Pain1

PAIN SENSATIONPAIN SENSATION PAIN SENSATIONPAIN SENSATION

According to The International Association for the Study of Pain (IASP):

Definition: Pain is an unpleasant sensory and emotional experience

associated with actual or potential tissue damage.

Significance

1) warning signal against tissue damage. Pain is one of the most

prominent symptoms of tissue damage.

2) Initiate protective reflexes which causes the subject to get rid of the

painful stimulus, or at least, to minimize tissue injury or damage

If persistent, physiological pain may progress to a If persistent, physiological pain may progress to a pathological condition pathological condition

itself, often referred to as itself, often referred to as maladaptivemaladaptive pain, in which case pain is pain, in which case pain is

dissociated from the original noxious stimulation or the healing process dissociated from the original noxious stimulation or the healing process

and thus does not represent anymore a symptom of disease but rather and thus does not represent anymore a symptom of disease but rather

abnormal sensory processing due to abnormal sensory processing due to damage to tissuesdamage to tissues (inflammatory (inflammatory

pain) or pain) or the nervous system the nervous system (neuropathic pain), or to (neuropathic pain), or to abnormal function abnormal function

of the nervous system itself (functional pain)of the nervous system itself (functional pain)..

pain resulting from activation of pain receptors may be referred to as pain resulting from activation of pain receptors may be referred to as

adaptiveadaptive or or physiologicalphysiological pain, because it minimizes tissue damage and pain, because it minimizes tissue damage and

promotes healing. promotes healing.

Classification:

Pain is classified into nociceptive, neuropathic and psychogenic; all

can be either acute or chronic. Pain is defined as chronic if persists more than 7 weeks.

1. Nociceptive is pain caused by tissue damage (inflammation) which stimulate

pain receptors (nociceptors).

1. Nociceptive is pain caused by tissue damage (inflammation) which stimulate

pain receptors (nociceptors).

2. Neuropathic: (pain due to injury of nerve pathway)

site of injury: CentralCentral Central pain (thalamic infarct).

MixedMixed Plexus avulsion, Post herpetic neuralgia.

PeripheralPeripheral Neuroma, nerve compression, phantom, neuralgias.

character: burning, tingling, numbness, pressing, squeezing, itching, constant +/-

intermittent shooting, lancinating, electric.

2. Neuropathic: (pain due to injury of nerve pathway)

site of injury: CentralCentral Central pain (thalamic infarct).

MixedMixed Plexus avulsion, Post herpetic neuralgia.

PeripheralPeripheral Neuroma, nerve compression, phantom, neuralgias.

character: burning, tingling, numbness, pressing, squeezing, itching, constant +/-

intermittent shooting, lancinating, electric.

3. Psychogenic: (difficult to differentiate whether secondary to or actual cause

of pain), anxiety, depression (30% of depressives complain of pain on initial

presentation).

3. Psychogenic: (difficult to differentiate whether secondary to or actual cause

of pain), anxiety, depression (30% of depressives complain of pain on initial

presentation).

Pain Receptors (Nociceptors)

Types of Pain Receptors

Free nerve endings which are morphologically similar but functionally

specific. They are classified according to their sensitivity into:

Polymodal Pain Receptors (most pain receptors)

These respond to a combination of mechanical, thermal, and chemical

noxious stimuli.

Mechanical Pain Receptors

respond to strong mechanical forces, such as cutting, crushing, pricking, or

even firm pressure on tissues.

Thermal Pain Receptors

respond to excessive changes in temperature (above 45oC and below 10oC).

Chemical Pain Receptors

respond to noxious chemical stimuli.

Distribution of Pain Receptors

Pain receptors are found in most tissues of the body but varies in their density.

They are abundant and widely spread in the skin and some internal

tissues such as: - the periosteum of the bone,

- arterial walls,

- joint surfaces,

- the dura of the falx and tentorium in the cranial cavity,

- the skeletal muscle,

- the parietal layer of serous membranes.

Many of the other deep tissues and viscera are poorly supplied with pain

receptors. So,

- for pain to occur, painful stimulus must by intense and widespread.

- The deep & visceral pain are poorly localized.

On the other hand, the brain itself and also the parenchymal tissues of the

liver, kidneys, and lungs have no pain receptors. They are called “pain

insensitive structures”

N.B.: Serious diseases in these structures don’t produce pain till they

extend to a pin sensitive structure like arterial wall or serous covering.

Pain Threshold:

Pain threshold is the lowest intensity of stimulus that can cause pain when

the stimulus is applied for sufficient period of time.

Pain threshold can be measured in many ways.

One of the accurate methods to quantify the threshold is heating the skin with

measured amounts of radiant heat from a calibrated electric lamp.

It has been shown that the majority of subjects begin to perceive pain when the

skin temperature reaches 45oC, and almost everyone perceives pain before the

temperature reaches 47oC.

So, it seems that the great majority of people do not show significant

differences in their sensitivity to painful stimuli. However, they differ widely in

their reaction to pain.

Stimulation of Pain Receptors:

noxious stimuli are strong enough

-----> tissue damage ------> release of

chemical agents from destructed cells

into the surrounding interstitial spaces

which are called “pain producing

compounds” (PPCs) ------> stimulate

pain receptors in the affected tissues.

PGE2

IL-1

Both threshold of pain receptors facilitating their

stimulation

The PPCs may be classified into:

1- Direct stimulators1- Direct stimulators

Substances which when reach

specific threshold directly stimulate

pain receptors ----> pain, as:

- K+ ions. - H+ ions

- Serotonin. - Histamine

- Bradykinin

2- Sensitizers2- Sensitizers

Substances which lower the threshold for

stimulation of pain receptors by direct

stimulators ----> facilitate pain

production. They include:

a) Substances released by the injured

tissues as: PGE2 & IL-1

b) Substances released by pain

receptors through antidromic impulses

as: substance P

N.B.: Substance P also stimulate mast

cells to release histamine which is a

direct stimulator.

The surface membrane of pain receptors contain several molecular receptors

which can be activated by various PPCs.

Molecular receptorFunction

Acid sensing receptor (H+)Stimulation

Purinergic receptor (ATP)Stimulation

Transient receptor potential rec. (thermal)

Stimulation

Bradykinin receptorStimulation

& sensitization

Histamine recptorStimulation

& sensitization

Serotonin receptorStimulation

& sensitization

Prostaglandin receptorsensitization

Interleukin-1 receptorsensitization

Substance P receptorsensitization

Cannabinoid receptorInhibition

Opioid receptorInhibition

Pain Tolerance:

It is the maximum intensity of pain can be tolerated by the subject without obvious

complaint.

Pain tolerance is affected by a number of factors:

- Anxiety, depression & fatigue ------> pain tolerance.

- rest, sever exercise & strong emotional excitement ------> pain tolerance.

Non Adaptation of Pain Receptors

Pain receptors do not adapt to continuing noxious stimuli.

Non adaptation to pain serves a protective function to keep the individual

trying to remove the damaging stimulus or to get away from it.

THE CHARACTER (QUALITY) of pain

1) Pricking or Cutting Pain

2) Burning Pain

3) Aching Pain

4) Throbbing Pain

5) Colicky Pain

A sharp and localized pain. It is of cutaneous origin and is caused by pricking or

cutting the skin by a sharp object.

A less well localized pain. It is usually of cutaneous origin and is caused by burns

or inflammations of the skin .

A dull-aching nature. It is more diffuse and felt coming from deeper tissues, e.g.

rheumatic pains.

is characterized by fluctuation of its intensity with arterial pulsations. It results

from localized inflammation in deep tissues, as in abscess formation.

Pain results from spasm of plain muscles in the walls of hollow viscera.

SomaticVisceral

sitesitecutaneous, deep tissuessympathetically innervated organs can be transferredto body surface

charactercharacterconstant, localisedaching, throbbing, gnawing

vague distribution and Quality deep, ache, dragging, squeezingacute: colic, paroxysmal, +/- N/V, sweating, BP and heart rate changes

Acute nociceptive pain:Acute nociceptive pain:

Fast (Immediate) physiological pain

Slow (delayed) pathophysiological pain

onset: during application of the stimulus

Duration: short duration.

Nature: pricking

Localization: well-localized

Afferent: A-delta fibers

Higher center: CC

Neurotransmitter: glutamate

Significance: * determine site & severity.

* Initiate withdrawal reflexes.

Abolished by deep pressure and not

abolished by morphine.

Shortly after application if tissue

damage occurs

Longer duration

Burning

Poorly-localized

C-fibers

Thalamus

Substance-P

* Associated with arousal, autonomic &

emotional reactions

Abolished by local anaethesia &

morphine