Central sensitization: causes, and associated symptoms and diseases.

Let's see what central sensitization is, and the characteristics of this pathological process.

Pain is an unpleasant sensory experience that can be experienced by all living beings with a central nervous system.. At an evolutionary and ethological level, in the natural world, pain is self-explanatory: this emotion keeps us alert and teaches us what we should not approach if we want to survive in the long term.

In our case, it is the nociceptors (free nerve endings of primary sensory neurons) that are the receptors that mainly respond to noxious stimuli. These send impulses via afferent neurons to the spinal cord, which transmits them to the brain, which interprets the pain signal and triggers the relevant response in the organism.

As counterintuitive as it may sound, without pain there is no life, because without the perception of danger there is no concept of "survival". Can you imagine what life must be like for a human being with modified pain perception? You don't have to think too far ahead, because below we tell you all about a term that is closely related to this idea: central sensitization.

What is central sensitization?

Central sensitization can be defined in the first instance as a pathological process of the nervous system that consists of a lowering of the pain threshold, which causes hypersensitivity to noxious stimuli. This event, known as CS, is caused by the increased excitability of the neurons of the central nervous system (CNS), especially in the second order medullary neurons.

In this state of hyperexcitability, the neurons involved are more readily activated by potentially noxious signals and tend to amplify the information received as it is transmitted along the neural tree.. This gives rise to a series of pathological conditions that we will see below.

When one responds in such an exaggerated way to a peripheral stimulus, two specific terms are experienced. We will tell you about them briefly.

1. Hyperalgesia

Central sensitization is inconceivable without hyperalgesia, as they are two sides of the same coin. The latter term refers to the patient's increased sensitivity to pain and extreme reaction to it. Something that was already painful before now becomes unbearable.

According to the National Cancer Institute (NIH), hyperalgesia is typical of processes that damage nerves or produce chemical changes in the nerve pathways involved in pain perception.. Postherpetic neuralgia is a clear example of this, as the site affected by herpes (shingles) may be hypersensitive for 3 months or more after its onset, due to damage to the superficial nerve fibers.

2. Allodynia

This term is slightly different from the previous one, but is also indivisible from central sensitization. Allodynia occurs when the patient abnormally perceives pain in response to a process that does not normally cause it, such as rubbing against a muscle.such as rubbing against a surface or simple palpation of the affected surface.

There are 3 types of allodynia: static mechanical, dynamic mechanical and thermal. The first is defined by the perception of pain with light manual pressure, such as touching the surface of the skin.

On the other hand, dynamic mechanical allodynia is produced by the repeated application of mild stimuli, such as the passing of a cotton swab or a brush. Finally, thermal allodynia occurs when the patient is particularly averse to mildly hot or cold climatic events.

Both terms are distinct, but fall under the umbrella of central sensitization. The following idea should be clear to you:

Central sensitization: hyperalgesia + allodynia.

Causes of central sensitization

The nickname "central" comes from the fact that, effectively, the problem is located in the Central Nervous System (CNS): the brain and spinal cord.. As we have said, the neurons in charge of pain perception amplify the signal and make it stronger, which is why the patient perceives atypical pain in the face of routine events.

In any case, neurons not only send amplified signals, but also transmit "erroneous" information, producing abnormal responses by the organism to common environmental stimuli. It should be noted that, in order to understand central sensitization, it is necessary to factor in that there are predisposing and precipitating factors.

Predisposing factors

Although this series of pathologies are very little known, it is known that there is a certain genetic inheritance for their occurrence. Central sensitization tends to run in families and, in addition, it seems that people who have suffered abuse or traumatic events during childhood are more likely to present it..

2. Precipitants

In predisposed individuals, there is usually a precipitant or trigger that causes the onset of central sensitization. Something as simple as a viral infection can cause it, but it generally occurs after accidents and serious injuries, both physical and emotional..

As studies indicate, a painful stimulus of great intensity can be the prelude to a series of functional and morphological alterations in the central nervous system (CNS), which translates into the general picture of central sensitization that concerns us here.

Some examples of central sensitization

We have spoken so far of central sensitization as a kind of pathology, but this is not exactly so. It is a common event in several diseases characterized by chronic pain. We will briefly tell you about some of them.

Fibromyalgia

It is a disorder characterized by the presence of a disorder characterized by the presence of a generalized musculoskeletal painwhich interferes with the patient's ability to rest, memory and general mood.

Fibromyalgia is usually caused by hereditary factors, infections and physical and emotional trauma. Something like a car accident or a traumatic situation can lead to the appearance of this chronic pathology, although it may not seem so. It is estimated that in certain countries the total prevalence is located in 2.4% of the general population over 20 years of age (generally between 35 and 55 years of age), being 6 to 8 times more common in women than in men.

Chronic fatigue syndrome

Chronic fatigue syndrome is a serious condition that simultaneously affects many body systems. It is characterized by severe fatigue that makes it impossible to makes it impossible to perform routine actions, sleep problems, difficulty concentrating, generalized pain and recurrent dizziness..

In the United States alone, an estimated 836,000 to 2.5 million people have chronic fatigue syndrome. It occurs mostly in people between 40 and 60 years of age and, again, is more common in women than in men.

Irritable Bowel Syndrome (IBS)

Perhaps this pathology is more familiar to you, right? Well, yes, even if you did not suspect it beforehand, irritable bowel syndrome and central sensitivity are widely correlated..

In a society increasingly dominated by stress and worries, IBS occurs in up to 20% of the global population, depending on the population focus consulted and the socioeconomic conditions there. Several factors codify its occurrence: genetic, health, dietary, cultural and emotional, among others.

The most common symptom of this pathology is chronic abdominal pain, accompanied by colic and bloating. which is usually totally or partially relieved when evacuating the bowels (defecating or passing gas). It also causes the appearance of diarrhea or constipation and mucus in the stool, among other things.

Untreated diseases

Unfortunately, diseases that present with central sensitization are largely unknown and, as a result, treatments are limited. In most cases, the aim is for the patient to learn to manage their pain and minimize it, but it is never completely eliminated..

For example, drugs such as analgesics, antidepressants and anticonvulsants (drugs for epilepsy) have been shown to be effective for some cases of chronic pain, although this is not a universally effective treatment. All of this must be accompanied by robust psychological care for the patient, dominated by physiotherapy, occupational therapy and psychological counseling, among other things.

Summary

As you may have noticed, central sensitization is a very poorly understood a very poorly understood general pathological condition, since the causes are never elucidated in many patients.. If anything, it is known that it manifests itself in the form of hyperalgesia and allodynia and that it is also part of a large number of diseases.

Unfortunately, in most cases little is left beyond resistance and resignation in the patients who suffer from it. Sometimes chronic pain cannot be dealt with and, therefore, all that remains is to live with it and learn to manage it.

Bibliographic references:

• Azkue, J. J., Ortiz, V., Torre, F., & Aguilera, L. (2007). Central sensitization in the physiopathology of pain. Gaceta Médica de Bilbao, 104(4): pp. 136 - 140.
• Fleming, K. C., & Volcheck, M. M. (2015). Central sensitization syndrome and the initial evaluation of a patient with fibromyalgia: a review. Rambam Maimonides medical journal, 6(2).
• M Adams, L., & C Turk, D. (2015). Psychosocial factors and central sensitivity syndromes. Current rheumatology reviews, 11(2): pp. 96 - 108.
• Serrano-Muñoz, D., Gómez-Soriano, J., Ávila-Martín, G., Galán-Arriero, I., Romero-Muñoz, L. M., Taylor, J. S., & Barriga-Martín, A. (2016). Central pain sensitization in patients with whiplash syndrome: a review. Latin American Journal of Orthopaedic Surgery, 1(3): pp. 102 - 107.
• Solà, J. F. (2018). Central sensitization syndromes: towards the structuring of a multidisciplinary concept. Medicina Clínica, 151(2): pp. 68 - 70.