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`Cysteine, cystine or N-acetylcysteine supplementation in
`parenterally fed neonates (Review)
`
`Soghier LM, Brion LP
`
`SoghierLM, BrionLP.
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates.
`Cochrane Database of Systematic Reviews 2006, Issue 4. Art. No.: CD004869.
`DOI: 10.1002/14651858.CD004869.pub2.
`
`www.cochranelibrary.com
`
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
`
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`T A B L E O F C O N T E N T S
`HEADER.........................................................................................................................................................................................................
`ABSTRACT.....................................................................................................................................................................................................
`PLAIN LANGUAGE SUMMARY.......................................................................................................................................................................
`BACKGROUND..............................................................................................................................................................................................
`OBJECTIVES..................................................................................................................................................................................................
`METHODS.....................................................................................................................................................................................................
`RESULTS........................................................................................................................................................................................................
`DISCUSSION..................................................................................................................................................................................................
`AUTHORS' CONCLUSIONS...........................................................................................................................................................................
`ACKNOWLEDGEMENTS................................................................................................................................................................................
`REFERENCES................................................................................................................................................................................................
`CHARACTERISTICS OF STUDIES..................................................................................................................................................................
`DATA AND ANALYSES....................................................................................................................................................................................
`Analysis 1.1. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 1 Weight gain during the study (g/
`kg/day)...................................................................................................................................................................................................
`Analysis 1.2. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 2 Increase in length (cm/ 6 days)........
`Analysis 1.3. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 3 Increase in head circumference (cm/
`6 days)....................................................................................................................................................................................................
`Analysis 1.4. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 4 Nitrogen retention (mg/kg/day).......
`Analysis 1.5. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 5 Plasma level of free cyst(e)ine
`(micromoles/100 ml).............................................................................................................................................................................
`Analysis 1.6. Comparison 1 Cyst(e)ine vs. placebo in added to cysteine-free PN, Outcome 6 Total plasma cyst(e)ine level
`(micromoles/ 100 ml)............................................................................................................................................................................
`Analysis 2.1. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 1 Weight gain during the
`study (g/kg/day)....................................................................................................................................................................................
`Analysis 2.2. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 2 Weight (g) at 36 weeks
`of postmenstrual age............................................................................................................................................................................
`Analysis 2.3. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 3 Death by 36 weeks of
`postmenstrual age................................................................................................................................................................................
`Analysis 2.4. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 4 Bronchopulmonary
`dysplasia among survivors at 36 weeks of postmenstrual age.........................................................................................................
`Analysis 2.5. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 5 Death or
`Bronchopulmonary Dysplasia (36 weeks postmenstrual age)...........................................................................................................
`Analysis 2.6. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 6 Retinopathy of
`prematurity (any stage) among those examined................................................................................................................................
`Analysis 2.7. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 7 Severe (stage 3 or more)
`retinopathy of prematurity among those examined..........................................................................................................................
`Analysis 2.8. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 8 Necrotizing enterocolitis
`(operated)..............................................................................................................................................................................................
`Analysis 2.9. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 9 Periventricular
`leukomalacia in all examined..............................................................................................................................................................
`Analysis 2.10. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 10 Intraventricular
`hemorrhage (any grade).......................................................................................................................................................................
`Analysis 2.11. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 11 Severe intraventricular
`hemorrhage (grade 3-4)........................................................................................................................................................................
`Analysis 2.12. Comparison 2 N-acetylcysteine vs. placebo added to cysteine-containing PN, Outcome 12 Plasma level of free
`cyst(e)ine (micromoles/ 100 ml)..........................................................................................................................................................
`FEEDBACK.....................................................................................................................................................................................................
`WHAT'S NEW.................................................................................................................................................................................................
`HISTORY........................................................................................................................................................................................................
`CONTRIBUTIONS OF AUTHORS...................................................................................................................................................................
`DECLARATIONS OF INTEREST.....................................................................................................................................................................
`SOURCES OF SUPPORT...............................................................................................................................................................................
`INDEX TERMS...............................................................................................................................................................................................
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
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`[Intervention Review]
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally
`fed neonates
`
`Lamia M Soghier1, Luc P Brion2
`
`1Pediatrics, Albert Einstein College of Medicine, Children's Hospital at Montefiore, Bronx, New York, USA. 2Division of Neonatal-Perinatal
`Medicine, University of Texas Southwestern at Dallas, Dallas, Texas, USA
`
`Contact address: Luc P Brion, Division of Neonatal-Perinatal Medicine, University of Texas Southwestern at Dallas, 5323 Harry Hines
`Boulevard, Dallas, Texas, 75390-9063, USA. Luc.Brion@UTSouthwestern.edu.
`
`Editorial group: Cochrane Neonatal Group
`Publication status and date: New search for studies and content updated (no change to conclusions), published in Issue 1, 2010.
`
`Citation: SoghierLM, BrionLP. Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates. Cochrane Database
`of Systematic Reviews 2006, Issue 4. Art. No.: CD004869. DOI: 10.1002/14651858.CD004869.pub2.
`
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
`
`A B S T R A C T
`
`Background
`Cysteine is a precursor of glutathione, an antioxidant that may reduce oxidation injury. The addition of cysteine to parenteral nutrition (PN)
`allows for the reduction of the amount of methionine in PN, thereby limiting hepatotoxicity and acidifies the solution, thereby increasing
`calcium and phosphate solubility and potentially improving bone mineralization.
`
`Objectives
`To determine the eIects of supplementing PN with cysteine, cystine or its precursor N-acetylcysteine on neonatal growth and short and
`long-term outcomes.
`
`Search methods
`The standard search method of the Cochrane Neonatal Review Group was used. MEDLINE, EMBASE, the Cochrane Central Register of
`Controlled Trials (The Cochrane Library) and recent abstracts from the Society for Pediatric Research/ American Pediatric Society, Eastern
`Society for Pediatric Research, and Society for Parenteral and Enteral Nutrition were originally searched in 2005. In August 2009 updated
`searches were done of The Cochrane Library, MEDLINE (search via PubMed), CINAHL and EMBASE from 2006 to 2009.
`
`Selection criteria
`All randomized (RCTs) and quasi-randomized trials that examined the eIects of cysteine, cystine or N-acetylcysteine supplementation of
`neonatal PN were reviewed.
`
`Data collection and analysis
`The standard methods of the Cochrane Collaboration and its Neonatal Review Group were used. Statistical analysis included relative risk,
`risk diIerence, and weighted mean diIerence (WMD).
`
`Main results
`Six trials fulfilled entry criteria. The majority of patients in these trials were preterm. Five small trials evaluated short-term cysteine
`supplementation of cysteine-free PN. One large multicenter RCT evaluated short-term N-acetylcysteine supplementation of cysteine-
`containing PN in extremely low birth weight infants (≤ 1000 grams).
`
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
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`Growth was not significantly aIected by cysteine supplementation (1 trial) or by N-acetylcysteine supplementation (1 trial). Nitrogen
`retention was significantly increased by cysteine supplementation (4 trials) (WMD 31.8 mg/kg/day, 95% confidence interval +8.2, +55.4, n
`= 95, including 73 preterm infants).
`
`Plasma levels of cysteine were significantly increased by cysteine supplementation but not by N-acetylcysteine supplementation.
`N-acetylcysteine supplementation did not significantly aIect the risks of death by 36 postmenstrual weeks, bronchopulmonary
`dysplasia (BPD), death or BPD, retinopathy of prematurity (ROP), severe ROP, necrotizing enterocolitis requiring surgery, periventricular
`leukomalacia, intraventricular hemorrhage (IVH), or severe IVH.
`
`Authors' conclusions
`Available evidence from RCTs shows that routine short-term cysteine chloride supplementation of cysteine-free PN in preterm infants
`improves nitrogen balance. However, there is insuIicient evidence to assess the risks of cysteine supplementation, especially regarding
`metabolic acidosis, which has been reported during the first two weeks of cysteine chloride administration. Available evidence from a large
`RCT trial does not support routine N-acetylcysteine supplementation of cysteine-containing PN in extremely low birth weight infants.
`
`P L A I N L A N G U A G E S U M M A R Y
`
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates
`
`Sick or preterm newborn infants may require intravenous nutrition, including intravenous administration of solutions containing amino
`acids. Newborn infants need cysteine (an amino acid) for growth under certain conditions. Cysteine may decrease the chance of liver
`disease and brittle bones. This systemic review was done to analyze whether adding cysteine (or related compounds) to intravenous
`nutrition aIects growth and other outcomes in newborn infants. Five trials studied the eIects of adding cysteine to intravenous nutrition
`that did not contain cysteine. Addition of cysteine significantly improved the babies' ability to build body proteins (analyzed in four studies);
`however, it did not improve growth (analyzed in one study); no other outcomes were available. One large randomized trial studied the eIect
`of adding another chemical, N-acetyl-cysteine, to intravenous nutrition that already contained cysteine. This study showed no benefit and
`no toxicity of this intervention. We conclude that present data are insuIicient to justify routine addition of cysteine to the intravenous
`nutrition of newborn infants that does not contain cysteine. Available evidence does not support routine addition of N-acetylcysteine to
`intravenous nutrition of newborn infants containing cysteine.
`
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
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`B A C K G R O U N D
`
`Description of the condition
`The inability of sick term and preterm infants to feed enterally
`frequently leads to the use of total parenteral nutrition (TPN)
`in order to provide adequate calories and nutrients to promote
`growth. Most commercial parenteral solutions contain a mixture of
`both essential and non-essential amino acids. Low plasma levels of
`any essential amino acid indicate a relative deficiency state and this
`may be detrimental to nitrogen balance and growth (Rose 1955).
`
`Description of the intervention
`Several facts suggest that the addition of L-cysteine, cystine or N-
`acetylcysteine to TPN may be beneficial in neonates, especially very
`low birth weight (VLBW) infants (infants with birth weight < 1500 g) .
`
`Several publications have suggested that L-cysteine could be a
`conditionally essential (i.e., essential under certain conditions)
`amino acid for very preterm but not for borderline preterm or
`full-term neonates (Uauy 1993; Brunton 2000; Heird 1998; Riedijk
`2006). In contrast, one randomized trial comparing various intakes
`of cyst(e)ine in enterally fed very low birth weight infants did not
`support the hypothesis that cyst(e)ine is a conditionally essential
`amino acid in these infants. (Riedijk 2007; Riedijk 2008).
`
`Cysteine is incorporated into proteins and is a precursor of taurine
`and glutathione. In adults, cysteine can be synthesized from
`ingested methionine via the trans-sulfuration pathway and hence is
`considered one of the non-essential amino acids (Rose 1955). In the
`fetus, hepatic cystathionase, the rate-limiting enzyme that converts
`cystathionine to cysteine, is either absent or barely detectable,
`resulting in decreased synthesis of cysteine from methionine and
`high serum concentration of cystathionine (Sturman 1970; Pascal
`1972; Gaull 1972; Heinonen 1974). Preterm infants have higher
`plasma cystathionine levels than term infants (Viña 1995). Hepatic
`cystathionase activity increases with gestational age (Sturman
`1970; Heinonen 1974; Zlotkin 1982) and increases rapidly aSer birth
`(Zlotkin 1982) reaching mature levels at about three months in
`term infants. In vivo studies have shown that cysteine synthesized
`in preterm liver may be present in apolipoprotein B but not in
`plasma, suggesting that endogenous cysteine is used preferentially
`for hepatic protein synthesis (Miller 1996). In contrast, a recent
`study showed that cysteine synthesized in the liver was found in
`plasma of VLBW infants in amounts that increased with maturation
`(Shew 2005). Extrahepatic cystathionase, which matures earlier
`than the hepatic enzyme, might account for some conversion of
`methionine into cysteine even in preterm infants (Uauy 1993).
`
`How the intervention might work
`Parenterally fed infants or adults who do not receive cysteine
`supplements in TPN have low plasma levels of cystine (Zlotkin
`1981; Uauy 1993), the oxidized, disulfide form of cysteine.
`Regression analysis suggests that an intravenous intake of cysteine
`of 500 micromoles per kg per day is required in preterm infants
`to reach plasma cysteine levels comparable to those in full-
`term, breast-fed infants (Van Goudoever 1994). Snyderman found
`that enteral supplementation of cysteine improved growth in
`preterm infants at two to four months of age (Snyderman
`1971). These data support the possibility that cysteine may
`be a conditionally essential amino acid in preterm infants;
`supplementation of cysteine could improve growth and nitrogen
`
`retention under certain conditions. Liver toxicity may be induced
`by high methionine content of commercial parenteral nutritional
`solutions that do not contain cysteine (Moss 1999; Brunton 2000).
`Because of this, some manufacturers have added cysteine to
`commercial TPN and reduced methionine content accordingly.
`Supplementation of an essential amino acid (in this case cysteine)
`may not result in any improvement in growth if caloric intake or
`intake of other essential or conditionally essential nutrients (e.g.,
`tyrosine) is limiting (Helms 1987; Heird 1993).
`
`Studies have suggested that preterm infants have a poorly
`developed antioxidant system, making various organs more
`susceptible to oxidation
`injury, thereby
`increasing the risk
`for
`intracranial hemorrhage, periventricular
`leukomalacia,
`retinopathy of prematurity, chronic lung disease, and necrotizing
`enterocolitis (Thibeault 2000). Cysteine deficiency may be an
`etiological factor in the limited ability of preterm babies to produce
`glutathione, a natural antioxidant. The first step of synthesis of
`glutathione involves gammma-glutamylcysteine synthetase, which
`mediates a peptide linkage between glutamate and cysteine.
`Studies have demonstrated a decrease in glutathione synthesis in
`erythrocytes of neonates when methionine was used as a cysteine
`precursor (Viña 1995). The eIect was more pronounced in preterm
`infants less than 32 weeks of age. Supplementation of parenteral
`nutrition with cysteine increases endogenous synthesis of taurine
`and glutathione (Shew 2000a).
`
`Taurine is present in all cells and is conjugated to biliary
`pigments, which are excreted into bile as soluble biliary salts. In
`adults, taurine can be synthesized from cysteine and methionine.
`However,
`in preterm
`infants, taurine
`is also considered a
`conditionally essential amino acid. Taurine needs to be included in
`parenteral solutions to prevent cholestasis (Howard 1992).
`
`Adding cysteine-hydrochloride (cysteine-HCl) to TPN solutions
`increases calcium and phosphate solubility. Cysteine-HCl reduces
`the pH of TPN significantly (Laine 1991; Parikh 2005), thereby
`increasing calcium and phosphate solubility and allowing
`increased mineral intake, a limiting factor in mineral accretion in
`preterm infants receiving TPN. Increased mineral accretion could
`reduce the incidence of osteopenia of prematurity and fractures
`in VLBW infants. However, acidification of the TPN by cysteine-
`chloride in the smallest infants (especially those < 1250 g) may be
`associated with metabolic acidosis for the first two weeks (Uauy
`1993; Laine 1991; Heird 1988). Such metabolic acidosis can be
`prevented by the administration of base (acetate or lactate) at a 2:1
`molar (base:cysteine hydrochloride) ratio (Uauy 1993; Laine 1991).
`
`3
`
`Stability of cysteine in TPN
`The poor stability of cysteine in commercial amino acid solutions
`has led to the development of cysteine substitutes with greater
`stability. Cysteine is easily oxidized to cystine, which precipitates
`rapidly and is virtually insoluble in TPN solutions. In solutions
`containing both cysteine and glucose, the cysteine content of
`the fluid decreases by 40% within the first 10 hours from both
`oxidization and formation of D-glucocysteine (Bjelton 1990). N-
`acetyl-cysteine, a soluble precursor, improves stability in solution,
`thereby allowing higher total concentrations of cysteine, cystine
`and its precursor in TPN. However, plasma cystine levels have been
`shown to be less than one third the lower limit of the reference
`range when cyst(e)ine intake consisted of small doses and no
`correlation was found between plasma cystine levels and N-acetyl-
`cysteine intake at these doses (Van Goudoever 1994).
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
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`Review Authors' note:
`In this review, international units (micromoles) were used. The
`molecular weights of cysteine-hydrochloride and N-acetyl-cysteine
`are, respectively, 157.7 and 163.2. Therefore, to convert micromole
`to mg, multiply by 0.1577 and 0.1632, respectively.
`
`intervention. This included any dose, formulation, or duration,
`but excluded studies in which the intervention included not only
`supplementation of cysteine, cystine or N-acetylcysteine but also
`supplementation of other nutrients. Starting age needed to be
`within 28 days of birth.
`
`Why it is important to do this review
`This updates the review "Cysteine, cystine or N-acetylcysteine
`supplementation in parenterally fed neonates (Soghier 2006a)".
`
`O B J E C T I V E S
`
`The primary aim of this review was to determine the eIect
`of supplementation of parenterally fed neonates with cysteine,
`cystine or its precursor N-acetylcysteine on growth assessed by
`weight gain, length, head circumference and nitrogen retention.
`
`The secondary aims were to determine the eIect of cysteine,
`cystine or its precursor N-acetylcysteine supplementation on
`mortality, morbidity secondary to oxidation injury (neurological,
`respiratory, ophthalmologic, gastrointestinal), bone accretion,
`acidosis or cholestasis.
`
`Primary comparisons:
`
`1. Cysteine or cystine vs. placebo supplementation to cysteine-free
`PN
`2. N-acetylcysteine vs. placebo
`3. Cysteine or cystine vs. N-acetylcysteine
`
`Planned subgroup analyses included the following:
`
`1. Enteral versus parenteral (cysteine, cystine or N-acetylcysteine)
`route
`2. Gestational age: preterm (< 37 weeks of gestation) versus full-
`term infants.
`3. Adequacy of intake of calories and other amino acids, or lack of
`adequate ((Kleinman 2004; Roberts 2001) intake of at least one
`other element
`4. TPN versus partial parenteral nutrition (PN)
`5. RCTs versus quasi-randomized trials
`
`M E T H O D S
`
`Criteria for considering studies for this review
`Types of studies
`in
`All randomized and quasi-randomized controlled trials
`which cysteine, cystine or N-acetyl cysteine was used for
`supplementation in comparison to placebo, other interventions or
`no supplementation in neonates. Cross-over were eligible if they
`assessed short-term outcomes. Cluster trials were eligible.
`
`Types of participants
`Infants 28 days postnatal age or less receiving no more than
`minimal enteral nutrition (maximum 15% of the daily calorie
`intake) were included.
`
`Types of interventions
`Intravenous or enteral supplementation of cysteine, cystine or
`N-acetylcysteine versus no supplementation, placebo or other
`
`Types of outcome measures
`Primary outcomes
`1. Growth: weight gain (g), head circumference gain (cm) and
`length gain (cm), both short-term (during the intervention
`period aSer one week and at the end of the intervention)
`and long-term (aSer the intervention period at the time of
`discharge or at 36 weeks of postmenstrual age [obtained by
`adding gestational age and postnatal age]) and at one year of
`age
`2. Nitrogen retention (mg/kg/day).
`
`Secondary outcomes
`1. Mortality before discharge
`2. Chronic
`lung disease defined as continuous positive
`airway pressure or oxygen requirement (with or without
`characteristic radiographic findings) persisting beyond 36 weeks
`postmenstrual age
`3. Retinopathy of prematurity of any stage or severe (stage 3 or
`more) (ICROP 2005)
`4. Necrotizing enterocolitis defined as clinical evidence of
`abdominal distension and feeding intolerance plus radiological
`evidence of pneumatosis intestinalis with or without portal
`venous gas and pneumoperitoneum (greater than stage 1, using
`Bell's modified criteria) (Kanto 1994)
`in
`5. Cystic periventricular
`leukomalacia defined as cysts
`the periventricular area on ultrasonogram, computerized
`tomography or magnetic resonance imaging scan
`6. Intracranial hemorrhage of any grade or severe (grade 3 or 4)
`(Papile 1978)
`7. Incidence of osteopenia of prematurity (defined as decrease
`bone mineral content and biochemical abnormalities), fractures
`8. Incidence of metabolic acidosis (defined based on normal values
`[mean minus 2 standard deviations] of pH and base deficit for
`gestational age and postnatal age)
`9. Incidence of cholestasis (serum level of direct bilirubin > 20% of
`total serum bilirubin, or serum level of direct bilirubin > 1 mg/dl
`if total level of bilirubin is < 5 mg/dl) (AAP 2004)
`10.Calcium and phosphate retention (mmol/kg/day)
`11.Plasma cyst(e)ine levels (mol/100 ml), single measurement on
`the last day of the intervention period. This must be obtained
`before completion of the intervention because of rapid decrease
`in plasma level aSerwards (Gomez 1993). Unless indicated,
`levels were assumed to correspond to free cyst(e)ine, because
`the bound fraction is not measured by conventional methods
`(Uauy 1993). Plasma levels of cyst(e)ine were calculated by
`adding the levels of cysteine and hemicystine.
`
`In the protocol there was a typographical error. There was
`a mismatch between aim, objectives and outcomes: nitrogen
`retention was listed as primary in aim and objectives and as
`secondary in outcome measures. This mismatch was corrected in
`the review.
`
`Cysteine, cystine or N-acetylcysteine supplementation in parenterally fed neonates (Review)
`Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
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`Search methods for identification of studies
`Electronic searches
`Searches were conducted using MEDLINE (1966 to December 2005),
`EMBASE (1974 to December 2005), the Cochrane Central Register
`of Controlled Trials (CENTRAL, The Cochrane Library, Issue 1,
`2006) and personal files. The MeSH headings used were cysteine,
`cysteine, cystine, N-acetyl cysteine and parenteral nutrition, limited
`to infant, newborn. Reports in any language were examined.
`
`In August 2009, the search was updated as follows:
`The Cochrane Library, MEDLINE (search via PubMed), CINAHL and
`EMBASE were searched from 2006 to 2009.
`Clinicaltrials.gov was searched with no date restriction.
`Search terms: cysteine OR cystine OR n acetyl cysteine AND
`parenteral nutrition. Limits: human, newborn infant and clinical
`trial. No language restrictions were applied.
`
`Searching other resources
`The proceedings of Society for Pediatric Research/ American
`Pediatric Society, Eastern Society for Pediatric Research, and
`Society for Parenteral and Enteral Nutrition (1991 to December
`2005) were hand searched for abstracts using the headings:
`cysteine and N-acetylcysteine. For the 1992 and 1993 editions
`of the Society for Pediatric Research/ American Pediatric Society
`(no subject headings) the sections on developmental biology,
`developmental pharmacology, gastroenterology and nutrition, and
`neonatal nutrition and metabolism were searched.
`
`Data collection and analysis
`The criteria and standard methods of the Cochrane Collaboration
`and its Neonatal Review Group were used.
`
`Selection of studies
`Studies identified in the search were included if they met the
`inclusion criteria. All randomized and quasi-randomized controlled
`trials fulfilling the selection criteria described in the previous
`section were included. All investigators reviewed the results of the
`search and separately select the studies for inclusion. The review
`authors resolved any disagreement by discussion.
`
`Data extraction and management
`The review authors separately extracted, assessed and coded all
`data for each study using a form that was designed specifically
`for this review. DiIerences of opinion between review authors
`were resolved by discussion. Additional data were sought from the
`authors of all the trials included in the systematic review.
`
`Where justified clinically, two or more subgroups of a published
`study, using standard formulae to calculate the weighted mean and
`to estimate the weighted standard deviation were merged (Altman
`1991; Brion 1990; Zahr 1984).
`
`Assessment of risk of bias in included studies
`The standard methods of the Cochrane Neonatal Review
`Group were employed. The methodological quality of the
`studies were assessed using
`the
`following key criteria:
`allocation concealment (blinding of randomization), blinding of
`intervention, completeness of follow-up, and blinding of outcome
`measurement/assessment. For each criterion, assessment was yes,
`
`no, can't tell. Two review authors separately assessed each study.
`Any disagreement was resolved by discussion. This information
`was added to the Characteristics of Included Stud