12th April 2013: Electrolyte Profile of Paediatric Patients with Hypertrophic Pyloric Stenosis

120413 title

Where can I find this paper?

http://www.ncbi.nlm.nih.gov/pubmed/23528507

What is this paper about (what is the research question)?

In paediatric patients with hypertrophic pyloric stenosis (HPS), what is the prevalence of abnormal laboratory results?  Are these results related to the duration of illness (by duration of vomiting), and is there any time trend in these results?

Summary of the Paper

Design: Retrospective chart review

Objective: To investigate the incidence and prevalence of abnormal laboratory results in patients with a radiological and operative diagnosis of HPS

Outcomes:

Primary – prevalence of high, low and normal CO2, K and Cl in HPS cases

Secondary – trend in prevalence of metabolic alkalosis and acidosis in HPS cases over the study period

Tertiary – association between days of vomiting and abnormal CO2, K and Cl

Reference Standard: Normal range laboratory results for the facility

Participants: Patients younger than 6 months, with HPS confirmed on ultrasound or Upper GI series, who underwent pyloromyotomy at a tertiary regional paediatric centre from 2000-2009.

Results:

205 patients were included in the study.  Their age varied from 1.4 to 13.9 weeks (SD 2.2), with a weight range of 2.1 to 4.9kg (SD 0.5).  88.3% were male.  74.3% were of non-Hispanic ethnicity.  80.5% white race, 1.5% African-American, 1.5% Asian and 16.5% other.

The proportion of HPS cases with normal serum CO2 was 62%, low 20%, and high CO2 18%.  Potassium was normal in 57%, low in 8% and high in 35% of cases.  Chloride was normal in 69%, low in 25% and high in 6% of cases.

Logistic regression analysis of the proportion of normal, low and high CO2 over the study period showed an increased in the prevalence of metabolic alkalosis (p=0.009) and a decreased in metabolic acidosis (p=0.002).

Advancing age was associated with presence of metabolic alkalosis on presentation with HPS (data not provided).

There was no correlation between the number of days of vomiting and abnormalities in electrolytes in this study population.

Authors’  conclusions

We observed that normal laboratory values are the most common finding in HPS and that metabolic alkalosis was found more commonly in the latter part of the decade and in older infants.

On the study design

This was a retrospective chart review for a 10 year period from 2000-2009.  Data from 2000-2002 was combined to increase power because the case numbers in single years were “small and unstable.”  It’s not clear what they mean by “unstable” as the raw data is not provided.

The authors do not comment on the total number of presentations over the study period, so it’s unclear if any cases were excluded, and reasons for any such exclusions.

There is demographic data missing with respect to birth weight (138/205), days of vomiting (196/205), heart rate at presentation (203/205) and weight at presentation (204/205).  The latter categories are unlikely to have been affected by this, and it is unclear whether additional data on duration of vomiting would have changed the analysis.

Prospective studies have the advantage of more complete data sets, and potential for further variables to be included, however can introduce observation/measurement bias.

What were the results and what does this mean?

Normal laboratory values are the most common finding in HPS and therefore  serum electrolytes are a poor marker for the presence or absence of HPS.

CO2 normal 62% low 20% high 18%

K normal 57% low 8% high 35%

Cl normal 69%  low 25%  high 6%

The incidence of metabolic alkalosis increased over the study period, and its prevalence is higher in older infants. 

They have no explanation for the increase in metabolic alkalosis over the decade of the study.

The authors postulate that the latter finding may demonstrate that advanced age at diagnosis serves as a marker for the duration and severity of stenosis.

What can we take from this paper into clinical practice?

This paper agrees with previous studies that the “typical metabolic picture” of hypochloraemic hypokalaemic metabolic alkalosis in paediatric HPS is no longer seen in the majority of presentations.

For us this means that we cannot rely on laboratory results as a marker for hypertrophic pyloric stenosis in infants.  We must continue to have a high index of suspicion for this condition in infants presenting with persistent vomiting and proceed to ultrasound for diagnosis.

Although laboratory results don’t help us decide which children need ultrasound, it is important to look for metabolic derangements and correct them as indicated.

What this study adds is that contrary to previous beliefs, there is no relationship between the duration of illness, and particularly vomiting, on the severity of metabolic derangements in these children.  This seems counter-intuitive, and perhaps the more important factor is not the duration of vomiting, but whether the infants are able to keep down an adequate amount of fluids – i.e. The severity of dehydration.

Unfortunately there was insufficient data in the patient charts to enable analysis of trends between dehydration, vomiting and abnormal laboratory results.  Only 43/205 (21%) charts mentioned hydration status, however 42% of the patients whose charts noted dehydration (36/205) had metabolic alkalosis at presentation, compared to 44% with normal CO2.

More questions to ask

  • Were there a higher proportion of males in this group than other populations?
  • Why was delayed presentation (60days vomiting in one case) not associated with more severe illness??
  • An insufficient number of charts contained information about hydration status – is this more relevant for laboratory abnormalities than days of vomiting?

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