Other than injury, most abnormalities of skeletal muscle or the neuromuscular junction are usually diffuse, and most often cause diffuse weakness. A diffuse abnormality of skeletal muscle is called myopathy. The most common myopathy syndromes start with weakness of the bilateral proximal limb muscles, around the hips and shoulders, and then may progress to weakness of muscles of the distal limbs, torso, neck, or head. Myopathy may be caused by many types of pathology, including genetic, idiopathic, immune, metabolic, nutritional, and toxic disorders.
An example of a genetic myopathy is Duchenne muscular dystrophy, which usually affects young boys because of a gene mutation on the X chromosome that causes the gradual loss of skeletal muscle cells. There are multiple autoimmune myopathies, most of which cause skeletal muscle inflammation, which is also called myositis. Some of the autoimmune myositides only affect skeletal muscle, while some also involve inflammation of other tissues, such as the skin, leading to certain kinds of rashes. Toxic disorders, such as from corticosteroid drugs that are used for many inflammatory conditions, are a common cause of myopathy.
A diffuse abnormality of neuromuscular junctions usually causes weakness of muscles of the head and neck initially, particularly those that move the eyelids and eyes. Weakness of muscles of the limbs and torso may also develop. Most of these disorders involve weakness that fluctuates over the course of each day, or from day to day or week to week. The weakness often worsens with muscle use, which is called fatigability, and strength improves with rest. The most common cause of neuromuscular junction dysfunction is an autoimmune disorder called myasthenia gravis. Most of these patients have antibodies in the bloodstream that bind to neurotransmitter receptors in the neuromuscular junction, which causes poor communication between the lower motor neurons and skeletal muscles.