Muscle weakness is a sensation that can range from simple muscle “fatigue” to loss of strength in the muscles and difficulty moving limbs, for example.

It can affect all the muscles of the body, whether it is the muscles of the limbs, the respiratory muscles, the muscles of the neck, etc.

Doctors generally distinguish:

• Real muscle fatigue: reduction in strength, regardless of the effort provided. The requested muscle does not respond normally, and this as soon as the effort begins.
• Muscle fatigue: feeling of exhaustion when a muscle is used, although the muscle is not necessarily weaker to begin with.
• Muscle “fatigability”: the muscle responds and acts normally, but it gets exhausted quickly and recovers slowly after an effort.

The difference between these three sensations is not always clear.

The causes of muscle weakness are multiple and are not always obvious at first glance. They can be infectious, endocrine, neurological, inflammatory, genetic, rheumatological, linked to a drug, etc. The weakness can start in childhood (genetic myopathy, for example) or much later in life.

The doctor will first assess the severity of the attack, the associated symptoms, the family and medical history and of course the age of onset of the symptoms.

A test called electromyography, which assesses the transmission of the nerve message in the muscles, may be indicated when the presence of myopathy, neurological or neuromuscular damage is suspected.

Muscle biopsies, among other tests, may be done.

Note that muscle weakness may simply be related to age or a lack of physical exercise. Another common situation: after pregnancy, the muscles are weakened (especially the muscles of the legs), in particular because of hormones and the decrease in physical activity. The gradual resumption of training will restore muscle tone.

Among the most common infectious causes of muscle fatigue are:

• Epstein-Barr virus infections (infectious mononucleosis)
• HIV / AIDS
• influenza
• Lyme disease
• syphilis
• meningitis, etc.

On the side of neurological causes, we find, among others:

• amyotrophic lateral sclerosis
• a cerebral vascular accident
• multiple sclerosis
• the myasthénie
• various myopathies, etc.

There are many drugs that can cause muscle weakness. Here are a few :

• antiretroviral drugs (zidovudine, lamivudine)
• statins
• chemotherapy agents
• antithyroid drugs
• corticosteroids
• interferon, etc.

Other disorders can cause muscle fatigue, such as:

• thyroid disease
• an inflammatory or autoimmune disease (dermatomyositis, lupus, etc.)
• chronic fatigue syndrome
• fibromyalgia

Muscle weakness can be mild or severe, temporary or permanent, and may or may not get worse. In most cases, it is transient, for example in connection with influenza or infectious mononucleosis, and there are no long-term consequences.

On the other hand, if it is a sign of a neurological or neuromuscular disease, muscle weakness is often irreversible and can even progress to potentially fatal damage (amyotrophic lateral sclerosis, for example). It affects motor or respiratory capacity and therefore poses significant problems.

It is crucial to make an appointment with your doctor to rule out serious causes and benefit from the best possible care.

Muscle weakness is often mild, but can also be the result of severe damage. In the case of most neurological or neuromuscular diseases, there is no effective treatment to suppress the symptoms or stop the progression of the disease.

Symptomatic management, including physiotherapy, psychological support, rehabilitation and occupational therapy will be necessary.