Pain perception

Some people are stoic and some aren't. … Maybe there's a plausible biological reason for that difference.
---Dr. Geoffrey Woods, Medical geneticist, Cambridge University

My patient's face was all pinchy with pain, an expression not generally seen in a 20-something year old lady. She'd visited Walmart the previous day (an outing that always makes me pinchy with impatience as my husband meanders through every last aisle). While there, she bent over to examine some wonderful find and skewered her forehead on a metal hook.

"Ew," you're thinking, which is what I would've thought if there had been a ragged tear in her face, or any kind of mark whatsoever. Her forehead, however, was completely smooth and unblemished.

"So," I said, "where did it snag you?" And I touched the indicated spot, sending her recoiling in pain, pinchier than ever. The area wasn't red, abraded, swollen, or bruised. I told her that while I appreciated she was in a lot of pain, my examination indicated no serious injury. I then assured her that her pain should diminish greatly in the following days.

As usual, when confronted with pain reports that seem to far outstrip apparent injury, I wished I could walk a mile in her forehead. What was she really feeling in there? Real pain, real drama, a need for sympathy, worried sick, or what?

Dr. Woods and company also wondered why humans demonstrate such a wide spectrum of pain perception. They already knew from previous research that three rare mutations in the SCN9A gene are a serious problem insofar as such genetic changes can cause persons to be either too sensitive or completely oblivious to painful stimuli. In the first case, the faulty SCN9A gene allows sodium ions to flood through channels in the surface membranes of cells in the central nervous system. As a result, these neurons painfully overfire in response to minor daily occurrences such as bumping a shoulder against a wall when taking a corner too close. At the opposite extreme, SCN9A gone haywire in a different way prevents the perception of pain altogether. Affected persons, consequently, receive no signals say from a fracture or sprain and, therefore, have no cue to stop moving and get off the injured joint.

The Cambridge team went looking for subtle, more common mutations in the SCN9A gene, namely substitutions of a single nucleic protein (SNP) in the gene's sequence(1). They studied a group of subjects with osteoarthritis of comparable severity to see if these SNP variations correlated with differing levels of pain perception. One variation present in a small subset of the group was indeed associated with the highest pain perception in those who carried it. The scientists then looked in other people with other common pain syndromes such as sciatica and lumbar disk disease, again finding that "the statistical link between having the rare SNP and feeling more pain was impressively strong." In other words, those persons with lower pain thresholds were significantly more likely to have this particular SNP variant.

The researchers found that just under 20% of their subjects carried one copy of this particular SCN9A mutation, but only 2-3% had two defective copies, and these were the people most likely to react strongly to their painful conditions. Further testing suggested that this mutation renders them incapable of squeezing their sodium channels shut so wave after wave of pain news just keeps rolling in.

Obviously, this genetic test is not available for clinical use, and there's nothing to be done about it anyway. Those of us in the medical business and everyone involved in the sympathy business can now appreciate, however, that some persons are hard-wired for more pain than others; for such persons, that 'stiff upper lip' advice may be harder to follow. And my hapless Walmart shopper was back two days later, her forehead no longer tender but her head now clamped in the vise of a secondary tension type headache brought on by scrunching her brow over the original hooking.
_____
1) Woods, GC, et al. "Pain perception is altered by a nucleotide polymorphism in SCN9A," Proceedings of the National Academy of Sciences.