Neuropathic pain

Neuropathic pain is pain caused by a lesion or disease of the somatosensory nervous system. Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allodynia). It may have continuous and/or episodic (paroxysmal) components. The latter resemble stabbings or electric shocks. Common qualities include burning or coldness, "pins and needles" sensations, numbness and itching.

Up to 7–8% of the European population is affected by neuropathic pain, and in 5% of persons it may be severe. The pain may result from disorders of the peripheral nervous system or the central nervous system (brain and spinal cord). Neuropathic pain may occur in isolation or in combination with other forms of pain. Medical treatments focus on identifying the underlying cause and relieving pain. In cases of peripheral neuropathy, the pain may progress to insensitivity.

Diagnosis of pain conditions relies on the character of the pain with a sharp stabbing character and the presence of particular features such as mechanical allodynia and cold allodynia. Neuropathic pain also tends to affect defined dermatomes and there may be limits to the area of pain. For neuropathic pain, clinicians look for an underlying lesion to the nervous system or an inciting cause consistent with the development of neuropathic pain. The obvious presence of an underlying feature or cause is not always detectable, and response to treatment may be used as a surrogate particularly in cases where diagnosis of the underlying lesion leaves the patient in pain for a prolonged period of time. MRI may be helpful in the identification of underlying lesions, reversible causes or serious underlying conditions such as primary presentation of a tumor or multiple sclerosis. Quantitative sensory testing (QST), a system of detailed analysis of the somatosensory system, is frequently used in research situations to identify neuropathic pain and a more detailed analysis of its components. It has been suggested by some authorities that QST may have a future role in the diagnosis of neuropathic pain and in particular the identification of neuropathic pain subtypes. Neuropathic pain can occur alone or in combination with other types of pain. The identification of neuropathic pain components is important as different classes of analgesic are required.

The gold standard for diagnosing small fiber neuropathy as the etiology of neuropathic pain is skin biopsy. Sudomotor assessment, through electrochemical skin conductance, an accurate objective technique, could be considered as a good screening tool to limit skin biopsy in patients in whom it is not suitable.

Neuropathic pain may be divided into peripheral, central or mixed (peripheral and central) types. Central neuropathic pain is found in spinal cord injury and multiple sclerosis. Peripheral neuropathies are commonly caused by diabetes, metabolic disorders, herpes zoster infection, HIV-related neuropathies, nutritional deficiencies, toxins, remote manifestations of malignancies, immune mediated disorders and physical trauma to a nerve trunk. Neuropathic pain is common in cancer as a direct result of cancer on peripheral nerves (e.g., compression by a tumor), or as a side effect of chemotherapy (chemotherapy-induced peripheral neuropathy), radiation injury or surgery.

Neuropathic pain has profound physiological effects on the brain, which can manifest as psychological disorders. Rodent models in which the social effects of chronic pain can be isolated from other factors suggest that induction of chronic pain can cause anxiety-depressive symptoms and that particular circuits in the brain have a direct connection. Depression and neuropathic pain may have a bidirectional relationship, and relief of co-morbid depression may underlie some of the therapeutic efficacy of antidepressants in neuropathic pain. Neuropathic pain has important effects on social well-being that should not be ignored. People with neuropathic pain may have difficulty working exhibiting higher levels of presenteeism, absenteeism and unemployment, exhibit higher levels of substance misuse (which may be related to attempted self-medication), and present difficulties with social interactions. Moreover, uncontrolled neuropathic pain is a significant risk factor for suicide. Certain classes of neuropathic pain may cause serious adverse effects necessitating hospital admission. For instance, trigeminal neuralgia can present as a severe crisis wherein the patient may have difficulty talking, eating, and drinking. As neuropathic pain may be comorbid with cancer, it can have important dose limiting effects on certain classes of chemotherapeutic.

Neuropathic pain can be very difficult to treat, with only some 40–60% of people achieving partial relief.

First-line treatments include certain antidepressants (tricyclic antidepressants and serotonin–norepinephrine reuptake inhibitors) and anticonvulsants (pregabalin and gabapentin). Opioid analgesics are recognized as useful in some cases but not recommended as first-line treatments. A broader range of treatments is used in specialist care. There is limited data and guidance for the long-term treatment of pain. Notably, strong evidence from randomized controlled trials is not universal for all interventions.^{[citation needed]}