In utero beta 2 adrenergic agonist exposure and adverse neurophysiologic and behavioral outcomes

Basic science data that support receptor sensitization 

Terbutaline and similar drugs stimulate the B2AR, which is part of the catecholamine system of neurotransmitters. Cell signaling that is associated with B2AR stimulation results from the binding of the ligands norepinephrine in the central nervous system (CNS) and norepinephrine and epinephrine in peripheral tissues. The B2AR is expressed on mammalian oocytes and preimplantation embryos. 1 Beta adrenergic receptors are expressed widely in mammalian tissues, which includes the brain during gestation. 2 In addition to coupling with the stimulatory G protein to activate adenylyl cyclase, generating cyclic adenosine monophosphate and protein kinase A, and increasing intracellular calcium as second messengers, activation of the B2AR also stimulates or inhibits mitogen-activated protein kinases, which regulate basic cell processes such as growth, differentiation, apoptosis, and migration. 3, 4 Early in fetal life, B2AR stimulation coupled with cyclic adenosine monophosphate generation provides signals for growth and later promotes important mechanisms, such as axonal outgrowth in neural cells and differentiation in diverse tissues. 5, 6, 7

B2AR signaling is regulated during postnatal life by desensitization (decreased signaling) and down-regulation (decreased numbers of receptors on the cell surface). Both processes terminate cell signaling if excessive input occurs and are essential homeostatic mechanisms that protect the cell from overstimulation and metabolic acidosis. Animal studies have shown that these protective regulatory mechanisms for B2AR signaling are not intrinsic properties of cells but are acquired during prenatal development. Fetal and newborn tissues are not only resistant to B2AR desensitization but actually show the opposite: agonist stimulation of the fetal receptor enhances net physiologic responses, instead of producing desensitization, as in adult tissues. 8, 9 The beta 2 agonist terbutaline crosses the placenta and blood brain barrier and stimulates B2ARs in all tissues of the fetus. 2, 10, 11 Thus, exposures during pregnancy that increase B2AR signaling or overstimulate the receptor could have widespread effects in light of the function of these receptors during pre- and postnatal life. The severity of effects depends on the dose and duration of the exposure and, most importantly, the stage of development of specific brain regions and organs during the time of insult. 12, 13

Rodent studies have shown that daily subcutaneous injections of terbutaline during postnatal days 2-5 result in abnormalities in brain development and behavior, compared with control rats. Differences include changes in microarchitecture in the cerebellum, hippocampus, and cortex in juvenile rats (postnatal day 30), functional differences in cell signaling in juvenile and adolescent (postnatal day 45) and adult rats (postnatal day 60), behavioral changes in juvenile rats, and neuroinflammation of the brain in juvenile rats. These findings are similar to those found in autism. 8, 12, 13, 14, 15, 16 The amount of terbutaline that is used (10 mg/kg) leads to robust beta adrenergic receptor stimulation in the neonatal rat. 8 The dosage of terbutaline in human pregnancy is 0.5-2 mg/kg/d 17, 18; however, because the drug has a much shorter half-life in the rat, 19 the dose that was used by the researchers cited earlier was proportionately higher. 14 The 4-day duration for terbutaline administration (postnatal days 2-5) is equivalent to 3-4 weeks in human pregnancy. 20, 21 This early postnatal period in the rat correlates with the mid-to-late second and early third trimester in human gestation, 20, 21 which are periods during which pregnant women may be treated with a B2AR agonist for preterm labor. Previous research in rodents has also shown that the period of postnatal days 2-5 is a critical window in CNS and tissue development. Administration of terbutaline during the later period of postnatal days 11-14 leads to fewer and different abnormalities than those induced by the early treatment. 8

Data that support sympathetic overactivity in selected diseases 

Overactivity of the sympathetic nervous system has been implicated in the cause of certain disease states and contributes to abnormal function in others. Increased catecholamine levels are part of the disease process in congestive heart failure and cause chronic stimulation of beta adrenergic receptors, which results in tachycardia and increased contractility. 22, 23 This overstimulation leads to receptor desensitization, abnormal downstream cellular signaling, and maladaptation that eventually results in myocyte hypertrophy, ventricular chamber enlargement, and fibrosis. By decreasing beta adrenergic receptor stimulation, beta blockers have become part of the basic treatment for this condition.

Hypertension, like congestive heart failure, often is treated with beta adrenergic receptor antagonists, although overstimulation of beta receptors is not thought to be the basis for this condition. 24 The exact mechanism for beta blockers antihypertensive effects is unknown but is thought to be due to several modes of action, which include antagonism of beta 1 adrenergic receptors in the renal vascular bed. Stimulation of these receptors normally produces renin; blocking these receptors decreases renin levels and conversion of renin to angiotensin II, which is a potent vasoconstrictor. Other mechanisms may involve blocking beta adrenergic receptors that control sympathetic outflow in the CNS and changes in arterial baroceptor sensitivity.

Ming et al 25 measured baseline cardiovascular autonomic function in children with autism (ages, 4-14) and in age-matched healthy control subjects. They found that measures of parasympathetic activity, cardiac vagal tone, and cardiac sensitivity to baroreflex were significantly lower in children with autism, compared with control subjects, and were associated with significant elevations in indices of sympathetic tone: heart rate, mean arterial blood pressure, and diastolic blood pressure. Low levels of cardiac vagal tone and cardiac sensitivity to baroreflex suggest impaired cardiac parasympathetic activity, with unrestrained and hyperactivity of the sympathetic nervous system.

Rodent work has shown that overstimulation of the B2AR by terbutaline during postnatal days 2-5 results in effects that can be related indirectly to sympathetic hyperactivity, as in the study of Ming et al. 25 Acetylcholine receptors that are found in the cardiovascular system that normally act to balance the catecholamine system are decreased in number and function in the heart after neonatal terbutaline administration. 8, 26 This would lead to a loss of parasympathetic balance and overactivity in sympathetic responses.

Human data that support poor neurophysiologic and behavioral outcomes after beta 2 adrenergic agonist exposure in utero 

The association of poor neurophysiologic and behavioral outcomes in children who are exposed in utero is seen with >1 agent of this class. Terbutaline has been associated with delayed development of expressive language. 27 Continuous terbutaline treatment for >2 weeks in the treatment of preterm labor was associated with an increased concordance of autism spectrum disorders in dizygotic twins. 28 In utero exposure to terbutaline by intravenous use for tocolysis or aerosol for asthma has also been correlated with autism. 29 A reported association of maternal asthma and autism spectrum disorders in the offspring might be explained by treatment with beta 2 adrenergic agonists for treatment of asthma during pregnancy. 30 A recent case control study that involved 398 children with an autism spectrum disorder diagnosis and 110 normal control children who were matched for sex, birth year, and birth hospital found that beta 2 adrenergic agonist exposure in the first and second trimester may be associated with a modest increase in the risk of having a child with an autism spectrum disorder (L.A. Croen, personal communication, 2009). In this study, albuterol was the most frequently used beta 2 adrenergic agonist in the first and second trimester, which indicates that asthma treatment may have been the reason for beta 2 adrenergic agonist use.

Increased psychiatric disorders together with poorer cognitive and motor performance have been seen in the largest long-term prospective study of infants who are born after in utero exposure to beta 2 adrenergic agonists. 31 In this study, the agent was intravenous fenoterol, followed by unspecified oral beta 2 adrenergic agonists. Only those children who were exposed to a longer duration of therapy were affected significantly.

Ritodrine exposure for tocolysis has also been associated with poorer school performance, compared with matched control subjects. 32 It is remarkable that this finding in school performance occurred despite the lack of difference between the groups in developmental delay, neurologic abnormalities, or behavioral differences.

Alvarez et al 33 found a hypertensive effect in adolescents (ages, 13-16 years) who had been exposed in utero to the beta sympathomimetic ritodrine and who were full-term births, compared with control subjects without the exposure. Heart rates and blood pressures were measured for 48 hours with the use of an ambulatory monitor. Teenagers who had been exposed to ritodrine in utero had higher heart rates, wider ranges in systolic blood pressure, and higher diastolic blood pressure than did control subjects.

Not all follow-up studies of patients who were exposed to beta 2 adrenergic agonists have shown these adverse outcomes. Other studies with isoxuprine 34 and ritodrine 35, 36 have shown no adverse effects when compared with control subjects. One case series that included no control subjects also showed no adverse long-term sequellae at 2 years of age after in utero exposure to terbutaline. 37 The ritodrine studies began with approximately 2 days of parenteral therapy followed by oral treatment. 35, 36 Given ritodrine's relatively short half-life and the listed oral dosing, it is likely that continuous exposure at a sufficient level to cause functional teratogenesis was not present in the studies. Likewise, the isoxsuprine study involved only short-term exposure, and it is likely that insignificant exposure occurred and may have avoided functional teratogenisis. 34 The terbutaline study lacked control subjects and involved the initiation of therapy between 27 and 36 weeks of gestation. 37 Thirty-one percent of the patients in this study started terbutaline therapy between 35 and 36 weeks of gestation. Animal studies would suggest that the mid-to-late third trimester may be too late to see a major effect from beta 2 adrenergic agonists. Thus, a sizable proportion of the exposed subjects in this study might not have been at the gestational age of maximal risk.

Genetic predisposition 

Single nucleotide substitutions or polymorphisms of the B2AR gene have been described. Three of these (Gly16, Glu27, and Ile164) code for changes in the receptor's amino acid sequence that lead to physiologic differences in receptor signaling. Stimulation of the Gly16 and Glu27 receptors in vivo results in decreased desensitization and down-regulation and is associated with enhanced signaling, 38, 39 compared with the wild-type variants Arg16 and Gln27. The Ile164 polymorphism results in reduced affinity for ligand binding and lower levels of second messenger formation. 40

Polymorphisms of the B2AR gene have been associated with susceptibility to and prognosis in diverse diseases and conditions that include medication response in asthma, 41 occurrence of type 2 diabetes mellitus, 42 outcome in congestive heart failure, 43, 44 Grave's disease, 45 myasthenia gravis, 46 rheumatoid arthritis, 47 obesity, 48 and psychologic coping. 49 Connors et al 28 found an increase in the more active polymorphisms of the B2AR in twins with autism spectrum disorders, and Cheslack-Postava et al 50 found an increased prevalence and transmission of these polymorphisms in singleton autism births. Thus, genetic polymorphisms can change the physiologic condition of receptor function and contribute to predispositions for several disease states. Considering the importance of the B2AR in normal brain and organ development, it is likely that polymorphisms that increase or decrease signaling are genetic risk factors for abnormal brain development during gestation, in a similar way as they are linked to disease in other organs. The presence of polymorphisms that are associated with enhanced signaling would then become a susceptibility factor for exposures to sympathomimetic drugs and may be a factor in defining the “population at risk” for later neurodevelopmental disorders in children after exposure to beta 2 adrenergic agonists during gestation.

Implications for clinical practice 

Bronchodilator therapy with beta 2 adrenergic agonists remains a mainstay of asthma treatment. Data from 1995-1999 linked the diagnosis of maternal asthma to autism spectrum disorders. 30 In 2005, the updated National Asthma Education Report of the working group on asthma and pregnancy removed oral beta 2 adrenergic agonists from the recommended pharmacologic therapy for asthma during pregnancy. 51 Inhaled beta 2 adrenergic agonists remain a part of the medically indicated therapy for this potentially life-threatening condition. Injected beta 2 adrenergic agonists are not recommended because there is no proven benefit of systemic therapy over inhalation therapy. 51

Albuterol is the preferred short-acting inhaled beta 2 adrenergic agonist because of the extensive data on the safety of this agent during pregnancy. 51 It should be used for quick relief of symptoms and to treat mild intermittent asthma. Care should be taken to avoid excessive treatment in light of the association between albuterol exposure in the first and second trimesters and a modest increase in autism spectrum disorders in the child (L.A. Croen, personal communication, 2009).

In patients with mild intermittent asthma with symptoms on >2 days per week and <2 nights per month, the working group recommends low-dose inhaled corticosteroid therapy as the preferred treatment during pregnancy. 51 Because of more data being available on its use in pregnancy, budesonide was the recommended inhaled steroid. Low-dose budesonide inhalation is recommended in the range of 200-600 μg daily.

If the patient's symptoms occur daily or >1 night a week, her asthma has progressed to moderate persistent asthma. For this severity, the working group has 2 alternate preferred treatments. 51 To avoid increased beta 2 adrenergic agonist exposure, the treatment with medium-dose inhaled corticosteroids should be selected because the 2 regimens are equally preferred. This would correspond to 600-1200 μg of budesonide. With the use of moderate-dose inhaled corticosteroid, an additional benefit of avoiding increased polypharmacy would be achieved.

If the pregnant asthmatic woman has continued daily symptoms and frequent night symptoms, she has severe persistent asthma. For these patients, the working group recommends high-dose inhaled corticosteroids and a long-acting inhaled beta 2 adrenergic agonist. 51 This would correspond to an inhaled dose of budesonide of >1200 μg daily. There are 2 long-acting beta 2 adrenergic agonists available: salmeterol and formoterol. The working group recommends salmeterol as the preferred long-acting beta 2 adrenergic agonist for pregnancy because there was greater experience in pregnancy with this agent. 51 Long-acting beta 2 adrenergic agonist use can lead to tachyphylaxis for the B2ARs and is associated with an increased risk of serious adverse events and of asthma-related deaths. 52, 53, 54 Whether this increase in asthma-related death is attenuated by the concomitant use of inhaled steroid cannot be determined from the available data. 54

An alternate, but not preferred, treatment for severe persistent asthma is high-dose inhaled corticosteroid therapy plus sustained release theophylline to a serum concentration of 5-12 μg/mL. 51 Salmeterol has been shown to be more effective in this group of asthmatic patients when compared with sustained release theophylline as maintenance therapy. 55 Although salmeterol plasma levels after inhalation are low to nondetectable, 56 to avoid the risk of salmeterol, a trial of theophylline may be worthwhile in pregnant asthmatic women.

Beta 2 adrenergic agonist use as a tocolytic 

Subcutaneous maintenance therapy 

Based on case series, terbutaline that is given by continuous subcutaneous pump has gained popularity as a maintenance therapy after threatened preterm labor to prevent preterm birth. However, because of the 20-50% placebo effect that is seen in threatening preterm labor, 60 these reports are insufficient to assess this therapy, and controlled trials are required. There were 2 placebo controlled trials of this therapy, both of which failed to show any benefit. 67, 68 Both ACOG and Cochrane reviews of subcutaneous terbutaline pump maintenance tocolytic therapy agree that it is ineffective. 64, 69 The Agency for Healthcare Research and Quality assessed beta 2 adrenergic agonists as “high” in probability of maternal risk, ineffective when used for maintenance tocolysis, and advised against any further research on maintenance tocolytic use. 70

In a large case series, 71 cardiopulmonary problems were seen in 0.54% of the patients, and pulmonary edema was seen in 0.32% of the patients. There are additional reported cases of pulmonary edema, 72 terbutaline hepatitis, 73 and sudden death. 74 The incidence of abnormal glucose tolerance has been reported in 1 case series, 75 but not in another. 76 However, in the study that did not report an increase in glucose intolerance, the need for insulin therapy was increased in those patients receiving terbutaline. 76

Adverse events with subcutaneous terbutaline pump therapy are not limited to the mother. Neonatal myocardial necrosis also has been reported with long-term subcutaneous terbutaline pump therapy. 77 Continuous exposure to terbutaline for tocolysis by any route for at least 2 weeks has been associated with autism spectrum disorders in the child. 28

Because prolonged subcutaneous beta 2 adrenergic agonist treatment is not effective 66 and has significant maternal and fetal risks, its use should be abandoned. There is no setting in which the benefit has been shown to justify the risk. Unfortunately, it continues to be prescribed by 2% of maternal-fetal medicine specialists. 78 The actual rate of prescribing may be higher because 31% of maternal-fetal medicine specialists who would not recommend maintenance tocolysis will prescribe maintenance tocolysis on patient request, but data are not available about how often this occurs and what maintenance tocolytics are prescribed. 78

Oral maintenance therapy 

Although an early small placebo controlled trial favored oral terbutaline over placebo, 79 a larger trial later showed a lack of efficacy. 80 The Cochrane Collaborative Reviews recently reviewed 11 randomized controlled trials of oral beta 2 adrenergic agonists and concluded that the evidence did not support their use for maintenance therapy after treatment of threatened preterm labor. 81 An ACOG 66 practice bulletin also assesses prolonged oral treatment as not effective. Studies cited earlier show that adverse functional abnormalities in rats were at doses that were designed to mimic oral or subcutaneous administration of terbutaline. 8, 17, 18 Because there is no potential benefit from this therapy and because of potential risk based on animal data, the use of oral maintenance therapy for threatened preterm labor should be abandoned. Unfortunately, it continues to be prescribed by 4% of maternal-fetal medicine specialists. 78 As noted earlier for subcutaneous therapy, the actual rate of oral tocolysis may be higher than reported; however, data are not available for the frequency, types, or duration of exposure. 78

Acute treatment for fetal distress 

Beta 2 adrenergic agonists, primarily terbutaline, have been advocated for use in intrauterine resuscitation for nonreassuring fetal heart rate patterns and for episodes of uterine tachysystole. Intravenous ritodrine that is given continuously during the second stage of labor abolished the progressive fetal respiratory acidosis that is seen during normal labor in 1 double-blind controlled trial. 82 Single bolus injections of 250 μg of terbutaline were reported in case series to improve fetal pH in cases of nonreassuring fetal status that included bradycardia that lasted for 2 minutes; fetal pH was unresponsive to other methods. 83, 84 Additionally, this single injection therapy has been shown to be effective in temporary inhibition of uterine activity at term. 85 A controlled trial confirmed these findings. 86 Based on these data, the use of intravenous boluses of 250 μg of terbutaline has been established as the standard temporizing treatment for a nonreassuring status in utero, especially when associated with uterine tachysystoly. 87

Terbutaline crosses the placenta after a single intravenous bolus of 250 μg and reaches a maximum fetal umbilical plasma concentration of one-half of the maximum maternal plasma concentration and ranges from 0.75-3.46 μg/L. 10 A single dose of this magnitude is unlikely to result in injury to the fetal nervous system because of the short duration of exposure. Therefore, the balance of risk and benefit for this form of beta 2 adrenergic agonist therapy is in favor of its continued use in clinical practice.

Conclusion 

The permanent shift in the balance of sympathetic-to-parasympathetic tone, as a result of B2AR overstimulation during critical periods of prenatal development, is a biologically plausible mechanism whereby beta 2 adrenergic agonists can induce functional and behavioral teratogenesis. Although the exact dose and duration of exposure to achieve these results is not yet established, currently available data concerning increased risk for autism in the offspring suggest that the duration is likely to be ≥2 weeks of continuous high-dose exposure. The period of maximum teratogenic risk is likely related to the time of maximal brain development from the mid-to-late second trimester through at least the early third trimester. In addition to autism spectrum disorders, conditions that may be related to this teratogenesis include increased psychiatric disorders, poor cognitive and motor function and school performance, and changes in blood pressure.

Given the risk of long-term neurophysiologic and behavioral impairment, the use of beta 2 adrenergic agonists should be limited to proven indications when alternate drugs are ineffective or unavailable and when the risks of the untreated disease to the mother and fetus are greater than the risk of the beta 2 adrenergic agonist. Treatment duration should be as short as clinically feasible. Further ongoing surveillance of the use of these agents in pregnancy is needed to refine the parameters for their safe use in pregnancy. Future pharmacogenetics research is also needed to better characterize the highest risk group for teratogenesis from these agents.