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Summary Elective Pain
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Case 11: Pain1. How is pain defined and what kinds of pain exist?
The International Association for the Study of Pain defines pain as an “unpleasant sensory and
emotional experience associated with actual or potential tissue damage, or described in terms of such
damage” (International Association for the Study of Pain, 1994, pp. 209–214)
This definition acknowledges that pain and nociception are different, with nociception referring to the
physiological processes caused by tissue damage. Although nociception normally results in pain, this is
not compulsory, and vice versa, pain may be experienced without nociception. This definition implies
that pain is not only a sensory experience but might also be associated with affective and cognitive
responses.
Pain can be classified in different ways:
1. Pathophysiological classification:
a. Nociceptive pain: due to tissue injury that stimulates
nociceptors.
- somatic pain: due to the stimulation of nociceptors in the superficial Picture from: Rohkamm R. Color atlas of neurology.p109
tissues (skin, mucosa). Well localized, sharp or burning. Also deep tissues (muscles, joints, bones, tendons,…)
dull or arching.
- visceral pain: due to the stimulation of nociceptors in the viscera ( internal organs). Poorly localized, diffuse
and dull, aching or cramping. This pain can be referred to skin parts supplied by same sensory roots as the
diseased organ(1). Visceral pain is not felt in the internal organ from where it originates, but it is referred to a
cutaneous zone specific to that organ. This can be explained by the arrival of sensory impulses from the internal
organ and its related zone of Head at the posterior horn at the same level of the spinal cord; the brain
(mis)interprets the visceral pain as originating in the related cutaneous zone(2).
b. Neuropathic pain: due to the injury of nerve cells in the peripheral or central nervous system. Sensory
features associated with neuropathic pain include allodynia, hyperalgesia, hypoalgesia, paresthesia and
dysesthesia. Pain is usually poorly localized and diffuse. It is describes as needles, tingling, burning,
sharp or shooting in nature. (1)
c. Mixed pain: when both nociceptive and neuropathic pain are experienced.
2. Duration classification:
Acute pain: severe pain of sudden appearance, immediately after an injury.
Chronic pain: pain that continues beyond the normal healing process. It can start as an acute pain
but lasts for more than 3 months
3. Etiology classification:
Malignant pain: pain associated with cancer. Pain can be caused by the malignancy or the related
treatment (chemotherapy, radiotherapy and surgery).
Non-malignant pain: pain that is not related to malignancy.
4. Anatomical classification: based on the anatomical localization of pain.
Allodynia: is a condition in which pain is caused by a stimulus that does not normally elicit pain.
Hyperalgesia: is an abnormally increased sensitivity to pain, which may be caused by damage
to nociceptors or peripheral nerves and can cause hypersensitivity to stimulus.
, Analgesia:
Is the loss of sensation of pain that results from an interruption in the nervous system pathway between sense
organ and brain. An analgesic drug, also called simply an analgesic, pain reliever, or painkiller, is any member of
the group of drugs used to achieve relief from pain such as aspirin, morphine, codeine.
Nociceptors:
1. Mechanoceptors: receptors that respond to pinch and pin prick sensations.
2. Silent nociceptors: activated during inflammation.
3. Polymodal mechano-heat nociceptors: the most prevalent receptors. These receptors respond to excessive
pressure, extremes of temperature, and alogens (substances that cause pain).
2. What is the anatomy of the nociceptive system?
1. Stimulus perception:
Transduction:
‘’Transduction refers to the process by which noxious stimuli
are transformed into electric signals at the sensory nerve
endings. ‘’(1)
Takes place in the peripheral terminals of primary
afferent neurons, where different modes of stimuli
( mechanical, heat, chemical, or cold) induce a
generator potential through activation of transduction
channels on the axonal membrane.
Transmission:
If the generator potential is sufficient, it will induce
action potentials that are transmitted through the
nervous system. There are three major components of
the transmission system.
- First component:
Primary afferent neurons (first order neurons) are located
in the dorsal root ganglia. Each neuron has a single axon
that bifurcates, sending one end to the peripheral tissues it
innervates, and the other into the dorsal horn of the spinal cord where
it synapses with the second order neuron. Afferent fibers enter the
spinal cord segregates into medial large myelinated fibers and
lateral small unmyelinated fibers.
Picture from Benzon H, Raja S, Fishman S, Liu S, Cohen S, Hurley R.
Essentials of Pain Medicine E-Book. Philadelphia: Elsevier Health
Sciences;2017.p4
- Second component:
The spinal neurons send projections to the thalamus and structures located in the brainstem and
diencephalon. Ascending pain signals are sent to the brain via the spinothalamic tract (the major pain
pathway, lays anterolaterally in the white matter of the spinal cord), whose fibers project to the
intralaminar and ventroposterior nuclei of the thalamus.
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