Temperature sensitivity and Fibromyalgia
Habituation is basically when you get used to something. The tag in the back of your shirt might drive you insane when you first put it on, but after a few minutes you don’t notice it. However, that may not be the case for those who suffer from Fibromyalgia.
Instead, the sensation increases with duration, this is called Temporal Summation (Smith et al, 2008). This Temporal Summation may be responsible for the increased sensitization that many Fibromyalgia patients endure. This sensitization rather than habituation may play an important role in the development of chronic pain among those suffering from Fibromyalgia and other related disorders.
The study by Smith and colleagues sought to understand the difference in habituation between women suffering from Fibromyalgia and healthy controls. It is not uncommon for Fibromyalgia studies to only consider women because women make up the majority of those diagnosed with Fibromyalgia (>90%).
The study examined the temperature at which the women reported pain during repeated trials. The intervals between trials were elongated in hopes of increasing the likelihood of habituation.
The study authors expected that those with Fibromyalgia would shower lower rates of habituation than the healthy controls. Pain thresholds were established prior to the trials, and all participants filled out the Mental Health Inventory, and Fatigue Severity Scale; pain levels were established using a visual analogue scale (1-100).
The procedure began by introducing the participant to a temperature and increasing it by 5 degrees until they reported pain.
The researchers examined two questions. The first was whether the different groups would report different temperature thresholds; also, would different symptom levels result in different threshold levels? The second was during which trial would a person report a higher threshold? In other words, would the threshold increase or decrease over trials?
The findings of this study by Smith et al found a strong correlation between the temperature thresholds for heat and cold. They also found a negative correlation between heat threshold and reported levels of pain, fatigue, anxiety, and depression (meaning that as these factors increase the threshold for heat decreases).
This could indicate that those who are experiencing worse symptoms of Fibromyalgia may have lower thresholds for heat tolerance. The average threshold for heat pain overall was about 110° F.
There were very large differences in threshold between the Fibromyalgia group and the healthy controls, supporting the prediction that those with Fibromyalgia would have lower heat pain thresholds.
When considering the different groups it was again clear that the Fibromyalgia group showed a lower threshold to heat pain. Specific demographic variables did not correlate with heat threshold.
When they considered trial number, there was no correlation between the health variables and trial number in regards to heat threshold. This again indicated that the healthy controls required larger increases in heat to induce pain.
The healthy controls also had endured longer trials because it took longer to reach their pain threshold. This supported the hypothesis that the Fibromyalgia group would show less habituation.
The study (Smith et al) also showed an increase in sensitization to cold pain. The average cold temperature that induced pain for all participants was about 50°F, however there was a large degree of variation between the Fibromyalgia group and the healthy control group.
The healthy control group reported pain at around 44° on average, while the Fibromyalgia group reported pain at an average of 57° –more than a 10° difference. This again supported the researcher’s hypothesis that those with Fibromyalgia would report cold pain at higher temperatures.
Both Fibromyalgia and the symptoms all positively correlated to cold pain threshold – as the symptoms worsen, the threshold for pain in cold temperatures is lowered.
There was again a significant difference across trials for those with Fibromyalgia. There was a decrease in cold pain threshold across the trials. The healthy control group showed habituation to cold, while the Fibromyalgia group showed sensitization.
Overall the Fibromyalgia group showed greater sensitization to both heat and cold pain stimulation, the opposite of what is typically expected in repeated trials.
Habituation is typically increased by repeated stimulation. The more we are exposed to a stimuli the easier it should be to ignore it. Therefore, this study proposed that increased trials would increase habituation in those with Fibromyalgia, as well as healthy controls.
However, that was not the case. Repeated trials increased sensitization to heat and cold pain in Fibromyalgia patients, rather than habituation.
Sensitization is the opposite response to habituation. Instead of learning to ignore the stimuli through repeated measures, the stimuli elicits an increased response. This has been found to be the case with painful stimuli for those with Fibromyalgia.
Unlike habituation, sensitization is hetero-synaptic, meaning that the increased response is not limited to just the single synapse that is initially activated. This study supported the suggestion that Fibromyalgia may be characterized by increased sensitization and decreased habituation.
What do you say? Has fibromyalgia made you more sensitive to extreme temperatures?
Smith, B. W., Tooley, E. M., Montague, E. Q., Robinson, A. E., Cosper, C. J., & Mullins, P. G. (2008). Habituation and sensitization to heat and cold pain in women with fibromyalgia and healthy controls. Pain, 140420-428. doi:10.1016/j.pain.2008.09.018