Weakness: Finding the Cause

How Neurologists Think About Weakness

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.

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The following guidelines are just a rough outline of what neurologists are trained to do when they evaluate someone who is weak. While these can help you understand what a doctor is trying to accomplish by asking you questions and doing an exam, this article is in no way intended to replace an adequate neurological examination! Weakness can become a very serious problem, especially if it spreads to include the muscles involved in breathing. While some neurological problems like numbness can frequently be benign, true unexplained weakness should always be investigated by a qualified physician.

When discussing weakness with a neurologist, it's important to know exactly what is meant by the word "weak." Some people use the word "weak" to mean "tired" or "fatigued," but even when someone is tired and she tries as hard as she can to lift something, she's still able to do so. The weakness that most concerns neurologists is when the body can no longer lift or resist something that it once could, e.g. if a gallon of milk suddenly seems to weigh 50 pounds. This is an important distinction, because while almost any illness, including the common cold, can make someone feel fatigued, fewer disorders make someone truly physically weak — and many of those disorders can be life-threatening.

How the Brain Tells Muscles to Contract

Moving with full strength depends on an electrical signal traveling from the surface of the brain down through the spinal cord, where the nerves communicate (synapse) in the anterior horn of the cord with a peripheral nerve that will leave the spine and travel to the muscle. There the nerves synapse again at the neuromuscular junction, sending the neurotransmitter acetylcholine to tell the muscle to contract. Calcium flows into special ion channels, and the muscle spindles shorten, resulting in flexion of that particular muscle.

Simple neural signals relay information about that contraction back to the spinal cord to prevent the opposite muscle from simultaneously contracting as well, in order to maximize the power of flexion. For example, if the bicep is trying to flex the arm at the elbow, it would be counterproductive if the tricep simultaneously tried to straighten the arm — so usually a neural loop signals the tricep to relax during bicep flexion.

The nerves in the spine are usually under some degree of constant inhibition from the brain, keeping the muscles relaxed. For this reason, if the signal between the brain and peripheral nerves is cut off, after a while there may be increased rigidity and brisk reflexes in the affected limb. These are known as upper motor neuron findings. In contrast, lower motor neuron findings include flaccidity and fasciculations. It's important to recognize, though, that in an acute injury or stroke, upper motor neuron findings may not be immediately present, and the doctor may still have to suspect damage to the brain or spinal cord.

In short, 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).

Localizing a Lesion in 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.

Localizing a Problem in 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. 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.

The neuromuscular junction can be impacted by toxins or autoimmune diseases that prevent normal signaling by the neurotransmitter. For example, botulinum toxin prevents neurotransmitter release from the nerve terminal. In myasthenia gravis, the receptor molecules on the muscle tissue are attacked by the body's own immune system, and therefore cannot bind the neurotransmitter acetylcholine after it has been released.

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.

Other Information Used by Neurologists

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.

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  • Hal Blumenfeld, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002

  • Ropper AH, Samuels MA. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.

By Peter Pressman, MD
Peter Pressman, MD, is a board-certified neurologist developing new ways to diagnose and care for people with neurocognitive disorders.