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Weakness is usually defined as a lack of or decrease in muscle strength. It is different from fatigue, which is a loss of energy.

What is going on in the body?

Weakness can be used to describe a mental and physical state in which someone lacks the muscle strength, for example, to walk. It is common and sometimes difficult to evaluate. Weakness has many causes.

What are the signs and symptoms of the condition?

Weakness itself may be a symptom. When someone complains of weakness, the healthcare professional will need to know more details such as:

• when the weakness started
• if the weakness is in one area or affects the whole body
• if the weakness is constant or only happens sometimes
• if the weakness is more physical or mental
• if the weakness came on slowly or happened suddenly
• whether weakness runs in the family
• whether the person has been or is currently sick
• how severe the weakness is

Other symptoms may also be important, such as weight loss, fever, depression, or pain.

What are the causes and risks of the condition?

• muscle problems, such as deconditioning or a lack of exercise, muscle injuries or inherited muscle defects, such as muscular dystrophy
• nerve problems, possibly nerve damage from injury or from toxins such as lead poisoning or alcohol dependence
• nerve damage such as diabetic neuropathy
• vitamin deficiencies, such as lack of vitamin B12
• spinal cord injuries or other disorders
• salt imbalances, such as a low sodium level, called hyponatremia, or a high potassium level, called hyperkalemia
• brain problems, such as a stroke or a condition called Parkinsonism, which affects the ability to move
• autoimmune disorders, which occur when people's immune systems attack their own bodies for unknown reasons. Some examples are multiple sclerosis, which causes inflammation and damage to the brain and myasthenia gravis, which causes muscle weakness that often gets worse toward the end of the day
• hormone imbalances, such as low thyroid hormone levels, called hypothyroidism, or low adrenal hormone levels, called hypoadrenalism
• any infection, especially infectious mononucleosis, flu, poliomyelitis, botulism or pneumonia
• any serious diseases, such as congestive heart failure, cirrhosis, chronic renal failure, or cancer
• psychiatric conditions, especially depression
• chronic fatigue syndrome and fibromyalgia, two poorly understood conditions with no known cause that commonly make people feel weak and tired

Other causes are also possible. Sometimes, no cause is found.

What can be done to prevent the condition?

Prevention depends on the cause. For example, weakness due to lack of exercise can be prevented with regular exercise. Weakness due to alcohol or diabetic neuropathy can be prevented by not drinking alcohol or by controlling diabetes with a proper diet and medications. Many cases of weakness cannot be prevented.

How is the condition diagnosed?

Sometimes the cause of weakness is obvious from the medical history and physical exam. In other cases, further testing is needed, depending on the suspected cause. For example, blood chemistry tests can be used to diagnose salt and water imbalances. Chest x-rays may show pneumonia, and cranial CT scans can help detect a stroke.

Special nerve and muscle tests, such as an electromyogram (EMG) or nerve conduction velocity test, may help diagnose myasthenia gravis or diabetic neuropathy. Other tests may be needed as well.

What are the long-term effects of the condition?

Weakness, when severe, may prevent people from doing normal activities. Most of the serious long-term effects are due to the cause. For example, people who have had a stroke may become paralyzed for life. People who are weak from an infection may have no long-term effects after treatment. People with cancer may die if treatment fails.

What are the risks to others?

Weakness is not contagious and poses no risk to others. But if the cause of weakness is an infection, the infection may be contagious.

What are the treatments for the condition?

Treatment depends on the cause. For example, an infection may be treated with antibiotics. An autoimmune disorder may be treated with corticosteroids or other drugs to suppress the immune system. People with cancer may need surgery, chemotherapy, or radiation therapy. An individual with a muscle injury may need to apply ice to the muscle, take pain medication and rest.

What are the side effects of the treatments?

Side effects depend on the treatments used. All medications have possible side effects. For example, antibiotics may cause allergic reactions and stomach upset. Corticosteroids can cause weight gain, a puffy-looking face, and weak bones. Surgery can be complicated by bleeding, infection, or a reaction to the anesthetic.

What happens after treatment for the condition?

If the weakness goes away or the cause is "fixed," people can usually resume normal activities fairly soon. In other cases, treatment may not end. For example, those with severe heart, liver, or kidney disease usually need treatment for life.

How is the condition monitored?

People can monitor their weakness and how it responds to treatment at home. Further monitoring depends on the cause. For example, those with diabetes need regular check-ups and blood tests.