Chronic Pain
Chronic pain can be due to a number of causes
Causes
Symptoms
- spontaneous pain
- allodynia - pain in response to touch
- hyperalgesia - enhanced pain in response to noxious stimuli
Mechanisms
- modulation of pain receptors increases sensitivity yo external stimuli
- increased expression of receptors, sodium channels can occur
- A-fibres can undergo a phenotype switch and express C-fibre markers
- sympathetic nervous system can become involved, with NE inducing sensory impulses
- central sensitization in the dorsal horn can occur
- the synapse becomes transformed, with changes in NMDA and neurokinin levels
- allows Aβ stimuli to be perceived as pain
Treatments
- pre-emptive strategies are important to prevent neuroplasticity from occurring in situations where chronic pain is a possibility
Antidepressants
- moderate pain relief independent of effect on mood, especially for TCAs such as amitryptiline
- NNT 2-4; NNH 15
- inhibits NA/5-HT reuptake, strengthening descending inhibition
- blocks NMDA receptors
- venlafaxine, duloxetine also effective
Anticonvulsants
- gabapentin and pregabalin useful for neuropathic pain
- partial efficacy: NNT 5; NNH 25
- acts by binding Ca2+ channel, perhaps on sensory neurons...