Continuation of pain series . Part 1 is here
Nociceptive pain is the most common type of pain; experienced due to pain signals from the periphery. Peripheral pain receptors are part of the somatosensory system and are present throughout skeletal muscle, bone, organ, cardiovascular and connective tissue. They are activated in response to actual or potential tissue damage and nociceptive signals are sent brain-bound; nociceptive pain signals are modulated by opioid receptors within the spine and dorsal horn area up higher on the spine near the brainstem - This type of pain is typically well responsive to opioids given orally, parenterally, etc. Often described as dull/throbbing, tearing, sharp/cutting, or aching. Nociceptive pain is felt when you break a bone, stub a toe, burn yourself on a stove, tear or pull a muscle, and with certain degenerative diseases such as rheumatoid arthritis, osteoarthritis, and cancer. In addition to their euphoric effects via the lymbic system, opioids can relieve this pain by reducing the 'action potential' of ascending pain pathways within the CNS; Meaning that opioid activity in this area reduces the frequency/intensity of pain signals traveling (ascending) to the brain.
This may be caused by actual injury to the somatosensory nervous system itself, or by abnormal changes in neurotransmission, commonly involving NMDA receptors, sensitization/central sensitization, or abnormal sympathetic/somatic nervous system interactions. Often described as burning, tingling, eliectrical-like shock sensations. Neuropathic pain is often experienced with fibromyalgia, phantom limb pain, peripheral neuropathy, postherpetic neuralgia, or psychosomatic related conditions. Opioids may be somewhat helpful in treating neuropathic pain, but generally with less efficacy. Specific drugs which antagonize NMDA receptors or enhance the inhibitory effect of GABA are sometimes useful. Ketamine works by antagonizing NMDA receptors reducing transmission of excitatory signals; while Gabapentin is often effective for neuropathic pain but not well understood.
Opioids Better Suited for Neuropathic Pain
Certain opioids may be more effective than traditional opiates in treating neuropathic pain; methadone shows NMDA activity as an antagonist and has a good track record for treating neuropathic pain that has been resistant to morphine, etc. levorphanol is sometimes used in resistant forms of pain such as neuropathic, and is believed to have NMDA activity in addition to acting as a norepinephrine reuptake inhibitor. Tramadol and Tapentadol are mild strength opioids which also possess norepinephrine activity which may be useful in difficult cases of pain.
Additionally; pain can be classified by which physiological system is involved:
- Myofascial Pain: Pain originating from skeletal muscles and the tissues surrounding them.
- Rheumatic Pain: Pain of the joints and other surrounding connective tissues.
- Vascular Pain: Pain originating from the blood vessels - heart, arteries, veins.
- Visceral Pain: Pain which originates in the organ tissue.
- Neuropathic Pain: By injury or illness of the somatosensory nervous system.