It’s always a pleasure to catch up with friends. Even if you travel half the country to see people from your own backyard that you haven’t seen in 10 years! Yes, it was a Canadian Pain Society Meeting. I had not attended for several years, and it was a wonderful surprise to see the growth and enthusiasm on display. Likely due to the wonderful leadership of the president: Dr John Pereira. Apart from seeing all these wonderful scientists from across Canada, I also had the chance to do some speaking and present a poster of recent research that a very bright Masters student by the name of Colin Wasylynuk completed. It is that research I will focus on here.
Central Sensitization – The Journey Continues…
I have spent most of my research life intrigued by pain that people experience, with features suggestive of central sensitization. What is that you might be asking – well, when the body experiences actual or apparent tissue damage, receptors in the tissue signal messages to the brain to be interpreted. Unfortunately, for a whole wack of different reasons, those messages can be misinterpreted, and the central nervous system relay messaging system does not necessarily accurately reflect the messages incoming or outgoing after brain interpretation. Essentially, the central nervous system can become hyperexcitable – just like The Pointer Sisters were in the 80s.
How Is This Measured?
That’s a good question. In fancy laboratories, a whole lot of different measurement techniques are used to pick up the presence of features suggestive of central sensitization. Most of these involve application of a known stimulus intensity – generally noxious in nature, and requires a person’s response, such as a pain rating. Alternatively, you can measure the threshold at which pressure or thermal or electrical stimulus become painful. Now, all of these require a patient’s response and attention to the stimulus. It is generally assumed that many people following whiplash injury who do not recover demonstrate these clinical features. The challenge is determining what threshold is clinically relevant as all these measures are subjective.
What Other Options Exist?
Well, that is where Colin came in. If you’re sensitive to the above stimuli, then possibly you are also sensitive to other stimuli, such as light or sound. Well, Colin thought, we could measure that. So, we went and found a convenient app (Reflex) that was available for iPhones that measured light sensitivity in the form of pupillometry. And this measure is not quite as reliant on a person’s determination of ‘what is painful’.
What Is Pupillometry I Hear You Ask?
Pupillometry involves a brief stimulus of light (the flash of a cell phone), and then measures the pupil’s response to this stimulus. The pupil initially constricts, which is under the power of the parasympathetic nervous system (‘rest and digest’) and then accommodates and re-dilates under the power of the sympathetic nervous system (‘fight or flight’).
What Did We Find?
Firstly, we measured a sample of healthy people to determine what ‘normal’ levels of pain sensitivity were. And concurrently, what measures of pupillometry were ‘normal’ for this healthy sample. Then we found an age/sex matched sample of people with whiplash who were sensitive to pressure. Let’s call them the sensitized sample. Then we measured their pupillometry recordings. Two pupillometry measures stood out between the groups. The sensitized sample with whiplash symptoms did not constrict as much as the healthy sample, and then more quickly re-dilated until their pupil regained 75% of its diameter. Thus, dysfunction of both the parasympathetic (PNS) and sympathetic system was evident. This was not expected, and we think it is the first time that PNS dysfunction has been observed in people with whiplash.
The other unique and unexpected finding was the lack of relationship between the measures of pain sensitivity and pupillometry measures. Thus, this dysfunction of the autonomic sensitivity appears to be independent of central nervous system hyperexcitability. What does this mean – we’re not sure exactly! We certainly can’t let this measure replace other measures of CNS hyperexcitability, that’s for sure. If you have any ideas, then message us!!! Until then, the journey continues…
