Lecture 28 - Neuropathic Pain I Flashcards
How prevalent is neuropathic pain?
Around 20% of Australians suffer from it
In 5%, the pai has signficiant impact on ability to function and quality of life
List some 2° physical consequences of chronic pain
- Deconditioning & postural changes
- Changes to:
- Psyche
- Sleep patterns
- Behaviour
- Appetite
- Thought
What is the definition of chronic pain?
Persists for more than 3 months
Outline the three broad groups of chronic pain
-
Defined nociceptive basis
- eg Chronic arthritis
-
Well-defined neuropathological basis
- eg Post-herpetic neuralgia
- Peripheral neuropathy
-
Idiopathic
- Pathogenesis not well accepted
- eg Chronic muskuloskeletal pain, esp. spinal pain
- Some headaches
List the classification groups of pain
Give examples of each
A. Nociceptive
-
Superficial somatic
- Malignant ulcers
-
Deep somatic
- Bone metastases
- Liver capsule distension or inflammation
-
Visceral
- Deep abdominal/chest masses
B. Neuropathic
- Trivial injury to the CNS or PNS
- Tumour related:
- Spinal cord compression
- Non-tumour related:
- Post-herpetic neuralgia
- Phantom pain
Outline the origin of stimulus for the various classifications of pain
A. Nociceptive
- Superficial somatic
- Skin
- Subcutaneous tissue
- eg mucosa of mouth
- Deep somatic
- Muscles
- Bones
- Joints
- Organ capsules
- Pleura
- Visceral
- Solid or hollow organs
- Deep tumour masses
B. Neuropathic
- Damage to nocicpetive pathways
Describe the sensation of the various classifications of pain
A. Nociceptive
- Superficial somatic
- Hot, burning, stinging
- Deep somatic
- Dull aching
- Visceral
- Dull deep
B. Neuropathic
- ‘Pins and needles’
- Tingling & burning
- Allodynia
- Phantom pain
List things that can initiate neuropathic pain
Trivial injury to the PNS or CNS:
- Infection
- Trauma
- Surgical intervention
Describe the persistence of neuropathic pain
- Persists after the initial triggering stimulus has been removed
- Lasts indefinitely
- May escalate over time
Describe the effect of analgesics in neuropathic pain
Poor response to conventional analgesics (<50%)
List conditions which often lead to neuropathic pain
- Diabetes: **Diabetic neuropathy **(20-25%)
- Herpes zoster infection: Post-herpetic neuralgia (25-50%)
- Post-masectomy pain (20%)
-
Cancer patients: neuropathic pain (33%)
- +/- nociceptive pain
List the various characteristics of neuropathic pain
-
Spontaneous pain
- Shooting, burning, electric shock-like
-
Hypersensitivity / hyperalgesia
- Increased pain derived from minimally painful stimuli
-
Allodynia
- Pain experienced due to usually innocuous stimuli
- Tactile
- Thermal
- Pain experienced due to usually innocuous stimuli
What can we learn from studying pain in humans?
Mostly characterise pain states
Very few studies directly test mechanisms of pain (anatomical, biochemical, physiological)
Outline the value of animal models in pain research
- Animal models were used in 1/3rd of articles in Pain
- Allow investigation of:
- Neurochemistry and anatomy
- Samples from pain relevant tissues
- eg mRNA
- Pain conditions
- Genetic and environmental conditions
- Physiological mechanisms
- Comparative value
- Molecule / pain related phenomenon in man always found in rodents
- Failures:
- However, there are notable cases where antagonists/drugs developed in murine models have failed to translate into humans (ie not worked in humans)
- Successes
- Ziconitide
- First found to be efficacious in mice
Describe the tail flick test
- Application of heat stimulus to tail
- Time recorded until spontaneous ‘flick’ withdrawal
- A spinal reflex
Describe the animal model of neuropathic pain
- Tight ligation of spinal nerves L5 and L6 on LHS
- Supply Sciatic nerve
- However, sciatic nerve is not completely ligated, the rat can still ambulate as normal.
- Neuropathic signs within days:
- Themal & tactile allodynia
- Persists for at least five weeks