Muscle weakness is a common symptom of many disease conditions. Usually it is just a temporary reaction to excessive physical exercise but long-lasting muscle weakness that reduces life quality should be examined by a doctor.
Fatigue and muscle weakness occurring after physical is probably the most common cause of muscle weakness. After rest the feeling disappears. Muscle pain and weakness during exercise is caused mainly because of accumulation of lactic acid in muscle tissue.
Infectious processes, especially viral infections, are associated with pain and weakness of muscles and joints.
Hypothyroidism can cause many symptoms including cold sensitivity, fatigue, inefficiency and muscle weakness.
Muscle weakness may exist due to a generalized inflammatory reaction occurring in expanded cancers. There is also a special pathological called Lambert-Eaton syndrome, in which the tumor tissue produces substances disrupting the neuromuscular transmission.
This autoimmune disease causes to disruption of neuromuscular transmission that manifests by rapidly emerging muscle fatigue following even minimal exertion. Resting leads to relatively rapid relief but the problem quickly recurs. The disease significantly impairs life quality and by disruption of function of respiratory muscles it may be even fatal.
Serum ions imbalance
Changes in serum concentration of calcium and potassium may be associated with muscle weakness, especially hypokalemia (low potassium) and hypercalcemia (high calcium). I personally examined a female patient with severe hypokalemia who was unable to raise her arms because of muscle weakness. In addition, muscle weakness is described in severe magnesium deficiency.
This autoimmune disease leads to muscle damage, weakness and sometimes even muscle pain. There is elevated substance called creatine kinase (CK) in serum, which is released from damaged muscle tissue. Quite important fact is that polymyositis is often (but not always) connected to a simultaneous malignant disease.
Congenital muscle diseases
This group of diseases is represented mainly by Duchenne muscular dystrophy. This congenital disease leads to progressive atrophy and muscle weakness. There is no efficient treatment, the patient gradually loses ability to walk and dies around the age of twenty.
Peripheral nerve disorders
Damage done to peripheral nerves ensuring muscle movement (or their spinal neurons) leads to muscle weakness and paralysis. Muscle strength may be reduced also in severe stages of polyneuropathy, for example in Guillan-Barré syndrome. A rare but fatal neurological disease that manifests as muscle weakness is so-called amyotrophic lateral sclerosis.
Body muscles have not enough oxygen and therefore get easily tired. Overall muscle weakness and fatigue are common symptoms of anemic syndrome.
Diagnosis should be based on medical history and physical examination supported by other diagnostic methods like blood tests. A neurologist should evaluate muscle strength and presence of muscle reflexes. Blood tests serve to evaluation of serum minerals, thyroid hormones and creatine kinase (CK) levels. Neuromuscular signal transmission can be investigated by electromyography. When we suspect an autoimmune process, we can try to serologically diagnose special autoantibodies from blood samples. Further diagnostic approach can be directed according to outcome of previously mentioned examination methods.