Neuropathic pain has been defined by the International Association for the Study of Pain as:
Pain caused by a lesion or disease of the somatosensory system (McMahon 2013).
This replaces the previous definition which defined neuropathic pain as arising due to dysfunction of the nervous system.
Under normal circumstances, acute pain is normal and “good” as it warns the person of imminent or actual danger. Chronic pain has less of a purpose and the body has various mechanisms to reduce the pain. The brain has several inhibitory mechanisms so that the pain impulses being sent to the brain are inhibited and not felt. The brain releases “endorphins” which act on the same receptors that morphine act on. However, certain diseases can interfere with this inhibitory process.
Neuropathic pain can be classified into 2 broad groups and examples of each group follow. Note that this is not a complete list.
Central Neuropathic Pain
- spinal cord injury
- multiple sclerosis
Peripheral Neuropathic Pain
- Traumatic – phantom limb pain, post-surgical
- Metabolic: diabetic, alcoholic, vitamin deficiencies, renal failure
- Infectious : post herpetic neuralgia, AIDS, hepatitis C
- Drugs/ toxic: chemotherapy drugs, antituberculosis medication, anti AIDS medications, arsenic, lead
- Hereditary – CIDP, Charcot -Marie-Tooth
Neuropathic pain arises when there is damage or irritation of nerve fibres. Peripheral nerve damage, if not too prolonged, can be reversible. Also severage of a nerve, as with limb amputation and postsurgical trauma often take chronic courses and may be challenging to treat. Metabolic, drug causes are usually more self-limiting, and depend most importantly on reversal of the offending cause.
Pain arising from a neural source typically has typical characteristics. It is usually difficult to tolerate, though one cannot discern absolutely from the history if the pain is neuropathic. Various questionnaires have been designed to try and more accurately pinpoint those patients with neuropathic pain. Though it is impossible to find words that can definitely differentiate pain of neuropathic origin from pain of musculoskeletal or visceral (for example stomach, pancreas) origin, certain verbal characteristic descriptions have been found to be more associated with neuropathic pain:
burning, freezing, cold, hot, pain with light touch (allodynia), itching, tingling and pins and needles, numbness
Pain of neuropathic source can be spontaneous, that is, may have no relation at all to movement or to internal organ function.
True neuropathic pain can be extremely challenging to treat. Various medications can be prescribed which are not pure analgesics but actually modulate the pain and increase pain inhibition in the brain and spinal cord. These usually do not eliminate the pain altogether. In addition, depending on the region of the pain, epidural injections or nerve blocks can be performed. In recalcitrant cases, one can consider the insertion of a spinal cord stimulator.