Clinical characteristics.

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle and ornithine degradation pathway. Clinical manifestations and age of onset vary among individuals even in the same family.

Neonatal onset (~8% of affected individuals). Manifestations of hyperammonemia usually begin 24-48 hours after feeding begins and can include lethargy, somnolence, refusal to feed, vomiting, tachypnea with respiratory alkalosis, and/or seizures.

Infantile, childhood, and adult onset (~92%). Affected individuals may present with:

• Chronic neurocognitive deficits (including developmental delay, ataxia, spasticity, learning disabilities, cognitive deficits, and/or unexplained seizures);
• Acute encephalopathy secondary to hyperammonemic crisis precipitated by a variety of factors; and
• Chronic liver dysfunction (unexplained elevation of liver transaminases with or without mild coagulopathy, with or without mild hyperammonemia and protein intolerance).

Neurologic findings and cognitive abilities can continue to deteriorate despite early metabolic control that prevents hyperammonemia.

Diagnosis/testing.

The biochemical diagnosis of HHH syndrome is established in a proband with the classic metabolic triad of episodic or postprandial hyperammonemia, persistent hyperornithinemia, and urinary excretion of homocitrulline. The molecular diagnosis of HHH syndrome is established in a symptomatic individual with or without suggestive metabolic/biochemical findings by identification of biallelic pathogenic variants in SLC25A15.

Management.

Treatment of manifestations: Acute and long-term management is best performed in conjunction with a metabolic specialist. Of primary importance is the use of established protocols to rapidly control hyperammonemic episodes by discontinuation of protein intake, intravenous infusion of glucose and, as needed, infusion of supplemental arginine and the ammonia removal drugs sodium benzoate and sodium phenylacetate. Hemodialysis is performed if hyperammonemia persists and/or the neurologic status deteriorates.

Prevention of primary manifestations: Individuals with HHH syndrome should be maintained on an age-appropriate protein-restricted diet, citrulline supplementation, and sodium phenylbutyrate to maintain plasma concentrations of ammonia, glutamine, arginine, and essential amino acids within normal range. Note: Liver transplantation is not indicated when metabolic control can be achieved with this regimen as liver transplantation may correct the hyperammonemia but will not correct tissue-specific metabolic abnormalities that also contribute to the neuropathology.

Surveillance: Routine assessment of height, weight, and head circumference from the time of diagnosis to adolescence. Routine assessment of plasma ammonia concentration, plasma and urine amino acid concentrations, urine organic acids, and urine orotic acid based on age and history of compliance and metabolic control. Routine developmental and educational assessment to assure optional interventions. Attention to subtle changes in mood, behavior, and eating and/or the onset of vomiting, which may suggest that plasma concentrations of glutamine and ammonia are increasing. Periodic neurologic evaluation to monitor for neurologic deterioration even when metabolic control is optimal.

Agents/circumstances to avoid: Excess dietary protein intake; nonprescribed protein supplements such as those used during exercise regimens; prolonged fasting during an illness or weight loss; oral and intravenous steroids; and valproic acid, which exacerbates hyperammonemia in urea cycle disorders.

Evaluation of relatives at risk: Once the pathogenic variants in a family are known, use molecular genetic testing to clarify the genetic status of at-risk relatives to allow early diagnosis and treatment, perhaps even before symptoms occur.

Pregnancy management: In general, pregnant women should continue dietary protein restriction and supplementation with citrulline and ammonia-scavenging medications based on their clinical course before pregnancy. Due to increased protein and energy requirements in pregnancy and, oftentimes, difficulty with patient compliance, weekly to every two-week monitoring of plasma amino acids and ammonia is recommended, especially in the first and third trimester, and close monitoring immediately after delivery.

Genetic counseling.

HHH syndrome is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the SLC25A15 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible. However, the identification of familial SLC25A15 pathogenic variants cannot predict clinical outcome because of significant intrafamilial phenotypic variability.

Suggestive Findings

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome should be suspected in symptomatic individuals with the following age-related clinical, laboratory, and neuroimaging findings.

Establishing the Diagnosis

Genotype-Phenotype Correlations

The SLC25A15 genotype does not correlate with the clinical or biochemical phenotype of HHH syndrome [Fiermonte et al 2003, Camacho et al 2006, Tessa et al 2009].

Nomenclature

The name "hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome" was coined by Vivian Shih, MD in 1969 for the disorder in which a "block in the ornithine metabolic pathway" has biochemical findings "not concordant with those in patients with proven hepatic ornithine transcarbamylase deficiency" [Shih et al 1969].

In 1999 after ORNT1 (now known as SLC25A15), the gene encoding ORNT1, was identified, the term ORNT1 deficiency was introduced and used interchangeably with HHH syndrome [Camacho et al 1999].

HHH syndrome may also be referred to as "ornithine transporter deficiency" or "ornithine translocase deficiency."

Clinical Description

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is characterized by variable clinical presentation and age of onset ranging from the neonatal period to adulthood. Those with neonatal onset are normal for the first 24-48 hours, followed by onset of symptoms related to hyperammonemia (poor feeding, vomiting, lethargy, low temperature, rapid breathing). Those with later onset may present with chronic neurocognitive deficits and/or unexplained seizures, spasticity, acute encephalopathy secondary to hyperammonemic crisis, or chronic liver dysfunction. Neurologic findings and cognitive abilities can continue to deteriorate despite early metabolic control that prevents hyperammonemia.

Unless otherwise indicated, the data used in this chapter are from a total of 122 individuals with HHH syndrome: Martinelli et al 2015 (n=111), six subsequent case reports [Qadri et al 2016, Guan et al 2017, Ono et al 2018, Silfverberg et al 2018, Ho et al 2019, Wild et al 2019] and three unpublished affected individuals [Author, personal observation].

The overall survival rate in individuals with HHH syndrome is 94% (109/116). Of the 21% (13/62) of individuals with HHH syndrome who manifested in the neonatal period, the mortality rate is 15% (2/13). Treatment with a protein-restricted diet resolves hepatic dysfunction (elevated transaminases and coagulopathy). Since hyperammonemia in HHH syndrome responds quickly to treatment, early diagnosis leads to an overall improved long-term outcome regardless of the age of onset.

The long-term neurodevelopmental outcome usually (but not always) correlates with the severity and duration of the hyperammonemic insult.

Pyramidal tract findings and intellectual disability, which range from mild to severe, are generally evident by childhood since almost 70% of HHH syndrome manifests in infancy and childhood [Martinelli et al 2015]. While treatment with a protein-restricted diet prevents postprandial and acute hyperammonemia, outcomes vary. For example, individuals with HHH syndrome with limited clinical manifestations throughout life have been reported [Filosto et al 2013; Silfverberg et al 2018; L Merritt, MD & E Font-Montgomery, MD, personal communication (see Outcomes and Presentation; pdf). Conversely, three of four adults with HHH syndrome who maintained normal levels of plasma ammonia for 11 to 38 years exhibited progressive neurologic and cognitive deterioration with serious outcomes [Kim et al 2012].

Neonatal diagnosis (birth – 1 month). About 8% (9/116) of affected individuals were diagnosed during the neonatal period, usually following an uncomplicated pregnancy and delivery. The clinical course is indistinguishable from that of other neonatal-onset urea cycle disorders: the infant is asymptomatic for the first 24-48 hours and, thereafter, has episodes of poor feeding, vomiting, lethargy, low temperature, and/or rapid breathing related to hyperammonemia (see Table 1). If left untreated seizures, coma, and even death may ensue. Hepatic dysfunction and coagulopathy in the neonatal period are common [Martinelli et al 2015, Wild et al 2019].

Given the small number of case studies published to date, little is known about the long-term outcome of individuals with neonatal onset of HHH syndrome. One child died from hyperammonemic encephalopathy at birth and another at age two months. Survivors ranged in age from one to 23 years – one demonstrated normal development at age six years, four had progressive pyramidal signs, and one with significant neuromotor impairment underwent liver transplantation at age seven years. In eight survivors with neonatal onset, cognitive abilities in four ranged from normal to mild deficiency, three exhibited severe cognitive impairment, and one was a recently reported premature infant [Martinelli et al 2015, Wild et al 2019].

Click here (pdf) for more details about outcomes in the neonatal-onset cases described above.

Infantile (>1 month – 1 year) age at diagnosis. Approximately 10% (12/116) of individuals with HHH syndrome present in this timeframe. Highly variable manifestations in infancy can include hypotonia, lethargy, failure to thrive, seizures, psychomotor delay, hepatomegaly, hepatic dysfunction (coagulopathy and elevated transaminases), hyperammonemia, feeding difficulties, and recurrent vomiting. Some children come to medical attention only after experiencing environmental stressors, most commonly infection.

Unique case studies include an affected individual who presented with fulminant hepatic failure and a recent example of the clinical progression of symptoms in an undiagnosed infant.

Click here (pdf) for more details about the presentation of infantile-onset HHHS described above.

Childhood (>1 year to 12 years) presentation accounts for almost half (56/116) of all HHH syndrome. Children in this group come to medical attention either for findings related to mild hyperammonemia with or without liver dysfunction or for evaluation of developmental and speech delay, dysarthria, intellectual disability, learning disabilities, hyperactivity, recurring vomiting, academic difficulties, spasticity, ataxia, and/or unexplained seizure activity. Environmental triggers (i.e., surgery, infection, medication) may also induce manifestations in previously healthy children.

A salient characteristic of affected individuals diagnosed in childhood who harbor the same SLC25A15 pathogenic variants is marked phenotypic variability.

Click here (pdf) for more details about outcomes in childhood-onset HHHS.

Liver dysfunction, a predominant feature at time of diagnosis, generally manifests as mild coagulopathy and elevated liver enzymes (AST and ALT) with or without hyperammonemia. In a few reports acute liver failure prompted consideration of liver transplantation [Fecarotta et al 2006, Mhanni et al 2008]. However, the liver dysfunction that may occur during the initial clinical presentation does not appear to cause long-term complications. Once the hyperammonemia is treated in a standard manner (see Treatment of Manifestations), the liver dysfunction subsides [Martinelli et al 2015, Ono et al 2018].

Despite early detection and adequate metabolic control (i.e., absence of hyperammonemia), some individuals with HHH syndrome continue to worsen neurologically with pyramidal tract involvement and cognitive decline [Camacho et al 2006, Debray et al 2008, Tessa et al 2009, Martinelli et al 2015]. Subclinical hyperammonemia is thought to be a major factor in neurocognitive decline, but not in the cause or progression of pyramidal syndrome. In some individuals with early-childhood onset, gait abnormalities and spasticity are the predominant findings.

The Urea Cycle Disorders Consortium reported developmental quotients (DQ) in four preschool children (age 4-5 years) with HHH syndrome: two were in the normal range and two were <71. Two also exhibited anxiety and acting out behaviors [Waisbren et al 2016].

Adolescence/adulthood (>12 years) accounts for about one third (39/116) of persons with HHH syndrome. After infancy, these individuals quickly learn to self-restrict their protein intake to avoid the malaise and vomiting that accompanies protein-rich meals. The milder form of the disease and self-adherence to a low-protein diet allow these individuals to lead a relatively symptom-free life and remain undetected until they inadvertently overwhelm their compromised ability to detoxify harmful elevations of plasma ammonia. Ammonia overload may result from catabolic events (i.e., surgery, infection, prolonged fasting, pregnancy, internal bleeding), deviation from a protein-restricted diet, or certain medications (e.g., valproate, steroids).

Individuals diagnosed in adolescence/adulthood may present with recurrent encephalopathy secondary to hyperammonemia (lethargy, disorientation, episodic confusion, unexplained seizures), intellectual disabilities, recurrent vomiting, chronic behavioral problems, cerebellar signs (dizziness, loss of balance, poor coordination, abnormal gait/posture), and pyramidal tract dysfunction (inability to perform fine movements, positive Babinski reflex, muscle weakness, ataxia, hyperreflexia, and spasticity).

Click here (pdf) for more details about outcomes in adolescent/adult-onset HHHS.

Cognitive development in persons with HHH syndrome ranges from normal (34%) to severe impairment (34%), with the majority having normal to mild neurocognitive deficit (59%). In some reports, affected individuals have significant neurologic findings such as spasticity and ataxia without cognitive impairment [Martinelli et al 2015]. Of note, pyramidal signs of the lower extremities (hyperreflexia, clonus, tip-toe gait, and/or spastic ataxia) may develop years after the initial diagnosis [Salvi et al 2001b, Debray et al 2008, Tessa et al 2009].

The Urea Cycle Disorder Consortium reported findings of two successive neurocognitive evaluations given to one adult with HHH syndrome: full scale IQ was 100 and 84 at ages 21 years and 26 years, respectively; performance was significantly diminished across all neuropsychological tests. No cognitive or behavioral issues were noted [Waisbren et al 2016].

Additional clinical biochemical abnormalities in HHHS can include the following:

• Mildly elevated plasma glutamine concentration (1.5- to 2-fold the upper limits of control values)
• Plasma lysine can range from normal to moderately decreased
• Increased urinary excretion of:
  • Orotic acid (2.5- to 12-fold the upper limit of control values)
  • Organic acids. An increase in the urinary excretion of components of the Krebs cycle (succinate, citrate, fumaric, α-ketoglutaric), methylcitrate, and lactate has been documented in a few reports [Korman et al 2004, Fecarotta et al 2006].
• Statistically significant elevation of AFP and pronounced liver ultrasound abnormalities at follow up [Ranucci et al 2019]

Prevalence

Since the description of the first individual with HHH syndrome by Shih et al [1969], approximately 122 individuals with HHH syndrome have been reported [Martinelli et al 2015, Qadri et al 2016, Guan et al 2017, Ono et al 2018, Silfverberg et al 2018, Ho et al 2019, Wild et al 2019].

Summar et al [2013], using two large longitudinal registries in the US (NIH-sponsored Urea Cycle Disorders (UCD) Consortium and The National UCD Foundation) and one in Europe (European Registry and Network for Intoxication-Type Metabolic Diseases), calculated an incidence for HHH syndrome of 1% and 3% of all UCDs in the US and Europe, respectively. In the US, the frequency of HHH syndrome is less than 1:2,000,000 live births [Summar et al 2013].

The incidence of HHH syndrome is highest in individuals of French-Canadian ancestry because of the SLC25A15 founder variant c.562_564delTTC (p.Phe188del) in this population (see Table 8) [Camacho et al 1999, Debray et al 2008]. A study of this founder variant in an isolated northern Saskatchewan population of mixed French-Canadian and Aboriginal descent suggested that the incidence of HHH syndrome in this population is 1:1,550 live births [Sokoro et al 2010].

Another common pathogenic variant, c.535C>T (p.Arg179Ter) (see Table 8), seen in 14% of individuals with HHH syndrome, is frequent in persons with HHH syndrome of Japanese and Middle Eastern descent [Tsujino et al 2000, Salvi et al 2001a].

Hyperammonemia

Most commonly, neonates with hyperammonemia and neonatal-onset HHH syndrome are initially suspected of having sepsis. See Häberle et al [2019] for a comprehensive algorithm for the differential diagnosis of neonatal hyperammonemia based on plasma and urine metabolites.

Like other urea cycle disorders (UCDs), HHH syndrome should be included in the differential diagnosis of any individual with hyperammonemia, including women who experience hyperammonemia during or following pregnancy. The onset and severity of findings in HHH syndrome are more variable and less severe when compared to UCDs like ornithine transcarbamylase (OTC) deficiency or carbamyl phosphate synthase (CPS-I) deficiency (OMIM 237300). UCDs usually present with isolated elevation in plasma ammonia concentration and metabolic alkalosis. Plasma amino acid analysis and acylcarnitine profile, urine amino acid analysis, urine organic acid analysis, and urine orotic acid measurements allow diagnosis of the specific UCD (see Urea Cycle Disorders) or HHH syndrome. Neonates with a UCD may have hypoglycemia.

A complete chemistry panel (CMP), CBC and differential, lactate determination, arterial blood gases, serum creatine kinase (CK), and urinalysis (to check for ketonuria) should always be included in the evaluation of any person with an elevated plasma ammonia concentration to evaluate for conditions including the following:

• Organic acidemias (present with acidosis)
• Lysinuric protein intolerance (associated with low plasma ornithine, lysine, and arginine)
• Fatty-acid oxidation defects (associated with nonketotic hypoglycemia and liver dysfunction). See MCAD Deficiency.
• Pyruvate carboxylase deficiency (presents with lactic acidosis and hypoglycemia)
• Mitochondrial disease. Given that the neurologic nonacute presentation for HHH syndrome may be indistinguishable from primary mitochondrial disease, urine organic acid analysis should also be ordered. In some cases of HHH syndrome, urinary excretion of Krebs cycle components (succinate, fumarate, citrate, and α-ketoglutarate) and lactate have been reported [Korman et al 2004]. This pattern of excretion of organic acids, which is commonly seen in children and adults with defects in mitochondrial complex I or III, may create the impression that persons with HHH syndrome have a primary rather than a secondary mitochondrial defect.

Hyperornithinemia

The only other condition that causes chronic elevations in plasma ornithine concentration is deficiency of ornithine amino transferase (OAT) (OMIM 258870), a mitochondrial matrix enzyme involved in the ornithine degradation pathway. However, OAT deficiency never presents with the neurologic and clinical biochemical features of HHH syndrome (e.g., elevation in plasma ammonia concentration and glutamine, urinary excretion of homocitrulline and/or orotic acid). OAT deficiency presents mostly with ophthalmologic findings known as hyperornithinemia with gyrate atrophy of the choroid and retina that manifest as chorioretinal degeneration with loss of peripheral vision, night blindness, and often posterior subcapsular cataracts [Kim et al 2013].

Homocitrullinuria

Homocitrulline is a by-product of canned milk production that arises from the reaction of cyanate and the terminal ε-amino group of lysine. In canned formulas, cyanate is produced from heat-induced urea breakdown. When homocitrulline is consumed in the diet from sources such as these, it is absorbed in the small intestine via a transport system similar to that of cationic amino acids and excreted in the urine. In contrast, homocitrullinuria detected in neonates given IV glucose only (and no dietary source of protein) indicates the presence of a metabolic disorder.

Some individuals with lysinuric protein intolerance (LPI) have been shown to excrete homocitrulline [Habib et al 2013]. Although these individuals may also have hyperammonemia, their clinical biochemical profile demonstrates low concentrations of plasma ornithine, lysine, and arginine and persistent urinary excretion of lysine, ornithine, and arginine.

Neurologic Findings

In those individuals with early-childhood onset HHH syndrome in whom gait abnormalities and spasticity predominate, the differential diagnosis should also include early-onset inherited spastic paraplegia (see Hereditary Spastic Paraplegia Overview).

Arginase I deficiency is the only other urea cycle disorder (UCD) in which a prominent manifestation is progressive pyramidal tract involvement leading to spastic paraparesis. In general, spastic paraparesis in arginase I deficiency manifests earlier in childhood.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs of an individual diagnosed with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, the evaluations summarized in Table 4 (if not performed as part of the evaluation that led to diagnosis) are recommended.