Michael C. Levin, MD, Saskatchewan Multiple Sclerosis Clinical Research Chair and Professor of Neurology and Anatomy-Cell Biology;Adjunct Professor of Neurology, College of Medicine, University of Saskatchewan;University of Tennessee Health Science Center
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
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)
Using the Brain to Move a Muscle
Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to determine what something feels like, as when they feel the texture of fabric or reach in their pocket to find a nickel in an assortment of coins. This information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways:
The sensory nerve pathway to the brain
The motor nerve pathway to the muscle.
When sensory receptors in the skin detect a texture or shape, they transmit an impulse (signal), which ultimately reaches the brain.
The impulse travels along a sensory nerve to the spinal cord.
The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord.
The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord.
The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information located deep in the brain.
The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors).
The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse.
The nerve carrying the impulse crosses to the opposite side at the base of the brain.
The impulse is sent down the spinal cord.
The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord.
The impulse travels out of the spinal cord along the length of the motor nerve.
At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where it stimulates the muscle to move.
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.
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.
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 much alcohol, 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)
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.
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).
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.
In people with weakness, the following symptoms are cause for concern:
Weakness that becomes severe over a few days or less
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.
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.
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.
Botulism (due to the bacteria Clostridium botulinum)
At first, often a dry mouth, drooping eyelids, vision problems (such as double vision), difficulty swallowing and speaking, and rapidly progressive muscle weakness, often beginning in the face and moving down the body
When contaminated food is the source, nausea, vomiting, stomach cramps, and diarrhea
No changes in sensation
Blood or stool tests to check for toxins produced by the bacteria
Sometimes electromyography or examination of a stool sample to check for bacteria
(such as Duchenne muscular dystrophy and limb-girdle muscular dystrophy)
Progressive muscle weakness that
May start during infancy, childhood, or adulthood
Depending on the type, may progress rapidly, causing early death
In some types, an abnormally curved spine (scoliosis) and weakness of the spinal muscles, which often develop during childhood
A thorough family history to determine whether any family members have had a similar disorder
X-rays of the spine to check for scoliosis
*Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present.
†Symptoms vary depending on the location (level) of the damage. Areas that are supplied by the parts of the spinal cord below the damaged part are affected (see Figure: Where Is the Spinal Cord Damaged?).
‡In the United States, MRI is usually available. However, if MRI is unavailable, myelography with CT (x-rays taken after a radiopaque contrast agent is injected via a spinal tap) can be done.
§Sensation is usually not affected.
CT = computed tomography; MRI = magnetic resonance imaging.
If people have severe or rapidly progressing generalized weakness or any problems breathing, doctors first do tests to evaluate the strength of the respiratory muscles (pulmonary function tests). Results of these tests help doctors estimate the risk of sudden, severe malfunction of the lungs (acute respiratory failure).
Other testing is done based on where doctors think the problem is:
A brain disorder: Magnetic resonance imaging (MRI) or, if MRI is not possible, computed tomography (CT)
A spinal cord disorder: MRI or, when MRI is not possible, CT myelography and sometimes a spinal tap (lumbar puncture)
A peripheral nerve disorder (including polyneuropathies) or a neuromuscular junction disorder: Electromyography and usually nerve conduction studies
A muscle disorder (myopathy): Electromyography, usually nerve conduction studies, and possibly MRI, measurement of muscle enzymes, muscle biopsy, and/or genetic testing.
Occasionally, MRI is not available or cannot be done—for example, in people who have a pacemaker, another implanted metal device, or other metal (such as shrapnel) in their body. In such cases, another test is substituted.
For CT myelography, CT is done after a needle is inserted into the lower back to inject a radiopaque dye into the fluid that surrounds the spinal cord.
For electromyography, a small needle is inserted into a muscle to record its electrical activity when the muscle is at rest and when it is contracting.
Nerve conduction studies use electrodes or small needles to stimulate a nerve. Then doctors measure how fast the nerve transmits signals.
If people have no symptoms besides weakness and no abnormalities are detected during the examination, test results are usually normal. However, doctors sometimes do certain blood tests, such as
A complete blood cell count (CBC)
Measurement of levels of electrolytes (such as potassium, calcium, and magnesium), sugar (glucose), and thyroid-stimulating hormone
Erythrocyte sedimentation rate (ESR), which can detect inflammation
Blood tests are sometimes done to evaluate kidney and liver function and to check for the hepatitis virus.
If the cause is identified, it is treated if possible. If weakness began suddenly and causes difficulty breathing, a ventilator may be used.
Physical and occupational therapy can help people adapt to permanent weakness and compensate for loss of function. Physical therapy can help people maintain and sometimes regain strength.
Essentials for Older People
As people age, the amount of muscle tissue and muscle strength tend to decrease. These changes occur partly because older people may become less active but also because the production of the hormones that stimulate muscle development decreases. Thus, for older people, bed rest during an illness can have a devastating effect. Compared with younger people, older people start out with less muscle tissue and strength at the beginning of the illness and lose muscle tissue more quickly during the illness.
Drugs are another common cause of weakness in older people because older people take more drugs and are more susceptible to side effects of drugs.
When evaluating older people who report weakness, doctors also focus on conditions that do not cause weakness but interfere with balance, coordination, vision, or mobility or that make movement painful (such as arthritis). Older people may mistakenly describe the effects of such conditions as weakness.
Regardless of what is causing weakness, physical therapy can usually help older people function better.
Many people mistakenly say they feel weak when they really mean they are tired or their movement is limited because of pain and/or stiffness.
True muscle weakness results only when one part of the pathway necessary for voluntary muscle movement (from brain to muscles) malfunctions.
If weakness becomes severe over a few days or less or if people have any of the warning signs associated with weakness, they should see a doctor immediately.
Often, doctors can determine whether the problem is true muscle weakness and can identify the cause based on the pattern of symptoms and results of the physical examination.
Physical therapy is usually helpful in maintaining strength no matter what the cause of weakness is.