Chronic Pain Impairs Number Sense

pain-number-sense

By Palanisamy Vijayanand

It’s clearly a budget. It’s got a lot of numbers in it ~ a proud George W. Bush on his budget experience.

World over, millions of dollars are spent every year seeking a cure for chronic pain. Much on dubious alternative medicines, and much more on modern medicine which is thin on evidence. The numbers – from a cost-benefit analysis point of view, for the desperate sufferer – clearly, do not add up. A bet on Psychic Flame, on a pleasant February afternoon at Mumbai’s Mahalaxmi Race Course would, perhaps, have yielded better results. Like pain, could spending on pain treatments also be a biopsychosocial phenomenon? That is, somehow the changes in the brain due to chronic pain make the sufferers to not count their pennies. Recent advances in imaging techniques have shown that chronic pain results in structural and functional alterations in specific brain regions, including prefrontal (PFC) and parietal (PC) cortices, which are thought to accommodate the ‘number-sense’ – our intuitive skill of understanding numbers and dealing with sizes and proportions. A recent paper by Wolrich and colleagues looked at the feasibility that chronic pain patients might have an altered number sense, in the context of pain assessment. The spending angle on pain treatments is my own hypothesis. The aims of their study were:

1) To elucidate whether patients with chronic pain use number-based assessment tools differently from acute pain patients.

2) To investigate experimentally if patients with chronic pain are more inaccurate when faced with numerical–spatial tasks compared with controls.

3) To explore if the inaccuracy was associated with the presence of spatial neglect-like symptoms, a clinical sign of PC and PFC dysfunction.

First, an audit of the use of numbers in gold-standard pain assessment tools in patients with acute and chronic pain was undertaken. Next, were the experiments. Experiment 1 was designed to test participants’ abilities to translate abstracts numbers into spatial representations on straight lines similar to what is required when using visual analogue scales (VAS). In the first part, they were asked to mark on separate lines (anchored left and right with ‘0’ and ‘100’) where they thought a set of random numbers lay. In the second part (number naming), to exclude motor dysfunction as a cause for deviation in the previous experiment, number lines pre-marked with vertical lines were presented in a random order, and the participants were asked to indicate what number they thought each individual mark denoted. Experiment 2 (to diagnose spatial neglect) was designed to test the subjects’ abilities to judge spatial–numerical interactions. 8 cm long horizontal lines, anchored with the numbers ‘2’ or ‘9’ on either side were presented, and the participants were asked to mark where they thought the middle of the line was – first, with ‘2’ on the left and ‘9’ on the right, and next with ‘9’ on the left and ‘2’ on the right. From these, the expected numbers were subtracted to obtain the ‘deviation from the expected response’.

In the audit, Numerical Rating Scale (NRS-11) scores revealed that 61% of patients with chronic pain (CP) and 22% with acute pain (AP) chose numbers ≥7 to describe their pain intensity. Similar results were found for VRS-4. When numeric scores were categorized according to the respective verbal descriptor as given by each patient, CP patients used higher numbers to describe their pain when it was moderate or severe, but not when it was mild. In the first part of Experiment 1 ‘Mean Absolute Deviation from the Expected Response’ (MADER) was greater (P, 0.001) in CP compared with both controls and AP patients, this was mainly influenced by pain intensity. The second part showed similar results, but pain intensity played a lesser role. Put together, the results suggest an altered number-sense in CP but not AP patients, which was not influenced by motor dysfunction. In Experiment 2, the magnitude of departure from the midpoint was statistically not different between the groups, but the direction of deviation was. Patients with CP showed right sided deviation bias, reminiscent of the left-sided neglect of patients with right parietal stroke. Increased errors in number marking and naming in patients with chronic pain strongly suggest impairment of number-sense in this group in contrast to patients with AP and controls. Interestingly, not only was the absolute degree of inaccuracy greater in chronic pain so was its rate of occurrence.

Intact number-sense is essential for the use of VAS and NRS. However, a reduced number-sense was observed in one-third of CP patients, which might result in inaccurate measurement of pain. The results indicate that treatment effects too, when measured as a number, might be under or over-estimated. George W. Bush, while showing the Oval office for a German reporter, articulated his learnedness with the famous, ‘That’s George Washington, the first president, of course. The interesting thing about him is that I read three – three or four books about him last year. Isn’t that interesting?’ The pain sufferer might have a similar dilemma with deciding between a three and a four on the NRS. While it generates data that could be statistically analysed, previous studies have shown that there is a potential for error, and interpreting data from a pain-rating scale is not as straightforward as it might first appear.As Bush would say, one might end up ‘misunderestimating’ the issue, and that includes the cost of pain treatments. Furthermore, as acknowledged by the authors, the CP group mainly suffered from low back pain. Whether different pain conditions affect the number sense differently is an angle which remains to be explored.

Dr. Palanisamy Vijayanand

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FCARCSI, DPainMed (RCSI), MSc (Pain), FFPMCAI, FFPMANZCA | Hyderabad, India

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