This article includes discussion of HHH syndrome, hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, and triple H syndrome. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
HHH syndrome is an inherited urea cycle disorder caused by deficiency of the mitochondrial 1 ornithine transporter (ORNT1) transferring ornithine from the cytosol to hepatic mitochondria for the ornithine transcarbamylase reaction.
Thus, the ornithine transporter is essential for function of the urea cycle. Absence or deficiency of the transporter leads to a hyperammonemia disorder, specifically hyperornithinemia, hyperammonemia, and homocitrullinuria. Affected individuals suffer progressive neurologic sequelae and mental impairment or even early death in some severely affected individuals. Severe neonatal forms are rare but may result in hyperammonemic coma within the neonatal period (28 days or younger; neonatal- or early-onset), whereas presentations in later life (older than 28 days; late-onset) during infancy, childhood, adolescence, or adulthood cause variable progressive neurologic disease. Biochemical markers include elevated plasma glutamine and ornithine and normal to low-normal plasma lysine, citrulline, and arginine as well as orotic aciduria and especially homocitrullinuria. Diagnosis is confirmed by molecular genetic analysis or, rarely, by enzymatic testing. Treatment consists of long-term dietary therapy supplemented with L-citrulline/L-arginine and, if necessary, ammonia scavengers. Intercurrent infections, vaccinations, or conditions that are likely to induce a catabolic state might lead to metabolic decompensations and hyperammonemic crises. In case of hyperammonemia, transient stop of protein intake, adequate fluid, glucose substitution, and first-line medications must be administered. Currently, international networks for rare metabolic diseases (UCDC established in 2006; E-IMD established in 2011; JUCDC established in 2012) aim to more completely describe the natural history, especially the initial and evolving clinical phenotypes of urea cycle disorders (UCDs) such as HHH syndrome (53). These networks collect systematic data to improve the clinical knowledge, develop guidelines, and provide patients and professionals with reliable data on disease manifestation, complications, and long-term outcomes of urea cycle disorders.
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• HHH syndrome is one of the rarest disorders of the urea cycle with variable clinical phenotype.
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• Hyperammonemia must be avoided as much as possible.
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• Biochemical, enzymatic, or molecular genetic analyses are necessary for confirmation of the diagnosis.
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• Emergency and long-term treatment protocols, as suggested by Haberle and colleagues (27), should be available in all specialized pediatric hospitals.
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• Neurologic outcome depends on noninterventional parameters, eg, intrinsic disease severity (reflected by onset type and initial peak plasma ammonium concentration during the first metabolic decompensation). The impact of interventional parameters, eg, diagnostic and therapeutic interventions, on clinical outcome remains to be elucidated.
Historical note and terminology
A 3-year-old boy with cognitive impairment and myoclonic seizures as well as intermittent hyperammonemia, abnormally high plasma L-ornithine levels, and homocitrullinuria was described by Shih and colleagues in 1969 (51). The term “hyperornithinemia-hyperammonemia-homocitrullinuria syndrome” (HHH syndrome) was coined to characterize the specific biochemical pattern that has been observed.