**2. Definition of neuropathic pain**

Neuropathic pain has been defined by the International Association for the Study of Pain (IASP) as "pain arising as the direct consequence of a lesion or disease affecting the somatosensory system" [18]. This is distinct from nociceptive pain – which signals tissue damage through an intact nervous system – in underlying pathophysiology, severity, and associated psychological comorbidities [13]. Individuals who suffer from neuropathic pain syndromes report pain of higher intensity and duration than individuals with non-neuropathic chronic pain and have significantly increased incidence of depression, anxiety, and sleep disorders [13, 19].

Any trauma to the somatosensory system appears to have the capacity to cause a neuropathic pain syndrome; yet the presence of any individual pathology does not guarantee the develop‐ ment of neuropathic pain, highlighting the importance of genetic and environmental factors as well as individual disease pathogenesis. To further complicate matters, individuals with seemingly identical diseases who both develop neuropathic pain may experience distinct abnormal sensory phenotypes. This may include a loss of sensory perception in some modali‐ ties and increased activity in others. Often a reduction in the perception of vibration and light touchiscoupledwithpositivesensorysymptoms suchasparesthesia,dysesthesia,andpain[20]. Pain may manifest as either spontaneous, with a burning or shock-like quality, or as a hypersen‐ sitivity to mechanical or thermal stimuli [21]. This hypersensitivity takes two forms: allodynia, painthatisevokedfromanormallynon-painfulstimulus,andhyperalgesia,anexaggeratedpain response from a moderately painful stimulus. For a more extensive list of sensory signs and symptoms associated with neuropathic pain see Table 1. Ultimately, the path towards effica‐ cious treatment of chronic pain will include a clear understanding of how certain pathophysio‐ logic changes lead to specific sensory signs and symptoms. This will allow clinicians to translate measurable sensory abnormalities into underlying pathology. With a clear view of mecha‐ nism, targeted treatment and individualized medicine become conceivable.
