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Abstract
The first oral overdose of paracetamol in a neonate is reported. A 55 day old neonate, born 29 weeks premature, was accidentally given 136 mg/kg paracetamol. Treatment was with activated charcoal, supportive care, and N-acetylcysteine. There was no biochemical evidence of hepatotoxicity, and no long term sequelae. After modelling of the data, the following pharmacokinetic variables were calculated: absorption half life (tabs), 0.51 hours; volume of distribution (V/Foral), 0.80 litres/kg; clearance (CL/Foral), 0.22 litres/h; they were consistent with population pharmacokinetic studies. The increased plasma half life (Tβ) of 5.69 hours thus reflected normal slower metabolism in infants, rather than toxicity. The toxicity of paracetamol in neonates is unclear, but appears to be low because of slow oxidative metabolism and rapid glutathione synthesis. In an overdose, estimates of toxicity can be made from dose and Tβ in neonates, or from maternal toxicity in transplacental poisoning. Treatment includes N-acetylcysteine and supportive care, with activated charcoal for oral poisoning.
- paracetamol
- overdose
- preterm
- toxicity
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Paracetamol is a readily available antipyretic and analgesic agent with few side effects. It is the preferred simple analgesic for children and is increasingly being used in neonates.1Although paracetamol overdose is common, neonatal paracetamol overdose is rare. It has been reported as the result of transplacental transfer after maternal overdose2-6 but never from oral ingestion. Management of neonatal paracetamol poisoning is currently based on limited pharmacokinetic studies in neonates and adult treatment protocols. The toxicity of paracetamol in neonates is unclear.
We report the first case of acute oral paracetamol poisoning in a neonate. The toxicokinetics (pharmacokinetics of an overdose) of paracetamol poisoning in the neonate and an approach to treatment are discussed.
Methods
Plasma paracetamol concentration was estimated using a colorimetric test (Vitros ACET slides; ORTHO Clinical Diagnostics, Victoria, Australia). The lower limit of the reportable range was 10 mg/l. Kinetic variables were derived from plasma concentration versus time data using a first order input (no lag time), first order elimination, one compartment disposition model. The model variables were absorption half life (tabs, hours), the apparent volume of distribution (V/Foral, litres), and total body clearance (CL/Foral, litres/h). Plasma half life (Tβ) was calculated from these variables. Data were analysed using MKMODEL (Biosoft, Cambridge, UK).7 In two previous case reports, Tβ was calculated from the plasma concentrations and times supplied.6 ,8
Case report
A 2.2 kg, 55 day old male infant (corrected gestational age 37 weeks) was prescribed 30 mg (13.6 mg/kg) paracetamol before his first triple antigen immunisation. In error, a dose of 300 mg was given by nasogastric tube. The error was realised after four hours when the next dose was due. He was otherwise well at the time of the overdose.
The infant had been delivered at 29 weeks gestation weighing 740 g. He was symmetrically small for gestational age but had few problems of prematurity. He had mild hyaline membrane disease, transient asymptomatic neutropenia, phototherapy for jaundice (maximum bilirubin 119 mmol/l) for two days, and apnoea. At the time of the overdose, he was being fed with calorie supplemented breast milk and receiving multivitamins and iron.
As soon as the overdose had been noticed, the stomach contents were aspirated; no paracetamol was returned. The paracetamol level was 121 mg/l four hours after ingestion, below the “150 treatment line” on the adult nomogram. Nasogastric charcoal was administered, and intravenous fluids (10% dextrose) were begun.
The level at eight hours was 84 mg/l, not decreasing with a Tβ parallel to the treatment line (see fig 1).9 ,10 The baby was given intravenous N-acetylcysteine: 150 mg/kg over 15 minutes; 50 mg/kg over four hours; 100 mg/kg over 16 hours. Charcoal was stopped after 24 hours. N-Acetylcysteine was continued for 20 hours and was tolerated without any adverse effects.
Plot of logarithmic plasma paracetamol concentration against time. The solid line represents the curve modelled to the data points from this case. Black squares represent real data points. The dashed line is the original treatment line based on adult patient data,10 and the dotted line is the Rumack-Matthews nomogram, 25% below the other treatment line.9
Figure 1 shows a plot of paracetamol concentration against time. There was no increase in transaminases or bilirubin. Initially, the international normalised ratio was slightly elevated at 1.3, but decreased to 1.0 after 48 hours. The patient was discharged from hospital six days later. He was seen in outpatients and had no further problems.
Results and discussion
Acute paracetamol overdose is relatively innocuous in young children8 ,9; however, the sensitivity of neonates, both term and preterm, to paracetamol overdose is not clear. There are few toxicokinetic data,2 ,3 so pharmacokinetic studies need to be extrapolated to estimate kinetic variables during an overdose. Good pharmacokinetic data now exist for paracetamol in neonates and children, and show that, at birth, clearance is 62% and volume of distribution is 174% of those of older children.11 This means that Tβ is normally prolonged in neonates and slowly decreases to the adult range with increasing age. Only one study has determined Tβ in preterm infants,12 and it shows that 28–32 week neonates have a longer Tβ (mean (SD) 11 (5.7) hours) than 32–36 week neonates (4.8 (1.2) hours). The nature of this overdose, a prescribing error, means that the precise timing and quantity of the drug were known. Pharmacokinetic variables were calculated: tabs was 0.51 hours, V/Foral was 0.80 litres/kg, and CL/Foral was 0.22 litres/hour after the data had been modelled (fig 1). These are consistent with recent population pharmacokinetic studies of therapeutic paracetamol dosing.11 The calculated increased Tβ of 5.69 hours thus reflects the normal slower oxidative metabolism rather than toxicity.
Reported cases of neonatal paracetamol poisoning have all resulted from transplacentally acquired paracetamol.2-6 In transplacental overdose, the fetus has the added insult of maternal toxicity. If the baby is delivered within 24 hours of maternal overdose, it is less likely to be affected by maternal toxicity, and the case resembles that of an oral overdose. All reported neonatal/fetal deaths occurred when the baby was delivered more than 24 hours after the overdose; death was in utero or a spontaneous abortion occurred.4 ,5 The only series of paracetamol overdoses in pregnancy reported a 17.7% incidence of fetal death, which was not significantly different from national statistics of spontaneous abortions.4 Four cases of infants delivered within 24 hours of a maternal overdose have been reported,2 ,3 ,6 ,8and they are summarised in table 1. In the first, the potential for toxicity was greatest with maternal hepatotoxicity and a prolonged Tβ. In two cases, the authors suggested that the moderately elevated prothrombin time immediately after birth indicated hepatotoxicity.2 ,6 However, about half of patients suffering from a paracetamol overdose without hepatotoxicity have an abnormal international normalised ratio. This increase appears to be due to inhibition of activation of vitamin K dependent coagulation factors (mainly factor VII), and is independent of hepatotoxicity.13 The isolated increase in prothrombin time in these cases does not represent hepatotoxicity and is either due to prematurity or is the direct effect of paracetamol on coagulation factors. The good outcome in neonates born within 24 hours of maternal overdose, despite significant maternal toxicity in one case,2 suggests that acute paracetamol poisoning is relatively harmless in neonates. A combination of the slower production of toxic metabolite in fetal livers and an increased rate of glutathione synthesis may explain this reduced toxicity.1 ,3
Summary of four cases of transplacental neonatal paracetamol poisoning that all occurred within 24 hours of maternal overdose, and this case of oral neonatal paracetamol poisoning
Predicting potential toxicity in neonates is difficult. In oral paracetamol poisoning, the dose can be estimated. The safe oral dose in neonates is not known, but is unlikely to be less than 150 mg/kg and may be as high as 200 mg/kg.9 In transplacental overdoses, an estimate of toxicity is better made from maternal dose and severity of maternal hepatotoxicity. An increased Tβ of paracetamol in adult overdoses is a good predictor of hepatotoxicity and outcome,10 but this has not been validated in children or neonates.1 ,3 In five cases of neonatal paracetamol poisoning for which Tβ was calculated (including ours),2 ,3 ,6 ,8 it was longer than four hours. This is probably a reflection of a decreased V/Foral and increased CL/Foral at birth.11 In neonatal paracetamol poisoning, the plasma Tβ should be determined and compared with the expected normal values in neonates of similar age.11 The use of adult nomograms in neonatal poisoning has problems,10 and, being based on a Tβ of four hours, should only be used as a guide before Tβ determination.
Treatment of neonatal paracetamol overdose to date has been based on adult studies.10 Early treatment of the mother is essential with the more common transplacental overdose. In a series of 60 paracetamol overdoses in pregnancy, only gestational age and time to N-acetylcysteine treatment were shown to be significant predictors of fetal death.4 The benefit of maternal N-acetylcysteine treatment to the fetus is twofold: prevention of maternal toxicity prevents adverse effects on the fetus; N-acetylcysteine crosses the placenta5 and detoxifies the toxic metabolite N-acetyl-p-benzoquinone imine formed by the fetal liver. Activated charcoal has been used in neonates and appears to be safe for treatment of oral overdose.14
Oral absorption of paracetamol in term neonates is slow11and may continue for up to three hours. Activated charcoal is potentially beneficial within this time period. Treatment with N-acetylcysteine is based on adult studies,3 but is safe in pregnancy4 ,5 and neonates.6 We suggest a low threshold for N-acetylcysteine administration; table 2gives suggested indications in neonatal poisoning. These are probably excessive given the apparent safety of paracetamol in neonates, but appropriate until further data are available.
Suggested indications for N-acetylcysteine treatment in neonatal paracetamol poisoning
Acknowledgments
Thanks go to Dr Peter Loughnan for helpful comments on the manuscript.