Thursday April 9 2009
Neuropathic pain is commonly described as "burning" or "stabbing"
“A simple technique could help differentiate patients with different causes of back pain,” suggests BBC News. The new service says that researchers have devised bedside tests that distinguish between neuropathic pain (nerve damage) and other causes of pain. It said the tests are better than existing tests for neuropathic pain. The news service highlighted the importance of this by saying that different causes of pain have different treatments and quoted one of the researchers as saying, “if a diagnosis is wrong, patients may receive treatment, including surgery, that does not improve their pain".
This study indicates that a simple, quick diagnostic procedure can distinguish between the most common cause of back pain (axial) and pain caused by nerve damage (neuropathic). As the treatment for these can be very different, this is potentially a very useful tool. The subjects in this study all had long-term back pain and the tool will need to be tested in a more general population of people with back pain.
Where did the story come from?
The research was carried out by Dr Joachim Scholz from the Massachusetts General Hospital in Boston and colleagues from other institutions in the US, UK and Switzerland. The work was supported by a grant awarded by Pharmacia through The Academic Medicine and Managed Care Forum, with supplementary support from Pfizer. The study was published in the (peer-reviewed) medical journal PLoS medicine, a free journal from the Public Library of Science.
What kind of scientific study was this?
This was a diagnostic test validation study. It had two parts, the first of which involved the researchers devising a set of questions and bedside tests for distinguishing between two different types of back pain: neuropathic and axial. These diagnostic ‘tools’ were then tested on a separate group of participants to measure their accuracy.
Neuropathic pain is caused by damage to the nerves and is often difficult to formally diagnose. Sufferers commonly describe it as a ‘burning’ or ‘stabbing’ pain. A common form of neuropathic pain is ‘radicular’ low back pain, also called sciatica, which comes from a slipped disc and radiates from the back into the legs. The researchers compared this to the most common type of low back pain, ‘axial pain’, which is confined to the lower back and is non-neuropathic (not caused by nerve damage but is due to damage to joints, muscles or other tissue.
In the first part of the study, the researchers recruited 130 patients with several types of peripheral neuropathic pain and 57 patients with axial low back pain. These patients came from physician referrals or had answered advertisements. The patients had chronic back pain, painful diabetic neuropathy or pain following shingles. They were given a structured interview of 16 questions and a bedside examination of 23 tests. The interview involved asking the participants to choose words from a list that described their pain and to grade the intensity of particular aspects of their pain from zero (no pain) to 10 (the maximum imaginable pain). Bedside tests included measurements of responses to light touch, pinprick, and vibration. The patients were divided into smaller groups according to their responses to the interview and tests and statistical modelling was used to identify six questions and 10 physical tests that discriminated best between pain subtypes. The researchers combined these items into a tool that they called the Standardised Evaluation of Pain (StEP) tool.
The second part of the study was conducted at Addenbrooke’s hospital in Cambridge, UK between January 2006 and November 2007. The researchers applied the StEP tool to an independent group of 137 patients with back pain. These people had been recruited using similar methods and inclusion criteria to the first part. This included a requirement that they had moderate or severe back pain (more than six on a scale of zero to 10) for three months or more. Patients with a severe medical or psychiatric illness, another painful disorder or neurological disease were also excluded. The patients were allowed to continue their previously prescribed pain relief treatment during this time.
The StEP tools diagnostic accuracy was measured against clinical diagnosis as a ‘reference standard’. The researchers calculated the sensitivity and specificity of StEP and its positive and negative predictive values for the distinction between radicular and axial back pain.
What were the results of the study?
A number of patients were excluded from the study, including 32 patients in the first part and 11 patients in the second part. Reasons for exclusion were because the duration or intensity of pain did not meet the inclusion criteria, because the patients suffered from other painful disorders or had diseases that would have affected the evaluation of their pain. Another six patients with low back pain were excluded from the validation study because there was no unanimous decision between the attending physicians on the diagnosis.
In the second validation part, the StEP tool correctly identified the type of back pain (radicular or axial) in 129 out of the 137 patients. When a cut-off score of four was used, the tool identified patients with radicular pain with 92% sensitivity, meaning that the test detected 92% of all people with radicular pain, but missed the other 8%. The test had 97% specificity, meaning that it correctly identified 97% of people as having axial pain rather than radicular pain, but 3% of people with axial pain were incorrectly diagnosed as having radicular pain. The tool had a positive predictive value of 97%, i.e. 97% of all people identified by the test as having radicular pain truly had this type of pain.
What interpretations did the researchers draw from these results?
The researchers say that the diagnostic accuracy of the StEP tool “exceeded that of a dedicated screening tool for neuropathic pain and spinal magnetic resonance imaging”. They go on to claim that the tool offers a “unique opportunity to improve targeted analgesic treatment”.
What does the NHS Knowledge Service make of this study?
Diagnostic studies such as these are rarely reported in the news, though they form an important part of developing any potential test. There are a few points to consider about this study:
- The researchers also looked at the accuracy of the individual examination signs that make up the tool and showed that the best tests were tests for radicular pain known as a straight-leg-raising sign, a test for detecting cold, and a reduced response to pinprick test. The authors note that this is not surprising as they are routine parts of the examination of patients with back pain, and can be a diagnostic criterion of neuropathic pain. How much better this screening tool is than a more usual physical examination, needs further investigation.
- The relevance of this study to a more general population of people with back pain will need to be tested. The participants in this study were included because their back pain had suspected neuropathic causes. As such, they are a selected group and it is probable that the test will not work as well in the unselected populations that would be managed in primary care without referral to a specialist hospital clinic.
These findings indicate that a simple, quick diagnostic procedure can distinguish between radicular (neuropathic) and axial (non-neuropathic) low back pain in the selected group tested. Because the two types of back pain are treated in different ways, this is important when deciding who to refer for further tests such as an MRI scan. More testing of this tool’s diagnostic accuracy in primary care settings such as GPs surgeries would be valuable.