Neuropathic Pain

Question 1

What is neuropathic pain

Answer 1

pain due to damage or dysfunction of the nervous system

Question 2

Types of neuropathic pain

Answer 2

sharp, shooting pains
electric jolt
painful tingling

Question 3

Clinical causes of neuropathic pain (4)

Answer 3

• nerve compression
• nerve crush, stretching, incomplete transection
• neuropathies due to disease
• neuropathies due to amputation or transections

Question 4

Nerve compression

Answer 4

nerve entrapment - could be due to sport injury or carpal tunnel syndrome - bundle compressed due to weight, inflammation, etc

Question 5

Nerve crush, stretch, incomplete transection

Answer 5

• blunt force trauma to peripheral nerves
• nerve streching: likely due to accident or injury
  incomplete transection: nerve partially cut -> causes pain at that site

Question 6

Diabetic neuropathy

Answer 6

increased blood glucose levels decreases blood flow -> leads to vessel damage -> nerve damage

Question 7

Drug induced neuropathy

Answer 7

harsh toxins can damage nerves - cisplatin, vincristine. Unclear mechanism

Question 8

Consequences of nerve damage

Answer 8

• exaggerated electrical response in nociceptive fibers (hyper responsive)
• exaggerated pain response (hyperalgesia, allodynia)
• macrophage invasion at nerve injury site
• nerve hyperexcitability enhanced

Question 9

Wallerian degeneration

Answer 9

• normal process
• Damage or transection of nerve will lead to degeneration of distal nerve fiber
• axon later regenerates and projects to its proper location –> pain resolves
• patient may experience numbness/tingling as nerve regenerates

Question 10

abnormal axonal regeneration

Answer 10

nerve sends out collaterals to determine where to grow -> collateral cannot determine where axon should regenerate -> start to fold in on themselves -> nerve becomes hyperactive and fires a lot of AP

Question 11

Pharmacological tx

Answer 11

antidepressants
anticonvulsants (Na+ or Ca2+ blockers)
Opioids

Question 12

TCA

Answer 12

Amitriptyline (or other TCA)

• block NE and 5-HT uptake, anti-cholinergic
• block NE = stimulate descending pathway

Question 13

SNRI

Answer 13

Duloxetine (and other SNRI)
blocks NE and 5-HT reuptake
indicated for diabetic neuropathic pain

Question 14

Atypical antidepressant

Answer 14

Bupripion (Wellbutrin, Zyban)

DA and NE reuptake blocker

Question 15

Anticonvulsants - Sodium channel blockers

Answer 15

Carbamazepine (Tegretol)
Lamotrigine (Lamictal)
blocking Na+ channels will prevent progression of axonal nerve impulses -> blocks nerve transmission -> blocks pain signal from reaching brain

Question 16

Anticonvulsants - Calcium channel blockers

Answer 16

Gabapentin (Neurontin)
Pregabalin (Lyrica)
Calcium channel modulators
primary mechanism not through GABA -> can increase GABA synthesis and release but do not directly affect GABA receptors
used off label -> quazi first-line tx
use over Na+ blockers or other Ca2+ blockers
Not habit forming

Question 17

Opioids

Answer 17

controversial use
have to use at high levels
Some studies suggest morphine, buprenorphine, oxycodone, and tramadol can be analgesic
at some point, you’re not treating pain but making pt numb