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from ancient German: smerzo - pain
Greek: algos - pain
Synonym: algesia
German: Schmerz

1 Definition

Pain is a complex sensation that is triggered by the nociceptors of the peripheral nervous system. It is processed and interpreted in the CNS. However, there are tight interactions between pain perception and the psyche.

Pain can occur just as symptom, but also have its own illness value - especially in chronic pain.

2 Development of pain

Pain develops in the pain receptors nociceptors. They mostly are free nerve terminals that react to different stimuli, eg.:

  • thermal stimuli (heat, cold)
  • mechanical stimuli (pressure, injury)
  • chemical stimuli (inflammation, acids, poisons)

Nociceptors need a comparatively strong stimulus to be excited, and they do not adapt (a quickly repeated stimulus does not lead to a reduction of excitability). The activability of nociceptors is changed (modulated), generally increased, by substances called pain mediators. They comprise prostaglandins, bradykinins, serotonin. Other factors that lead to an increased excitability are lack of oxygen in the tissue (eg. caused by infarction), lowering of the pH value (CO2 increase), or a change in the electrolyte concentration in the blood.

Pain often accompanies illnesses or injuries, but they also can gain their own illness value as pain syndrome. There, pain persists over months, and the underlying illness is either difficult to treat or untreatable, or the cause for the pain cannot be found.

States of pain are learnable for the body. Repeatedly occuring pain leads to a more intensive and longer sensation of pain since the threshold of pain is lowered. Therefore, the early and sufficient pain relief with the help of medication is important. Investigations have shown that in Germany, as opposed to other countries, pain is often treated insufficiently. This probably goes back to the deeply rooted and unfounded fear pf dependency from pain medication.

3 Conduction of pain

The nerve fibers which conduct the pain information can be divided into fast (A-delta-fibers) and slowly conducting (C-fibers) fibers. C-fibers are evolutionary older. This explains the low velocity and the pain localization difficult to define ("somewhere in the lower leg"). On the one hand, in the spinal cord, there are reflex interconnections that trigger a flight movement. At this point, the pain has not been brought to awareness yet (pulling back the hand, even before the stovetop has been recognized as hot).

On the other hand, the information is forwarded to the brain via the anterior lateral tract (Tractus spinothalamicus). In the cortex, the pain becomes 'aware', and in the limbic system, it is evaluated emotionally. Higher brain centers can take influence on the intensity of pain perception via descending antinociceptive paths (pain modulation).

During the circuitery in the spinal cord, the pain sensation can be reduced by endogenic substances (endorphines). Some painkillers, such as opiates take effect exactly at this point.

4 Types of pain

The type of pain depicted until now is a physiological kind of pain. This means that the pain sensation is useful as a warning sign for the function of the body. Then, you talk about nociceptor pain. Neuropathic pain, which is caused by damage of the nervous system (eg. by amputation, paraplegia, viruses, or permanently high blood sugar) needs to be differentiated from that.

Pain also occurs as consequence of functional disorders. Partial systems of the body are faulty (eg. defective regulation of perfusion leads to migraine), or the reaction of the body to influences from the outside (stress, anxiety, disgust...) is inappropriate.

5 Classification

5.1 ...according to duration of pain

5.1.1 Acute pain

Acute pain is an important warning sign of the body. When it occurs, you need to find the cause and eliminate it. Then, the pain usually subsides within a few hours or days. Often itís already helpful to relieve the painful region and cool it (eg. in inflammations) or warm it (in osteoarthritis).
Medium and strong pain can be treated with pain medication or local anesthetics. For post-operative pain, it has even been shown by studies that a good pain treatment promotes healing and lowers the risk of complications. According to the intensity of the pain, pain medications or a combination of different pain medications is used.

5.1.2 Chronic pain

Under chronic pain, doctors understand pain that lasts longer than six months. Also recurring pain such as migraine is considered chronic pain when it occurs on more than 15 days a month. The pain persists independent of the original illness, i.e. it has los its signal effect. In case of an untreated or insufficiently treated chronic pain, a pain memory develops. There are also sleeping disorders, and the physical and mental resilience is massively restricted. Subsequently, also professional performance is reduced, and not so rarely, permanent invalidity and depression do occur. A sole medicinal pain treatment is often not enough in these cases to recover the quality of life of the affected.

An example for chronic pain is back pain.

5.2 ...according to quality of pain

The patient can describe the pain closer by its quality. The description of pain can have 2 aspects:

  • the affective quality of pain that characterizes the subjective meaning of pain, eg. "agonizing", "excruciating", "paralyzing", "terrible", "violent"
  • the sensory quality of pain that characterizes actual perception, eg. "stabbing", "pressing", "burning", "pulsating", "piercing", "dull", "sheer", "gnawing" etc.

The quality of pain is specifically asked when taking the patient history, so that you gain hints on the type and cause of pain.

5.3 ...according to the localization of pain

A special form of pain is transferred pain ("referred pain"), where the pain is felt at another place than where the triggering stimulus is set.

5.4 ...according to cause of pain

5.5 ...according to the pain conditions

5.6 ...according to the pain trigger

6 Pain measurement

As with every important clinical symptom, there is also an interest for pain to quantitatively determine it. Since pain is a subjective perception, the "objective" instrumental measurement is impossible at the time of writing (2015).

A widespread method of "pain measurement" by self-declaration of the patient is the structured inquiry of subjective pain sensation in the form of pain questionnaires oder pain scales (eg. VAS, VRS and NRS).

In infants and babies, due to the missing or limited possibilities of communication, you can use a scale with the acronym KUSS for the estimation of pain (Büttner et.a. 1998).

With chronic pain, you also need to consider the measurement over a longer period of time in the form of a pain diary.

Algesiometry tries to determine pain intensity by the targeted triggering of pain stimuli (eg. mechanical or thermal).

In the field of basic research, there are tries to visualize pain using methods such as functional MRI.

7 Pain therapy

7.1 Causal therapy

When the cause for the pain can be identified, therapy firstly is directed at the cause and aims to eliminate the pain trigger event. A lot of therapeutic measures comes into question, eg. immobilization in a fracture, or the surgical care of injuries.

7.2 WHO step-by-step plan

Drugs are an important part of pain therapy. Here, the WHO step-by-step plan of 1986 is used, where a step-by-step adaption of analgesics is intended.

  • Step 1: Non-opioid analgesics: non-steroidal anti-inflammatory drugs, metamizole dipyrone. Most of the painkillers in use are non-steroidal anti-inflammatory drugs (NSAIDs) that are proven especially in inflammatory pain. In long-term medication, especially in the elderly, multimorbid patients, there can be numerous side effects and interactions.
  • Step 2: Non-opioid analgesics + low-potency opioids Tilidine/naloxone, tramadol, and dihydrocodeine are used against moderately strong pain.
  • Step 3: Non-opioid analgesics and high-potency opioids: Tapentadol, morphine, hydromorphine, oxycodone, fentanyl, and buprenorphine are used against strong to very strong pain, if other step-1 and step-2 drugs are not enough anymore.
  • Step 4: Invasive therapies: Peridural or spinal injection, peripheral local anesthesia, ganglionic blocker etc.

7.3 Symptomatic therapy

You have to differentiate all further treatments from causal therapy that target the elimination or relief of the pain symptoms themselves, including:

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