Some people confuse weakness with fatigue. An example of true weakness is being unable to lift something no matter how hard you try, although you may have been able to do so earlier. Fatigue means that with full effort, you can get the strength you need to lift something, but it may feel more difficult and tiresome to do so. Weakness usually only affects some muscle groups, whereas fatigue affects all of them.

The difference between weakness and fatigue is important because while fatigue can be caused by benign problems like sleep loss or a mild illness, weakness can signify something more important, such as stroke or neuromuscular disease. Like numbness, weakness is especially concerning if it comes on suddenly or just affects one side of the body. This may be a sign of a stroke or other serious problem, and requires immediate attention.

When someone is weak, neurologists try to figure out exactly where the source of weakness lies. All other steps in determining the cause of weakness follow this crucial step. Accurately locating the source of the problem can be challenging and requires a degree of expertise, but by using some basic guidelines, asking proper questions and doing a detailed examination, a neurologist can usually localize the source of weakness. The first step in determining why someone is weak is to compare upper and lower motor neuron findings, and to determine if the problem is with the peripheral nervous system or central nervous system (the brain and spinal cord).

The Central Nervous System

If there are significant upper motor neuron findings on a neurological examination, doctors may want to further investigate the brain and spinal cord, as looking for other signs can shed more light onto the exact location of the problem. For example, if someone is numb below a certain level on the neck, this suggests they have a problem with the cervical spinal cord. If they have a problem that includes the face (especially if it's just the lower half of the face), the problem is more likely to be in the brainstem or the brain itself. Because of a quirk in the design of the nervous system, motor fibers cross at the bottom of the brainstem. So if someone's right leg is weak, it could be a problem with the right side of the spinal cord or the left side of the brain.

The Peripheral Nervous System

Weakness due to a problem with the peripheral nervous system can result from problems with the peripheral nerves, the neuromuscular junction, or the muscles.

The peripheral nerves can be damaged by infection, metabolic diseases, and most commonly by impingement in small passages such as the foramina, where they exit the spine. Perhaps the most common examples include radiculopathies, tennis elbow or carpal tunnel syndrome. Syndromes that just affect motor neurons without also causing numbness are rare, but can include certain forms of Guillain-Barré  syndrome, amyotrophic lateral sclerosis and multifocal motor neuropathy.

There is a wide variety of muscular disorders (myopathies) which can lead to weakness. Often the weakness affects both sides of the body equally, as is the case in polymyositis, but in other cases this may not be the case. For example, inclusion body myositis is a common cause of muscle weakness that is frequently asymmetric.


In addition to localizing the lesion, neurologists use information about the course of the weakness, and how it spread, in order to determine the cause. A stroke, for example, tends to come on very quickly, whereas a myopathy can take months to develop. The pattern of spread is also important: Guillain-Barre syndrome, for example, typically starts in the feet and spreads upwards, whereas botulinum toxin causes weakness that descends from the top of the body.

The number of medical problems that cause weakness is very large. Recognizing the location of the problem, and the pattern associated with the weakness, can help doctors to sort through the long list of potential problems to find the true culprit. Remember that unexplained weakness should always be investigated by a qualified medical professional.


Information courtesy of Very Well Health


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