a muscle disorder is suspected. Genetic testing has become increasingly important and can now confirm many dystrophies and metabolic myopathies without a biopsy at all.