Infection and preterm labour

Current”

OBSTETRICS&‘ ' GYNAECOLOGY. ,. Mini-symposium: Preterm labour and delivery

Infection and preterm labour

P. R. Bennett

Several studies have demonstrated an association between genital tract infection, chorioamnionitis and preterm labour. For example Lamont et al3 examined the genital tract flora of 74 women who delivered following idiopathic preterm labour and made comparisons with a group of 26 women who were delivered before term for other reasons, usually hypertensive disorders of pregnancy. The criteria for abnormal colonisation were strict. Any bacteria except Neisseria gonorrhoea was considered to be a normal commensal unless present in high numbers with an inflammatory response. Abnormal bacterial colonisation was found in 47% of those in preterm labour but in only 15% of controls. Histological evidence of chorioamnionitis was present in 56% of the preterm labour group but in only 10% of controls. Guziac and Winn4 examined 2774 placenta from women delivered at Johns Hopkin Hospital in one year. They found that the incidence of preterm labour was 5.4% when neither chorioamnionitis nor membrane rupture were present, rising to 12% when chorioamnionitis was present with intact membranes and to 56% when both membrane rupture and chorioamnionitis were present. Only 8% of women had clinical signs when chorioamnionitis was present and only 3% of their babies had early sepsis. Logistic regression analysis showed that 25% of preterm deliveries were attributable to chorioamnionitis. The association between non-bacterial infection of the genital tract and preterm labour is less well defined. Lamont et al3 reported higher rates of U. urealyticum and M. hominis colonisation in their preterm labour group when compared to controls, and C. trachomatis was found in 7% of those in preterm labour but none of the controls. The association between C. trachomatis infection and adverse

The association between infection and preterm labour Preterm labour occurs in up to 10% of all births but is associated with 85% of perinatal deaths in otherwise normal babies and an unknown proportion of physical and mental handicap. The cause of preterm labour is usually unknown. Cases associated with multiple pregnancy, polyhydramnios or abnormalities of the uterus or cervix are in the minority. Associations have been found between preterm labour and maternal age, weight and height, cigarette smoking, parity, and other socio-economic factors.’ Recently the role of infection has been recognised as a major factor in the aetiology of preterm labour. The evidence that preterm labour is associated with genital tract infection falls into three categories; the increased incidence of puerpural sepsis following preterm delivery; the increased incidence of infection in premature neonates; and the epidemiological association between preterm labour and genital tract colonisation by various organisms. The incidence of maternal puerpural sepsis is three-fold greater following preterm labour than delivery at term. The incidence of infection in premature neonates is 5% compared to O.l-0.5% in term infants.3 This is partly a reflection of their immaturity, their increased risk of instrumentation and exposure to hospital pathogens in the neonatal intensive care unit. It is likely, however, that many of these infections are acquired in utero or during vaginal delivery. Common causes of neonatal sepsis are the group B streptococci and Escherichia coli both of which are commonly found in the vagina. P. R. Bennett BSc, MD, MRCOG, Lecturer in Obstetrics and Gynaecology, Royal Postgraduate Medical School, Institute of Obstetrics and Gynaecology, Queen Charlottes and Chelsea Hospital, Goldhawk Road, London W6 OXG, UK Current Obstetrics 0 1992 Longman

and Gynuecology UK Ltd

Group

(1992) 2, 194-198

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pregnancy outcome has been the subject of a large number of studies in the past few years. Most have demonstrated an increase in the rates of premature membrane rupture and preterm labour in association with cervical carriage of C. trachomatis or have demonstrated a reduction in preterm delivery and neonatal morbidity when C. rrachomaris, discovered in pregnancy. is treated.’ Pathogenesis

Most cases of clinically overt chorioamnionitis associated with preterm labour are due to ascending infection following preterm premature rupture of membranes.‘j The risk is proportional to the duration of membrane rupture and the number of vaginal examinations. The mechanism of preterm rupture of membranes is not clearly understood. Whilst prematurely ruptured membranes are not generally weaker when tested for bursting strength they do appear to be thinner at the site of rupture.’ Preterm membrane rupture is associated with a similar history in previous pregnancies. It may result from abnormal membrane structure, cervical incompetence or uterine distention. It is likely that infection is a major aetiological factor although this remains a matter for debate. Bobbitt et al* found a high incidence of bacteria in the amniotic fluid immediately following membrane rupture. Naeye et al2 reported that among 6613 women who delivered before 37 weeks the incidence of genital tract infection was three times greater in those whose membranes ruptured before, rather than during, labour, suggesting that infection was a cause and not the effect of the membrane rupture. Alternative routes of infection include haematogenous spread in mothers with bacteraemia and the introduction of infection during invasive procedures. It is still not clear whether cervical cerclage is associated with an increased risk of intrauterine infection, the rates reported by most studies being around l-2% which is similar to the background incidence. When cerclage is used later in pregnancy, following prolapse of membranes into the vagina, the risk may be as high as _ 75%. The risk of chorioamnionitis following amniocentesis or chorion villus biopsy is low but this probably increases with more complex manipulations. Approximately half of the 1.5% of losses associated with antepartum fetal blood sampling are due to chorioamnionitis.’ Biochemical mechanisms by which infection may initiate preterm labour In the past few years considerable interest has been shown in the biochemical mechanisms by which infection might initiate preterm labour. In general, the aetiology of preterm labour, whether associated with infection or not. is unknown, essentially because the mechanism of the onset of normal labour at term is not understood.

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The endocrinological changes leading to labour in sheep have been established.” Although an understanding of human parturition is less advanced there is clear evidence for the roles of prostaglandins in human parturition. Prostaglandin levels rise in amniotic fluid, plasma and urine during labour. Prostaglandins are powerful oxytocic agents and may be used to induce labour and prostaglandin inhibitors prevent labour. Prostaglandins are formed from the precursor arachidonic acid, a substrate for at least three enzyme groups (Fig.). Metabolism via the cyclooxygenase pathway produces the classical prostaglandins, prostacyclin and thromboxane. Metabolism via the lipoxygenase enzyme pathways produces a series of hydroxyeicosatetraenioc acids (HETEs) including the leukotrienes, and metabolism via epoxygenase pathways produces a series of epoxyeicosatetraenioc acids. The principle candidates for the source of prostaglandins which initiate labour are the fetal membranes and decidua. Amnion contains large stores of arachidonic acid. Prior to labour metabolism is principally via the lipoxygenase enzyme pathways to produce 5and I2-HETE and leukotriene B4. There is also production of 14,15,epoxyeicosatetraenioc acid. ‘I In association with labour, there is an increase in overall arachidonic acid metabolism and a change in the ratio of cycle-oxygenase to lipoxygenase metabolism to favour synthesis of prostaglandin E2 which is known to mediate cervical ripening and to cause uterine contractions. The roles of the lipoxygenase metabolites of arachidonic acid are unknown but it appears that 5-HETE may play a role in prelabour (Braxton-Hicks) contractions. 12.i3 The mechanisms which control arachidonic acid metabolism in the fetal membranes and decidua, and which stimulate prostaglandin production at term are not understood. A fetal stimulus may act via the amniotic fluid. Candidates for this stimulus currently include platelet activating factor (PAF)14 and interleukin I (ILI).” which are found in increased concentration in the amniotic fluid during labour and which will stimulate prostaglandin production in amnion cells. PAF is a particularly strong candidate since its secretion by the fetus into the amniotic fluid is related to fetal lung maturity. Esterified

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Any stimulus to prostaglandin production in the fetal membranes must directly or indirectly increase the availability of substrate arachidonic acid either by providing an exogenous source or by increasing intracellular mobilisation through increased phospholipase activity. There would also need to be a simultaneous increase in the activity of the enzyme cyclooxygenase (Fig.). Addition of exogenous arachidonic acidI or phospholipase A2l’ to amnion cells in culture increases their production of prostaglandin E2. This suggests that an increase in free substrate arachidonic acid stimulates synthesis of the cyclooxygenase protein although this remains to be confirmed by gene expression studies. There is an in vivo increase in the expression of the cycle-oxygenase gene with labour.” Bacterial production of prostaglandins

The great majority of studies which examine biochemical mechanisms by which infection might initiate preterm labour presume that this will involve an increase in intrauterine prostaglandin concentrations which mediate cervical ripening and uterine contractions. One possible mechanism by which bacterial infection might initiate labour is by production of prostaglandins by the bacteria themselves. Gulbis et all9 have published conflicting reports of the production of prostaglandin E2 by E. coli but have concluded that E. coli do not synthesise prostaglandin E2. That common vaginal pathogens do not synthesise prostaglandins or other oxytocic eicosanoids was confirmed by Bennett et al.” They studied the metabolism of arachidonic acid in several bacterial strains by incubating the organisms with exogenous radiolabelled arachidonic acid and concluded that common genital tract pathogens were not able to metabolise arachidonic acid to any compounds which might stimulate the onset of preterm labour. Release of phospholipase from bacteria

Since an increase in phospholipase activity is a prerequisite for an increase in prostaglandin synthesis from intracellular arachidonic stores considerable interest has centred upon the role of phospholipase release from bacteria as a cause of preterm labour. Phospholipase activity in common vaginal pathogens was first demonstrated by Bejar et al.” They found that virtually all of the common genital tract pathogens contained significant phospholipase A activity and suggested that it might be release of phospholipase A from infecting organisms which increases arachidonic acid mobilisation and prostaglandin release from the fetal membranes, leading to preterm labour. They supported this hypothesis by the results of a single experiment in which purified snake phospholipase A was injected into the amniotic cavity of a pregnant rhesus monkey caused both an increase in free arachidonic acid in the amniotic fluid and the onset of preterm labour. Subsequently a series of

studies were published concerning the effects of intact bacteria upon prostaglandin synthesis. Lamont et al” demonstrated that bacteria release a substance which will increase prostaglandin synthesis in amnion cells. This study was later extended to demonstrate a similar effect with a wide range of potential pathogens.23 Bennett et al” showed that, at term, prior to labour, arachidonic acid metabolism in amnion cells is essentially via lipoxygenase and epoxygenase pathways with very little metabolism via the cycle-oxygenase enzyme to prostaglandins. In association with labour, either at term or preterm, they found a large increase in arachidonic acid mobilisation and an increase in metabolism via both cycle-oxygenase and non-cyclooxygenase pathways. Both the cycle-oxygenase : lipoxygenase ratio and prostaglandin synthesis increased. The addition of conditioned medium, in which bacteria had been grown and then removed, to amnion cells collected at elective Caesarean section, caused an identical change in the arachidonic acid metabolic profile. Later the same group” showed that purified phospholipase A2 caused the same change in arachidonic acid metabolism in amnion cells and that the ability of any organism to do so was strongly correlated with measured phospholipase activity in its conditioned medium. Bacterial endotoxins

An alternative mechanism by which bacteria might increase intrauterine prostaglandin concentrations is by the action of endotoxin. Endotoxin, or lipopolysaccharide, is a component of the cell wall of gram negative bacteria. Romero et al have shown that endotoxin from E. coli stimulates synthesis of prostaglandin E2 in amnion cells in monolayer culture and suggest that this may be another mechanism for preterm labour. The same group24 measured endotoxin concentrations at amniocentesis in 126 women with preterm premature rupture of membranes. Labour was significantly more likely when amniotic fluid culture was positive and when endotoxin was detected although in some of the labouring patients endotoxin concentrations were below those which stimulate prostaglandin synthesis, suggesting that other mechanisms must also be at work. The inflammatory response

Whilst the release of phospholipases or endotoxins from pathogens within the genital tract may be causes of increased prostaglandin production leading to preterm labour there are a number of reasons to suspect that these are not the sole mechanisms. In the experiments of Lamont et a122g23and Bennett et a1113i7 bacteria were grown in the conditioned medium for 18 h or more, producing a saturation density culture. This would be the equivalent of an overwhelming intra-amniotic infection. To produce changes in arachidonic acid metabolism similar to those seen with labour doses of conditioned medium very near to the toxic dose were required. The mere presence of

INFECTION

bacteria in the vagina or in the amniotic fluid, without a significant inflammatory reaction, is not necessarily associated with preterm labour. Attention has therefore turned to the possibility that substances released as part of the inflammatory response to infection increase prostaglandin production within the uterus. Interleukin-1 (IL-l), a macrophage/monocyte produced cytokine, is a powerful stimulator of prostaglandin synthesis in amnion cell with much greater potency than bacterial products. Romero et al” measured IL-l levels in the amniotic fluid of women in term and preterm labour. They found that IL-l was not detectable in women not in labour, or in those who were in preterm labour without evidence of infection. IL-l levels were elevated in those in preterm labour associated with infection as well as in those in labour at term. They also found a strong positive correlation between concentrations of prostaglandins and IL-l in the amniotic fluid. It is possible that the effect of inflammation upon arachidonic acid metabolism within the uterus is even more complex than the stimulation of metabolism within the amnion or other cell types by substances released from bacteria or inflammatory cells. Bennett et al26 incubated activated leukocytes with amnion cells whose arachidonic acid pools had been radiolabelled. They found that the leukocytes stimulated an increase in the mobilisation of arachidonic acid from the amnion cells and an increase in the synthesis of prostaglandin E2. The arachidonic acid released from the amnion cells was also taken up by the leukocytes and metabolised to other prostaglandins including prostaglandin F2a, the most potently oxytocic prostaglandin. It appears then, that there are several possible mechanisms by which infection, both bacterial and non-bacterial. might increase prostaglandin production within the uterus. Virtually all of the work on the biochemical mechanisms by which infection may cause preterm labour involves, as its end point, an increase in prostaglandin synthesis in cells within the uterus. However the studies of Lopez Bernal and Romero25 suggest that there is a group of women in preterm labour in whom there is no elevation in prostaglandin synthesis. Although in both studies these tended to be cases in which infection was not an aetiological factor it does appear that labour can be initiated without prostaglandin production. Other mechanisms must be acting in these cases and it remains possible that infection may also initiate labour by mechanisms which do not involve prostaglandins. A better understanding of the mechanisms of labour before term will follow once the enigma of labour at term has been solved. Clinical aspects Diagnosis

The diagnosis of the onset of labour is, in practice, retrospective, since the latent phase of labour may not be distinguishable from prelabour contractions.

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When dealing with labour at term various definitions for the establishment of labour have been proposed. Regular, painful, uterine contractions associated with progressive dilatation of the cervix clearly indicates active labour but this definition may be too stringent to be clinically useful in cases of preterm labour. If tocolysis is to be used it is probably more appropriate to begin treatment when contractions have established without waiting for cervical dilatation to take place. If there is a low index of suspicion for preterm labour then a proportion of women will begin treatment who may not have progressed to established preterm labour. Whilst a proportion of cases of preterm labour are obviously associated with chorioamnionitis, the studies of Lamont,3 Guzack and Winn4 and others have shown that a large proportion of cases of ‘idiopathic’ preterm labour are also associated with either abnormal genital tract colonisation or unrecognised chorioamnionitis. In some cases an exhaustive clinical search may be necessary to identify the presence of chorioamnionitis and often the diagnosis may not be made until postpartum microbiological and histopathological studies can be performed. Uterine tenderness and foul vaginal discharge are late signs of chorioamnionitis. Earlier clinical signs include fetal tachycardia, maternal tachycardia and pyrexia. Maternal leukocytosis associated with a left shift, is found in well over half of cases with clinical infection.(j The results of culture of a high vaginal swab may not be available until after delivery but, where available, a gram stain may be very valuable in demonstrating the presence of gonococci or streptococci. The frequency of failed tap and the poor sensitivity and specificity of amniotic fluid culture in demonstrating the presence of intra-amniotic infection has limited the use of amniocentesis.‘* Studies of fetal tone and breathing movements have suggested that at least one episode of body movement or fetal breathing for over 30 min indicates absence of chorioamnionitis. Although changes in fetal behaviour may be useful as an indication intra-amniotic infection this has not been clearly established at present and requires further study. Managemenr

There is a clear difference in the management of cases of preterm labour in which there are clinical signs of infection and those in which infection is not apparent. Where there is a strong suspicion that intra-amniotic infection is present, delivery should be allowed to take place as quickly as possible in the interests of both mother and baby, irrespective of fetal maturity. Corticosteroid therapy to prevent respiratory distress syndrome is contraindicated, both because of its risk in the presence of infection and because delivery is likely to take place before having any significant effect upon the fetus. The controversy as to whether antibiotics should be given to the mother before delivery has not been resolved by any controlled studies. The principle arguments against antibiotic

198 CURRENT OBSTETRICS AND GYNAECOLOGY treatment for the mother are that it may lead to resistant infection in the neonate and to difficulties in culturing organisms from the fetus to establish antibiotic sensitivities. However, it is usual practice to begin very broad spectrum antibiotic treatment of premature neonates with a very low index of suspicion for infection. There are no randomised controlled trials which have addressed this question although the results of two uncontrolled studies” suggest that early administration of antibiotics leads to a reduction in neonatal sepsis and an improvement in survival. In cases of ‘idiopathic’ preterm labour the possibility of subclinical infection needs to be borne in mind. There are no studies of the use of antibiotics in idiopathic preterm labour but some extrapolation can be made from the results of studies demonstrating no value in the use of antibiotics in preterm premature rupture of membranes. Antibiotic treatment is therefore probably of no benefit in idiopathic preterm labour and should be reserved for cases associated with vaginal chlamydia, group B streptococci or gonococci. The use of corticosteroids has been proven to reduce the incidence of neonatal respiratory syndrome.2g There is no evidence that corticosteroids increase the risk of either maternal or neonatal infection where intra-amniotic infection is not clinically apparent. Despite the theoretical risks of premature closure of the ductus arteriosis and neonatal pulmonary hypertension, antiprostaglandins are increasingly being used in the management of idiopathic preterm labour. Although they are more powerful inhibitors of uterine activity than intravenous p-mimetics there is currently insufficient evidence to recommend them as first choice treatment or for more than 1 or 2 days. 2g Similarly the value of intravenous p-mimetic tocolysis remains a matter for debate. Although they are clearly able to arrest premature contractions in the short-term and delay delivery, there is no evidence that of any improvement in perinatal mortality or morbidity. Many clinicians use intravenous Bmimetics solely to gain time, either to give corticosteroids or to effect transfer to a regional neonatal care centre. In cases where increasingly larger dose of intravenous p-mimetics are required to arrest preterm contractions the possibility of undiagnosed infection should be borne in mind. References

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labour. In: Elder MG, Hendricks CH, eds Preterm labour. London: Butterworths 1981 Bennett PR. The biochemical mechanisms of preterm labour. In: GVP Chamberlain, ed Contemporary Obstetrics and Gynaecology. London: Butterworth, 1988 Lamont RL, Taylor Robinson D, Newman M, Wigglesworth J, Elder MG. Spontaneous early preterm labour associated with abnormal genial bacterial colonisation. Br J Obstet Gynaecol 1986; 93: 804 Guziac DS, Winn K. The association of chorioamnionitis and preterm delivery. Obstet Gynecol 1985; 65: 11 Ryan GM, Abdella TM, McNeeley SG, Baselski VS,

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Drummond DE. Chlamydia trachomatis infection in pregnancy and effect of treatment on outcome. Am J Obstet Gynecol 1989; 162: 34 Gibbs RS, Duff P. Progress in pathogenesis and management of clinical Artal R, Sokal RJ, Stokjov J. The mechanical properties of prematurely ruptured membranes. Am J Obstet Gynecol 1976; 125: 655 Bobitt JR, Hayslip CC, Domato JD. Amniotic fluid infection as determined by amniocentesis in patients in preterm labour with intact membranes. Am J Obstet Gynecol 1981; 140: 947 Charles D, Edwards WR. Infectious complications of cervical cerclage. Am J Obstet Gynecol 1981; 141: 1065 Challis RG, Riley SC, Yang K. Endocrinology of labour. Fet Med Rev 1991; 3: 47 Bennett PR, Rose M, Myatt L, Elder MG. Preterm labor: stimulation of arachidonic acid metabolism in human amnion cells by bacterial products. Am J Obstet Gynecol 1987; 156: 649 Bennett PR, Elder MG, Myatt L. The effects of lipoxygenase metabolites of arachidonic acid upon contractility of human pregnant myometrium in vitro. Prostaglandins 1988; 33: 837 Walsh SW. 5-Hydroxyeicosatetraenioc acid, leukotriene C4 and prostaglandin F2a in amniotic fluid before and during term and preterm labor. Am J Obstet Gynecol 1989; 161: 1352 Hoffman DR, Romero R, Johnston JM. Detection of platelet activating factor in amniotic fluid of complicated pregnancies. Am J Obstet Gynecol 1990; 162: 525 Romero R, Parvizi ST, Oyarzun E et al. Amniotic fluid interleukin-1 in spontaneous labor at term. J Rep Med 1990; 35(3): 235 Rose MP, Elder MG, Myatt L. Arachodonic acid metabolism in the human placenta. Troph Res 1987; 2: 71-83 Bennett PR, Elder MG, Myatt L. Secretion of phospholipases by bacterial pathogens may initiate preterm labour. Am J Obstet Gynecol 1990; 163(l): 241 Bennett PR, Henderson DJ, Moore GE. Changes in the expression of cycle-oxygenase in human fetal membranes and placenta with labour. Am J Obstet Gynecol 1992 (in press) Gulbis E, Galand N, Dumont JE. Prostaglandin formation in bacteria; a reappraisal. Prostaglandins 1981; 21(3): 439 Bennett PR, Elder MG. Common genital tract pathogens do not synthesis prostaglandins or other eicosanoids. Am J Obstet Gynecol 1992 (in press) Bejar R, Curbello V, Davis C, Gluck L. Premature labor and bacterial sources of phospholipase. Obstet Gynecol 1981; 51: 473 Lamont RF, Rose M, Elder MG. Effects of bacteria1 products on prostaglandin formation in amnion cells. 1985; iancet 2: 1331 Lamont RF, Anthony F, Myatt L, Booth L, Furr PM, Tavlor Robinson D. Production of urostaalandin E2 bv human amnion in vitro in responsei addition of media conditioned by microorganisms associated with chorioamnionitis and preterm labour. Am J Obstet Gynecol 1990; 162: 819 Romero R, Roslansky P, Oyarzun E, et al. Bacterial endotoxin in amniotic fluid and its relationship to the onset of preterm labor. Am J Obstet Gynecol 1988; 158: 1044 Romero R, Brody DT. Oyarzun E, et al. Infection and labor; Interleukin I a signal for the onset of parturition. Am J Obstet Gynecol 1989; 160: 1117 Bennett PR, Elder MG. Preterm labour and infection: the interaction between leucocytes and amnion cells in the metabolism of arachidonic acid. Prostaglandins 1991; 43(l): 81 Lopes-Bernal A, Hansell DJ, Kong TY, Keeling JW, Turnbull AC. Prostaglandin E production by the fetal membranes in unexplained preterm labour and preterm labour associated with chorioamnionitis. Br J Obstet Gynecol 1988; 96(10): 1133 Gilstrap LC, Leveno KJ, Cox SM, Burris JS, Mashburn M. Rosenfeld CR. Intrapartum treatment of acute chorioamnionitis. Am J Obstet Gynecol 1988; 159: 759 Keirse MJNC, Grant A, King JF. Preterm labour. In: Enkin M, Keirse MJNC, Chalmers I, eds. Effective care in pregnancy and childbirth. Oxford: Oxford University Press