Pain Disrupts Working Memory

EDMONTON, Alberta, May 18 -- Chronic pain interferes with the brain's ability to keep information in mind while working on other tasks that require attention, researchers here found.

Although pain specialists have known for years that chronic pain interferes with attention, it hasn't been clear which functions are actually disrupted, according to Bruce Dick, Ph.D., and Saifudin Rashiq, M.B., M.Sc., of the University of Alberta.

It turns out that the function affected most is the so-called working memory trace -- the ability to maintain and mentally manipulate a memory trace while performing the spatial working memory task -- the researchers reported in the May issue of Anesthesia & Analgesia.

They enrolled 24 patients, 18 of them women, from the pain center at the University of Alberta Hospital and gave them a series of cognitive tests both before and after they had analgesic treatment for their pain.

Tests included:

• The test of everyday attention, which profiles sustained and selective attention.
• The reading span test, which reflects the efficiency of basic cognitive skills.
• And the spatial span test, which taxes the processing and storage components of spatial working memory.

Patients were eligible if they had a reduction of at least four points on a numerical pain score after treatment, which might have included epidural injection, somatic nerve blockade, pulsed radiofrequency rhizotomy of the medial branch nerves, and trigger point therapy.

Patients were excluded if they had a history of significant head injury, neurological disorder, or disease known to impair attention, the researchers said.

Using the test of everyday attention, the researchers found that a third of the patients were not clinically impaired by their pain, and divided the remaining 16 into two groups -- less and more disrupted.

All patients reported pain levels significantly reduced after their treatment (at P<0.0001).

But, the researchers reported, the pain relief had no significant effect on measures included in the test of everyday attention, including overall performance, selective attention, or sustained attention.

There was also no significant difference on the reading span test, they said.

But they did find differences on the "mirror task" of the spatial span test. In that test, patients are asked to look at a series of letters presented askew on a computer screen and to determine whether each letter is in its normal orientation or a mirror image.

Patients are also told to remember what direction the top of each letter is facing. At the end of the series, the patient must report the direction. The test relies on the working memory trace, the researchers said.

The unimpaired and less impaired groups performed equally well on the mirror task, the researchers found, but the more impaired group fared significantly worse (at P<0.05).

"Working memory trace (is) a specific cognitive process that appears to be disrupted by chronic pain," they concluded.

The study is limited by its small size, the researchers said, but the statistical findings were robust.