Date Published: April 7, 2009
Publisher: Public Library of Science
Author(s): Giorgio Cruccu, Andrea Truini
Abstract: Giorgio Cruccu and Andrea Truini discuss a new pain assessment tool published in PLoS Medicine called Standardized Evaluation of Pain and they review other tools to assess neuropathic pain.
Partial Text: According to the latest definition, the term neuropathic pain refers to pain arising as a direct consequence of a lesion or disease affecting the somatosensory system .
Large-size, non-nociceptive afferents (i.e., those that do not carry pain) have a lower electrical threshold than small-size, nociceptive afferents. Unless special techniques are used, i.e., experimental blocks or stimulation of special organs (cornea, tooth pulp, glans), electrical stimuli unavoidably also excite large afferents, thus hindering nociceptive signals. Hence standard neurophysiological responses to electrical stimuli, such as nerve conduction studies (NCS; see Glossary) and somatosensory-evoked potentials (SEPs), can identify, locate, and quantify damage along the peripheral or central sensory pathways, but they do not assess nociceptive pathway function , .
Quantitative sensory testing (QST) analyses perception in response to external stimuli of controlled intensity (Table 1). Detection and pain thresholds are determined by applying stimuli to the skin in an ascending and descending order of magnitude. Mechanical sensitivity for tactile stimuli is measured with plastic filaments that produce graded pressures, such as the von Frey hairs, pinprick sensation with weighted needles, and vibration sensitivity with an electronic vibrameter. Thermal perception and thermal pain are measured using a thermode, or other device that operates on the thermoelectric effect.
In patients with neuropathic pain, abnormal sensory findings should be neuroanatomically logical, compatible with a definite lesion site. Location, quality, and intensity of pain should be assessed. Proper assessment requires a clear understanding of the possible types of negative (e.g., sensory loss) and positive (e.g., pain and paresthesias) symptoms and signs. Neuropathic pain can be spontaneous (stimulus-independent or spontaneous pain) or elicited by a stimulus (stimulus-dependent or provoked pain). Spontaneous pain is often described as a constant burning sensation, but may also be intermittent or paroxysmal, and includes dysesthesias and paresthesias. Provoked pains (hyperalgesia and allodynia) are elicited by mechanical, thermal, or chemical stimuli.
Over recent years, several screening tools for distinguishing neuropathic from nociceptive pain have been validated . Some of them, i.e., the Neuropathic Pain Questionnaire (NPQ) , ID Pain , and PainDETECT , rely only on interview questions. PainDETECT was designed to detect neuropathic pain components in patients with low back pain; it has been validated in about 8,000 patients with low back pain, and reaches about 80% sensitivity and specificity .
In a new study reported in this issue of PLoS Medicine, Joachim Scholz and colleagues present a pain assessment tool called StEP (Standardized Evaluation of Pain) that combines six interview questions and ten physical tests . This novel tool assesses pain-related symptoms and signs and differentiates distinct pain phenotypes reflecting different mechanisms. Besides being diagnostically useful, a standardized approach for differentiating pain phenotypes independently from disease aetiology supports a mechanism-based concept of classifying and treating pain and thus offers an opportunity to improve targeted analgesic treatment , .
Despite intensive investigations, the cause of neuropathic pain often remains unknown, and careful assessment is needed before pain can be labelled idiopathic or psychogenic. Pain assessment has advanced enormously over recent years. But whereas the new laboratory tools help in diagnosing neuropathic pain and quantifying damage to the nociceptive pathways, they measure neither pain intensity nor response to treatment. As StEP shows, the most convenient approach is still to combine physical examination and patient’s report.