Results of the Search
Figure 1 provides a summary of the literature search results and screening process.
Included Studies
We identified 23 studies that could provide information on the effect of sulodexide in the management of CVD [42, 44, 45, 48, 54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72], with exclusion of the studies exclusively reporting the course of venous ulcers. Whenever possible, data were retrieved from the original tables; where data were reported as graphs only, numeric data were reconstructed by analysing the graphs at high magnification with a high definition software and applying standard algorithms in R. Studies were published in English, Italian, Spanish, Polish, Russian and Chinese but no limitation was imposed on the language. We instead excluded studies from the quantitative synthesis when no numeric data or graphs were presented in relation to the considered outcomes [48, 66, 72], when only mean data without an interpretable form of variance were reported [54, 55, 57, 58, 62, 69] and when data were reported but considered in one study only.
Overall, the studies considered in this review included 7153 participants (mean age 55 years; 68% female). The mean number of participants included per clinical trial was 300 (range 8–2263). Not all studies yielded data suitable for a quantitative synthesis; the studies providing quantitative information included 1901 participants (mean age 55.2 years; 65% female) with an average size of 140 patients per study (range 8–476).
All participants were reported to meet the respective CVD criteria of every study. Formal classification of CVD stage was reported only in seven of the most recent studies [43, 44, 66, 68,69,70, 72], according to the CEAP consensus [15]. The C3 and C4 classes accounted for almost 70% of classified cases: C0: 0.3%; C1: 3.0%; C2: 8.4%; C3: 31.8%; C4: 36.9% (with a prevalence of C4a where the stage was split); C5: 8.7%; C6: 10.9%.
Four studies specified that investigators recommended/prescribed compression treatment [43,44,45, 69], although one author specified that the compliance with compression was limited [69].
Symptoms and signs were almost exclusively monitored by means of four- or six-level scores. Only one study measured pain with a VAS [68]; the same study measured the distal oedema in cm3. Specialised techniques were used to measure the markers of inflammation in serum or tissue [43, 44, 68, 71].
Excluded Studies
We excluded 41 studies that were originally considered as potentially eligible. The reasons for exclusion were: no clinical data reported (n = 13) [12, 13, 29, 39,40,41, 73,74,75,76,77,78,79], indication other than CVD with data on CVD symptoms not available/not extractable (n = 13) [80,81,82,83,84,85,86,87,88,89,90,91,92], no extractable data on symptoms (n = 9) [36, 93,94,95,96,97,98,99,100,101], preliminary partial publication (n = 3) [102,103,104], data reported but not stratified by treatment (n = 1) [105], review (n = 1) [47] or duplicate (n = 1) [98].
Effects of Interventions
The effects of interventions were divided into three major groups: symptoms and signs of CVD, modifications of inflammatory markers and number of patients with adverse events. Except scoring of heaviness, biochemical markers and number of patients with adverse events, none of these variables were reported by at least two controlled studies. Consequently, the methods for combining single-arm data have been used for all variables.
Symptoms of CVD
Symptoms of CVD for which at least two studies provided interpretable data were: total symptoms score [63, 68], pain [56, 59, 61, 64, 65, 67, 68, 70], cramps [45, 56, 59, 63, 64, 67, 70], paraesthesia [45, 56, 59, 60, 64, 67] and heaviness [45, 60, 61, 67, 70]. All these variables were monitored by means of scales rating the intensity 0–3 except for one study where pain was measured with the VAS [68]. The total symptom score was computed by summing the individual score over a different number of symptoms in the two cases: investigator-defined number of symptoms [63] or VCSS symptoms and signs [68].
Total Symptom Score
The total symptom score was reported in two studies on a total 483 patients. One study used the sum of scores assigned to each of: heaviness; paraesthesias; cramps; pain that was orthostatic, clinostatic and on effort; oedema of the foot, leg and general; skin pigmentation; desquamation; eczema; alteration of annexes; hypodermitis; stasis ulcer; lymphangitis; cyanosis; venous ectasia [63]. The scale ranged from 0–3, so that the total score could range from 0–51. The other study instead used the VCSS scoring system, which could range from 0 to 21 (since patients with ulcers were excluded) [68]. We therefore used the standardised mean difference between baseline and month 2 [63] or month 3 [68] data and pooled the studies using the log-transformed means.
The result (Fig. 2) showed a definite effect associated with sulodexide, regardless of the measurement method, dosage and formulation used. The overall effect in terms of standardised mean difference was 1.63 SD [95% confidence interval (CI): 1.18; 2.25 SD] above zero. In absolute terms (raw change of total score), the effect can be indicated as a decrease of 7.57 (95% CI 5.92; 9.69) score points with significant heterogeneity (I2: 95.9%; P < 0.0001).
Pain
Pain was reported in 8 studies on a total of 927 patients [56, 59, 61, 64, 65, 67, 68, 70] monitored for 2 (N = 46) [56, 61], 3 (N = 866) [59, 64, 67, 68, 70] or 6 (N = 15) months [65]. Six studies used the conventional 0–3 scoring system [56, 59, 61, 64, 65, 67], one used a 0–5 scale [70] and one used the visual analogue scale (VAS) [68]. We used the standardised mean difference to estimate the effect, with the random effects model (Fig. 3).
The overall effect could be estimated as 2.51 SD (95% CI 1.20; 3.82 SD) with high heterogeneity (I2 100%; P < 0.0001). In absolute terms, the effect estimated after the exclusion of the trial using the VAS could be reported as a decrease of 1.74 (95% CI 1.27; 2.22) score points with high heterogeneity (I2 98.2%; P < 0.0001).
Cramps
Seven studies reported the intensity of cramps (frequently reported as “nocturnal” cramps) measured with the usual scale 0–3 [45, 56, 59, 63, 64, 67], except one that used the 0–5 scale [70]. Overall, 1427 patients were monitored, of whom 480 were treated for 2 months [56, 63] and the rest for 3 months. The overall effect estimate (Fig. 4) was 1.20 score points of improvement (95% CI 0.97; 1.44) with very high heterogeneity (I2 97.7%; P < 0.0001). In this case, however, large heterogeneity was expected since the dispersion measure within each study (homogeneous population and limited score span) was mostly smaller than the differences between populations across studies.
Paraesthesia
Six studies reported the intensity of paraesthesia, measured with the usual 0–3 score scale [45, 56, 59, 60, 64, 67]. Overall, 747 patients were monitored, of whom 79 were treated for 2 months [56, 59] and the rest for 3 months. The overall effect estimate (Fig. 5) was 1.06 score points of improvement (95% CI 0.65; 1.48) with high heterogeneity (I2 98%; P < 0.0001).
Heaviness
Five studies reported the intensity of heaviness, measured with the 0–3 score scale in four [45, 60, 61, 67] and 0–5 in the fifth [70]. Overall, 948 patients were monitored, treated for 2 (95 patients) [60, 61] or 3 months. The overall effect estimate (Fig. 6) was 1.32 score points of improvement (95% CI 1.05; 1.69) with high heterogeneity (I2 94%; P < 0.0001).
Two of the studies reporting this variable were comparative vs. calcium heparin [61] or placebo [45]. The result of the comparison showed a significantly greater effect with sulodexide (P = 0.01; Fig. 7), corresponding to a decrease of 0.28 score points (95% CI 0.07; 0.50).
Signs of CVD
Signs of CVD for which at least two studies provided interpretable data were: oedema [45, 56, 61, 63,64,65, 68, 70], discoloration [56, 60, 64, 70] and skin temperature [45, 65, 67]. The variables were scored 0–3 except for one study using a 0–5 scale [70] and one in which oedema was measured as volume [68].
Oedema
Eight studies reported the intensity of oedema, measured with a 0–3 score scale in six [45, 56, 61, 63,64,65], with a 0–5 score scale in one [70] and as volume in one [68]. Overall, 1005 patients were monitored, of whom 522 were treated for 2 months [56, 61, 63, 65] and the rest for 3 months. The overall effect estimate used the standardised difference (Fig. 8) and yielded 1.54 SD of improvement (95% CI 0.97; 2.10 SDs) with high heterogeneity (I2 99%; P < 0.0001). The effect size in terms of score points could be estimated after exclusion of the study using actual volume and resulted in an improvement of 1.15 (95% CI 0.78; 1.52) score points with high heterogeneity (I2 98%; P < 0.0001).
Discoloration
Four studies reported data on skin discoloration over a total of 418 patients, of whom 87 were treated for 2 months [56, 60] and the rest for 3 months [64, 70]. The overall effect estimate yielded an improvement of 1.06 score points (95% CI 0.16; 1.96; Fig. 9); however, the heterogeneity was high (I2 97.6%; P < 0.0001).
Skin Temperature
Three studies reported data on skin temperature over a total of 571 patients treated for 2 (N = 15) [65] or 3 months [45, 67]. The overall effect estimate yielded an improvement of 1.11 score points (95% CI 0.48; 1.74; Fig. 10) with high heterogeneity (I2 98.5%; P < 0.0001).
Modifications of Inflammatory Markers
The only markers examined under the same conditions in at least two trials were the serum level of MMP-2 [68, 71] and MMP-9 [43, 68, 71]. The effect of exposure to sulodexide on IL-6, MCP-1, s-ICAM-1 and the generation of free radicals were tested under different conditions in 3–4 different experiments in two trials [43, 44].
Matrix Metalloproteinase-2
Two studies examined the level of MMP-2 in the serum of 50 patients treated for 3 (22 cases) [68] or 6 months (28 cases) [71]. Since both measuring methods used are known to be remarkably precise, the pooled analysis is better suited to the fixed effect model and yielded an estimate indicating a significant decrease of the MMP-2 of 30.5 ng/ml (95% CI 20.7; 40.3) (Fig. 11).
Matrix Metalloproteinase-9
Three studies examined the level of matrix metalloproteinase-9 in serum of 61 patients treated for 2 (N = 11) [43], 3 (N = 22) [68], or 6 (N = 28) months [71]. In view of the different levels of precision of the techniques used, we analysed the standardised mean difference. The combined outcome indicated a mean decrease of 2.43 (95% CI 1.33; 3.52) SDs (Fig. 12) or, in original units, 1.14 ng/ml (95% CI 0.95; 1.32) using the fixed effect model.
Interleukin-6
The release of IL-6 was examined in three different experiments in the same study [44]. The three tests can be considered as three independent assays of the same phenomenon and can therefore be combined in a meta-analysis. The distribution of data suggested using the fixed effect model on the standardised mean difference to avoid heterogeneity. The outcome indicated a significant (P < 0.0001) decrease of IL-6 release in the cells exposed to serum from treated patients compared with the cells exposed to the serum from the same patients before treatment (controls with CVD; Fig. 13). In absolute terms, the treatment reduced the release of IL-6 by 611 (95% CI 517; 705) pg/105 cells.
Monocyte Chemoattractant Protein-1
The release of MCP-1 was examined in two different experiments in the same study [44]. The outcome, based on the same approach as before (fixed effect model on the standardised mean difference, Fig. 14), indicated a significant (P < 0.0001) decrease of MCP-1 release in the cells exposed to serum from treated patients compared with the cells exposed to the serum from the same patients before treatment (controls with CVD). In absolute terms, the treatment reduced the release of MCP-1 by 156 (95% CI 105; 207) pg/105 cells.
Soluble Intercellular Adhesion Molecule-1
The release of s-ICAM-1 was examined in two different experiments in the same study [44]. Using the fixed effect model on the standardised mean difference, the outcome (Fig. 15) indicated a significant (P < 0.0001) decrease of s-ICAM-1 release in the cells exposed to serum from treated patients compared with the cells exposed to the serum from the same patients before treatment (controls with CVD). In absolute terms, the treatment reduced the release of s-ICAM-1 by 104 (95% CI 61; 148) pg/105 cells.
Generation of Free Radicals
The release of free radicals into the medium was examined in two different experiments in the same study [44]. The outcome, according to the fixed effect model on the standardised mean difference (Fig. 16), indicated a significant decrease (P < 0.0001) of the generation of free radicals in the cell cultures exposed to serum from treated patients compared with the cells exposed to the serum from the same patients before treatment (controls with CVD). In absolute terms, the treatment reduced the generation of free radicals by 140 (95% CI 92; 186) events/105 cells.
Number of Patients with Adverse Events
The number of patients reporting adverse events, regardless of their potential correlation with treatment, was reported in two comparative trials [45, 56] and in six non-comparative trials [63, 64, 66, 67, 69, 70]. We decided not to examine the data of routine haematology, clinical chemistry and vital signs, since in the few studies in which such data have been reported, only mean data have been published, whereas no mention was made of individual clinically relevant changes that could configure an adverse event.
The analysis of all available data on the proportion of subjects reporting adverse events indicated a very low incidence: 3% (95% CI 1; 4; Fig. 17). However, the high heterogeneity (I2 93%) and the distribution of the events across studies suggested that the overall result was the composite of two different conditions. Stratifying the proportion of patients reporting adverse events by type of study (interventional or observational; Fig. 18) clearly showed two significantly different (P < 0.01) rates of reporting: 13% (95% CI 8; 18) in the interventional studies and 1% (95% CI 0; 1) in the observational studies.
The significant difference between observational and interventional studies suggests that the observational studies can be useful to record rare adverse events but, unless well monitored, are likely to underestimate the incidence of the common adverse events.
Among the considered studies, two were comparative vs. heparan sulphate and placebo, covering a total of 270 patients [45, 56]. The incidence of adverse events with sulodexide in these interventional trials, one of which was double-blind vs. placebo [45], did not differ from that among controls (Fig. 19): RR 1.31 (95% CI: 0.74; 2.32).