Hyperammonaemia in classic organic acidaemias: a review of the literature and two case histories

Organic acidaemias (OAs), a group of rare, inherited, metabolic disorders, typically present in neonates or infants as acute, potentially life-threatening, metabolic decompensation [1‐3], which follows an initial symptom-free period of a few days [3, 4]. An increase in plasma ammonia to toxic levels is often a key feature of metabolic decompensation [5, 6], which needs to be addressed promptly since it can lead to life-threatening neurological complications [7]. Paediatricians, who do not have specialist knowledge of metabolic disorders, have the challenging task of initiating treatment while obtaining a timely diagnosis. This article provides practical advice to help with the decision-making and clinical management of hyperammonaemia during early, acute metabolic decompensation in neonates and infants with OAs.

Although more than 65 different OAs have been described, even the more common ones are rarely encountered by paediatricians, as the collective incidence is about 1 in 3000 live births [8]. The initial signs and symptoms of metabolic decompensation – such as poor feeding, vomiting, dehydration and lethargy – are non-specific and can mimic other conditions, such as bacterial sepsis [3], thereby making rapid identification and intervention to prevent irreversible neurological sequelae [3, 4] particularly challenging. Elevated plasma ammonia levels have a direct impact on neurological outcomes [3, 9], with levels above 200 μmol/L usually being associated with drowsiness and lethargy (impaired vigilance) [5].

This article describes the three OAs most likely to result in hyperammonaemia during acute metabolic decompensation: propionic acidaemia (PA; MIM# 606054), methylmalonic acidaemia (MMA; MIM# 251000) and isovaleric acidaemia (IVA; MIM# 243500). Together with maple syrup urine disease, PA, MMA and IVA are sometimes referred to as ‘classic’ OAs [10]. Other OAs cause typical biochemical changes identified through newborn screening and have different clinical implications. While hyperammonaemia is an important manifestation of PA, MMA and IVA, its clinical significance needs to be considered together with other abnormalities, such as acidosis, ketosis, and lactic acidaemia, as well as distinguishing it from the hyperammonaemia caused by urea cycle disorders (UCDs) [11], the main group of conditions from which OAs must be differentiated [3, 12, 13]. Not all patients with PA, MMA or IVA present with hyperammonaemia, but most develop it in the newborn period, and continue to have recurrent episodes of hyperammonaemia during metabolic decompensation [3, 14].

This article considers the pathophysiology and underlying biochemical pathogenesis of OAs, and the diagnostic and clinical management of hyperammonaemia in PA, MMA and IVA. Two case studies illustrate the clinical presentation, key diagnostic evaluations and treatment in a neonate with acute decompensation and in a child with chronically elevated ammonia levels.

The development of an acute, life-threatening illness in a previously apparently healthy neonate is a strong indicator of an inherited metabolic disorder [15]. At initial presentation of neonates, the normal diet of breast milk or formula, together with postpartum catabolism, causes acute decompensation. In patients with OAs beyond the neonatal period, acute attacks are usually triggered by conditions that increase catabolism, such as fasting, vomiting or febrile infections (Table 1) [5, 13, 16].

In the newborn period, poor feeding, vomiting, dehydration, lethargy, tachycardia and hypothermia (Table 1) usually develop within 2–7 days after birth [3, 13] and, if left untreated, can quickly progress to respiratory distress, coma and death [16, 17]. However, as these symptoms are also observed in more common conditions, such as infection, a high index of suspicion should be maintained [18]. If there is neurological involvement [3, 19], routine laboratory studies for OAs (chemistry profile, ammonia level) should be performed at the same time as the septic work-up.

After the neonatal period, OAs may present with delays in development, failure to thrive, altered mental state, abnormal tone, seizures and motor dysfunction [16, 20, 21]. Other signs and symptoms of acute or chronic decompensation can include weight loss (often seen in newborns) and bone-marrow suppression in PA and MMA, and the characteristic ‘sweaty feet’ odour in patients with IVA [16, 22‐24].

All the classical OAs are inherited as autosomal recessive traits [17] and are caused by a deficiency in the enzymes involved in breaking down amino acids (isoleucine, valine, methionine and threonine in PA and MMA, and leucine in IVA) and additional substrates (odd-chain fatty acids, cholesterol, nucleotides in PA and MMA) [14, 16, 17, 25, 26] (Fig. 1). PA is caused by a deficiency in propionyl-coenzyme A (CoA) carboxylase, composed of alpha and beta subunits encoded by the PCCA and PCCB genes, respectively [17]. MMA can be caused by a deficiency of multiple enzymes affecting either the transformation of cobalamin to adenosylcobalamin, or the activity of methylmalonyl-CoA mutase, or racemase [16, 17, 27, 28]. IVA is caused by a deficiency in isovaleryl-CoA dehydrogenase [16, 17]. Each enzyme deficiency leads to a build-up of its precursor(s): propionyl-CoA (P-CoA) in PA, P-CoA and methylmalonyl-CoA in MMA, and isovaleryl-CoA in IVA [16, 29], and the corresponding organic acids once released from CoA (Fig. 1).

Hyperammonaemia likely results from the interaction of the substrates for the defective enzymes with other biochemical pathways, including the urea and tricarboxylic acid (TCA) or Krebs cycles (Fig. 1) [2, 17, 29]. In the urea cycle, a series of enzymes and transporters facilitate the incorporation of ammonia (as the amino group of carbamyl phosphate and aspartate) into urea for urinary excretion (Fig. 1). The complete urea cycle is mostly active in the liver, where, under physiological conditions, ammonia is removed in the mitochondrial matrix and cytosol of periportal hepatocytes [30]. N-acetylglutamate synthase (NAGS; EC 2.3.1.1) catalyses the formation of N-acetylglutamate (NAG) from glutamate and acetyl-CoA; NAG is an essential activator for the first enzyme in the urea cycle (carbamyl phosphate synthetase 1; CPS-1). Ammonia that escapes the urea cycle in periportal hepatocytes conjugates with glutamate to form glutamine in pericentral hepatocytes, explaining the increase in glutamine seen in all UCDs [30]. The urea cycle interacts with other mitochondrial pathways such as the TCA cycle, fatty acid oxidation and amino acid catabolism (Fig. 1) [2, 17, 29]. The TCA cycle enables the extraction of reducing equivalents from acetyl-CoA that fuel the mitochondrial electron transport chain to generate adenosine triphosphate through oxidative phosphorylation. The TCA and urea cycles are linked at oxaloacetate, the start and end point of the TCA cycle (Fig. 1), where oxaloacetate can be converted to aspartate via transamination [2, 29]. In addition, glutamate can be generated from 2-oxoglutarate and ammonia during physiological cataplerosis [2, 17, 29]. The oxidation of fatty acids, carbohydrates and amino acids generates acetyl-CoA, one of the substrates necessary for NAG synthesis [31, 32].

In OAs, the accumulated metabolites (P-CoA, methylmalonyl-CoA and isovaleryl-CoA) compete with acetyl-CoA to inhibit the activity of NAGS [12, 33, 34], thereby diminishing the synthesis of carbamyl phosphate [35]. This secondary impairment of the urea cycle can lead to elevated ammonia levels with neurotoxic effects during metabolic decompensation in OAs [12, 29, 36]. In contrast, the hyperammonaemia observed in UCDs is caused by primary defects in urea cycle enzymes or transporters [32].

The end point of propionic and methylmalonic acid metabolism is the generation of succinyl-CoA, an important intermediate of the TCA cycle [16]. This degradation pathway (anaplerosis) represents an important mechanism for replenishing TCA cycle intermediates [13]. Since anaplerosis is disrupted in PA and MMA, TCA cycle intermediates must be derived from other pathways. For instance, glutamine can be cleaved to form ammonia and glutamate; this in turn splits into ammonia and 2-oxoglutarate, which can then enter the TCA cycle (Fig. 1) [13]. A by-product of these reactions is the release of ammonia, which may contribute to the chronic hyperammonaemia observed in these disorders. In addition, the reduced concentration of glutamate might further impair synthesis of NAG by NAGS. Finally, the flow of glutamine to α-ketoglutarate to replenish the TCA cycle results in a decrease, rather than an increase, in glutamine levels in PA as ammonia levels increase [13].

Hyperammonaemia toxicity in the developing brain is due to multifactorial mechanisms, including disruption of amino acid and cerebral energy metabolism, and increased oxidative stress [36]. Excess propionic and methylmalonic acid levels also appear to contribute directly to progressive neurological deterioration by acting through protein kinases and phosphatases to interfere with cytoskeletal assembly in neuronal and glial cells [8], as well as disrupting the mitogen-activated protein kinase and p53 signalling pathways that promote apoptosis of neuronal cells [8, 37].

In addition to propionic acid and methylmalonic acid, other toxic metabolites such as propionylcarnitine and methylcitric acid can also disrupt the normal functioning of the TCA cycle [8, 12]. There is increasing evidence to suggest that the significant disruption of mitochondrial function – involving multiple deficiencies in oxidative phosphorylation via the mitochondrial electron transport chain, secondary to inhibition of the TCA cycle [25, 38] – culminates in a synergistic impairment of energy metabolism and depletion of cellular energy stores [2, 29]. Elevated levels of propionic acid, methylmalonic acid and isovaleric acid appear to directly mediate oxidative stress by increasing levels of reactive oxygen species (ROS) [8], and reducing ROS-protective levels of glutathione [39]. This further contributes to the intra- and extracerebral disease manifestations of the classical OAs [2]. Mitochondrial electron transport chain dysfunction has been linked to damage in the basal ganglia [1, 19, 25]; it has been suggested that methylcitric acid may be more neurotoxic than methylmalonic acid [40]. In addition, intracellular metabolite accumulation leads to CoA sequestration and its subsequent depletion, resulting in detrimental effects primarily in mitochondria [41].

Developmental delays are common in OAs, with some patients showing progressive neurological deterioration with increasing age. In a retrospective study of 55 patients with PA (median age 5.2 years), 75.5% showed some degree of intellectual disability, with a median IQ of 55 [42]. Another study in 80 patients with classical OAs, found that PA was associated with the worst neurological prognosis: 37% of patients had abnormal neurological findings, 61% had abnormal psychometric scores, and 56% had basal ganglia lesions. In contrast, all patients with IVA had normal neurological examinations, only 18% had psychometric impairment, and only 17% had basal ganglia lesions [43].

OAs are also associated with long-term systemic effects in other organs with high energy requirements, such as the heart, kidneys, eyes [12, 25, 44, 45], pancreas [22] and bone marrow [23]. These extracerebral complications are apparently related to the synergistic effects of toxic metabolites and impaired mitochondrial oxidative phosphorylation [2, 25], often occur despite good metabolic control, and can appear in asymptomatic individuals [2, 12, 42, 46]. This may be related to sequestration of CoA within mitochondria, depletion of mitochondrial DNA, increased production of ROS, and alterations in gene expression [2]. Dysfunction of the mitochondrial electron transport chain may contribute to late multi-organ complications in PA and MMA [25], including the formation of megamitochondria in kidney proximal tubules, seen in MMA patients with chronic kidney disease [38]. For patients with PA, the depletion of carnitine and the sequestration of CoA by excess organic acids are further responsible for decreases in fatty acid oxidation and acetyl-CoA production [29, 41], in turn leading to further secondary inhibition of the urea and TCA cycles [29], and contributing to long-term cardiomyopathy [12].

Table 2 provides a clinical checklist aimed at paediatricians who are unfamiliar with OAs. This checklist complements current guidance [3] and outlines symptoms that may relate to an underlying diagnosis, laboratory testing requirements, clinical laboratory findings (Table 1), and immediate management recommendations for patients with OAs.

The following cases illustrate: (1) the diagnostic work-up and initial management of acute decompensation in a 2-day-old neonate; (2) the diagnostic work-up and initial and long-term management of chronically elevated blood ammonia in an infant aged 9 months at diagnosis.

Hyperammonaemia, frequently observed in the classical OAs (PA, MMA and IVA), is most often associated with metabolic decompensation in the neonatal period, and contributes to the risk of neurological damage. Timely diagnosis and intervention is essential, as prognosis is strongly linked to elevated blood ammonia levels and duration of coma, as well as to the increased levels of specific organic acids. Prompt evaluation from a metabolic specialist is required, but should not delay intervention, diagnostic work-up and initial management.

While the diagnosis of OAs remains challenging because of the low incidence of these disorders, as well as their variable and non-specific presentation, recent advances in diagnostic techniques, such as newborn screening by tandem mass spectrometry [11], which allow earlier identification, are encouraging. Screening of neonates for OAs and other inherited metabolic disorders offers the potential for early diagnosis and therapy [63]; however, newborn screening is not offered in all countries, and evidence that screening improves long-term outcomes is currently limited [1, 14, 50, 63, 69]. In many cases of OAs, results become available after newborns are already symptomatic.

With the diverse range of interventions in OAs, timely diagnosis by the identification of elevated ammonia levels and metabolic acidosis can allow their rapid correction, which is critical for the prevention of brain damage [46, 50].