The paediatric studies from which evidence on the incidence of hyponatraemia is based, are heterogeneous in terms of inclusion criteria and conceptualization of the phenomenon under study. There is variation in the cut-off point for hyponatraemia between the studies and they are not classified according to its severity[9–11]. Most of them were designed with the objective of describing incidence and severity of this phenomenon only in specific diagnostic categories[9–13]. Hence, our study involved an analysis of hyponatraemia in a diverse hospitalized paediatric population of all clinical diagnostic groups.
Incidence of CAH and HAH has been reported as 14.3% and 8.1% by Hoorn et al[19], while 36% and 9% by Al Shibli[20] et al respectively. These figures are far from our data, 15.8% and 1.4%, especially HAH. Both studies defined hyponatraemia at or below the plasma sodium of 135 mEq/L. Inclusion of children with plasma sodium levels of 135 mEq/L could have contributed to the increase in the frequency of hyponatraemia. In addition, the study by Al Shibli et al included children only with bronchiolitis[20].
In a prospective study, 27.7% of children admitted to the paediatric ward and intensive care unit, were reported with HAH[21]. There were 46.8% of children who were hyponatraemic on admission but further analysis on these children was not undertaken. The high incidence of HAH was attributed to use of hypotonic intravenous fluids in 96.7% of children. Children with HAH were categorized in various clinical disorders, the basis of this classification had not been described[21]. There is no surgical category even though 37.5% of the children with HAH underwent surgery. In view of these different considerations, it is difficult to compare with our study.
It is evident from these studies that the incidence of HAH was much lower than CAH, which is also especially evident in our study. Therefore, CAH is quantitatively more relevant than HAH in a secondary level paediatric setting.
In the studies that report hyponatraemia classified by severity, majority were mild forms[8,10,11,20–22] as is also reflected in our study. In our investigation, there was higher incidence of CAH in toddlers and early childhood age groups. In contrast, HAH was more frequent in early childhood, middle childhood and early adolescence (Table 2). Other studies have reported higher rates of hyponatraemia in younger paediatric patients[8,9,23] but similar age group analysis has not been conducted. Moreover, higher incidence of CAH was encountered in our patients with longer stays, as described in literature[8,24]. This is attributed to the severity of clinical illness in patients with hyponatraemia, although further research is needed to prove this hypothesis. The highest incidence of CAH in our study was in infections, cardiovascular, and haematological and immune disorders groups. On the contrary, HAH was more frequent in allergy and toxins, endocrine and metabolic, and gastrointestinal disorders (Table 3).
From the multivariate analysis it can be concluded that the highest risk of CAH was in children included in diagnostic groups: infections, cardiovascular and gastrointestinal disorders. To our knowledge, a combined analysis of the variables age, LOS, period of the year and diagnosis at discharge has not been reported in literature.
The incidence of HAH in our study was not only low but also all cases were mild forms in contrast to severe cases of HAH reported in literature[25]. We explored the possible reasons for the same. In our paediatric general ward, before October 2014 all children who needed intravenous fluids, received hypotonic fluids (sodium chloride [NaCl] 0.20% + dextrose [D] 5% or NaCl 0.33% + D 5%). Gradually after October 2014, this was replaced with isotonic fluids (NaCl 0.9% + D 5%)[26]. However, there was no difference in incidence of HAH before and after October 2014 (Table 5). In a recent retrospective study, of the change in maintenance fluids from hypotonic to isotonic on the tertiary care paediatric surgical ward the prevalence of hyponatraemia decreased from 29% to 22% for all paediatric age groups (adjusted OR 0.61, CI 0.51-0.82). However, it is interesting to note that the prevalence of hyponatraemia was still as high as 22% even with isotonic fluids[27], in contrast to that of HAH in our analysis and literature reports of HAH[1].
Our centre like most other secondary centres is not commissioned for surgical care of children under 3 years and for complex surgeries in children. Also, we transfer children to tertiary intensive care unit after initial stabilisation, which are limitations of our study. Exclusion of data from all these children may be contributory towards low incidence of HAH in our study. We would like to emphasise that more research is needed at secondary care level to assess the true incidence of HAH and identify at risk children in medical and surgical diagnostic groups.
Our study has some limitations due to its single-centre nature and retrospective design. Data on the tonicity, amount and rate of intravenous fluids administration was not recorded, hence could not be analysed. There was no provision to assess the relationship of hyponatremia with clinical severity scoring systems due to the data protection laws.
We selected a representative sample of paediatric hospitalized patients, by including a very broad range of age, diagnosis and method of admission (emergency or scheduled). About 60-70% of children in the most common diagnostic groups had their bloods analysed for plasma sodium levels (Table 1). Our study design is based on conceptual classification previously proposed by others[6]. In addition, the frequency and severity of hyponatraemia was compared with diagnostic groups derived from the coding of discharge diagnosis. This approach is used previously for comparison of child mortality[14]. The quality (validity, reliability, completeness and timeliness) of retrospective coded data depends on the documentation of discharge diagnosis and treatment by health care physicians, and on accuracy and consistency of the coders. In our hospital, the health record coders are professionally trained through an accredited training programme and supervised by a coordinator for consistent coding practice. The study by Hennessy et al suggested that coder characteristics do not influence the validity of hospital discharge data which can be achieved with appropriate training and management of the coding staff[28]. Therefore, we consider that our results are representative of a true incidence of the phenomenon at study.
To our knowledge this may be the first study with conceptual classification (CAH and HAH) of hyponatraemia in the hospitalised paediatric population. In addition, comparison with a broad set of ICD coded diagnoses grouped with a clinically and practically relevant classification, provides valuable information on the phenomenon under study in the paediatric population.