Pain Pathways

Dr Sameer Gupta

Consultant in Anaesthesia and Pain Management, NGH

Objective

• To give you a simplistic and basic concepts of pain pathways to help understand the complex issue of pain

Pain Definition

• Unpleasant SENSORY and EMOTIONAL experience associated with actual or potential tissue damage

Pain

Is having pain Any good ?

– Evolutionary process.

Physiological Effects of acute

pain

• Positive role:

• warning of tissue damage

• immobilization for healing

• protection of the species: establishment of memory

Pain Physiological responses

Affect

–HR, BP, RR, GIT

–Learned Behaviour response

–Memory

Classification of Pain

• Acute Pain : Post operative Pain, non surgical cause

• Cancer Pain :Palliative Care

• Chronic Non Cancer Pain –Visceral Pain –Musculoskeletal pain

Classification of Pain

• Nociceptive: inflammatory

• Neuropathic: nerve related

• Mixed

First attempts (cancer pain)

Acute Pain

• Generally last less than a week

• Results from activation of sensory nerve fibres called nociceptors

Pain Pathways (nociceptors)

Myelinated A delta Fibres

Unmeyelinated C afferent fibres

Location: most body tissues except brain, liver

Pain

• Experience of pain is a complex mix of activation of different afferent fibre types

• Eg A-beta fibres

First attempts 1664 Descartes,

Pain a specific sensation

Pain Pathway

Stimulus Nociceptors Spinal Cord, Modulation happens

Perception

Brain Experience,

learned

behaviour

Memory

Alternatively

• Sensors and conduction: Stimulus and nociceptive receptors

• Central Processing and modulation: Spinal cord and Brain

• Effectors: behaviour, reaction, emotions

Stimulus

• Noxious Stimulus

–Mechanical

–Thermal

–chemical

Tissue Injury

Cyclooxygenase (Cox)

Phospolipase A2

Break down of membrane lipids

Arachidonic Acid

Prostaglandins

Products of Arachidonic Acids

• Act directly on the nociceptors ( peripheral terminals of Aδ and c fibre) and lower their threshold to thermal stimuli.

• Results: a sensation of burning at room temperature

Nerve Fibres Transmitting Pain

• C Fibres: unmyelinated fibres, Characterised by diffuse dull intense pain

• A Delta Fibres: small myelinated fibres conduct localised sharp sensation

Noxious Stimulus

Excitatory Neurotransmitters

Eg Glutamate

Ascending pathw ay

Cortical Areas

Somatosensory

cortex

Prefrontal cortex Thalamus

PAG

At the level of spinal cord

Experience of pain

Wind Up (Central Facilitation)

• Poorly understood, ? Clinical relevance in pain

• Mediated via C fibres

• Repeated frequency dependent stimulation ( usually electrical stimulus) results in increase in amplitude of the neurons in the spinal cord.

• Possibly a role of NMDA receptors and substance P

• May result in central sensitisation

Are there any safeguards or

protection ?

• Mechanisms

• Descending Pathways

• Gate control Theory

Noxious Stimulus

Excitatory Neurotransmitters

Eg Glutamate

Central Processing

Past experiences, Mood,

Memory, other sensory inputs

Ascending pathw ay Descending pathways

Modulation

Gate control theory

• Balance of activity between large and small fibres

• Interneurons of the substansia gelatinosa regulate the input in Lamina V

Gate control theory

Gate control Theory

• Aβ Fibres Dorsal Horn Spinal Cord Brain

Nociceptive

Stimulus

Gate control Theory

Aβ Fibres Dorsal Horn Spinal Cord Brain

Nociceptive

Stimulus

What does that mean?

• Low intensity stimulation of the skin or peripheral nerves or vibration will generate analgesia

Is there any practical application of

this theory

• Rubbing, application of heat , counter

irritation creams

• TENS ( trans cutaneous nerve stimulation)

• Spinal Cord Stimulation

• Possibly Acupuncture

Descending Pathways

• Modulation of gate control are influenced by Higher Centre inputs

• Higher centre inputs are modulated by various neurotransmitters

Noxious Stimulus

Excitatory Neurotransmitters

Eg Glutamate

Central Processing

Past experiences, Mood,

Memory, other sensory inputs

Ascending pathw ay Descending pathways

Modulation

Endogenous Pain Control

System

Role of Higher Centres

• Peri Aque-ductal Grey, Locus Cerulus

– Key role

– Inhibits the firing of the dorsal horn neuron that respond to noxious stimulus (gate control theory).

– Changing the levels of neurotransmitters at the level of synapses

Basis of Pharmacological

treatment of some pain Reducing Excitatory Neurotransmitters

and excitation of the nerve (with respect to pain transmission)

Basis for using anti epileptics, Local Anaesthetics (Na channel blockade)

Enhancing Inhibitory

Neurotransmitters

Noradrenaline and Serotonin

Basis for using Anti depressants

Endogenous Pain Control System

• Opioids Receptors

• High concentrations of Opioid receptors in Spinal cord and Brain

• Body produce endogenous opioids

Endogenous opioids peptides

• Enkephalin

• Dynorphine

• Beta endorphine: effects similar to morphine

endorphines

Released in the presence of pain

Practical utilities

1) acupuncture: bearable pain

2) Placebo

3) Psychological modulation of pain

knowing pathway helped in controlling pain?

Medication

Prostaglandins act directly on the nociceptors reduces there threshold and normal stimuli can activate these producing a sensation of pain (peripheral sensitization)

– NSAIDS and possibly Paracetamol utitlises this

Medications

• Opioids: morphine

• Local anaesthetic: block conduction of the nerve

Definition Chronic Pain Ongoing persistent pain greater than 3-6 months

“pain experience persists beyond the usual course of an acute disease”

or

“after healing/cure has taken place”

or

“recurs at intervals for months or years”

or

“due to persisting stimulation in areas of ongoing tissue damage”

Chronic Pain

serves no useful biological function, but has profound effects on the patient and their family

Physical : Immobility

Emotional: distress

social : little social interaction/ isolation

economical : job issues

Day to day activity is severely affected.

Pain presentation

Treatment

consist of therapies aimed at all these areas and not focused to one particular area

Chronic Pain

Principles of Treatment

Improve Pain Perception

Improve Function / Mobility

Improve Sleep

Improve Emotional and Psychological Consequences of Pain

IMPROVE QUALITY OF LIFE