Before the covid-19 pandemic, we had already become accustomed to the annual waves of flu in which hospitals appeared on the news packed with people in the corridors suffering the ravages of the worst symptoms of the flu, including pneumonia.
Fortunately, for most patients, flu symptoms are more mild and do not require hospitalization.
Among these symptoms the most common are fever that causes chills and cold sweat, headache, persistent or dry cough accompanied by sore throat, stuffy nose, weakness and… terrible muscle and joint pain .
Why, if the flu virus – like the covid-19 virus – enters our body through the respiratory tract, affecting the mucous membranes of the nose, throat and lungs?
The immune reaction is a double-edged sword.
In most respiratory viral infections, the symptoms are not due directly to the action of the virus but to the response of the immune system to eliminate it.
Sore throat, cough, increased mucus, fever are nothing more than symptoms associated with the body’s response to the action of the immune system . That is why they are common to countless viral and bacterial diseases.
In general, almost any disease that causes inflammation ends up giving rise to these common symptoms, which also include muscle weakness and pain, and joint pain.
It is the inflammatory response that causes all these symptoms, regardless of the pathogen that causes them.
Muscle inflammation accompanied by weakness is known as myositis, while myalgia is the name given to pain in muscles and joints.
When an immune response is produced against a pathogen, the different cells involved release a series of small proteins known as cytokines.
Cytokines are from different families that regulate multiple activities of the cells of the immune system and other organs.
Some serve to signal the place where the infection is found, others regulate the type of response that should be produced depending on the pathogen that has infected, others control the intensity of the immune response and others even regulate the repair of damage to organs and tissues.
Cytokines are part of a complex signaling system that cells of the immune system need to control the immune response but that also influence other tissues and organs.
Many of these cytokines produce inflammation not only at the site of infection but also in other organs including muscles and joints.
Hence, in flu and other respiratory infections, including covid-19, muscle pain occurs, quite similar to that suffered with mild arthritis.
Among these cytokines, interleukin 1-beta (IL-1b), which produces fever, and interleukins 6 (IL-6) and 17 (IL-17), together with tumor necrosis factor-alpha (TNF-a ), are the most relevant to explain muscle pain.
In cases of influenza and other viral infections, high levels of IL-6 in muscle have been associated with pain.
This pain is probably associated with the loss of muscle caused by the flu, but also with other viral infections capable of causing muscle fiber breakdown or rhabdomyolysis. These muscle tears have been associated with high levels of IL-6 or TNF-a after these infections.
In response to these cytokines, muscle produces elevated levels of prostaglandin E₂ (PGE₂), a lipid molecule associated with inflammation and pain.
For this reason, against pain and muscle weakness, non-steroidal anti-inflammatory drugs are prescribed whose mechanism of action consists of inhibiting the production of PGE₂. Like ibuprofen or paracetamol .
Stop the pain, yes or no?
We have just explained that both muscle and joint pain are secondary effects of the inflammatory response of the immune system in its attempt to stop the infection.
And that regardless of the origin that causes the inflammation, it is the presence of inflammatory cytokines that causes the most serious symptoms in the muscle and joints. Symptoms that include not only pain but also the loss of muscle fibers.
Now there is a paradox .
Inflammation is the first strategy the immune system uses when trying to fight off a viral infection. Furthermore, the inflammatory response is essential to keep the infection under control , and also to repair the damage caused by viruses and bacteria.
If these responses are part of the battery of weapons that our body uses in its continuous fight against pathogens, does it make sense to cancel them?
Muscle pain and weakness in a viral infection can be bearable collateral damage.
A very different thing is when they become a chronic symptom associated with an imbalance in the immune system that is causing chronic muscle inflammation with no clear origin.
And possibly that is what happens in other types of myalgias and fibromyalgias, although there is still a lot of discussion about it.
It is necessary to find the keys that affect chronic muscle pain accompanied by weakness, and an uncontrolled release of pro-inflammatory cytokines could be the cause.
Hence the importance of controlling the levels of these cytokines in the treatment of this type of disease.