Types of pain sensation

Types of pain sensation

a/ cutaneous pain:

•       It’s pain produced by stimulation of pain receptors in skin or body surface

•       Transmitted by both Adelta, C fibers

•       It occurs in two phasesà fast pricking followed by slow burning pain

•       Unlike others types of pain it’s accurately localized due to:

–      The high density of pain receptors in the skin

–      The fast pain fibers reach sensory cortex

–      Besides touch & vision help greatly in localization

–      b/ deep pain:

–      Deep or muscluskeletal pain is the pain produced by injury to muscles, ligaments, tendons, joints & bones. It’s conducted along C fibers

–      Causes:

–       inflammation of deep tissues or joints eg rheumatic arthritis

–      Muscle spasm eg injury to bones, tendons & joints à this pain is associated  with reflex contraction of nearby skeletal muscles à ischemia à more pain à spasm à ischemia à pain ..etc (viscous circle)

–      Ischemia eg  1/ anginal pain of cardiac muscles                       11/intermitent claudication  of leg muscle

–      c/Neuropathic pain:

–      This is a chronic type of pain occurring due to damage to or pathological changes of nerve fiber either in the peripheral or central nervous system. Examples:

–      Trigeminal neuralgia

–      Herpes zoster

–      Phantom limb pain

–      Diabetic neuropathy

–      Sciatica

–      Characters:

–      1- described as excruciating, electric or shooting pain

–      2- Characterized by occurring in bouts or paroxysms

–      3- It is accompanied by hyperplasia &/or parasthesia

–      4- It is partially responsive to opioid therapy

–       

–      d/ visceral pain:

–      This is pain from internal viscera of the thorax, abdomen & pelvis.

–      Conducted along C fibers

–      Due to sparse distribution of pain receptors in the viscera à a sharp cut in the viscera does not cause pain, while diffuse stimulation of pain nerve ending throughout a viscous cause sever burning pain

–      Parietal wall of the viscera (pleura, peritonium, pericardium) are richly supplied by pain receptors attached to A delta fibers. Thus if a disease spread from a viscus to its wall it causes sharp acute pain (parietal pain)

–      Pain innervation of the viscera: afferent fibers from visceral structure reach CNS via sympathetic & parasympethetic nerves

–      Causes of visceral pain:

–      1/ ischemia: causes pain due to accumulation of acidic metabolic products as bradykinin & proteolytic enzymes

–      2/ spasm of hollow viscus: spasm of gut, gall bladder, ureter, uterus  etc causes pain by:

–      a/ mechanical stimulation of mechanical pain receptors

–      b/ obliteration of the blood vessels by spasmodic  contraction of the smooth muscles causing ischemic pain

–      3/ overdistention of hollow viscus: extreme overfilling or overdistention causes mechanical & ischemic stimulation of pain nerve endings.

–      4/ inflammation or chemical irritation of peritoneal covering of the internal organs: as in case of perforated peptic ulcers due to accumolation of HCL & proteolytic enzymes à this cause well localized parietal pain

–      5/ compression or infiltration of viscera by tumours

–      Characters of visceral pain:

–      it’s slow pain characterized by:

–      Dull aching or rhythmic cramps (colic)

–      Diffuse & poorly localized

–      Accompanied by exaggerated autonomic changes, nausea, vomiting, decrease heart rate, decrease blood pressure

–      Gaurding: reflex spasm of skeletal muscles over the affected area

–      Usually referred to surface area

c- Referred pain

•       This is the pain which is not felt in the diseased structure itself, but at another area in the body far a way from the site of its origin

•       It’s usually felt on skin area originating from the same embryonic segment or dermatome as the diseased viscous (or deep structure) & therefore supplied by the same dorsal root (dermatomal rule)

•       Mechanism of referred pain:

•       1/ conversion projection theory:

•       Afferent pain fiber from the skin & a diseased viscous converge on the same spinothalamic neuron in the dorsal horn that will finally activate the same cortical neuron. Whatever maybe the source of pain the cortex will project it to a skin area as the skin is the commonest source of pain due to:

•       Skin is richer in pain receptors

•       Skin is more exposed to stimulation

•       Skin is topographically  represented in the cortex while the viscera are not

•       So somatosensory cortex is more accustomed  to receive impulses from skin than from viscera. So pain impulse carried to the cortex in spinothalamic neuron shared by afferent from the skin & others from the diseased viscus are misinterpreted as coming from the skin ie the cortex will project the sensation to the skin area instead of projecting it to the diseased viscous

•       2/ facilitating theory:

•       Pain fibers from skin are always carrying impulses but these are normally subliminal (subthreshold) to spinothalamic neuron. When the viscus is diseased , impulses passes through afferents which give collateral to spinothalamic neurons receiving pain fibers from the skin(same dermatome for that segment) à increasing their excitability ie facilitate them to reach threshold à pain felt in this skin area + hyperalgesia

•       It’s possible sometime to abolish mild referred pain by local anesthesia applied to dermatome of the reference . This confirms that facilitation is responsible for referred pain

•       Example of referred pain:

•       Cardiac pain: is felt in retrosternal region, root of the neck, outer parts of the chest & inner part of the left arm & also in epigastrium

•       Gastric pain: is felt between the umbilicus & xiphoid process

•       Gall bladder pain: is felt as a back pain that radiate to the inguinal region & testicles

•       Appendicitis pain: is felt around the umbilicus

Pain control

•       Several observation have led to believe that pain transmission & perception can be inhibited in the central sensory pathway by ascending & descending impulses. These observation include:

–      The degree that a person react with pain varies widely

–      Injuries caused in accident or in battle field may ignored at the time (stress analgesia). The same injury if inflicted for example in surgery without anesthesia would cause agonizing  pain

–      Irritation of the skin overlying a diseased viscus eg with mustard plaster relief pain

–      Acupuncture has been used for years to prevent or relieve pain

–      Strocking the area around the injury or rubbing or shaking is often reduce the pain of injury

•       This results from the capability of body itself to suppress pain signals to CNS by activating pain control system