This study aimed to examine serum CGRP levels in participants with PPCS including PTH 2–6 months after a concussion. A novel finding is that elevated serum CGRP concentrations were observed in PPCS participants compared with healthy individuals. At follow-up, we observed a statistically significant reduction in CGRP levels, which correlated with a reduction in symptom levels. Although elevated CGRP levels have not previously been reported in PPCS, it is a common finding in migraine [
31], and the clinical similarities suggest a potential role for CGRP in the pathophysiology of PTH. In fact, a recent study showed that CGRP infusion in patients with PTH could induce migraine-like headache [
32]. In our study, > 70% reported headache with migraine-like features, suggesting the possible involvement of CGRP in this cohort as well. Interestingly, treatment with a CGRP antibody (erenumab) reduced the intensity and frequency of headache in a recent open-label study in PTH patients, suggesting its potential as a future treatment for PTH, similar to its use in migraine [
33]. However, the evidence is limited, and placebo-controlled RCT studies are needed to draw firm conclusions.
The increased CGRP levels at baseline, and the reduction in CGRP levels at follow-up in PPCS participants were driven by females in this study. Interestingly, this is in accordance with a recent animal study showing increased serum CGRP concentrations after 7 days in female rats, but not in males after an induced head injury [
34]. Females have a greater risk of developing PPCS/PTH [
35], prompting speculation about a potential link between CGRP and this sex difference, similar to observations in migraine [
36]. However, because of the limited sample size of males with PPCS in the present study (n = 19), firm conclusions on CGRP differences between PPCS males and healthy males are not possible due to a power issue, as evidenced by the wide confidence interval (Table
3).
Our findings of higher CGRP concentrations are in contrast with our primary hypothesis predicting lower CGRP concentrations in PPCS patients than those in healthy individuals. Our hypothesis was based on a previous study, which demonstrated lower CGRP plasma concentrations in PTH patients than in healthy controls [
14]. The difference could be attributed to the inclusion criteria. In the present study, the mean disease duration was 4 months and only young participants were included compared to a mean disease duration of 49 months and a mean age of 36 in the previous study.
Exploratory analyses – baseline data
Unexpectedly, we observed that higher CGRP levels correlated with fewer headache days, shorter headache duration and reduced current headache pain at baseline (rho ≈ – 0.23). Similarly, elevated CGRP levels correlated with a lower symptom burden, although bodily pain measured by the SF-36 was the only finding that remained statistically significant after Bonferroni correction. The fact that high CGRP concentrations are correlated with a more positive outcome, contradicts findings in migraine [
31] and the majority of pain-related studies [
37]. However, it could align with the findings in the previously mentioned PTH study [
14]. This earlier study included patients refereed to a specialized headache clinic, indicating more severe headaches. Interestingly, their study demonstrated lower CGRP levels in PTH than controls and the same tendency toward an inverse relationship with monthly headache days although it was not statistically significant (rho = – 0.11, p = 0.27). This tendency was shown in a recent study conducted in post-deployment soldiers as well (rho = – 0.12, p = 0.063) [
38]. It is thus possible that more severe PTH are associated with lower CGRP levels. An explanation for lower CGRP given in the previous PTH study was that constant headache may result in the depletion of CGRP in trigeminal afferents [
14]. This speculation was based on the finding that CGRP tissue levels were depleted in rats following capsaicin injection in the paw skin and sciatic nerve [
39]. Whether capsaicin-mediated pain is comparable to the pain in headache is unknown.
However, other explanations can account for our findings as well. CGRP is a neuropeptide with several physiological functions unrelated to headache [
13]. In the aforementioned animal study, CGRP inhibition with antibodies in concussed female rats did not alleviate cephalic pain hypersensitivity, raising questions about the role of peripheral CGRP in headache in females [
34]. In contrast, there are several studies showing that CGRP inhibition could alleviate symptoms in rodents with migraine-like behavior in both males and females [
40]. Furthermore, in a recent study on severe traumatic brain injury in humans, high CGRP concentrations were correlated with a lower risk of mortality, indicating a potential beneficial role for CGRP [
41]. Additionally, an animal study showed that CGRP may have a favorable impact on peripheral nerve regeneration [
42]. Further studies are needed to establish the role of CGRP in concussion and PTH.
A noteworthy observation, unrelated to PPCS, in our study was that healthy males had higher serum levels of CGRP than healthy females. This align with a recent study in post-deployment soldiers [
38], but contrasts with a previous study that showed the opposite in females, particularly among those using oral contraceptives [
43]. Studies investigating sex differences in CGRP among healthy individuals are limited, and it was not the primary aim of this study; further research is needed to draw definite conclusions.
Exploratory analyses – follow-up data
At follow-up we showed that a reduction in CGRP levels was correlated with improved symptom levels measured by the BDS questionnaire. The post hoc analysis revealed that the reduction in CGRP correlated especially with an improvement in headache, dizziness and autonomic/cardiopulmonary symptoms (Suppl. Table 2). This suggests that a CGRP reduction could be linked to a positive physiological response. A major limitation in the follow-up data was that the questionnaire data and blood samples were collected at different time points, with a median time difference of 27 days. This could have contributed to the lack of an association between delta CGRP and change in headache days, duration, and pain, particularly when considering the limited response rate in the follow-up headache questionnaire (Suppl. Figure 2). Finally, the RCT intervention did not show any effect on CGRP concentrations at follow-up. This was expected and stated in the preregistered analysis plan since the RPQ-difference (symptom levels) was only 7 points between the intervention arms [
15].
In conclusion of our exploratory analyses at baseline and follow-up, we found an association between CGRP levels and patient-reported outcomes measured by questionnaires, which is a rare finding. Future studies should replicate these findings, and additional research is needed to establish the role of CGRP in PPCS/PTH and concussion.
Strengths and limitations
This study had several strengths. The cohort was well-characterized, the concussion diagnosis was validated using the WHO criteria, and we had a relatively large sample size.
We used an ELISA-kit with no cross-reactivity with calcitonin, a peptide fragment of CGRP (CGRP position 8-37), amylin, and substance P according to the manufacturer. Furthermore, the assay was based on two antibodies for CGRP (sandwich ELISA) indicating its specificity for CGRP. In contrast, most other commercial ELISA kits have not investigated cross-reactivity and are only based on one antibody. Moreover, the QC’s and the calibration curves were constructed in study representative serum matrix, which should minimize the risk of matrix effects. Finally, we checked freeze–thaw stability of CGRP in serum to ensure that the reported concentrations in PPCS participants were accurate.
This study also had limitations. The QC’s systematically produced higher than nominal concentrations (170 pg/mL vs. 125 pg/mL), which indicates a slight overestimation of the concentrations in general. However, more importantly, the QC’s were reproducible with an inter-assay and intra-assay CV ≤ 15%. Furthermore, although the manufacturer reported no cross-reactivity between similar molecules, it cannot be ruled out that cross reactivity exists with a related peptide biomarker. However, this seems unlikely since CGRP was completely depleted in CGRP-free serum which was a pool of samples from both patients and healthy individuals. Since samples from patients and healthy individuals were evenly distributed on each ELISA plate, we do not expect these assay related factors to affect the conclusion of this study.
Preanalytical stability of CGRP is another point of concern. Prolonged storage and lack of protease inhibitors may decrease CGRP concentrations [
45], although there is conflicting evidence on this matter [
46]. In our study, patient samples underwent one freeze–thaw cycle and had a longer storage duration (up to 8 years), in contrast to the samples from healthy individuals (1 year of storage). Despite these possible limitations, our study still revealed significantly higher CGRP levels in PPCS patients than in healthy individuals. A further limitation of the study was the presence of multiple symptoms in addition to headache among PPCS participants, a consequence of the inclusion criteria requiring an RPQ score of 20 or higher. This diversity of symptoms prevented a clear identification of the exact causes of increased CGRP levels in PPCS participants.
Finally, the healthy individuals consisted of anonymous blood donors, for whom no demographic data were available. Danish blood donors tend to have a higher self-reported health and healthier lifestyle than non-donors, which may introduce a selection bias known as “the healthy donor effect” [
47]. Although the mechanisms are poorly understood, lifestyle-related factors, such as higher weight, blood pressure [
48], and exercise [
49], might increase CGRP-concentrations. Since we lacked data on lifestyle factors in both groups, it cannot be ruled out that the observed differences might be partly explained by variations in lifestyle factors rather than PPCS/PTH. The potential confounding effects of preexisting migraines must also be considered since the headache questionnaire utilized did not allow accurate classification of headaches prior to the concussion [
19]. However, 75% of the PPCS participants reported less than 15 headache days a year pre-trauma, and none reported using migraine medications before the concussion (Table
1). Furthermore, since migraine is generally not a contraindication for becoming a blood donor, the group of healthy individuals may also include individuals with migraine, thereby reducing the likelihood that this condition skews our findings. Regardless of potential confounding variables, the observed fivefold increase in median CGRP concentration in females with PPCS, compared with healthy individuals, is substantial. For this difference to be solely attributed to confounding factors, these factors would have to exert a strong influence on CGRP levels.
In conclusion, our data are strongly suggesting a role for CGRP in PTH. Future studies should aim to independently verify whether this is the case, preferably in a population with blood samples available before the head trauma (which can be done in athletes), to clearly establish a causal link between CGRP and concussion/PTH in humans. Furthermore, future studies should investigate whether CGRP targeted therapies are effective in PTH in a placebo-controlled RCT design.