PHYSIOPATHOLOGY OF PAIN
INTRODUCTION
In dentistry, pain is a daily concern; it is the main motivator for consultations. It can be acute (often), easy to treat, or chronic (neurogenic, psychogenic, idiopathic and even atypical), difficult to diagnose and treat.
In order to optimize therapy, we must understand the pathophysiology of pain.
I.DEFINITIONS: Pain is a completely subjective manifestation and therefore its exact definition is difficult. It is likely to negatively affect the patient’s behavior or well-being.
Pain remains a subjective personal experience, but the enormous plasticity of the nervous system helps us understand how each of us responds to the environment in a unique way.
1. According to Larousse Médical : pain is a painful sensation manifesting itself in different forms (burning, stinging, stretching, cramps, etc.) of varying intensity and extent.
2. According to IASP 1979 ( I nternational A ssociation for the
Study of Pain ; International Association for the Study of Pain): Pain is an unpleasant sensory and emotional experience associated with actual (existing) or potential (virtual) tissue damage (lesion) or described in terms of such damage.
Although pain is usually secondary to a very obvious physical cause, this definition avoids closely linking pain to its sole causal nociceptive stimulus. It also highlights the close intertwining between the organicity of pain and its emotional and affective consequences on the individual reactions specific to each patient.
The assessment of the intensity of pain is eminently variable: it depends on the emotional structure of the subject who suffers from it, which makes any attempt at calibration between the intensity of the painful stimulus and suffering illusory.
II. TERMINOLOGY:
1. Sensitive or nociceptive receptor (nociceptor) : it is a part of a nerve cell capable of responding to a specific category of stimulus. At the level of the skin and the mucous membrane, they are found in the form of free nerve endings (corpuscles or discs) such as the Meissner, Pacini and Ruffini receptors. These receptors receive and then transform the different types of energy (stimuli) of thermal, electrical, mechanical and even chemical origin into a nerve impulse to the brain.
The skin, oral mucosa, dental pulp and periodontal ligament are very rich in nociceptive free nerve endings, called nociceptive exteroceptors.
Open-ended receptors represent universal receptors, capable of receiving versatile sensations.
2. Nociception : it refers to the process that is at the origin of the nervous message that causes painful stimulation. It therefore corresponds to all the functions of the organism that allow to detect, perceive and react to internal or external stimuli that are potentially harmful to the organism.
3. Nerve cell or neuron : it is the functional unit of the system
nervous, it is excitable and conductive. It includes the body, the axon, the dendrites, the synapses and the free endings.
4. Pain intensity and sensitivity: The intensity of pain experienced from a given stimulus depends on two main factors:
* the strength of the stimulus on the receptor;
* the sensitivity of the individual.
III. COMPONENTS AND CONCEPTUAL BASES OF PAIN
Pain is a multidimensional phenomenon, including not only the sensations evoked by noxious stimuli but also the body’s reactions to such stimuli. These reactions, the psychological repercussions of which constitute suffering, vary according to the overall state of the personality, its past experience and present motivations.
We currently recognize 4 components or dimensions of pain:
1. Sensitive or sensori-discriminative component : this is the decoding of pain on a qualitative and quantitative level. It corresponds to the perception of the characteristics and nature of the pain (stinging, burning, blows, electric shocks, etc.), the duration of the harmful stimulation, its intensity and its location.
2. Affective and emotional component : affective and emotional alterations aggravate and complicate the pain. It reflects the impact (unpleasant, difficult to bear, painful, etc.) determined by the pain itself but also by its context. This aspect can also be prolonged by states of anxiety and depression.
3. Cognitive-evaluative component : the cognitive state or mental processes modulate the perception of pain (this is the anticipation of pain). The cognitive state concerns the attention paid to the pain, its interpretation, its intensity, its unpleasantness and the anticipation of its arrival.
4. Behavioral component : this is the set of verbal and non-verbal manifestations observable in the person in pain (fear, screaming, movements, speech, etc.).
IV. ANATOMICAL AND FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM
The human nervous system consists of two major parts.
1. Central nervous system : it includes
*spinal cord
*the encephalon (cerebral exchange + cerebellum + brain)
2. Peripheral nervous system : It is made up of afferent and efferent nerves and nerve fibers (neurons).
* Afferent fiber: it is sensitive, responsible for the transmission of the nociceptive message from the periphery to the brain.
* Efferent fiber: it is motor, responsible for the transmission of the response made by the brain, after integration and analysis of the nervous message from the latter to the limbs.
V. PHYSIOPATHOLOGICAL MECHANISM OF NOCICEPTIVE PAIN
This mechanism goes through three stages:
1. A resting membrane potential.
2. An action membrane potential: in which the electrical signal (nerve impulse) is generated.
3. The alternation of phases: resting potential/action potential is at the origin of the conduction of the nerve impulse.
Among the neurotransmitters we can cite:
-Acetylcholine; Dopamine; Noradrenaline; Serotonin…
Conduction of nociceptive nerve impulses : nociceptive stimulation of a region of the body (the skin for example) results in the generation of a nerve impulse or nociceptive message, which will be transmitted to the brain for integration and analysis. How?
The nociceptive message passes through 3 successive sensory neurons:
1st neuron : from the receptor to the posterior horn of the spinal cord, this is the first relay ;
2nd neuron : from the spinal cord to the thalamus, this is the second relay ;
3rd neuron : from the thalamus to the cerebral cortex where the message is perceived as pain, this is the third relay .
The transmission of nerve impulses occurs at the level of axons and via synapses.
a- at the level of the axons
a.1. unmyelinated axon : slow and continuous conduction; ensured by small diameter sensory afferent fibers C.
a.2. myelinated axon : faster conduction, ensured by the medium and large diameter A alpha and A beta fibers. The A delta fibers, which are poorly myelinated, ensure the transmission of the slow nociceptive message.
b. at the synaptic level : the pre-synaptic electrical signal (the nerve impulse) is converted into a chemical signal (neurotransmitters mentioned above), the post-synaptic neuron receives the chemical signal and in turn generates the electrical signal.
The perception of the painful symptom requires a complex process in which several stages can be distinguished:
- development of the influx in the receiver;
- The nociceptive impulse is then transmitted to the spinal cord by two types of nerve fibers:
*myelinated fibers of group (A)
*unmyelinated fibers (C)
3. The spinal cord processes and modulates the nociceptive message, which it then transmits to the supraspinal structures (the brain stem) then the thalamus and other structures of the brain (cortical) where it will be processed, analyzed and transmitted in the form of pain.
Knowledge of these different stages is necessary to understand the appearance of lesion pain, among which we distinguish pain due to excess nociception, pain secondary to tissue damage other than the nervous system (inflammatory pain), and neurogenic pain secondary to damage to the nervous system.
VI. PHYSIOLOGICAL CONTROL OF PAIN : spinal transmission of nociceptive messages is dependent on excitatory influences but also on inhibitory influences. The inhibitory influence occurs at several levels:
- at the periphery: in the lesion site, Endorphin inhibits the secretion of Algogenic substances ( which give rise to pain).
- at the medullary level : this is the gate control. This is the inhibition of the ascending pathway of the nociceptive message. This is the segmental control of pain (Melzack theory of 1965)
- at the supramedullary level: this is the inhibition of the descending pathway. It occurs at the level of the thalamus and at the level of the brain stem.
VII. CLASSIFICATION OF PAIN:
1. according to the classification of JEFFRY OKESON from 1995 we distinguish:
- Acute and chronic (temporal) pain
- Nociceptive and neuropathic or neurogenic pain
- Somatic and visceral pain
- Localized and generalized pain.
1.1. Acute pain : or symptomatic pain; it generally appears following trauma or an illness. This pain is accompanied by vegetative signs (sweating, vasodilation, tachycardia, etc.)
The most common acute pain in the orofacial region occurs in the teeth and periodontium. It responds very well to conventional analgesics and to the elimination of the cause.
1.2. Chronic pain : there is often no clear relationship with tissue damage. Vegetative signs are absent, but psychosocial and behavioral signs associated with pain take precedence .
However, we should not lose sight of the fact that chronic post-injury pain has a defined starting point (inflammation, trauma, etc.), i.e. the pain persists longer than the cause.
Chronic pain is neurogenic, due to a dysfunction of the peripheral or central nervous system. It does not respond to usual analgesics. It sometimes responds to antidepressants and antiepileptics.
1.3. Nociceptive pain : this pain is transmitted by irritation and stimulation of superficial nociceptors (oral mucosa, dental pulp, periodontal ligament, skin, etc.) or deep nociceptors (muscles, bones, joints, etc.) and is diffuse.
1.4. Neuropathic pain : this neurological or neurogenic pain is an interpretation error made by a faulty nervous system. The pain is perceived in the absence of any nociceptive stimulation and any somatic pathology (allodynia). It can be peripheral or central, spontaneous or provoked, occurring in response to a stimulation that does not normally have a painful character: thermal variation, superficial brushing (Essential Facial Neuralgia).
At the orofacial region level we find:
* Episodic or paroxysmal neuropathic orofacial pain ( NFE: Essential Facial Neuralgia of the trigeminal nerve and ZONA). They are peripheral.
* Persistent neuropathic orofacial pain e.g. atypical odontalgia, atypical facial pain, idiopathic burning sensations in the mouth (stomatodynia) and (glossodynia)
* Central neurological pain: thalamopathies (eg: migraine).
1.5. Somatic pain: these are nociceptive pains (cutaneous, bone, joint, tendon, muscle, etc.)
1.6. Visceral pain : the existence of nociceptive receptors within the viscera has not been demonstrated. These are interoceptors.
The description of visceral pain is difficult and imprecise, their localization is imperfect and their irradiations are misleading.
1.7. Localized pain: this is pain felt at the site of the injury; it is a point or a small site of injury.
1.8. Generalized pain: in this case the site of injury is different from the site of pain perception:
* referred pain (irradiated or diffuse): this is pain that follows the path of the nerve related to the lesion, in the case of acute total pulpitis.
* referred pain (projected): this is synalgia: it is a diffuse pain felt at a distance from the territory of the initial lesion. In dentistry, the most frequent case of this type of pain is dento-dental and dento-cutaneous synalgia.
PHYSIOPATHOLOGY OF PAIN
2. Topographic classification
2.1. primary pain: this is localized pain.
2.2 Secondary pain: this is diffuse pain.
3. According to the pathophysiological mechanism (IASP 1995)
3.1 Nociceptive or somatic pain
3.2 Neuropathic or neurogenic pain
3.3 Psychogenic or non-somatic pain
3.4 Idiopathic pain: this is semiological pain whose etiology is not identified (glossodynia, stomatodynia)
VIII. SPECIFICITY OF ORO-FACIAL PAIN: the dental pulp is particularly rich in free nerve endings (nociceptive receptors of type C , unmyelinated). The sensory segment of the trigeminal nerve is generally similar to a medullary segment and finally, the extent of the thalamic or cortical representation of the oro-facial area makes this region the most sensitive in the body.
IX. CHARACTERISTICS OF ORO-FACIAL PAIN
Chronic orofacial pain is sometimes very difficult to diagnose because of the large number of body structures, skin, mucous membranes, dental, muscular, bony, glandular, vascular, nervous and articular.
The richness and complexity of the innervation at this level make pain frequent. It can give rise to very varied painful manifestations ranging from simple discomfort to uncontrollable expressions.
CONCLUSION: Pain is a complex phenomenon that must be carefully analyzed and understood (causes and mechanisms) so that it can be treated correctly.
END
PHYSIOPATHOLOGY OF PAIN
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