Patients
A convenience sample of patients with diabetes mellitus and a foot ulcer was recruited from outpatient podiatry clinics in secondary care between 2002 and 2008. Patients were recruited into a longitudinal research programme examining psychological and behavioural aspects of diabetic foot ulceration. The clinical team identified eligible patients and introduced them to the research. Patients willing to find out more about the study and/or participate were then introduced to a member of the research team who, in turn, provided further written and verbal information. Written informed consent was obtained from all participating patients.
A total of 169 patients were recruited into the programme. Of these, 24 withdrew during follow-up and a further 33 were lost to follow-up. In the present article we report on data from 93 patients who contributed complete data for the primary analysis, i.e. patients for whom data on predictor variables were available at baseline and data on ulcer status at 24 weeks). Reasons for exclusion from the analysis were: (1) no clinically verified information on whether or not the ulcer had healed by the census date (n = 25); and (2) missing data across one or more variables included in the primary analysis (n = 51). Details of the original cohort and the subgroup who provided complete data for the primary analysis are given in Table 1. One-way ANOVAs and χ
2 analyses revealed (data not shown) that no differences were evident between the groups on the variables reported in this manuscript. Of the patients with complete data for the primary analysis, 56 had a healed ulcer by week 24 and 37 remained unhealed. Of those whose ulcer was unhealed, 14 had an amputation and three died during the follow-up period.
Table 1 Demographic and clinical characteristics of patients with and without complete data for primary analysis
All hospital trusts approached regarding this study agreed to participate and ethical approval was obtained from all trusts. All clinics subscribed to a standard regimen of foot care, i.e. aggressive debridement at each visit, treatment of infections with antibiotics and the use of removable Scotch-casts and other footwear/devices for off-loading ulcers on weight-bearing areas, minimising the likelihood of between-centre variations in treatment outcomes. Inclusion/exclusion criteria ensured the population consisted of patients with neuropathic or neuroischaemic ulcers. Patients were not eligible if they had: no palpable pulses on the affected foot; a history of major amputation (i.e. any lower limb amputation greater than a single digit); known large vessel peripheral vascular disease (e.g. previous bypass surgery, angioplasty); advanced diabetic retinopathy with severe visual impairment; advanced nephropathy (e.g. on dialysis); other severe disabling medical conditions (e.g. stroke); or were being treated with platelet-derived growth factor, tissue engineered skin or total contact casts.
Patients participated in a prospective observational study conducted over 24 weeks and were assessed at baseline (week 0) and weeks 6, 12 and 24. Baseline measurements included: ulcer assessment, i.e. present/absent and ulcer area measured in square millimetres, glucose control, psychological distress and coping style. Data on the following were also recorded: neuropathy, ischaemia, presence/absence of ulcer infection (as reported in medical notes), BMI, number of previous ulcers, diabetes type, patients’ self-report of other chronic conditions, age, sex, income, smoking and marital status. Ulcer assessments were repeated at weeks 6, 12 and 24.
Ulcer assessment
Ulcer assessments involved determining: (1) whether or not the ulcer had healed; and (2) the size of unhealed ulcers at each follow-up. The former was determined with reference to clinical records. The latter involved using a transparent film and tracing the topical area of the wound. The tracing was then placed on a digital tablet (Visitrack: Smith and Nephew, London, UK) and the area of the ulcer was re-traced with a stylus to produce a measurement of absolute ulcer area. The timing of the ulcer assessments was dictated by patients’ referral to the specialist podiatry clinics and entry into the study.
Glucose control
Glycated haemoglobin (HbA1c) was measured to provide a surrogate marker of disease control 2 to 3 months prior to study entry. HbA1c was measured by cation exchange high-performance liquid chromatography using an analyser and associated reagents (HA-8140; A. Menarini Diagnostics, Wokingham, UK). The assay was maintained in line with the Diabetes Control and Complications method [21], with no significant assay drift and a between-batch imprecision (CV) of 1.8% (at mean HbA1c 5.5%). All assays were performed on the same instrument.
Neuropathy, ischaemia and ulcer classification
Neuropathy was assessed by applying a 10 g nylon monofilament to a number of sites on the affected foot and patients reporting the presence/absence of sensation. Level of neuropathy was based upon the number of tested sites with sensory loss. Consistent with established criteria [22], individuals were classified according to per cent of sensory loss as follows: 30% to 50% of tested sites ‘mild neuropathy’, 60% to 80% ‘moderate neuropathy’ and 90% to 100% ‘severe neuropathy’. Percentage rather than absolute values were used, as the number of sites assessed varied between podiatrists. Ischaemia was assessed by measuring palpable pulses at the dorsalis pedis and posterior tibial areas of the affected foot. Ulcers were classified as neuropathic, i.e. sensory loss in 60% or greater of sites tested, or neuroischaemic (neuropathy combined with evidence of ischaemia).
Psychological distress and coping
The Hospital Anxiety and Depression Scale assessed anxiety and depression [23]. Scores were converted into binary outcomes to reflect the presence (i.e. scores of 11 or greater) or absence (i.e. scores of 10 or less) of clinical anxiety and depression [24]. Cronbach’s alpha reliability coefficients in the present study were 0.86 and 0.54 for anxiety and depression respectively.
The Medical Coping Modes Questionnaire [25] assessed confrontation, avoidance and acceptance-resignation coping. Confrontational styles are characterised as being more controlling, competitive and extroverted; avoidant styles demonstrate less knowledge and attention to their condition; acceptance-resignation styles exhibit greater helplessness and less affective expression. Cronbach’s alpha reliability coefficients in the present study were 0.71 (confrontation coping), 0.54 (avoidance coping) and 0.73 (acceptance-resignation coping).
Salivary cortisol and MMPs
Samples of saliva and wound fluid were collected at baseline for the measurement of cortisol and both MMP2 and MMP9. For cortisol, participants collected four saliva samples into tubes (Salivette; Sarstedt, Nümbrecht, Germany) over a single day, i.e. on waking, 30 min later, between 11:00 and 13:00 hours (before lunch), and between 20.00 and 22:00 hours (at least 2 h after evening meal). Participants were asked to not brush their teeth before providing the first two samples and not to consume food or caffeinated drinks in the hour before providing all samples. They were also asked to refrigerate samples until returned to the study site. Salivary cortisol was determined by ELISA (assays carried out by Cultech, Swansea, Wales, UK). The sensitivity of the assay was 0.3 nmol/l with an inter-assay variation of 3.43 nmol/l and an intra-assay variation of 2.66 nmol/l.
Wound fluid was collected at baseline. Sterile, pre-weighed filter paper was applied to the wound to absorb fluid using methods described previously [18], and then re-weighed and stored at −80°C until processing. Levels of MMPs were normalised to the mass of fluid collected. The presence of MMP2 and -9 was detected in wound fluid samples using gelatine substrate SDS-polyacrylamide gel electrophoresis (zymography) as described previously [26–28].
Salivary samples for the measurement of cortisol and wound fluid samples for the measurement of MMPs were not collected from all patients. Their collection was dictated by whether appropriate funding and ethics approval had been obtained for these aspects of the study. Accordingly, 89 patients provided saliva samples for the measurement of cortisol (data available from 82, 81, 82 and 85 patients for samples 1, 2, 3 and 4 respectively). One-way ANOVA and χ
2 tests examined whether the cortisol subgroup was representative of the original cohort for the variables reported in this manuscript. This subsample only differed in the measure of acceptance coping, baseline HbA1c and marital status, with patients who provided saliva samples having marginally lower HbA1c levels (8.34 vs 9.10, p = 0.011), a slightly greater propensity towards acceptance-resignation coping (6.83 vs 5.77, p = 0.014) and being more likely to be single (22% vs 9% of patients). Cortisol awakening response was only calculated in patients who provided information on the specific times at which they had provided cortisol samples and who reported adherence to the protocol, i.e. first sample was taken on waking and the second one 30 min later (n = 45). One-way ANOVA compared the cortisol awakening response with that of patients providing all four cortisol samples. No statistically significant differences were observed.
Finally, wound fluid was collected from 21 patients at baseline. One-way ANOVAs and χ
2 tests compared patients who provided samples with those who did not. The groups were largely comparable on all variables reported in this manuscript, except that patients who provided wound fluid samples were less likely to have an infected ulcer (20% vs 43% of patients; p = 0.020) and more likely to report high blood pressure (69% vs 42% of patients; p = 0.013).
Statistical methods
For the primary analysis it was estimated that a sample size of 90 participants would provide 80% power to detect an odds ratio of 1.9 associated with a 1 standard deviation change in a normally distributed predictor variable, where the response probability at the mean of the predictor is 0.5, using alpha = 0.05. Testing the primary aim involved logistic regressions in which the binary outcome captured whether or not the ulcer had healed by the end of the observation period, i.e. by 24 weeks. For these analyses, patients were designated as ‘not healed’ if: (1) they were deemed to have an active ulcer at 24 weeks, i.e. one where clinical notes indicated that complete healing of the topical surface of the ulcer had not been achieved by the 24 week census point; (2) they had had an ulcer-related amputation during follow-up; or (3) they had died during follow-up. In contrast, patients for whom the podiatrists reported complete healing of the topical surface of the ulcer at some stage during follow-up were designated as ‘healed’. The classification of deceased patients and those undergoing amputation as ‘unhealed’ is a convention reported by other groups [29].
Separate preliminary analyses were first conducted for each potential clinical and demographic determinant of ulcer healing, as measured at baseline, i.e. age, sex, marital status, income, ulcer infection, number of previous ulcers, type 1/type 2 diabetes, HbA1c, BMI, neuropathy, ischaemia, smoking, chronic co-morbid conditions and ulcer area. A conservative cut-off of p ≤ 0.1 was adopted for these variables to qualify for inclusion in the final model. Thus, the final model for the primary analysis included only those clinical and demographic factors that met the p ≤ 0.1 criterion, and the measures of anxiety, depression and coping. Due to the intercorrelation between the measures of anxiety and depression (r = 0.674, p < 0.0001), the model was run separately for these variables.
The secondary aim of exploring the effect of psychosocial factors on change in ulcer size over the observation period, involved repeated measures ANOVA. The dependent variables were the ulcer area measures at 0, 6, 12 and 24 weeks, and thus only included patients for whom ulcer area data were available at each time point. For patients who died or who had an amputation during the follow-up period, the last available ulcer measurement was used for all observations after death or amputation occurred. This approach is statistically conservative as it assumes no further change would have occurred in these patients.
Finally, one-way ANOVA analyses compared levels of cortisol and both MMP2 and -9 between patients who had healed vs not healed over the observation period. For cortisol, this involved comparisons across all four time points at which samples were collected and also looking at the difference between the first and second samples. This latter variable, referred to as the cortisol awakening response, is a widely used measure of HPA reactivity [30, 31].
All analyses were conducted using SPSS version 15 and non-normally distributed variables were subjected to logarithmic transformation.