Neuropathic pain is caused by damage to- or dysfunction of- the peripheral and central nervous system, rather than stimulation of pain receptors. It can involve any level of the nervous system including the sympathetic nervous system (sympathetically-maintained pain). Neuropathic pain does not occur in all patients and it is unclear what mechanisms predispose to the development of neuropathic pain in individual patients. In neuropathic pain the nerve fibres may be damaged, dysfunctional or injured. The impact of injury includes a change in nerve function both at the site of injury and areas around the injury. This leads to incorrect signals being sent to the brain. The brain perceives that these signals are coming from pain receptors in the skin or organs where in fact this is not the case. Phantom pain following limb amputation is a good example of this. Certain conditions or factors are implicated in the development of neuropathic pain, including: diabetes, chemotherapy, shingles, surgery, alcoholism and HIV infection.
Neuropathic pain may be puzzling to those who do not understand the mechanisms underpinning this pain, including the sufferer. Patients commonly use abnormal adjectives to describe painful and non-painful sensations such as ‘shooting’, ‘burning’, ‘tingling’ and ‘numbness’. Lack of knowledge in health care professionals who are unaware of the pathophysiology may cause them to label patients as having psychosomatic pain. In patients with neuropathic pain, there are three types of changes that can occur: painful symptoms, visible skin changes and loss of sensation.
Neuropathic pain commonly results in ‘spontaneous’ pains. Some of these sensations appear to have a ‘life of their own’ and are bizarre. Sensations include abnormally painful responses to an ordinary physical stimulus (evoked pain); and spontaneous pain – which occurs in the absence of a stimulus. There are two types of spontaneous pain: continuous and paroxysmal.
- Continuous pain– is a steady, ongoing sensation which is often felt in the skin. It is sometimes described as burning, cutting, pricking, tingling, “pins and needles” and stabbing. If it is felt in the deep tissues, it is usually described as cramping, throbbing, crushing or aching.
- Paroxysmal pain – this is an intermittent pain that usually is not associated with any precursor and described as shooting, lancinating, jabbing or stabbing in nature.
Evoked pains are usually exaggerated responses to innocuous events that do not cause pain in people with ‘normal’ pain pathways:
- Allodynia - pain that comes on from simple contact that is not normally painful. This is divided into mechanical allodynia (e.g. clothes touching the skin) and thermal allodynia (e.g. cool breeze against the skin).
- Paraesthesia anddysaesthesia – absence or impairment of the senses especially touch
- Hyperpathia - a prolonged duration of pain following a painful event.
- Hyperalgesia - hypersensitivity to mildly painful events – knocks, for instance.
Other sensations can include physical contact in one area of the skin resulting in painful sensations in another (trigger zone pain), or pain that radiates down a whole leg or arm (referred pain).
In patients with neuropathic pain, there may be visible changes in the skin in the area that overlies the painful area. Mechanisms underlying these changes that may accompany neuropathic pain are not well understood and are complex. The expected changes include the skin being pinker or redder than other areas; or more blue, mottled and dusky-looking. The changes that occur are largely due to alterations in blood flow. As well as colour changes; the skin can look waxy, dry, puffy or swollen with a reduction in hair and nail growth. The affected area not only appears different; but patients sometimes also ‘divorce’ themselves from the area, especially if it is an affected limb.
Loss of sensations
Areas affected by neuropathic pain can be numb to touch. Nerve damage from injury or disease processes can lead to loss of normal sensation. The nerves that are the most prone to damage (from diseases like diabetes) are those carrying temperature sensations; and hence the assessment involves testing for sensation, pain and temperature. The loss of normal input from the anaesthetic area may cause a failure of the normal gating process and promote central sensitisation in an effort to gain information from the affected area. Such amplification may result in the paroxysmal presentation of pain in a non-sensate area. This is a confusing and distressing condition for the patient, often compounded by a lack of belief by health care professionals that a numb area can be painful. The analogy of phantom pain can be a helpful teaching metaphor for patients and health care professionals.