Weakness refers to loss of muscle strength. That is, people cannot move a muscle normally despite trying as hard as they can. However, the term is often misused. Many people with normal muscle strength say they feel weak when the problem is fatigue or when their movement is limited because of pain or joint stiffness. Muscle weakness can be a symptom of nervous system dysfunction.

For a person to intentionally move a muscle (called a voluntary muscle contraction), the brain must generate a signal that travels a pathway from

  • The brain
  • Through nerve cells in the brain stem and spinal cord
  • Through nerves from the spinal cord to the muscles (called peripheral nerves)
  • Across the connection between nerve and muscle (called a neuromuscular junction)

Also, the amount of muscle tissue must be normal, and the tissue must be able to contract in response to the signal from the nerves. Therefore, true weakness results only when one part of this pathway―brain, spinal cord, nerves, muscles, or the connections between them―is damaged or diseased.

Weakness may develop suddenly or gradually. Weakness may affect all of the muscles in the body (called generalized weakness) or only one part of the body. For example, depending on where the spinal cord is damaged, spinal cord disorders may cause weakness only of the legs.

Symptoms depend on which muscles are affected. For example, when weakness affects muscles of the chest, people may have difficulty breathing. When weakness affects muscles that control the eyes, people may have double vision.

Complete muscle weakness causes paralysis. People may have other symptoms depending on what is causing the weakness. Weakness is often accompanied by abnormalities in sensation, such as tingling, a pins-and-needles sensation, and numbness.

Causes

Because malfunction in the same part of the signal pathway causes similar symptoms regardless of cause, the many causes of muscle weakness are usually grouped by the location of the cause (see Table: Some Causes and Features of Muscle Weakness). That is, causes are grouped as those that affect the brain, spinal cord, peripheral nerves, muscles, or connections between nerves and muscles. However, some disorders affect more than one location.

Common causes

Causes differ depending on whether weakness is generalized or affects only specific muscles.

For generalized weakness, the most common causes are

  • A decrease in general physical fitness (called deconditioning), which may result from illness and/or a decrease in physical reserves (frailty), such as muscle mass, bone density, and the heart’s and lungs’ ability to function, especially in older people
  • Loss of muscle tissue (wasting, or atrophy) due to long periods of inactivity or bed rest, as occurs in an ICU
  • Damage to nerves due to a severe illness or injury, such as severe or extensive burns
  • Certain muscle disorders, such as those due to a low level of potassium (hypokalemia), consumption of too muchalcohol, or use of corticosteroids
  • Drugs used to paralyze muscles—for example, to keep people from moving during surgery or while on a ventilator

For weakness in specific muscles, the most common causes are

  • Strokes(the most common cause of weakness affecting one side of the body)
  • A pinched nerve, as occurs incarpal tunnel syndrome
  • Aruptured or herniated disk in the spine
  • Pressure on (compression of) the spinal cord, as can result from cancer that has spread to the spinal cord
  • Multiple sclerosis

Less common causes

Many other conditions sometimes cause weakness (see Table: Some Causes and Features of Muscle Weakness). For example, electrolyte abnormalities (such as a low level of magnesium or calcium) can cause weakness that sometimes comes and goes, as well as muscle cramping and twitches.

In people with a seizure disorder, one side of the body may become weak after a seizure stops (called Todd paralysis). The weakness usually subsides over several hours.

A low blood sugar (hypoglycemia) can also cause weakness, which resolves when hypoglycemia is treated.

Fatigue

Many people report weakness when their problem is actually fatigue. Common causes of fatigue include a severe illness, cancer, a chronic infection (such as HIV infection, hepatitis, or mononucleosis), heart failure, anemia, chronic fatigue syndrome, fibromyalgia, and mood disorders (such as depression).

Evaluation

First, doctors try to determine whether people are weak or simply tired. If people are weak, doctors then determine whether the weakness is severe enough or worsening quickly enough to be life threatening.

Warning signs

In people with weakness, the following symptoms are cause for concern:

  • Weakness that becomes severe over a few days or less
  • Difficulty breathing
  • Difficulty raising the head while lying down
  • Difficulty chewing, talking, or swallowing
  • Loss of the ability to walk

When to see a doctor

People who have any warning sign should go to an emergency department immediately. Immediate medical attention is crucial because weakness accompanied by a warning sign can worsen quickly and cause permanent disability or be fatal.

If people have symptoms suggesting a stroke (see Table: Some Causes and Features of Muscle Weakness), they should seek medical attention immediately because early treatment can help limit loss of function and sensation.

People without warning signs should call their doctor. The doctor can decide how quickly they need to be seen based on their symptoms and other disorders they have. For most of these people, a delay of a few days is not harmful.

If the weakness worsens gradually (over months to years), people should discuss the problem with their doctor at their next routine visit.

What the doctor does

Doctors first ask questions about the person’s symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause and the tests that may need to be done (see Table: Some Causes and Features of Muscle Weakness).

Doctors ask people to describe in detail what they are experiencing as weakness. Doctors ask

  • When the weakness began
  • Whether it began suddenly or gradually
  • Whether it is constant or is worsening
  • Which muscles are affected
  • Whether and how the weakness affects the ability to do certain activities, such as breathing, brushing their teeth or hair, speaking, swallowing, standing up from a seated position, climbing stairs, and walking
  • Whether they have other symptoms that indicate malfunction of the nervous system, such as speech or vision problems, loss of sensation or memory, or seizures
  • Whether any activity or condition (such as heat or repetitive use of a muscle) makes the weakness worse

What seems to be sudden weakness is sometimes gradual weakness, but people do not notice it until they can no longer do something, such as walking or tying their shoes.

Based on the description of weakness, doctors can often identify the most likely causes, as for the following:

  • A muscle disorder: Weakness beginning in the hips and thighs or the shoulders (that is, people have difficulty standing up or lifting their arms overhead) and no effect on sensation
  • A peripheral nerve disorder: Weakness beginning in the hands and feet (that is, people have difficulty lifting a cup, writing, or stepping over a curb) and loss of sensation

Doctors also ask about other symptoms, which may suggest one or more possible causes. For example, if people with back pain and a history of cancer report weakness in a leg, the cause may be cancer that has spread and put pressure on the spinal cord.

People are asked about symptoms that suggest fatigue or another problem, rather than true muscle weakness. Fatigue tends to cause more general symptoms than true muscle weakness, and fatigue does not follow a particular pattern. That is, it is present all the time and affects the whole body. People with true muscle weakness often report difficulty doing specific tasks, and the weakness follows a pattern (for example, becomes worse after walking).

Doctors ask about recent or current disorders that commonly cause fatigue, such as any recent severe illness or a mood disorder (such as depression).

Doctors ask about past and current use of drugs, including alcohol and recreational drugs. Whether family members have had similar symptoms can help doctors determine whether the cause is hereditary.

During the physical examination, doctors focus on the nervous system (neurologic examination) and muscles.

Doctors observe how the person walks. How people walk may suggest the disorder that is causing symptoms or its location. For example, if people drag a leg, do not swing one arm as much as the other when walking, or both, their symptoms may be caused by a stroke. Doctors also check for other signs that the nervous system is malfunctioning, such as loss of coordination or sensation.

Cranial nerves (which connect the brain with the eyes, ears, face, and various other parts of the body) are tested—for example, by checking eye movements, the ability to speak clearly, and the ability to rotate the head (see Table: Viewing the Cranial Nerves).

Muscles are checked for size and unusual unintended movements (such as involuntary twitches and shaking). Doctors note how smoothly muscles move and whether there is involuntary resistance to movement (detected when doctors try to move a muscle that they have asked the person to relax).

Reflexes are checked. Reflexes are automatic responses to a stimulus. For example, doctors test the knee jerk reflex by gently tapping the muscle tendon below the kneecap with a rubber hammer. Normally, the knee then jerks involuntarily. This evaluation helps doctors identify which part of the nervous system is probably affected, as for the following:

  • The brain or spinal cord: If reflexes are very easy to trigger and are strong
  • The nerves: If reflexes are hard to trigger and are slow or absent

Muscle strength is tested by asking the person to push or pull against resistance or to do maneuvers that require strength, such as walking on the heels and tiptoes or standing up.

A general physical examination is done to look for other symptoms that may suggest a cause, such as difficulty breathing.

Generally, if the history and physical examination do not detect specific abnormalities that suggest a brain, spinal cord, nerve, or muscle disorder, the cause is likely to be fatigue.

 

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