Research Notes: Hyperammonemia

From eMedicine:

Hyperammonemia is not a true disease; it is a sign that specific abnormalities that cause blood ammonia levels to become elevated may be present. Elevated blood ammonia levels cause a constellation of signs and symptoms that may appear to be a single disease.

Normal blood ammonia levels range from 10-40 µmol/L, compared with a BUN level of 6-20 mg/dL. The total soluble ammonia level in a healthy adult with 5 L of circulating blood is only 150 mcg, in contrast to approximately 1000 mg of urea nitrogen present. Because urea is the end product of ammonia metabolism, the disparity in blood quantities of the substrate and product illustrates the following 2 principles:

• The CNS is protected from the toxic effects of free ammonia.
• The metabolic conversion system that leads to production of urea is highly efficient.

An individual is unlikely to become hyperammonemic unless the conversion system is impaired in some way. In newborns, this impairment is often the result of genetic defects, whereas, in older individuals, the impairment is more often the consequence of a diseased liver. However, a growing number of reports address adult-onset genetic disorders of the urea cycle in previously healthy individuals.

Pathophysiology

The true mechanism of neurotoxicity in hyperammonemia is not yet fully determined. Irrespective of the underlying cause, the clinical picture is relatively constant. This implies that the pathophysiologic mechanism, focusing on the CNS, is common to all individuals with hyperammonemia.

The normal process of removing the amino group present on all amino acids produces ammonia. The alpha-amino group is a catabolic key that protects amino acids from oxidative breakdown. Removing the alpha-amino group is essential for producing energy from any amino acid.

Under normal circumstances, both the liver and the brain generate ammonia in this removal process, substantially contributing to total body ammonia production. The urea cycle is completed in the liver, where urea is generated from free ammonia.

The hepatic urea cycle is the major route for disposal of waste nitrogen chiefly generated from protein and amino acid metabolism. In the same context, low-level synthesis of certain cycle intermediates in extrahepatic tissues also makes a small contribution to waste nitrogen disposal. Two moles of waste nitrogen are eliminated with each mole of urea excreted. A portion of the cycle is mitochondrial in nature; mitochondrial dysfunction may impair urea production and result in hyperammonemia. Overall, activity of the cycle is regulated by the rate of synthesis of N-acetylglutamate (NAG), the enzyme activator that initiates incorporation of ammonia into the cycle.

The brain must expend energy to detoxify and to export the ammonia it produces. This is accomplished in the process of producing adenosine diphosphate (ADP) from ATP by the enzyme glutamine synthetase, which is responsible for mediating the formation of glutamine from an amino group. Synthesis of glutamine also reduces the total free ammonia level circulating in the blood; therefore, a significant increase in blood glutamine concentration can signal hyperammonemia.

The biologic requirement for tight regulation is satisfied because the capacity of the hepatic urea cycle exceeds the normal rates of ammonia generation in the periphery and transfer into the blood. Hyperammonemia never results from endogenous production in a state of health.

An elevated blood ammonia level, although it may be secondary, must never be ignored. Moreover, since the normal ureagenic capacity of the liver is so great in relation to physiologic load, such a finding points directly to an impairment of the urea cycle in the liver.

The CNS is most sensitive to the toxic effects of ammonia. Many metabolic derangements occur as a consequence of high ammonia levels, including alteration of the metabolism of important compounds, such as pyruvate, lactate, glycogen, and glucose. High ammonia levels also induce changes in N-methyl D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) receptors and causes downregulation in astroglial glutamate transporter molecules.

As ammonia exceeds normal concentration, an increased disturbance of neurotransmission and synthesis of both GABA and glutamine occurs in the CNS. A correlation between arterial ammonia concentration and brain glutamine content in humans has been described. Moreover, brain content of glutamine is correlated with intracranial pressure. In vitro data also suggest that direct glutamine application to astrocytes in culture causes free radical production and induces the membrane permeability transition phenomenon, which leads to ionic gradient dissipation and consequent mitochondrial dysfunction. However, the true mechanism for neurotoxicity of ammonia is not yet completely defined. The pathophysiology of hyperammonemia is that of a CNS toxin that causes irritability, somnolence, vomiting, cerebral edema, and coma that leads to death.

[...]

Genetic causes of hyperammonemia manifest as a wide variety of conditions. The different presentations are categorized as catastrophic newborn, late-infantile, and adult. Each inherited disorder is reported in various clinical presentations. In some patients with adult-onset disease, no precedent sign of intellectual dysfunction was present, leading to the assumption that the disorder was truly latent until the first acute presentation.

Age of onset depends on the age and rate of progression of the underlying disease process. Impairments that must be considered range from hepatic necrosis with hepatocellular damage to inborn genetic disorders of the urea cycle. Although history and age of the patient are helpful to diagnosis, genetic causes must never be disregarded, irrespective of the stage of life.

History

The multiple primary causes of hyperammonemia, specifically those due to urea cycle enzyme deficiencies, vary in presentation, diagnostic features, and treatment. For these reasons, the members of the family of urea cycle defects are individually considered in this article. However, the common denominator, hyperammonemia, can be clinically manifested by some or all of the following: anorexia, irritability, heavy or rapid breathing, lethargy, vomiting, disorientation, somnolence, asterixis [Involuntary jerking movements, especially hands] (rarely), combativeness, obtundation, coma, cerebral edema, and death, if treatment is not forthcoming or effective. As a consequence, the most striking clinical findings of each individual urea cycle disorder relate to this constellation and roughly temporal sequence of events.

The most helpful diagnostic information of history in a patient with suspected hyperammonemia is intercurrent illnesses with exaggerated lethargy and vomiting.

Physical

• General
  • Poor growth may be evident.
  • Hypothermia is occasionally seen.
• Head, ears, eyes, nose, and throat (HEENT): Papilledema may be present if cerebral edema and increased intracranial pressure have ensued.
• Pulmonary
  • Tachypnea or hyperpnea may be present.
  • Apnea and respiratory failure may occur in latter stages.
• Abdominal: Hepatomegaly is usually mild, if present.
• Neurologic
  • Poor coordination
  • Dysdiadochokinesia [inability to perform rapid, alternating movements]
  • Hypotonia or hypertonia
  • Ataxia
  • Tremor
  • Seizures
  • Lethargy that progresses to combativeness to obtundation to coma
  • Decorticate or decerebrate posturing

Causes

• Urea cycle defects with resulting hyperammonemia are due to deficiencies of the enzymes involved in the metabolism of waste nitrogen. The enzyme deficiencies lead to disorders with nearly identical clinical presentations. The exception is arginase, the last enzyme of the cycle. Arginase deficiency causes a somewhat different set of signs and symptoms.
• Genetic defects of the urea cycle include the following:
  • NAG [N-Acetylglutamate] synthetase deficiency (eMedicine)
  • Carbamyl phosphate synthetase (CPS) deficiency (eMedicine)
  • OTC (ornithine transcarbamylase) deficiency (eMedicine)
  • Citrullinemia (argininosuccinic acid synthase deficiency, citrullinuria) (eMedicine)
  • Argininosuccinate (ASA) lyase deficiency (argininosuccinic aciduria, argininosuccinase deficiency) (eMedicine)
  • Arginase deficiency (hyperargininemia, familial argininemia, argininemia) (eMedicine)

See also:

• Hyperammonemia-Hyperornithinemia-Homocitrullinemia Syndrome (eMedicine)
• Hyperinsulinemia (eMedicine)
• Methylmalonic Acidemia (eMedicine)
• Propionic Acidemia (Propionyl CoA Carboxylase Deficiency) (eMedicine)

Other Problems to be Considered

• Organic acid disorders (eg, isovaleric acidemia)
• Lysinuric protein intolerance
• Transient hyperammonemia of the newborn
• Hepatic insufficiency/dysfunction
• Mitochondrial diseases and pyruvate carboxylase deficiency
• Valproate ingestion
• L-asparaginase ingestion
• Reye syndrome

Lab Studies

• Plasma ammonia level - Obtain this measurement when clinical signs and symptoms are suggestive of hyperammonemia. No other laboratory test can substitute for this measurement, nor does any other test indicate need for it. Only clinical suspicion indicates need.
• Liver function studies (ie, serum transaminases, prothrombin time [PT]/activated partial thromboplastin time [aPTT]), alkaline phosphatase levels, bilirubin levels): Severe liver disease can cause hyperammonemia; therefore, evaluating the function of the liver is always appropriate as a first approximation to etiology.
• Plasma amino acid level quantitation - Certain primary genetic causes can be suspected based on specific increases in amino acid levels, such as increased citrulline or argininosuccinic acid levels. By contrast, severe liver disease tends to cause a generalized increase in plasma amino acid levels.
• Urinary organic acid profile - Disorders that involve metabolic intermediates of amino acid catabolism can cause mild-to-moderate inhibition of the urea cycle, resulting in hyperammonemia as a secondary phenomenon. This test can help to identify level increases in such intermediates as propionic acid, methylmalonic acid, isovaleric acid, or other organic acids and aid in diagnosis.
• Urine amino acid levels: These are helpful in confirming argininosuccinic aciduria; lysinuric protein intolerance; or hyperornithinemia, hyperammonemia, and homocitrullinuria (HHH) syndrome.
• Blood lactate levels: This is useful in ruling out mitochondrial diseases.
• Blood gas levels - Patients with urea cycle disorders may have alkalosis due to stimulation of the respiratory drive by ammonia. Patients with urea cycle disorders are rarely acidotic. Severe refractory acidosis suggests organic acid disorder or mitochondrial disorder.
• BUN level: This is often very low (<3 mg of urea/100 mL) in persons with urea cycle disorders.
• N-carbamoyl-L-glutamic acid: In infants with confirmed hyperammonemia, oral loading with N-carbamoyl-L-glutamic acid has been advocated as both diagnostic and therapeutic for patients with NAG synthetase deficiency.

Imaging Studies

• Some authors advocate baseline MRI studies in patients with confirmed genetic causes of hyperammonemia; this is because some suggestive data indicate an elevated risk for stroke in these patients.

Medical Care - Hyperammonemia is a medical emergency because of the neurotoxicity, which is a direct effect of ammonia on the CNS. Initial management should consist of protein intake cessation with the provision of as many nonprotein calories as is practical via intravenous routes, oral routes, or both (if possible). More specific therapy depends on the etiology of the hyperammonemia. Hemodialysis, intravenous sodium phenylacetate/benzoate (Ammonul), or both may be needed.

Consultations

• Geneticist - The role of the biochemical geneticist is to assist in interpretation of available laboratory tests toward a diagnosis of a specific genetic entity. Treatment must be guided by a professional experienced in the treatment of urea cycle disorders. This professional may be a biochemical geneticist, clinical geneticist, endocrinologist, or pediatrician, depending on the expertise available at a specific institution. If such a diagnosis is confirmed, the patient requires long-term follow-up with a geneticist.
• Neurologist - The role of the neurologist is to provide a basic status evaluation for later reference during follow-up care. This evaluation is especially useful in the primary genetic entities, in which recurrence is a virtual certainty and the risk of additional nervous system compromise exists.
• Gastroenterologist - A gastroenterologist may be of assistance in evaluation of liver disease or when hepatic transplant is considered as therapy for a urea cycle defect.

Diet - Dietary therapy greatly depends on the etiologic diagnosis. Protein restriction is helpful in most cases, and restriction of specific amino acids may be imperative in treatment of particular entities. Dietary treatment of urea cycle disorders is highly specialized and usually requires consultation with a registered dietitian who works in a metabolic disease clinic.

Mol Genet Metab. 2004 Apr.
Ammonia toxicity to the brain and creatine.
Bachmann C, Braissant O, Villard AM, Boulat O, Henry H.
Laboratoire Central de Chimie Clinique, Centre Hospitalier Universitaire Vaudois, University of Lausanne, LCC, CHUV, Bugnon 46, 1011 Lausanne, Switzerland.

Symptoms of hyperammonemia are age-dependent and some are reversible. Multiple mechanisms are involved. Hyperammonemia increases the uptake of tryptophan into the brain by activation of the L-system carrier while brain glutamine plays a still undefined role. The uptake of tryptophan by the brain is enhanced when the plasma levels of branched-chain amino acids competing with the other large neutral amino acids are low. Hyperammonemia increases the utilization of branched-chain amino acids in muscle when ketoglutarate is low, and this is further enhanced by glutamine depletion (as a result of therapy with ammonia scavengers like phenylbutyrate). Anorexia, most likely a serotoninergic symptom, might further aggravate the deficiency of indispensable amino acids (e.g., branched-chain and arginine). The role of increased glutamine production in astrocytes and the excitotoxic and metabotropic effects of increased extracellular glutamate have been extensively investigated and found to differ between models of acute and chronic hyperammonemia. Using an in vitro model of cultured embryonic rat brain cell aggregates, we studied the role of creatine in ammonia toxicity. Cultures exposed to ammonia before maturation showed impaired cholinergic axonal growth accompanied by a decrease of creatine and phosphocreatine, a finding not observed in mature cultures. By using different antibodies, we have shown that the phosphorylated form of the intermediate neurofilament protein is affected. Adding creatine to the culture medium partially prevents impairment of axonal growth and the presence of glia in the culture is a precondition for this protective effect. Adequate arginine substitution is essential in the treatment of urea cycle defects as creatine is inefficiently transported into the brain.

See also: Creatine synthesis and transport disorders

Eur J Pediatr. 2003 Nov.
Hyperammonaemia as a cause of psychosis in an adolescent.
Bélanger-Quintana A, Martínez-Pardo M, García MJ, Wermuth B, Torres J, Pallarés E, Ugarte M.
Unidad de Enfermedades Metabólicas, Servicio de Pediatría, Hospital Ramón y Cajal, Ctra de Colmenar Km 9.1, Madrid, Spain.

Diseases that cause hyperammonaemia usually appear during the neonatal period or during the first months of life as severe neurological metabolic distress. In some cases, as the one reported here, the age of onset and initial symptoms are non-specific and the episodes of acute metabolic encephalopathy may be attributed to encephalitis, poisoning or psychiatric problems. Our patient had N-acetyl glutamate synthetase deficiency due to a lack of activation by L-arginine. Treatment with N-carbamylglutamate was successful in maintaining normal ammonia levels. CONCLUSION: We emphasise the importance of measuring ammonia levels in patients with neurological or psychiatric symptoms as part of their diagnostic work-up.

J Neurosci Res. 1998 Jan 15.
Roles of neuroactive amino acids in ammonia neurotoxicity.
Albrecht J.
Department of Neurotoxicology, Medical Research Centre, Polish Academy of Sciences, Warsaw.

Many neurologic disorders are related to congenital or acquired hyperammonemia (HA). Advanced symptoms of HA range from seizures in acute stages to stupor and coma in more chronic conditions, manifesting variable imbalance between the inhibitory and excitatory neurotransmission. Evidence obtained with the use of experimental HA models suggests that acute neurotoxic effects of ammonia are mediated by overactivation of ionotropic glutamate (GLU) receptors, mainly the N-methyl-D-aspartate (NMDA) receptors, and to a lesser degree the KA/AMPA receptors. NMDA receptor-mediated neurotoxicity may be potentiated by impaired control of their function by metabotropic GLU receptors, which are inactivated by ammonia. Prolonged overactivation of the NMDA receptors upon extended ammonia exposure causes their downregulation. The GLU receptor changes may be related to their excessive exposure to extrasynaptic GLU. Ammonia promotes GLU accumulation in the extrasynaptic space by enhancing its release from neurons, and/or by decreasing its reuptake to the nerve endings and astrocytes, where the effect results from inactivation (downregulation) of the astrocytic glutamate transporter GLT1. Excitotoxic effects of ammonia are augmented by increased synthesis of nitric oxide (NO), which is associated with NMDA receptor activation and/or increased synaptic transport of arginine (ARG). A shift toward neural inhibition is promoted by positive modulation of the gamma-aminobutyric acid (GABA)ergic tone resulting from excessive accumulation in the brain of endogenous central benzodiazepine receptor agonists, and from upregulation of astrocytic peripheral benzodiazepine receptors leading to elevated levels of prognenelone-derived neurosteroids, which positively modulate the GABA(A) receptor complex. Inhibitory neurotransmission may also be favored by enhanced release from astrocytes of an inhibitory amino acid, taurine.

Adv Exp Med Biol. 1994.
Molecular mechanism of acute ammonia toxicity and of its prevention by L-carnitine.
Felipo V, Kosenko E, Miñana MD, Marcaida G, Grisolía S.
Instituto de Investigaciones Citológicas, Fundación Valenciana de Investigaciones Biomédicas, Valencia, Spain.

In summary, we propose that acute ammonia intoxication leads to increased extracellular concentration of glutamate in brain and results in activation of the NMDA receptor. Activation of this receptor mediates ATP depletion and ammonia toxicity since blocking the NMDA receptor with MK-801 prevents both phenomena. Ammonia-induced metabolic alterations (in glycogen, glucose, pyruvate, lactate, glutamine, glutamate, etc) are not prevented by MK-801 and, therefore, it seems that they do not play a direct role in ammonia-induced ATP depletion nor in the molecular mechanism of acute ammonia toxicity. The above results suggest that ammonia-induced ATP depletion is due to activation of Na+/K(+)-ATPase, which, in turn, is a consequence of decreased phosphorylation by protein kinase C. This can be due to decreased activity of PKC or to increased activity of a protein phosphatase. We also show that L-carnitine prevents glutamate toxicity in primary neuronal cultures. The results shown indicate that carnitine increases the affinity of glutamate for the quisqualate type (including metabotropic) of glutamate receptors. Also, blocking the metabotropic receptor with AP-3 prevents the protective effect of L-carnitine, indicating that activation of this receptor mediates the protective effect of carnitine. We suggest that the protective effect of carnitine against acute ammonia toxicity in animals is due to the protection against glutamate neurotoxicity according to the above mechanisms.