Plasma tryptophan and kynurenine in females with temporomandibular disorders and fibromyalgia, 2019, Barjandi et al

[Andy]

Both temporomandibular disorders myalgia (TMDM) and fibromyalgia (FM) have been linked to central and peripheral changes in serotonin availability. The precursor of serotonin, tryptophan (TRP), is mainly catabolized via another pathway to produce kynurenine (KYN), but whether changes of this pathway are present in TMDM and FM are still unclear. The aim was to explore blood plasma concentrations of TRP and KYN in TMDM and FM in an attempt to identify novel associations for future research.

Plasma of 113 female participants (17 TMDM, 40 FM and 56 healthy pain-free controls) were analyzed for TRP and KYN concentrations. The degradation of TRP via the KYN pathway was indicated by the KYN to TRP ratio (KYN/TRP). Pain intensities were assessed with the Graded Chronic Pain Scale (GCPS) and Visual Analogue Scale (VAS). Psychological symptoms were evaluated using the Hospital Anxiety and Depression Scale (HADS), Patient Health Questionnaire (PHQ-9) and General Anxiety Disorder scale (GAD-7). In TMDM there was a negative correlation between TRP and pain intensity (rs =-0.55 p=0.023) and positive correlations between KYN/TRP and pain intensity (rs =0.59 p=0.013). In FM, KYN/TRP was negatively correlated to anxiety symptoms (rs =-0.36 p=0.022) and a trend towards significantly lower TRP levels was found compared to controls (p=0.05). The association between KYN/TRP and pain intensity as well as anxiety ratings in this small exploratory study may indicate that KYN/TRP could be a relevant indicator for symptom severity in TMDM and FM. Further investigations of the KYN pathway in chronic myalgia are warranted.

Pay wall, https://onlinelibrary.wiley.com/doi/abs/10.1111/joor.12892
Not currently available on Sci Hub.

 

[Hutan]

Sounds like a splatter-gun approach - shoot and see what sticks.

TMJ - more TRP=less pain
higher KYN/TRP ratio = more pain
Fibromyalgia - higher KYN/TRP ratio = less anxiety

- a trend to lower TRP levels than controls, but not statistically significant even though the sample size was decent.
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Presumably everything else was unremarkable. Which is surprising given the number of things they looked at.

The association between KYN/TRP and pain intensity as well as anxiety ratings in this small exploratory study may indicate that KYN/TRP could be a relevant indicator for symptom severity in TMDM and FM.

I don't see how they can conclude that.

• A higher KYN/TRP ratio in TMJ was associated with more pain, but presumably was not associated in any way with anxiety.
• A higher KYN/TRP in Fibro was associated with less anxiety, but presumably was not associated in any way with pain.

Neither association found had a fantastic P value, so it's looking pretty random. I don't know if they adjusted the statistics for the multiple comparisons.