Background
In the lower limb, peripheral arterial disease (PAD) may remain asymptomatic or progress to such a degree that symptoms manifest as ischaemic pain, impaired wound healing and tissue loss [
1]. Even in the absence of symptoms, PAD is associated with reduced physical capacity and functional decline [
2,
3]. Furthermore the condition is responsible for reduced health-related quality of life (HRQoL) comparable to that of cardiovascular disease due to pain, sleep disturbance and reduced mobility [
4‐
7]. Early diagnosis and timely risk factor modification are necessary to prevent the direct and indirect complications of PAD. Given the varied presentation and often-asymptomatic nature of the disease process, early detection relies on routine clinical screening and non-invasive tests.
The traditional office-based assessment for PAD is the ankle-brachial index (ABI), calculated as the ratio of ankle to brachial blood pressure. In the general population the ABI has been shown to have high specificity for the presence of PAD, with a low ABI considered prognostic of future cardiovascular morbidity and mortality [
8,
9]. In addition, the ratio has been has been shown to correlate well with lower limb function and walking ability [
10,
11] but weakly with HRQoL [
12]. There is also growing evidence demonstrating limited diagnostic accuracy of this test in specific populations including those of older age and people with diabetes or significant renal disease [
1,
13,
14].
The toe brachial index (TBI), calculated from the ratio of toe and brachial blood pressure is an alternative non-invasive test which has been shown to have better diagnostic accuracy for detecting PAD than the ABI in specific patient populations especially those with diabetes [
15,
16]. While recent review has demonstrated there is currently insufficient literature to conclude a specific cut-off value for PAD diagnosis, the TBI was found to be useful in detecting disease regardless of the diagnostic limit used [
17]. As a more distal measure of blood flow the TBI is less affected by the complications that may invalidate the ABI including medial arterial calcification [
18]. Furthermore, low toe pressures have been shown to be significantly associated with higher likelihood of non-healing wounds and amputation [
19]. However, there is currently limited evidence regarding the predictive capacity of the TBI for morbidity and mortality, and there has been no investigation of relationships with other measures of health including HRQoL. As a diagnostic test for PAD with greater accuracy than the ABI in at risk populations the TBI may also reflect changes in HRQoL associated with the condition and possibly to a greater extent than the ABI.
The aim of the present study was to explore the relationship between the TBI and components of HRQoL as measured using the generic Short Form 36 version 2 questionnaire (SF-36v2) in older people with and without diabetes.
Discussion
This study of 100 participants is the first investigation of the relationship between the TBI and HRQoL. The mean TBI value in this population was 0.79 (±0.21), a value above the threshold for the diagnosis of PAD that suggests the population as a whole was generally without severe disease of the peripheral vasculature. Despite this, people with lower TBI values were found to have reduced HRQoL for the SF-36v2 domains Social Function, Role Physical, and the Physical Component Summary.
People with lower TBI values had greater and more frequent interference with normal social activities due to physical and emotional problems as indicated by poorer scores for the SF-36v2 Social Function domain (
r
s = 0.219,
p = 0.028) [
25]. Reduced social functioning has previously been shown in people with intermittent claudication symptoms from significant PAD, which is proposed to lead to feelings of inadequacy and may result in social isolation [
6,
26]. In this present study PAD symptoms were not found to correlate significantly with the Social Function domain. However this is likely to have been confounded by the very small number of participants recruited who reported painful PAD symptoms. Had the participant cohort had a lower mean TBI, indicative of more frequent and more severe pathology it is possible a similar relationship between PAD symptoms and reduced Social Function would have been demonstrated. Furthermore, the TBI is reflective of lower limb vascular health generally and may be affected by disease other than PAD including microvascular diseases such as peripheral neuropathy. It is possible that investigation of a study population with a lower mean TBI would demonstrate associations with other or atypical symptoms responsible for physical or emotional limitations of social function, including neuropathy and microvascular disease.
This study also demonstrated a significant relationship between lower TBI values and the SF-36v2 Role Physical domain (
r
s = 0.219,
p = 0.029). This scale reflects problems with work or other daily activities that result from physical health and our results indicate people with lower TBI values are more likely to be limited in the type and amount of work they perform [
25]. This domain also demonstrated significant inverse correlations with foot complications (
r
s = −0.222) and PAD symptoms (
r
s = −0.206). The TBI has recently demonstrated significant correlations with PAD symptoms and foot complications in older people and may additionally reflect physical limitations associated with these [Unpublished observations Sonter, J. and Chuter V].
While the painful symptoms of PAD are known to limit activity and to correlate significantly with reduced Role Physical HRQoL, many people with PAD have atypical symptoms or are asymptomatic. Indeed previous studies have reported approximately 30–45 % of people with PAD will experience atypical symptoms [
27‐
30]. Even in these cases, the level of impairment may greater than for those without PAD [
3,
4,
31]. In the present study the population was generally without PAD and yet a modest significant association between the TBI and lower Physical Component Summary score was found (
r
s = 0.203,
p < 0.05). The median score for the Physical Component Summary was the lowest of all SF-36v2 domains assessed in this population [46 (37–52)]. Low scores on this scale encompass limitations in self-care, physical, social and role activities, body pain, tiredness, and a poor health rating [
32]. Given the similarly lower TBI values associated with reductions in both Social Function and Role Physical, a significant association with this composite score is unsurprising. In a study of people with peripheral vascular disease, the ABI also demonstrated a small correlation with the SF-36 Physical Component Summary score [
12]. As both the ABI and TBI are used to assess the peripheral vasculature, the similar results are likely due to both reflecting PAD-associated physical impairment. More specific physical impairments including reduced walking speed, activity level and standing balance have been associated with the ABI [
10]. As a similar measure with potentially greater utility in at-risk populations, the TBI could also reflect reductions in these physical functions and activities. In the present study 28 % of participants were found to have a TBI of less than 0.70, a suggested diagnostic threshold for PAD. As only 5 % of participants reported traditional symptoms the correlations found may underestimate the relationship between the TBI and HRQoL. Further investigation of the functional limitations occurring at lower TBI values would provide a clearer understanding of the specific lower limb impairments associated with the index and the effect of these on HRQoL.
Limitations
The results of this study should be interpreted in the context of its limitations. It is important to acknowledge that many factors influence HRQoL and causality was not investigated in this study. This was a cross-sectional study and as such only correlations have been reported. Given the older age of participants in the present study and generic SF-36v2 tool used it is likely the results are affected by other factors unrelated to PAD or the TBI. The overall mean TBI indicated most of the population was without significant pathology, therefore it is likely that the correlations would be stronger in a more symptomatic population. However, this population is reflective of community-based populations attending podiatry services that require vascular screening. The TBI values obtained in this study may have been falsely low due to normal day to day variation in blood pressure and temperature however this methodology has been utilised previously with good diagnostic efficacy [
33]. Lastly, a generic HRQoL was chosen for the present exploratory investigation, however, future studies may benefit from the inclusion of disease-specific and functional assessment tools.
Conclusions
The study provides the first evidence of a relationship between the TBI and HRQoL. In an older population lower TBI values were associated with role limitations due to physical health, with impaired social function. The results presented here indicate the TBI may be useful beyond its role as a non-invasive assessment of peripheral arterial health. In addition it may indicate impaired aspects of HRQoL that are not reflected by signs and symptoms of PAD. Future investigations to determine the association between the TBI and HRQoL using PAD-specific HRQoL tools are now warranted. Additional information is required to determine whether the TBI can be used clinically to identify patients at risk of impaired HRQoL.
Acknowledgements
We thank all the participating investigators for their assistance with data collection for this study.
The authors wish to acknowledge the financial support of an Establishment grant from the Clive and Vera Ramacotti Foundation. The funding sources had no involvement in this study. We also wish to acknowledge the assistance of all participating investigators.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JS developed the protocol, collected and analysed data and drafted the manuscript. VC developed the protocol and edited the manuscript. Both authors read and approved the final manuscript.