greek.doctor

57. Adjuvant analgesics. Centrally-acting muscle relaxants

Analgesics for neuropathic pain

Neuropathic pain is caused by damage to peripheral or central pathways. This pain is not easily treated as it responds poorly to conventional analgesics like NSAIDs, and even to opioids. Other drug types are necessary.

Compounds

  • Tricyclic antidepressants
    • Amitriptyline
    • Nortriptyline
  • Antiepileptics
    • Gabapentin
    • Pregabalin
    • Carbamazepine
    • Oxcarbazepine
  • Local anaesthetics (as patch)
  • Capsaicin (as patch)
    • Capsaicin is the molecule in spicy foods that make them spicy.
  • Nabiximols (cannabis preparations)

Nabiximols are described in “centrally acting muscle relaxants”. The other drugs have been described in other topics. Only capsaicin will be covered in this section.

Mechanism of action

Capsaicin binds to receptors in nociceptive nerve endings and reversibly damages them, preventing them from causing neuropathic pain.

Administration

It’s applied as a dermal patch. It should be placed on the painful area for one hour. It takes 1 – 2 weeks for the analgesic effect to develop, but it lasts for up to 3 months.

Side effects

When placing the patch there is a painful response, but this can be alleviated with local anaesthetics.

Centrally acting skeletal muscle relaxants

These drugs act on the CNS to reduce spasticity (pathologically increased muscle tone), without compromising voluntary muscle contractions (like peripheral muscle relaxants do).

Compounds

  • Baclofen
  • Nabiximols (cannabis extract)
  • Benzodiazepines
  • α2 adrenergic agonists
    • Clonidine
    • Tizanidine

Baclofen can be administered per os or by intrathecal administration by a pump. Benzodiazepines and α2 adrenergic agonists are covered in their own topic.

Indications

  • Spasticity – resistance to movement, involuntary muscle spasms and enhanced stretch reflex
    • Caused by a lesion of the upper motor neuron, which regulate the stretch reflex
    • Due to stroke, cerebral palsy, multiple sclerosis, etc.
  • Acute muscle spasms

Mechanism of action

Baclofen is a GABA-B receptor agonist. Benzodiazepines bind to an allosteric site on GABA-A receptors, stimulating them. Stimulation of GABA receptors inhibits the stretch reflex, which reduces muscle tone and spasticity.

Nabiximols bind to and activate cannabinoid receptors, which inhibit the stretch reflex.