Muscle Diseases are disorders affecting skeletal muscle structure, metabolism, electrical excitability, blood supply or function, producing symptoms such as weakness, pain, cramps, stiffness, fatigue or abnormal muscle bulk. Skeletal muscle contracts through interactions among actin and myosin within sarcomeres, with calcium release from the sarcoplasmic reticulum regulating contraction.
Muscles depend on intact motor nerves, neuromuscular junctions, energy metabolism, adequate circulation and structural integrity, so apparent muscle weakness can originate from several levels of the motor system rather than muscle tissue itself. Primary muscle diseases include inherited muscular dystrophies, inflammatory myopathies, metabolic myopathies and toxic or drug-induced myopathies. The distinction between true weakness and fatigue, pain-limited effort or neurological weakness is important because treatment depends on localisation and mechanism.
Muscular dystrophies are inherited disorders characterised by progressive muscle degeneration and weakness, although individual diseases differ considerably in inheritance, age of onset and affected muscles. Duchenne muscular dystrophy is an X-linked disorder caused by pathogenic variants affecting dystrophin and typically produces childhood-onset progressive weakness. Becker muscular dystrophy involves dystrophin abnormalities but often has a later or less severe course. Other muscular dystrophies involve proteins such as sarcoglycans, laminin-related structures or other components of muscle cells and their supporting tissues.
Genetic testing has become central to diagnosis, although muscle biopsy and other investigations may still be required in selected cases. Complications can extend beyond skeletal muscle to involve the heart, respiratory muscles, swallowing and mobility, demonstrating that muscular dystrophy is not simply a disorder of limb strength.
Inflammatory myopathies are a group of disorders in which immune-mediated processes damage skeletal muscle and produce weakness. Conditions within this group include polymyositis, dermatomyositis, immune-mediated necrotising myopathy and inclusion body myositis, which have different pathological and clinical characteristics. Weakness is often proximal, affecting tasks such as climbing stairs, rising from a chair or lifting objects overhead. Dermatomyositis also produces characteristic skin manifestations, while some inflammatory myopathies are associated with interstitial lung disease or malignancy.
Blood tests may show elevated creatine kinase and other muscle enzymes, although enzyme levels do not always correlate directly with clinical severity. Diagnosis can involve autoantibody testing, electromyography, magnetic resonance imaging and muscle biopsy. Treatment commonly involves immunomodulatory therapy, but the precise approach depends on subtype, severity and associated organ involvement.
Metabolic and mitochondrial Muscle Diseases interfere with the energy systems required for muscle contraction. Skeletal muscle has high energy requirements and relies on phosphocreatine, glycolysis, fatty-acid oxidation and mitochondrial oxidative phosphorylation according to activity intensity and duration. Glycogen-storage diseases can impair carbohydrate utilisation and produce exercise intolerance, cramps or episodes of muscle breakdown. Fatty-acid oxidation disorders can cause symptoms particularly during prolonged exercise or fasting.
Mitochondrial disorders may affect muscle alongside the brain, heart, eyes or other organs because mitochondria are required throughout the body. Some metabolic myopathies are inherited, while others can result from endocrine disorders, electrolyte abnormalities, medications or systemic disease. Identifying the metabolic pathway affected can influence dietary management, exercise recommendations and avoidance of specific triggers.
Rhabdomyolysis is an acute syndrome involving breakdown of skeletal muscle and release of intracellular contents into the circulation. Causes include severe exercise, crush injury, prolonged immobilisation, seizures, extreme temperatures, infections, toxins and certain medications or metabolic disorders. Myoglobin released from damaged muscle can contribute to acute kidney injury, while electrolyte disturbances such as hyperkalaemia can produce life-threatening cardiac complications. Creatine kinase is typically markedly elevated and is used to monitor muscle injury.
Treatment focuses on addressing the cause, maintaining appropriate fluid balance and detecting complications involving kidney function and electrolytes. Rhabdomyolysis differs from chronic muscular disease because it represents acute muscle breakdown, although an underlying inherited metabolic or structural myopathy can predispose a person to recurrent episodes.
Muscle weakness can also result from disorders outside muscle tissue, making anatomical localisation essential. Motor-neurone disorders damage the neural pathways that activate muscle, peripheral neuropathies interrupt signals through nerves, and neuromuscular-junction disorders impair communication between nerves and muscle. Myasthenia gravis causes fluctuating weakness due to autoimmune disruption of neuromuscular transmission, commonly affecting ocular, facial, bulbar and limb muscles. Unlike primary muscle disease, weakness in neuromuscular-junction disorders may worsen with repeated use and improve with rest, although clinical patterns vary. Electromyography and nerve-conduction studies can help distinguish these conditions, while antibody testing and other investigations can identify specific mechanisms. This distinction prevents the common error of treating every complaint described as “muscle weakness” as a primary muscular disorder.
Diagnosis of Muscle Diseases combines history, examination, laboratory testing, imaging, neurophysiology and genetic investigation. Creatine kinase can indicate muscle-cell injury but may also rise after strenuous exercise or trauma and therefore requires contextual interpretation. Electromyography can demonstrate patterns of muscle or nerve dysfunction, while nerve-conduction studies assess peripheral nerve physiology. Magnetic resonance imaging can identify muscle inflammation, oedema, fatty replacement and patterns of selective muscle involvement and can guide biopsy.
Muscle biopsy provides histological information and remains useful for selected inflammatory, metabolic or structural disorders. Genetic testing has transformed diagnosis for many inherited myopathies and can identify pathogenic variants that establish a molecular diagnosis without biopsy. A detailed family history, age of onset, progression pattern and associated organ involvement remain highly informative.
Management depends on the specific Muscle Disease and may include physical rehabilitation, disease-modifying medication, respiratory monitoring, cardiac surveillance, nutritional management and genetic counselling. Excessive immobilisation can worsen weakness and deconditioning, so appropriately supervised physical activity is often important, although exercise prescriptions must account for disease-specific risks. Corticosteroids and other immunotherapies can reduce inflammation in selected immune-mediated myopathies. Some inherited metabolic disorders have specific dietary or enzyme-replacement approaches, while emerging genetic therapies target particular molecular defects in selected conditions.
Respiratory and cardiac complications require active monitoring in several muscular dystrophies because respiratory muscle weakness and cardiomyopathy can become major determinants of morbidity. Muscle Disease therefore requires multidisciplinary management focused not only on muscle strength but also on mobility, breathing, swallowing, cardiac function, nutrition and long-term independence.
