ANCA-associated vasculitis (AAV) is a group of primary vasculitides associated with inflammation of the small and medium sized blood vessels. The most frequent forms are Granulomatosis with Polyangiitis (GPA, formerly Wegener’s Granulomatosis) and Microscopic Polyangiitis (MPA) .
Mortality has drastically decreased after introduction of immunosuppressive therapy and most patients can now be brought into remission. Unfortunately, the disease and its treatment are associated with damage that accumulates with prolonged disease duration, recurrent disease episodes and treatment exposure [2,3].
Quality of Life (QoL), especially physical QoL, is reduced in AAV patients compared to the general population [4–6]. It is important to identify modifiable factors associated with health-related QoL [5,7], as these factors will guide development of new treatments that improve the outcome.
One of the factors previously associated with reduced physical QoL is prednisolone use [4,5]. A well-known adverse effect of glucocorticoids (GCs) such as prednisolone is skeletal muscle atrophy . GC-induced skeletal muscle atrophy results from a combination of reduced protein synthesis and increased muscle proteolysis [8,9]. Mainly fast-twitch (type II) muscle fibres are affected . Proximal muscles are more severely affected than distal and cranial muscles . GC-induced skeletal muscle atrophy develops after approximately 4 weeks of therapy, and is most frequently seen with higher doses of GCs (prednisolone 40-60mg/d or equivalent doses of other GC) .
GC-induced skeletal muscle atrophy might partly explain the relation between prednisolone use and impaired physical QoL in AAV. In our clinical experience, many patients with AAV suffer from a significant loss of leg muscle strength during prednisolone treatment. They report difficulties rising from a chair and walking stairs. In several studies that focused on patient perspectives, patients reported muscle weakness as an important disease burden [10,11]. In a study by Newall et al, AAV patients had a reduced exercise capacity, which correlated with quadriceps force . These findings suggest an impact of leg muscle force on exercise performance in AAV patients, which might in turn affect QoL.
Muscle strength can be improved through exercise . Therefore, reduced muscle strength and an association of muscle strength with QoL in AAV patients would warrant intervention studies regarding exercise programs or improvement of muscle strength for this population.
The first goal of this cross-sectional study performed at the outpatient clinic of the Vasculitis Expertise Center Groningen was to investigate the relation between leg muscle force, physical activity and health-related QoL in AAV patients. The second goal was to study the relation of leg muscle force with disease duration and treatment exposure, especially GC treatment.
GPA and MPA patients were recruited from the outpatient clinic of the University Medical Center Groningen (UMCG) between July 2015 and October 2017. Patients were eligible for inclusion if they met the following inclusion criteria: age ≥18 years, diagnosis of Granulomatosis with Polyangiitis (GPA) or Microscopic Polyangiitis (MPA) according to the Chapel Hill criteria , first diagnosis or most recent relapse of disease activity within 3 years prior to inclusion, and treatment with a high dose of prednisolone (1mg/kg/day, tapered according to local protocol) and an additional immunosuppressive drug (e.g., cyclophosphamide, rituximab) as induction therapy. Patients with comorbidity causing reduced mobility or muscle strength and patients with active disease were excluded. More specifically, patients with neurological disease (cerebrovascular accident, hernia nuclei pulposi, peripheral motor nerve damage, critical illness neuropathy), pulmonary disease (dyspnea and activity limitations due to pulmonary involvement of AAV, chronic obstructive pulmonary disease, restrictive pulmonary disease), cachexia and fractures (vertebral fractures, radial fracture) were excluded. All patients signed written informed consent for participation in the study. The study was performed according to the principles of the Declaration of Helsinki and has received ethical approval by the local Medical Ethical Committee of the University Medical Center Groningen (METc 2015/184).
Isometric knee extension, elbow flexion and hip flexion strength were measured using a handheld dynamometer (CIT Technics, Groningen, The Netherlands) and expressed in Newton (N), as described by Van der Ploeg et al . Test positions for the muscle strength measurements are the same as described by Bohannon  and shown by Douma et al . The ‘break’ method was used, in which the researcher slightly overcomes the maximum force of the subject . The measurements were performed by four operators. All operators received instructions and training before performing measurements for the study. Hand-held dynamometry of hip and knee force has previously been shown to have a good to excellent (ICC > = 0.75) intra- and inter-rater reliability as well as a good to excellent concurrent validity compared to gold-standard measurement using an isokinetic dynamometer . The measurements were performed twice on each side, alternating measurements between sides. The highest measured value per muscle group was used as the maximum muscle force of that muscle group. Expected elbow flexion and knee extension forces for each patient were calculated based on age, sex, height and weight using a regression equation derived from a Dutch healthy population sample . Elbow flexion and knee extension force were expressed as a percentage of this expected value.
All analyses were performed using SPSS Statistics version 23 (IBM). A two-sided P-value <0.05 was considered statistically significant. Variables were presented as n (percent [%]) or median (interquartile range [IQR]). Reliability of muscle strength measurements was assessed by comparing repeated muscle strength measurements using intraclass correlation coefficients (ICC) with an absolute agreement definition. Point estimates of ICC were assessed as poor (<0.50), moderate (0.50–0.74), good (0.75–0.89) or excellent (> = 0.90) as described previously . Muscle strength and physical activity were compared with all subscales of the RAND-36, prednisolone dose and disease duration using Spearman Rank correlation. Disease duration was compared between patients with muscle strength below and above 100% of their predicted value. For patients who received paired measurements, muscle strength (N) and self-reported physical activity were compared between the first (prednisolone dose >30mg/d) and second (prednisolone dose < = 2.5mg/d) visits using the Wilcoxon signed rank test. Finally, linear regression was performed with the age-adjusted PCS and MCS of the RAND-36 as dependent variables and previously reported factors associated with these scores in AAV [5,7], as well as muscle strength and physical activity, as independent variables in univariable analysis. Subsequently, a multivariable model was built for age-adjusted PCS and MCS with statistically significant factors from univariable analysis, as well as previously reported potential confounders [5,7], included as independent variables in the model. For age-adjusted PCS, these potential confounders were: age >69 years, sex, current prednisolone dose >5mg/d, nervous system involvement and previous vasculitis relapse. For age-adjusted MCS, potential confounders were: age >69 years, sex, current prednisolone dose >5mg/d, ENT involvement and previous vasculitis relapse.
Out of all 48 patients, 46 had a successful measurement of knee extension. Overall intra-rater reliability was good to excellent. Intraclass correlations, overall and per assessor, are shown in S1 Table. Of 46 measured patients, 35 (76%) had a knee extension force less than 100% of their predicted value based on age, sex, height and weight. Elbow flexion force was below 100% of predicted for 30/45 measured patients (67%).
In this study, we found that the majority of AAV patients have a muscle strength below their predicted values based on age, sex, height and weight , and that physical QoL was significantly reduced in AAV patients compared to healthy norm values . We also identified an association of self-reported physical activity measured by the Baecke questionnaire with mental QoL in AAV.
The reduced leg muscle strength in AAV patients, found in this and previous studies [12,29], might be the result of steroid myopathy. Indeed, studies in several other disease populations have previously shown associations between chronic glucocorticoid use and reduced muscle strength [30–32]. While leg muscle strength did not show a correlation with prednisolone use in the present cross-sectional study, this might be explained by large inter-individual variation in muscle strength and GC sensitivity. Inclusion of muscle imaging to assess for typical signs of muscle atrophy, such as ultrasonography or MRI, might be interesting for a future study.
Interestingly, relapsing patients with longer disease duration more frequently had a muscle strength below norm values. This suggests that accumulating damage from relapses of AAV and treatment of these relapses results in a reduction of muscle strength over time.
Physical activity, as measured using an accelerometer and the Baecke questionnaire, showed a negative association with cumulative prednisolone exposure, as well as a positive association with follow-up time in newly diagnosed patients. This indicates that prednisolone therapy negatively impacts exercise capacity and that physical activity increases when tapering prednisolone. Alternatively, the association might be confounded my more recent disease activity in patients with a higher cumulative prednisolone exposure in the past 6 months, as disease activity might also result in reduced physical activity. Contrary to our expectations, we did not find an association of muscle strength with cumulative prednisolone dose.
In agreement with earlier studies [4,5], especially the age-adjusted PCS was reduced in AAV patients compared to general population norms . Leg muscle strength was independently associated with PCS. Therefore, muscle strength might be part of the explanation for reduced QoL in vasculitis patients. Leg muscle strength showed only a positive trend with physical activity. Also, leg muscle strength, in contrast to self-reported physical activity, was associated with PCS in multivariable linear regression. This suggest that muscle strength directly affects physical QoL, not (only) through physical activity. Physical activity was a main factor associated with mental QoL. Based on these results, interventions focusing on improving muscle strength and exercise capacity might improve both physical and mental QoL in AAV patients.
This study has several limitations. First, due to logistic reasons, only some patients received accelerometer and bioelectric impedance analysis (BIA) measurements, limiting the sample size for these measurements. For this reason, these measurements were not included in linear regression analyses. Furthermore, due to the cross-sectional nature of the study, inferences about causality cannot be made and statistical power is limited by inter-individual variation in variables measured. Also, no matched control group was included, requiring comparison to literature values from an unmatched general population. Lastly, generic questionnaires were used for the study. While this enables comparison with reference values from literature, the questionnaires are less sensitive to change than disease-specific questionnaires would be.
Our findings that most AAV patients have a lower muscle strength than expected based on healthy population norms and that muscle strength and self-reported physical activity are positively associated with QoL suggest that AAV patients might benefit from interventions aimed at improving muscle strength and physical activity. Studies in other disease populations have demonstrated clinically relevant improvements with simple interventions. For example, improvement of fatigue and physical function was achieved in Rheumatoid Arthritis patients by giving them a pedometer and a step-monitoring diary . As fatigue negatively influences QoL in AAV , and self-reported physical activity was positively related to QoL in the present study, physical activity interventions might also reduce fatigue and improve QoL in AAV patients.
In conclusion, the majority of AAV patients have reduced leg muscle strength and physical QoL compared to norm values. Knee extension strength is independently associated with physical QoL, while self-reported physical activity is independently associated with mental QoL. Therefore, interventions promoting leg muscle force and physical activity might improve both aspects of QoL and should be evaluated in clinical trials.
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