We thank Groom et al. for their comments regarding our pilot study1 on the use of transdermal glyceryl trinitrate (GTN) for treating preterm labour. As they have pointed out, recent basic2 and clinical3 data suggests that topical nitric oxide (NO) donors (i.e. GTN) may be used effectively for cervical ripening4,5. This effect may be due to the proinflammatory action of nitrous oxide.
Endogenous NO, acting synergistically with other factors, appears also to play an important role in maintaining uterine quiescence and uteroplacental perfusion6,7. As our, and others8,11, studies have suggested, NO donors, primarily GTN, can be used antepartum, intrapartum, and postpartum to attain uterine relaxation in a variety of clinical scenarios, including preterm labour and cervical incompetence12.
The apparent paradoxical effects of the involvement of nitrous oxide in maintaining uterine quiescence and normal cervical ripening may be explained by gestational age differences in tissue sensitivity or differential tissue concentrations that are required to attain a physiologic effect on the cervix and myometrium6,7. It is unlikely that transdermal application of GTN for treating preterm labour will produce pharmacologically adequate concentrations in the cervix, compared to direct topical cervical application, to exert proinflammatory effects. Indeed, we and others10 have used transdermal GTN as adjunctive treatment in the presence of cervical incompetence and emergency cerclage for hour-glass membranes, with good clinical effect; perhaps because of its effects on uterine quiescence. Close monitoring of cervical dilatation is still required when using transdermal GTN for preterm labour.
In our pilot study1 there was no difference in the number of women delivering at seven and fourteen days following treatment with transdermal GTN compared with placebo. However, the numbers are obviously small and no real comparison can be made. No follow up data regarding the cervix was obtained following the period of treatment. Until larger, multicentre trials are carried out to examine properly the use of GTN for preterm labour, its clinical use should be restricted to research, for example its effect on the cervix.