Pain is often classified as nociceptive or neuropathic. Each of these pain types has characteristics and features that enable physicians to distinguish between them. The value of classifying the pain lies in providing a more accurate diagnosis and, subsequently, the most appropriate treatments.
In this article, general overviews of both nociceptive and neuropathic pain are provided. Two types of pain states, hyperalgesia and allodynia, are also described.
Nociceptors are specialized types of nerve cells that react to noxious or unpleasant stimuli. There are a variety of classes of nociceptors responsible for alerting the brain to potentially harmful conditions. Heat, pressure, sharp mechanical, and chemical pain are relayed to the nervous system by the various classes of nerve fibers. Examples of nociceptive pain include:
- Intense heat, such as when a hand touches a hot stovetop surface.
- Sharp mechanical stimulation, such as a razor blade nicking the skin during shaving.
- Pressure experienced when force is applied, such as a pinch to the back of the arm.
- Chemical stimulation, such as when salt comes into contact with an open cut or wound.
Nociceptors play a key role in providing warning signals to the brain in order to protect the body from injury. For example, when a hand touches a hot stovetop surface, the person quickly withdraws his or her hand. The nociceptors rapidly transfer the sensation of the heat stimuli to the spinal cord, which then relays a withdrawal motor reflex to protect the hand.
Nociceptive pain can be further broken down to somatic and visceral pain:
- Somatic pain relates to pain experienced within more superficial and/or peripheral tissues, such as the skin, muscles, and bones.
- Visceral pain is often experienced as pressure within the abdomen or involving specific organ-related problems.
Neuropathic pain, or nerve pain, results from disease, malfunction, or injury of the central nervous system (the brain and spinal cord) or peripheral nervous system (everything else). Nerve pain is most often described as having the following characteristics:
- Prickling (pins and needles)
- Persistent tingling and/or numbness
- Burning sensations without a source of heat present
Neuropathic pain tends to be chronic and more difficult to treat. One of the most common examples of a neuropathic pain condition is diabetic peripheral neuropathy. Chronically high blood sugar levels lead to nerve damage that results in neuropathic pain in the feet. People with this condition report feeling sharp pain, prickling, tingling, numbness, and burning sensations in both feet.
Hyperalgesia and Allodynia
In addition to neuropathic and nociceptive pain types, pain is often described in other terms such as that being related to hyperalgesia and allodynia. Hyperalgesia and allodynia may be associated with either nociceptive pain or neuropathic pain, depending on the circumstances surrounding the development of the pain condition.
Hyperalgesia is said to be present when there is an increased and/or enhanced pain response to a normally painful stimulus. For example, a disproportionately high pain level following the application of firm pressure to an extremity that has recently undergone surgery may be considered hyperalgesia. In this instance, the extremity may be somewhat sensitized to the pressure stimulus which results in greater than expected pain levels from the application of pressure to the limb.
Allodynia is said to occur when a typically non-painful stimulus, such as contact with clothing or a cool breeze, evokes a painful response. This phenomenon is often seen in association with neuropathic pain conditions such as diabetic neuropathy and fibromyalgia. For example, a patient with diabetic neuropathy may find it painful to wear shoes or even to have bed sheets contact the skin of the feet.
Although research continues to advance understanding of pain as a disease, current theories regarding the complexities of the pain experience are incomplete. Contributions from numerous medical disciplines are pushing the frontiers of pain science forward.