Pain Pathway and AMP: Pathophysiology

Pain

Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (given by IASP).

6 important phrases related to pain

• It is always a personal experience, influenced to varying degrees by physiological, biological, and social factors.
• Pain and nociception are different phenomena.
  • Pain can't be inferred solely from activity in sensory neurons.
• Through their life experiences, individuals learn the concept of Pain.
• A person's report of an experience in Pain should be respected. 
• It may have adverse effects on normal functioning, social and psychological wellbeing.
• Verbal description is only one of several behaviors to express pain.

The inability to communicate does not negate the possibility that a human or a nonhuman animal experiences pain.

Pathophysiology of Pain

The pathophysiology of pain is mediated mainly through Pain receptors and Lateral Spinothalamic tract. The pain receptors have free nerve endings. Pain receptors are 

• Mechanoreceptors - Supplied by A δ nerve fibers.
• Thermal-mechanoreceptors - Supplied by A δ nerve fibers.
• Polymodal receptors - Supplied by C-type nerve fibers. 

The Lateral spinothalamic tract is the Pathway followed by stimuli to reach the pain signals to the brain. The lateral spinothalamic tract is subdivided into the Neospinothalamic tract and the Paleospinothalamic tract. 

These are also involved in pain pathophysiology. To learn more about their involvement, sign up for the prepladder app.

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Pain Fibers

Based on myelination and unmyelination, pain fibers are divided into myelinated and Unmylinated pain fibers.

• Myelinated
  • A α- Concerned with proprioception.
    • Thick nerves
  • A β - Concerned with fine touch
    • Thick nerves                                                Fast nerve conduction velocity 
  • A δ - Concerned with pain.
    • Fast, acute, and localized pain
    • Sharp, dull, and throbbing pain
    • Thin nerves
• Unmyelinated - C type.
  • Slow nerve conduction velocity
  • Concerned with pain.
    • Slow, Diffuse, and non-localized (dull) pain 
    • Thin nerves

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Classification of Pain

Classification based on duration

• Acute - < 3 months
• Chronic - > 3 months

It is important to remember that, In hyponatremia, the acute and chronic pain can be identified based on hours

• > 48 hours - chronic
• < 48 hours - Acute

Classification based on pathophysiology

• Nociceptive
  • Has nociceptive stimuli
  • It is Well-localized 
  • Nociceptive type of pain Responds well to NSAIDs.
  • Has the adaptive or protective response 
  • 2 types
    • Somatic
    • Visceral
• Neuropathic
  • No stimuli
  • Pathological pain
  • It is usually Diffused pain
  • No response to NSAIDs 
  • No protective response 

Assessment of pain 

• Numerical rating system
  • Rated from 0 to 10
  • 0 - Best
  • 10 - Worst
• Visual analogue scale
  • On scale numerical system is present.
  • Patient marks the point on the scale to describe the pain.
• Categorical scale 
  • Pain is expressed as Smiley images
  • Patients pain relates to the Smiley image
• Initial pain assessment tools
  • Certain images and questionnaires are present.
• McGill - Melzack pain questionnaire

Pain Assessment Tools

2 main types of Pain Assessment Tools:

• Unidimensional
  • Tells only about severity of pain.
  • Numerical Rating Scale (NRS)
  • Visual Analogue Scale (VAS)
  • Categorical Scale
• Multidimensional
  • Along with severity other dimensions are also observed.

• Initial Pain Assessment Tool (IPAT)
• McGill - Melzack Pain Questionnaire 
• Brief Pain Inventory (BPI)

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Management of pain

• Acute pain management 

Acute pain management is done with the help of NSAIDs and Opioids.

• Chronic pain management
  • TCAs - Most effective
    • Limiting factor:We can’t escalate too much due to its side effects.
    • Most used TCAs:

• Nortriptyline 
• Desipramine

• SSRIs - Alternative to TCAs
  • Poor analgesic action
  • Hence not preferred much.
• Serotonin norepinephrine reuptake inhibitors (SNRIs) - Better alternative to SSRIs
  • Duloxetine
  • Venlafaxine
• Anticonvulsants
  • Gabapentin
  • Pregabalin
  • Phenytoin
  • Carbamazepine 
• Opioids

Classification of Opioid analgesics and antagonists

• Strong agonists
  • Morphine
  • Heroin
  • Meperidine
  • Methadone
  • Fentanyl
• Moderate agonists
  • Codeine 
  • Oxycodone
  • Hydrocodone
• Weak agonists - Propoxyphene
• Mixed agonists-antagonists
  • Pentazocine
  • Nalbuphine
  • Buprenorphine
  • Butorphanol – can be given through the nasal route.
• Antagonists
  • Naloxone is used to reverse the opioid toxicity.
  • Naltrexone is used for de-addiction.
  • New antagonists - Blocks peripheral µ receptors.
    • Alvimopan - Helps in negating post-surgical paralytic ileus.
    • Methylnaltrexone - Helps in negating constipation caused by opioids.

Frequently Asked Questions

Q: Which receptor is supplied by A δ nerve fibers?

Answer: Mechanoreceptors

Q: What type of nerve fibers are supplied by Thermal-mechanoreceptors?

Answer: A δ nerve fibers.

Q: Which receptor is supplied by C-type nerve fibers?

Answer:  Polymodal receptors

Hope you found this blog helpful for your NEET SS General Medicine preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.