Background

Inborn errors of metabolism (IEM) are a group of inherited genetic metabolic disorders that lead to enzymatic defects in the human metabolism and form a large class of genetic diseases. The cause of IEM is the mutation in a gene that encodes for an enzyme, leading to defective synthesis of this enzyme or its activity. This affects the normal function of a metabolic pathway. In most disorders, problems arise from accumulation of substrates which are toxic to tissues, organs, or blood [1].

Till now, more than 500 IEMs have been detected. Although individual IEM is rare, the incidence of overall IEMs is high and varies dramatically in different countries and regions [2]. According to the Society for the Study of Inborn Errors of Metabolism (SSIEM) [1] and based on the main substrate, which is affected, IEM was classified as follows:

  • Disorders of amino acid (e.g., maple syrup urine disease, organic acidemias, and urea cycle defects)

  • Disorders of carbohydrate metabolism (e.g., galactosemia and glycogen storage disorders)

  • Disorder of lipid metabolism (e.g., fatty acid oxidation defects [medium chain acyl-CoA dehydrogenase deficiency])

  • Disorders in the metabolism of purines and pyrimidines

Many IEMs are amenable to treatment with early diagnoses. For others that are not amenable to treatment, establishing diagnosis in the index case is important for prenatal diagnosis in subsequent pregnancy [3].

  • Early diagnosis and treatment are very important to reduce the rates of morbidity and mortality related to IEMs. The first step is suspicion, followed by performing routine and specific laboratory analysis as soon as possible. Early neonatal screening is important especially in countries with high rates of consanguinity because early diagnosis and treatment are likely to result in better neurodevelopmental outcomes and lower mortality rates.

The study aimed for:

  • Early detection, diagnosis, and intervention to improve the neonatal outcome

  • Emphasizing the importance of expanded early neonatal screening

Methods

The study included 200 cases out of 5500 neonates admitted to Neonatal Intensive Care Unit. The study was conducted in the period from January 2015 to June 2018.

Inclusion criteria

  • Positive family history with affected siblings or unexplained infant/neonatal death

  • Persistent unexplained vomiting

  • Persistent metabolic acidemia

  • Convulsions

  • Hypoglycemia

  • Respiratory distress or apnea

  • Drowsiness or disturbed consciousness level

  • Poor oral intake and dehydration

Exclusion criteria

  • Multiple congenital anomalies

  • Birth anoxia

  • Birth trauma

  • Neonates with intracranial hemorrhage

  • Neonates with surgical problem

  • Neonatal sepsis based on clinical and laboratory finding

All the studied cases were subjected to (1) history taking provided by the care-giver: assessment of the following factors—sex, birth weight, gestational age, and consanguinity. The suggestive complaints of IEM were poor suckling, weak crying, convulsions, persistent vomiting, lethargy, previous sibling deaths, persistent metabolic acidosis, hypoglycemia, and/or hyperammonemia. (2) Full clinical examination: general, abdominal, cardiac, chest, and neurological examination. (3) Laboratory investigations include routine investigations (complete blood picture, C-reactive protein (CRP), liver function tests, renal function tests, serum ammonia, serum lactate, arterial blood gas, and serum glucose level), and specific metabolic investigation includes metabolic screen in blood and organic acid in urine.

Mass spectrometry analysis

Whole blood samples were drawn by heel prick or venipuncture from high-risk babies and spotted on Whatman 903 filter paper (Whatman Inc., USA). Amino acids and acylcarnitines were analyzed using a triple-quadrupole mass spectrometer, equipped with a (Acqulity, Water, USA) Waters Alliance 2790 HPLC pump.

The result of MS/MS includes levels of tyrosine, phenylalanine, citrulline, alanine, methionine, proline, leucine/isoleucine, arginine, valine, glycine, glutamic acid, aspartic acid, and argininosuccinic acid, and 29 acylcarnitines—C0, C2, C3, C3-DC, C4, C4-DC, C4-OH, C5, C5:1, C5-DC, C5-OH, C6, C6-DC, C6-OH, C8, C8-OH, C10, C10:1, C12, C12:1, C14, C14:1, C14:2, C16, C16-OH, C18, C18:1, C18-OH, and C18:1-OH—were quantified. Multiple analyte ratios were also calculated as Phe/Tyr, Leu/Phe, C3/C2, and C3-DC/C10. The internal standards were purchased from Cambridge Isotope Laboratories, and the Neolynx software (Neolynx Inc., CA, USA) was used for processing MS/MS data.

Organic acids in urine

Random urine samples were taken (5 ml) by using urine collecting bag and stored at – 20 °C for organic acid analysis by gas chromatography/mass spectrometry (GC/MS).

Test principle of organic acids in urine

Determination of organic acids in urine was done by GC/MS technique. Urine organic acid profile is an important second-line investigation in which up to 150 different disorders can be identified from a single analysis. However, some inborn errors of metabolism are clearly identifiable only during episodes of acute decompensation, e.g., medium chain acyl-CoA dehydrogenase deficiency.

Gas chromatography/mass spectrometry (GC/MS) is the most reliable method for urine organic acid analysis to detect organic acidemia and other metabolic disorders.

Statistical analysis

Data were verified, coded by the researcher, and analyzed using IBM-SPSS version 19 (@IBM-SPSS Inc., 2012, NY, USA). Data were presented as number and percentage or mean ± standard deviations. Chi-square test analysis was used to compare the proportions between qualitative variables. p value was considered statistically significant when p ≤ 0.05.

Results

The current study recruited 5500 neonates admitted to Neonatal Intensive Care Unit, in the period from January 2015 till June 2018. Highly suspected 200 cases with IEM were studied to be diagnosed.

The socio-demographic characteristics of the studied cases were 60% males (n = 122) and 78 (39%) females. Their age ranged from 1 to 30 days after birth (Table 1).

Table 1 Socio-demographic characteristics of the studied cases
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The most common manifestations of the studied cases were poor suckling (61%), followed by respiratory distress (57%), then convulsions (54%) and vomiting (52%) (Tables 2, 3, 4, 5, 6, 7, and 8).

Table 2 Frequency of the most common manifestation among cases
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Table 3 The difference between the commonly diagnosed cases in our study (AAD and FAOD) in relation to consanguinity, sibling affection, convulsions, DCL, vomiting, and poor suckling
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Table 4 The difference between the commonly diagnosed cases in our study (AAD and FAOD) in relation to hypotonia, spasticity, skin manifestations, and hepatomegaly
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Table 5 Routine screening investigations for diagnosis of the IEM of the studied patients
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Table 6 The difference between the commonly diagnosed cases in our study in relation to metabolic acidosis, hyperammonemia, hyperlactatemia, and hypoglycemia
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Table 7 The different disease types of the studied cases after routine and specific laboratory finding
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Table 8 The abnormal amino acids/acylcarnitines in dried blood spots and organic acids in urine of the different disease types in the studied cases
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The outcome of the cases with IEM shows that 36% of them were discharged on treatment and continue follow-up at genetic unit and the rest of the cases (64%) died due to complications.

Discussion

In the current study, consanguinity was positive in 122 patients (61%) of patients. It was higher than that reported in a study conducted by Shawky et al. [5] at Ain Shams University, Egypt, in which 17 patients (43.5%) had consanguineous parents. Also, our results were higher than a study carried out in Iraq by Arif et al. [6], in which 174 (9.9%) neonates had positive consanguineous marriage and 132 (7.5%) of their patients had positive family history of either metabolic disorders or sudden unexplained death of siblings. In this study, the presentation day ranged from 3 to 12 days with a mean of 6.5 ± 0.7 days after-birth. This was consistent with Shawky et al. [5] who reported that all patients were normal at birth with no complications during delivery, and then they started to develop symptoms at an age ranged from 1 to 10 days with a mean of 3.4 ± 2.2 days.

In our study, the most common manifestation at the time of presentation was sepsis-like manifestations (poor suckling, found in 122 out of 200 cases (61%), followed by respiratory distress in 114 out of 200 cases (57%), then convulsions in 108 out of 200 cases (54%), and vomiting in 104 out of 200 cases (52%)). Our results were in agreement with Shawky et al. [5] and Clarke [8] who reported that sepsis-like symptoms were the most common presentation (26.5% and 15%), respectively. Also, Lunda et al., 2002, reported that the predominant symptoms in neonates suggesting the possibilty of metabolic disorders were convulsions and lethargy [9].

The prevalence of IEM in the current study was similar with Shawky et al. [5] and higher than that found in a study carried out by Selim et al. [10], an Iranian study by Pishva et al. [11], two studies by Elsobky and Elsayed [12] and Riyami et al. [13], an Indian one by Nagaraja et al. [14], an Iraqi one by Arif et al. [6], and two Chinese studies by Sun et al. [15] and Huang et al. [16] (Fig. 1).

Fig. 1
figure 1

Diagnostic prevalence of the studied cases

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Shawky et al. [5] showed that 13/40 (35.5%) suspected neonates were admitted to the El Mahalla Hospital NICU, Egypt, over 1-year duration with sepsis-like symptoms, convulsions, persistent metabolic acidosis, persistent vomiting, and previous sibling death of unidentified causes confirmed to have IEM.

Moreover, Pishva et al. [11] reported that the prevalence of IEM was 14%, which was lower than in our study (Fig. 2). Another study in Karachi by Wajner et al. [17] reported the prevalence of 25.8%. Moreover, several other studies, in Egypt by Elsobky and Elsayed [12], in Brazil by Coelho et al. [18], and in India by Nagaraja et al. [14], reported lower detection rate 6.5%, 3.17%, and 3.2%, respectively.

Fig. 2
figure 2

IEM Subcategory prevalence among the studied IEM cases

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Additionally, a study done in Oman by Riyami et al. [13] found that 10.8% of their patients were diagnosed with IEMs in high-risk Omani neonates. Likewise, an Iraqi study by Arif et al. [6] with a total number of 1758 symptomatic Iraqi children from birth to early adolescence was analyzed for possible IEM; positive cases were 224/1758, with a rate of detection (12.7%) both of which were lower than in our study.

Amino acid disorder (AA) in our study was accounted as the most common disorder detected, and this was in accordance with Selim et al. [10]. Regarding the AA disorders, urea cycle disorders (UCD) was the most common type in our study, detected in 20/70 (28.5%) of diagnosed cases, with male predominance in 15/20 case (75%). This was in agreement with Shawky et al. [5] as UCD was the most common type with an incidence but with higher level than our study that was 53.8%, and 75% of the diagnosed cases were males. In Shawky et al.’s study [5], the most common IEM was UCD in 7/13, then MSUD in 2/13, followed by MMA and MCAD 1/13. Dissimilar, Gündüz et al. [4] found that UCD was detected only in six cases (11.8%), which were lower than in our study for both.

The second most common amino acid disease in our study was maple syrup urine disease (MSUD), which was detected in 18 cases (25.7%). Our results were higher than in Shawky et al. [5] who diagnose 2/13 (15.4%) cases with MSUD and Selim et al. [10] which was 7.4%. Similarly, Karam et al. [19] in Lebanon identified MSUD in 9.1%; in Fateen et al. [20] in Egypt, it was 1.2%; and Elsobky and Elsayed [12] reported that 10% of children with convulsions had MSUD. This can be explained by the larger number of patients in our study complaining from poor suckling and convulsions, and these presentations are considered as the common symptoms of MSUD.

Furthermore, in our study, organic acidemia was presented in 15/70cases (21.4%), which was higher than in Shawky et al. [5] who detect organic acidemia in 3/40 (7.5%) and in Elsobky and Elsayed [12] who reported organic acidemia in 1.07%.

In our study, one case was diagnosed as B-ketothiolase deficiency; this case was presented on the 1st day after birth with persistent vomiting, convulsion, metabolic acidosis, and hypoglycemia. This patient had one sister with the same disease diagnosed after birth with increased tiglylglycine in urine. In our study, fatty acid oxidation defect was presented in 15/70 cases (21.4 %). This was higher than Riyami et al. [13] in Oman, who detect FAOD in (16%) of cases; Shawky et al. [5] who found FAOD in 1/13 (7.6%); in Selim et al. [10] who found FAOD in 3.4% of patients; and in Arif et al.’s study [6] in Iraq (1.1%).

Our results were lower than that of Vargas et al. [7] who reported that FAOD was detected in 19 patients (28%). These higher results in their study can be explained by their selection criteria which were based on selection of patients with FAOD manifestations only.

In our study, galactosemia was suspected in 3/200 (1.5%) cases. Our results were lower than a study by Fateen et al. [20], who detected galactosemia in (2.5%) of cases. As well, a study by Shawky et al. [5] detected galactosemia in (7.7%) and in a study by Arif et al. [6], who detected galactosemia in 31/1758 (1.76%) of cases. This difference might be explained by that more neonates in their studies presented with clinically suspicious galactosemia and have facilities to do enzymatic assay for galactosemia that was not available in our hospital. Also, they included larger number of patients for longer time.

Conclusion

From this descriptive study, we concluded that IEM disorders are not rare disease in high-risk neonates with attentions to consanguinity which is a common tradition in our country. Regarding the metabolic disorders, manifestation presented in most cases, i.e., sepsis-like manifestations. These manifestations are common, and many physicians misdiagnose them as they are unaware about these disorders. IEMs represented a significant cause of sick neonate’s admission to NICU, and it should be considered as a differential diagnosis for any sick neonate. Most IEMs are inherited in an autosomal recessive pattern, so its incidence increases with positive consanguinity. The delay in diagnosis of IEM (due to lack of facilities) leads to high morbidity and mortality, so early detection and early intervention to the neonates at risk of IEMs before the onset of symptoms can prevent or, at least, reduce serious neurological and developmental squeal.