For this cross-sectional study, data from the TransplantLines Biobank and Cohort Study (ClinicalTrials.gov Identifier: NCT03272841) were used (20). This ongoing cohort study includes SOTR and donors (aged ≥18 years) visiting the University Medical Center Groningen (UMCG, Groningen, The Netherlands). More detailed information on the study design, inclusion and exclusion criteria has been described previously (20). The study was conducted in accordance with the guidelines laid down in the Declarations of Helsinki and Istanbul and approved by the Institutional Review Board (METc 2014/077). The study protocol was reviewed and approved by the TransplantLines Scientific Committee (TxL 2022/004).

For the current study, all enrolled SOTR with a functioning allograft for at least 6 months, with available data on tremor influence on ADL between September 2016 and November 2020, were included. A consort flow diagram is shown in Supplementary Figure S1.

All included SOTR attended the outpatient clinic of the UMCG and were treated according to standard immunosuppressive therapies, with revision of therapy effectiveness at least once per year. Immunosuppressive maintenance therapies were generally tacrolimus-based, with addition of mycophenolate mofetil (MMF), and prednisolone. Other immunosuppressive drugs used less frequently were cyclosporine, everolimus, sirolimus, and azathioprine. Although immunosuppressive regimens may be adapted on an individual basis, standard protocol target trough concentrations of tacrolimus and cyclosporine in the UMCG are shown in Table 1.

Blood was drawn in the morning after an overnight fasting period of at least 8 h, including no medication use. All tacrolimus and cyclosporine whole blood trough concentrations were determined by validated liquid chromatography mass-spectrometry analyses at the UMCG (21). Other laboratory parameters were measured using routine laboratory methods (Roche Diagnostics, Basel, Switzerland).

Tremor severity was assessed using a Dutch translation of the Fahn-Tolosa-Marin (FTM) tremor rating scale part C (TRS-C) (22). This questionnaire is recommended for use in clinical practice (23). The TRS-C consists of eight questions to assess patient-perceived tremor occurrence during ADL, including writing, speaking, and bringing food or liquids to the mouth, and is provided in Supplementary Table S1. Furthermore, the questionnaire assesses the influence of tremor on personal hygiene, dressing, working, and social activities. As the TRS-C is designed to evaluate the impact of tremor on ADL, by nature, this questionnaire assesses severity over the previous couple of days of the patient’s life. The interviewers were trained to conduct the questionnaire during routine follow-up visits, and to interpret the answers of patients. To every question, patients graded the impairment tremor has on ADL with a score ranging from 0 (no influence of tremor on ADL) to 4 (severe influence of tremor on ADL). A total score was calculated by summing the individual scores, with a theoretical maximum score of 32 points. Patients with a TRS-C total score of 0, or with a TRS-C total score of 1 acquired because of mild disability of speech (i.e., shaky voice only when nervous), were classified as having no tremor. For data visualisation, and statistical analyses, tremor severity was rated according to severity: no tremor (defined as 0 points or 1 point on speech (“mild tremor only perceived when nervous” on the total TRS C score)), mild tremor (1–3 points), and severe tremor (≥4 points).

HRQoL was assessed using the 36-Item Short Form health survey, which is a validated questionnaire to assess several health domains, including physical functioning, role limitations due to physical health, role limitations due to emotional problems, vitality, emotional wellbeing, social functioning, pain, and general health perceptions (24). Subsequently, physical component summary (PCS) and mental component summary (MCS) scores were derived, with a higher score meaning a higher HRQoL. The PCS includes items from physical functioning, role limitations due to physical health, vitality, social functioning, pain, and general health perceptions, whereas the MCS includes items from role limitations due to emotional problems, vitality, emotional wellbeing, social functioning, and general health perceptions.

Blood pressure and heart rate were measured with a semi-automatic device (Philips Suresign VS2+, Andover, Massachusetts, USA). Body weight and height were measured with participants wearing indoor clothing without shoes. Body mass index (BMI) was calculated as weight in kilograms divided by height in meters squared (kg/m²). Information on patients’ medical history was extracted from electronic patient records, and medication use was extracted from patient records and verified with the patient by a trained researcher. Diabetes was defined based on use of antidiabetic drugs, fasting glucose ≥7.0 mmol/L, and/or HbA1c ≥6.5%. Kidney glomerular filtration rate was estimated with the creatinine-based CKD-EPI equation (25). Anaemia was defined as a haemoglobin concentration <8.1 mmol/L for men and <7.5 mmol/L for women according to WHO criteria (26).

Dispersion of TRS-C scores in total, and per transplant type, were visualized by means of pie charts and bar charts. Patient characteristics are presented and compared in patients without tremor, with mild tremor, and with severe tremor. Continuous variables are summarized as mean ± SD or median [interquartile range], depending on distribution, whereas categorical or dichotomous variables are presented as count (%). To assess differences between tremor severity groups, Analyses of Variance were used for normally distributed variables, Kruskal-Wallis tests for non-normally distributed variables and Chi-squared tests for categorical variables. To assess associations of tremor severity with clinical, biochemical and pharmacotherapeutic parameters, multinomial logistic regression analyses, adjusted for sex, age, and log₂ time after transplantation were performed. Sensitivity analyses with additional adjustment for tacrolimus trough concentrations were performed. Also, exploratory subgroup analyses were performed to compare non-CNI users with no, mild, or severe tremor.

Differences in PCS and MCS between grades of tremor severity were visualised with boxplots. Analyses of variance were performed for testing significance of differences between the different grades. Furthermore, bivariable linear regression analyses with PCS and MCS as dependent variable were performed to assess associations of mild and severe tremor with HRQoL. In multivariable linear regression analyses we assessed the association of mild and severe tremor with HRQoL, while adjusting for potential confounders including age, sex, type of transplantation, log₂ time after transplantation, polypharmacy, diabetes, anaemia, eGFR, use of tacrolimus, use of cyclosporine, employment status, educational level, and the presence of a partner. Potential presence of effect modification by age and sex was assessed by adding interaction terms to the linear regression models.

Scatterplots and QQ-plots were visually evaluated to assess data distribution. Non-normally distributed variables were transformed using a binary logarithm (log₂) when necessary to meet the assumptions for regression. All statistical analyses were performed with IBM SPSS software (version 23.0, SPSS Inc., Chicago, IL, USA) and R (version 3.5.1, Vienna, Austria). In all analyses, a two-sided p-value <0.05 was considered statistically significant.

In total, 689 SOTR were included in the current study, including kidney (n = 395, 57.3%), liver (n = 168, 24.4%), lung (n = 95, 13.8%), heart (n = 29, 4.2%), and small intestine (n = 2, 0.3%) transplant recipients with a mean ± SD age of 58.0 ± 13.7 years, of whom 38.5% were female. A detailed overview of baseline characteristics in patients with either no, mild, or severe tremor, is presented in Table 2. In brief, patients with more tremor-related impairment were more often lung transplant recipients, less often liver transplant recipients, more frequently used tacrolimus and tended to have higher tacrolimus trough concentrations. Furthermore, tremor-related impairment was more frequently reported in patients with polypharmacy, patients with lower eGFR, and patients with higher fasting glucose concentrations. Notably, patients with mild or severe tremor tended to have higher cyclosporin trough concentrations, although differences between the groups were not statistically significant.

206 (29.9%) SOTR reported mild tremor and 81 (11.8%) reported severe tremor. Mild or severe tremor was reported in 39.7% among kidney transplant recipients, 33.9% among liver transplant recipients, 57.9% among lung transplant recipients, and 55.1% among heart transplant recipients. TRS-C scores of all SOTR and per organ type are shown in Figures 1A, B and 2 respectively. Additional bar charts of TRS-C total score per organ type are shown in Supplementary Figures S2A–D. Among patients that reported impairment because of tremor, median TRS-C score was 2.0 [1.0–4.0] [range 0–19]. Notably, patients reported that tremor-related impairment was most pronounced during writing and bringing food to the mouth. Furthermore, 18 (2.6%) patients reported changes in social activities because of tremor. Scores per TRS-C question are visualized by means of bar charts in Supplementary Figures S3A–H.

Results of multinomial logistic regression analyses adjusted for age, sex, and log₂ time after transplantation with presence of mild or severe tremor as dependent variable are shown in Table 3. Whole blood tacrolimus trough concentration was a determinant of mild tremor, independent of age, sex, and log₂ time after transplantation (OR per μg/L increase: 1.11, 95% CI: 1.02 to 1.21, p = 0.019). Notably, higher whole blood tacrolimus trough concentration also tended to be associated with more severe tremor, although this observation was not statistically significant (OR per μg/L increase: 1.11, 95% CI: 0.98 to 1.26, p = 0.1). Furthermore, lung transplantation was associated with mild tremor; heart transplantation, polypharmacy and higher creatinine were associated with severe tremor. These associations remained similar after adjustment for tacrolimus trough concentrations (Supplementary Table S2). Notably, of the 95 patients who did not use calcineurin inhibitors, 34 (36%) reported mild or severe tremor. Exploratory subgroup analyses showed that non-CNI users with tremor more often had diabetes (Supplementary Table S3). Use of concomitant medication that may affect tremor, such as beta blockers and bronchodilators, was not associated with mild of severe tremor.

SOTR that reported mild or severe tremor had significantly lower physical and mental HRQoL, as visualized in Figure 3 (p < 0.001 for both). In bivariable linear regression analyses, both mild and severe tremor were associated with a lower physical HRQoL, with the strongest association for severe tremor (PCS: β = −5.64, 95% CI: −9.34 to 1.94, p = 0.003 and β = −15.87, 95% CI: −21.26 to −10.48, p < 0.001 resp.). A similar pattern was found for mental HRQoL (MCS: β = −3.50, 95% CI: −6.68 to −0.31, p = 0.032 and β = −11.22, 95% CI: −15.86 to −6.57, p < 0.001 resp.). These associations remained similar after adjustment for age, sex, type of transplantation, log₂ time after transplantation, polypharmacy, diabetes, anaemia, eGFR, use of tacrolimus, and use of cyclosporine (Table 4). When we additionally adjusted for employment status, educational level, and the presence of a partner, results remained generally similar, although the association between mild tremor and physical HRQoL was no longer statistically significant. No interactions of age and sex were present for the association of tremor severity with HRQoL.