Neuropathic pain is caused by peripheral or
central nervous system injury. It is often described
as burning, shooting and may be associated with
altered sensation. Neuropathic pain is associated
with a loss of opioid receptors in sensory afferents
and an increased release of glutamate (a neuroexcitatory
amino-acid) in the dorsal horn. Activation
of glial cells, neuroma formation, increase in sodium
channels, and calcium channel activation results in
sensitization of the dorsal horn and higher centers.
This resultant hyper excitability causes spontaneous
pain, hyperalgesia, and allodynia in areas adjacent
to the nerve damage. An injured sensory nerve may
produce absent or abnormal sensation. These changes
thus lead to a variable response to opioids.