Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) syndrome is a rare urea cycle disorder caused by mutations in the SLC25A15 gene, leading to metabolic and neurological impairments. Liver transplantation (LT) may restore urea cycle function and prevent disease progression. This retrospective study analyzed six patients with HHH syndrome who underwent LT between 2016 and 2022. Pre- and post-transplant evaluations included biochemical tests, genetic analysis, neurological assessments, and quality-of-life measures. LT successfully normalized metabolic parameters (ammonia and amino acid levels) and allowed patients to resume normal diets. Early transplantation resulted in neurological improvement in 5 of 6 patients (83.3%), including reduced lower limb spasticity and improved walking ability. Two patients (33.3%) achieved nearly normal gait, and one patient (16.7%) recovered to normal motor function within three months after LT. Quality-of-life scores improved in 2 patients (33.3%). The overall survival rate was 83.3%, with one patient dying from unrelated causes 5 years post-transplant. No significant long-term complications were observed in the surviving patients. Liver transplantation is an effective treatment for HHH syndrome, halting neurological decline and improving quality of life. Early LT before irreversible damage provides the best outcomes, making it a viable option for patients with progressive symptoms unresponsive to conventional therapies. Not applicable.

Arginase 1 deficiency (ARG1-D) is a rare hereditary urea cycle disorder characterized by elevated arginine levels, resulting in progressive neurological impairment and severe physical and cognitive disability. Due to its low prevalence, overlapping symptoms with other neurological disorders, and current limitations in newborn screening tools, ARG1-D is often misdiagnosed or diagnosed late, limiting access to early interventions. This review and expert opinion aim to provide an overview of the clinical manifestations, diagnostic challenges, and treatment options for ARG1-D, offering a practical resource for specialists to recognize this rare, progressive, yet treatable disease. ARG1-D typically presents with progressive spastic paraplegia, developmental delays, cognitive impairment, and seizures, with symptom onset and severity varying by age. Differential diagnoses mainly include hereditary spastic paraplegia, cerebral palsy, and hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, each with distinct clinical features and biochemical markers. Potential red flags for ARG1-D include elevated plasma arginine levels, spasticity, seizures, and cognitive impairment. These should prompt further examinations to confirm the diagnosis, which is based on biochemical assays for hyperargininemia and on genetic testing. Once confirmed, early treatment is advised, including dietary protein restriction, ammonia scavengers, antiepileptic drugs, and novel therapies, such as pegzilarginase, which targets the disease's metabolic root. Experts stress the importance of increased awareness of ARG1-D characteristics, noting that early recognition and treatment are crucial to patient outcomes. Greater recognition of ARG1-D's distinctive features, differential diagnosis, and diagnostic tools, even among non-specialist clinicians, could help prevent misdiagnoses and facilitate the identification of this rare yet treatable condition. 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. 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. 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 adherence 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 adherence, 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. 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.

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive urea cycle disorder caused by defective hepatic ornithine transport, leading to hyperammonemia and progressive neurological complications. We report the case of a patient who was treated for hyperammonemic crisis at birth and subsequently diagnosed with HHH syndrome. Management, including ammonia-lowering therapy and a low-protein diet, was initiated; however, due to significant socioeconomic barriers, he was lost to follow-up from the age of two. He re-presented at the age of 12 in a severely debilitated state with global developmental delay and refractory epilepsy. Investigations demonstrated radiological evidence of neurological damage, including corpus callosal atrophy, alongside biochemical and ultrasonographic features of hepatic dysfunction. This case highlights the critical importance of sustained treatment, multidisciplinary follow-up, and adequate social support in preventing irreversible complications of HHH syndrome.

Urea cycle disorders (UCDs) are inherited metabolic conditions that lead to inadequate nitrogen detoxification due to defects in urea cycle enzymes or transporters. The severity of UCDs is classified into two types: neonatal onset (severe) and late onset (often milder). This cross-sectional study aimed to assess the levels of intelligence, developmental disabilities, and social functioning in adult patients with UCDs in Japan. A total of 116 adult patients with UCDs were enrolled in the study, including 10 with carbamoyl phosphate synthetase 1 deficiency, 69 with ornithine transcarbamylase deficiency (OTCD), 17 with argininosuccinate synthetase deficiency, 9 with argininosuccinate lyase deficiency, 4 with arginase 1 deficiency, and 7 with Hyperornithinemia-Hyperammonemia-Homocitrullinuria syndrome. Of these, 25 (21.6 %) developed symptoms during the neonatal period (within 28 days after birth), while 86 (74.1 %) presented with symptoms after 28 days of age. The age of onset was unknown in 5 patients. This study included 111 surviving patients and 5 deceased patients (3 with OTCD and 2 with CPS1D). Fifty-three patients (45.7 %) experienced intellectual disabilities, while 48 (41.4 %) had non-intellectual disabilities. Additionally, learning disorders and communication disorders were common among many of the study participants. Sixty patients (51.7 %) graduated from regular high school, and most patients with intellectual disabilities graduated from special education schools. Almost half of the patients (51, 44.0 %) were able to obtain jobs, including simple tasks in supported workplaces, and received compensation for their work. Notably, more patients with OTCD could demonstrate higher social performance including experience of higher education and marriage. However, even OTCD patients without intellectual disabilities often struggled with specific neurobehavioral issues. This study provides information on the social situation of adult UCD patients and underlines the importance for clinicians, as well as society and communities, to understand the ongoing challenges faced by patients with UCDs in order to provide better support.

Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome is an extremely rare disorder of urea cycle, with few patients reported worldwide. Despite hyperammonemia control, the long-term outcome remains poor with progressive neurological deterioration. We report the clinical, biochemical, and molecular features of two Lebanese siblings diagnosed with this disorder and followed for 8 and 15 years, respectively. Variable clinical manifestations and neurological outcome were observed. The patient with earlier onset of symptoms had a severe neurological deterioration while the other developed a milder form of the disease at an older age. Diagnosis was challenging in the absence of the complete biochemical triad and the non-specific clinical presentations. Whole exome sequencing revealed a homozygous variant, p.Phe188del, in the SLC25A15 gene, a French- Canadian founder mutation previously unreported in Arab patients. Hyperammonemia was controlled in both patients but hyperonithinemia persisted. Frequent hyperalaninemia spikes and lactic acidosis occured concomitantly with the onset of seizures in one of the siblings. Variable neurological deterioration and outcome were observed within the same family. This is the first report from the Arab population of the long-term outcome of this devastating neurometabolic disorder.