The review included 22 trials (1,445 participants). The median quality score was 2.5 (range 1 to 7). Fourteen studies were double blinded, randomisation was adequately described in 10 studies and allocation concealment was described in four studies.
Time to 2 segment regression (nine studies, 12 comparisons, 374 participants): 10 comparisons showed a significant increase with clonidine compared to control, with treatment effects ranging from 14 to 75 minutes. There was a statistically significant dose response effect, P=0.006.
Time to regression to L2 (seven studies, nine comparisons, 275 participants): seven comparisons showed a significant increase with clonidine compared to control, with treatment effects ranging from 11 to 128 minutes. There was a statistically significant dose response effect, P<0.001.
Time to complete sensory block (10 studies, 13 comparisons, 481 participants): one comparison showed a significant increase with clonidine compared to control, with treatment effects ranging from -6 to 28 minutes. There was weak evidence of a dose response effect.
Extent of cephalic spread (nine studies, 14 comparisons, 365 participants): five comparisons showed a significant increase in the number of blocked dermatomes, two showed a significant decrease. There was no evidence of a dose response effect.
Time to complete motor block (seven studies, 11 comparisons, 305 participants): there were no statistically significant differences between clonidine and control, WMD 0.72, 95% CI -0.04, 1.49.
Duration of complete motor block (10 studies, 13 comparisons, 402 participants): 11 comparisons showed a significant increase with clonidine compared to control, with treatment effects ranging from 6 to 131 minutes. There was no evidence of a dose response effect.
Time to first request for analgesia (nine studies, 13 comparisons, 472 participants): 12 comparisons showed a significant increase with clonidine compared to control, with treatment effects ranging from 35 to 310 minutes. There was no evidence of a dose response effect.
Intra-operative pain (seven studies, eight comparisons, number of participants not reported): clonidine was associated with a significantly reduced risk of intra-operative pain compared to control, RR 0.24, 95% CI 0.09, 0.64, NNT 13.
Intra-operative hypotension (17 studies, 23 comparisons, 858 participants): clonidine was associated with a significantly increased risk of intra-operative hypotension compared to control, RR 1.81, 95% CI 1.44, 2.28, NNH 8.
Intra-operative bradycardia (13 studies, 19 comparisons, 561 participants): clonidine was not significantly associated with risk of intra-operative hypotension.
None of the results were substantially altered in the sensitivity or subgroups analysis.