Pathophysiology of Pain - Anaesthesia Unravelled # 7

Hello there NAVE crew? In this "Anaesthesia Unravelled" video lesson we will be talking a little about the Pathophysiology of Pain.

Pain is one of the worst feelings there is. Anaesthesia itself has the primary purpose of preventing suffering during surgery. However, although avoiding pain it's obvious, recognising it is still a challenge, considering the differences between species, breeds and even individuals. In order to know how pain works, we need to understand the pain processes first. Physiological pain is important for the protection of the individual against potentially harmful situations. Another important function is learning, as the individual tends to avoid potentially harmful situations that it went through before.

We must remember that the peripheral stimulus follows a path, from the periphery to the brain. After the stimulus, there is the Transduction step, which transforms the electrical, pressure or chemical stimulus into an electrical one. They are carried by three types of fibres, called afferents: The Abeta fibres, which are formed by mechanoreceptors, carry the stimulus of touch and pressure to the CNS; the Adelta and C fibres, which have nociceptors, carry pain stimuli (thermal, mechanical or chemical). This step is called Transmission. In the spinal cord there is the Modulation step, in which the stimulus can be exacerbated or inhibited, depending on the situation. Finally, we have the Perception step, with the recognition of the stimulus by the brain.

Pain pathway, from the stimulus (transduction) to the recognition by the brain (perception).


Pain, when left untreated, sensitises this whole process. Thus, if there is an excess of stimulation, as during inflammation, for instance, the excitability threshold of the Adelta and C fibres decreases, culminating in an effect called Hyperalgesia. This process feeds a perpetuating cycle which, at a given moment, can cause the Abeta fibres, that usually don't carry painful stimuli, to begin to do so. In that case, allodynia occurs.

Pain can be classified as acute, subacute or chronic, depending on the duration. Obviously, it is much easier to treat acute pain than chronic pain; hence the need to carry out effective treatment as soon as possible. Pain can also be differentiated into somatic (occurs in the periphery) and visceral (in the cavities). There's also neuropathic pain, which is closely related to chronic pain.

For the recognition of pain, there are scales that can aid and guide us. The simplest one is the Visual Analogue Scale, which can be used in various situations. However, it is not specific to a given species. In this case, we have other scales, specific to each one. Virtually all of these scales are based on behavioural changes and facial expression has been on highlight recently.

Basic analgesic therapy can be divided into those used for mild, moderate and severe pain and the main pharmacological groups used are non-steroidal anti-inflammatory drugs, os opioids (partial and total agonists) and adjuvants.

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Further reading

Scales
Refs

Gaynor JS, Muir WW. Manual de controle da dor em medicina veterinária. 2a ed. Editora MedVet, 2009. 643p.
Taffarel MO, Luna SPL. Fisiopatologia e avaliação clínica e experimental da dor aguda e crônica. In: Luna SPL, Carregaro AB. Anestesia e Analgesia em Equídeos, Ruminantes e Suínos. Ed MedVet. 1a ed. 2019, 53-73.

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