This prospective, randomized, controlled, double-blinded pilot trial was completed between June 2019 and December 2022, in Calgary Alberta. The trial was registered on ClinicalTrials.gov (NCT04051203) and approved by the University of Calgary Conjoint Health Research Ethics Board (REB 18-1,369). The methods for this study have been published elsewhere by the authors as a protocol paper (16).

Participants were recruited from the Calgary Brain Injury Program, Calgary Chronic Pain Centre, Alberta Neurological Centre, through community neurologists and advertising on the University of Calgary Brain Neurorehabilitation Laboratory website.

Eligible participants were at least 18 years of age, met ICHD-3 criteria for headache attributed to trauma or injury to the head and/or neck and post-traumatic GON (6), as established by a physiatrist and/or neurologist, and had a previous beneficial diagnostic anesthetic injection to the greater occipital nerve. Participants were enrolled at least 3 months after their last GON injection and had an average pre-treatment daily headache intensity of ≥4/10 on Numeric Pain Rating Scale (NPRS) scale (on the days when headaches were present) and headache frequency of ≥10 days/month, chosen to reflect post-traumatic GON. All patients had a clinical diagnosis of mTBI, meeting the American Congress of Rehabilitation Medicine guidelines (28).

Exclusion criteria included: an inability to provide informed consent, a history of surgery in the occipital region, an unstable psychiatric or medical condition, uncontrolled rheumatologic or inflammatory disorders, widespread neurologic disorders (e.g., Multiple sclerosis), fibromyalgia/chronic fatigue syndrome, coagulopathy, immunosuppression, active cancer, herpes zoster infection within last 6 months, pregnancy, or if currently breastfeeding.

Demographic information and written/electronic consent were collected 2 weeks prior to study injection in person or by phone call (minimizing hospital visits during the COVID-19 pandemic). This included age, sex, height and weight, education, past-medical history, previous treatments, TBI history (date and mechanism of injury, previous head injuries, immediate and current symptoms) and headache history (frequency, intensity, headache type, medications, headache triggers and associated symptoms). Pre-injection Headache Impact Test-6 (HIT-6) (29) and Quality of Life After Brain Injury questionnaire (QOLIBRI) (30) were also administered at this time. Following initial assessment, 2 weeks of pre-injection daily headache diaries recording frequency and NPRS scores were completed. Headache diaries were completed daily for 3 months post-injection and HIT-6 and QOLIBRI were repeated at 1 week, 1 month, and 3 months post-injection. All questionnaires and any electronic consent forms were collected through online survey invitations using REDCap v.7 electronic data capture tools hosted at the University of Calgary (31, 32).

Feasibility of the study protocol was determined by recruitment (ability to recruit more than 30% of those screened), appointment attendance (greater than 70% of those eligible consent and receive study injection), and retention (at least 70% of daily headache diaries completed and at least 70% of participants completing the 3 month HIT-6 and QOLIBRI). We determined the safety of the PRP injections by monitoring adverse events documented by participants in the additional comments section of daily headache diaries and post-injection communication with the study team.

To determine changes in headache burden, we collected daily headache intensity NPRS scores (average and maximum) at 1 week, 1 month, and 3 months post-injection. Participants were asked “What was the worst level of headache-related pain you experienced today?” and “During these hours, what was your average level of headache-related pain?” on a scale of 1–10 (10 being the highest) for each day a headache was reported. The maximum and average intensity ratings were averaged for each study timepoint (2 weeks pre-injection and 1 week, 1 month, and 3 months post-injection). Additional exploratory outcomes included headache frequency (number of days headache reported divided to give a weekly average), HIT-6 and QOLIBRI scores at 1 week, 1 month, and 3 months post-injection.

Minimal clinically important difference (MCID) for the NPRS was set as a change of 2 points (33). The MCID for the HIT-6 was taken as a reduction in score of 6 or more points (34) and the QOLIBRI was set as an improvement of at least 30% (30).

Participants were randomized in a 1:1:1 fashion using sealed envelopes prepared by an uninvolved researcher blinded to the study protocol to one of three treatment arms: (1) autologous PRP injection, (2) steroid/anesthetic injection, or (3) normal saline.

All participants underwent a blood draw on their day of injection according to standard phlebotomy technique. Briefly, a 19-gauge needle was used to collect 60 mL of whole blood from the medial vein of the antecubital fossa to a syringe containing 5 mL of Sodium Citrate to prevent coagulation. Whole blood collected from participants in the steroid/anesthetic or saline groups were discarded appropriately. Two syringes (2 mL injectant in each) were prepared by a research assistant and covered to blind the participant and the physician delivering the injection. Following the injection, the physician and the participant were asked to guess which injection they received.

On injection day, participants received 2 mL of injectate (PRP, steroid/anesthetic, or normal saline) per side. To reduce pain at the injection site and improve blinding, topical lidocaine cream (5%) was applied approximately 15 min prior to injection. All participants received bilateral GON injections under ultrasound guidance along the superior nuchal line, given the reported advantages over conventional blind injection technique (35). Participants reported their pain using NPRS immediately before and after injection and were monitored for 30 min for immediate adverse reactions.

Recruitment, adverse events, and adherence data are reported descriptively. Headache diary adherence was rated as full (≥99% completed), partial (70–98% completed) or incomplete (≤69% completed) adherence.

Statistical analyses were performed using STATA (v 16.0) software. Demographic characteristics were presented using descriptive statistics (frequencies for count data, means and standard deviations for continuous data), one-way ANOVAs compared group means for continuous data and chi-square tests were performed for categorical data. Exploratory analyses employed multivariable linear regression to investigate mean differences between intake and post-injection follow-up scores in headache average, headache frequency, headache maximum, HIT-6, and QOLIBRI at each follow-up timepoint (1 week, 1 month, and 3 months). An injection type by timepoint interaction term was forced into all models to examine injection type specific score differences at each timepoint. Wald tests assessed the significance of the interaction term for each model but were retained regardless of significance to explore study outcomes stratified by injection type. Estimates and 95% confidence intervals are reported where appropriate. An alpha level of 0.05 was used to determine statistical significance.

Between June 2019 and December 2022 (recruitment was on hold from March 2020 to August 2020 due to COVID-19 restrictions), 81 potential participants were referred by physician or contacted the study team, 67 completed the screening phone call and 37 were considered eligible. Of these, two declined to participate, 35 consented and were enrolled into the study. Three participants, one from each group, were excluded from analysis (see Figure 1).

Characteristics of the 32 participants included in the analysis are presented in Table 1. The average age was 39.3 years (±11.1) and the majority were female (72%). The most common cause of injury was motor vehicle collision (50%) and participants were recruited on average 46.4 months (range 8–429 months) post-concussion. Of the 32 participants, 13 (41%) had suffered at least 1 previous mTBI and the majority (56%) had received 2–5 therapeutic GON injections. The most common past-medical conditions were previous fractures (20%), surgical interventions (20%), and depression (19%). Past medical history of cancer was significantly higher in the saline group (χ² (2) = 7.02, p = 0.030), otherwise there were no between group differences (Supplementary Table S1).

Of the 67 individuals who completed screening calls, 37 (55%) were considered eligible, exceeding our recruitment target of >30%. Of these, 35 (95%) consented and attended the study injection appointment, exceeding our attendance target (>70%). Of the 3 months of post-injection daily headache diaries, 82% were completed (exceeding the target of >70%). Overall retention at 3 months was also >70% for the HIT-6 and QOLIBRI questionnaires (91% completed). Full study adherence by timepoint and injection group is presented in Figure 2.

No serious adverse events related to any study injection were reported. General post-injection site pain was reported by one participant in the PRP group, two in the saline and two in the steroid/anesthetic group. Participants were advised to use cold pressure and analgesia (no NSAIDS for the first two weeks post-injection) as needed and monitored through communication with the study team and headache diaries.

Less than 50% of participants guessed the correct injection (PRP n = 5/11; steroid/anesthetic n = 1/10; and saline n = 4/11). Half (50%) of participants guessed that they had received the PRP injection with 41% guessing saline and only 9% guessing steroid/anesthetic. Physicians guessed correctly on 2/11 and 3/10 injections in the PRP and steroid/anesthetic group, respectively, and 11/11 following the saline injection. Although physicians were 100% correct following the saline injection, they guessed that the patient had received saline following 75% of the study injections.

Exploratory outcome analyses are presented in Table 2, 3 and Figures 3, 4. There were no significant injection type by timepoint interactions for any of the exploratory outcome measures.

Exploratory outcomes evaluated the effectiveness of a PRP injection compared to a steroid/anesthetic or saline injection. A full evaluation of the benefits of PRP over current treatment practices for post-traumatic GON requires a larger sample size. As the primary objective of this pilot study was to determine feasibility and safety of the protocol prior to larger trials, we did not complete a power calculation but assumed that a sample size of 30 would be sufficient for our aims and objectives (16, 43). Although the treatment groups were similar in baseline demographics, the majority of participants were female, which has been shown to be independent risk factor for poor symptomatic recovery following TBI (3) and may have affected therapeutic response. As well, we did not perform platelet counting for PRP samples, but we assume that concentrations were adequate with the injections being prepared according to manufacturer’s specifications. Further, persistent benefits from PRP injection beyond 3 months was not assessed but would be important to evaluate in future studies. Lastly, while we have demonstrated the feasibility of our study protocol for future study design, we did not assess the clinical or economic feasibility of PRP injections under ultrasound guidance. When compared to steroid/anesthetic and saline injections, PRP requires more equipment and is more expensive to administer, which may limit its clinical application.