Argininosuccinic aciduria

Argininosuccinic aciduria is the second most common urea cycle disorder, after ornithine transcarbamylase deficiency. The condition combines a urea cycle problem (hyperammonemia in the catabolic newborn period and beyond) with a separate set of complications that the urea cycle alone does not predict: chronic liver disease, hypertension, and a phenotype involving hair and trichorrhexis nodosa that is unique among the urea cycle disorders. The biology is the reason. Argininosuccinic acid lyase, the enzyme that fails in this condition, also has a role in nitric oxide synthesis. The clinical phenotype reflects both the nitrogen problem and the nitric oxide problem.

What ASA is

Argininosuccinic aciduria, ASA, is an autosomal recessive urea cycle disorder caused by deficiency of argininosuccinate lyase (ASL), encoded by ASL on chromosome 7q11. ASL splits argininosuccinate into arginine and fumarate as the fourth step of the urea cycle. When ASL activity is reduced or absent, argininosuccinate accumulates, the urea cycle cannot complete, ammonia rises, and arginine production from the cycle fails. Plasma argininosuccinic acid is elevated and the urinary excretion is dramatically increased.

The clinical presentation in classical ASA includes neonatal hyperammonemic encephalopathy in the first days to weeks of life, with poor feeding, vomiting, lethargy, hypotonia, and progression to coma if untreated. Survivors of the neonatal crisis face a chronic course that includes recurrent metabolic decompensations, developmental delay, intellectual disability of variable severity, hepatomegaly with elevated transaminases and progressive fibrosis or cirrhosis in some cases, hypertension, neurological features including movement disorders and seizures, and the characteristic hair finding of trichorrhexis nodosa, in which the hair shaft develops nodes that fragment easily.

The hepatic and vascular features of ASA are not predicted by hyperammonemia alone. Argininosuccinate lyase is the enzyme that produces arginine and that, in non-urea-cycle tissues, contributes to nitric oxide synthesis through arginine-citrulline cycling. The clinical picture in ASA reflects both the nitrogen accumulation and the secondary nitric oxide signaling problems, and management has evolved to include attention to both.

A milder late-onset form of ASA presents later in childhood with episodic hyperammonemia, learning differences, hepatomegaly, or hair findings without the dramatic neonatal crisis. The clinical course depends on residual enzyme activity.

Reported live-birth incidence in newborn screening programs runs roughly 1 in 70,000 to 1 in 200,000.

Detection

Newborn screening uses tandem mass spectrometry on the dried blood spot to flag elevated citrulline. The same marker flags citrullinemia type I. Second-tier testing distinguishes the two: ASA shows elevated argininosuccinic acid in plasma and urine, while citrullinemia type I shows elevated citrulline without argininosuccinate accumulation. Confirmation includes plasma amino acids, urine organic acids, plasma ammonia, and ASL gene sequencing.

What management looks like

Standard of care is lifelong protein-restricted diet using essential amino acid medical formula combined with measured natural protein, plus supplementation with arginine. Arginine supplementation in ASA serves two roles: it replaces the arginine the failing urea cycle no longer produces, and it drives the residual ASL activity in some variant combinations to consume more accumulated argininosuccinate. Sodium phenylbutyrate or glycerol phenylbutyrate is used as an ammonia scavenger to help control ammonia in cases where dietary management alone is insufficient.

Acute decompensations are managed with intravenous dextrose, fluid resuscitation, ammonia scavengers, and hemodialysis for severe hyperammonemia. The acute management of urea cycle disorders has evolved into a standardized protocol that ASA shares with the other urea cycle conditions.

Liver transplantation is offered in selected cases with severe disease, intractable metabolic decompensations, or progressive liver disease. Transplant addresses the hyperammonemia by providing a functional liver-based urea cycle, and it can stabilize the hepatic course. The systemic features that derive from nitric oxide deficiency are not fully addressed by transplant, and post-transplant management of hypertension and neurological surveillance continue.

Hepatic surveillance with periodic liver function testing, alpha-fetoprotein, and imaging is part of routine follow-up. Hypertension surveillance is part of routine follow-up. Hair management for trichorrhexis nodosa is a cosmetic and supportive concern rather than a medical one.

Nitric oxide pathway interventions, including L-citrulline supplementation in selected variants and inhaled nitric oxide in case-report contexts, have been explored but are not standard. Gene therapy and other disease-specific approaches are in preclinical development.

What this looks like for a family

A baby is born and the heel-prick is sent. On day 4, the state lab reports an elevated citrulline. On day 5, urine organic acids and plasma argininosuccinic acid confirm ASA. The metabolic team starts protein restriction, arginine supplementation, and ammonia scavenger therapy. Plasma ammonia is checked. The neonatal period is managed without the catastrophic hyperammonemic crisis that historically defined the disease.

The first decade involves recurrent metabolic clinic visits, dietary management, sick-day plans, and ongoing developmental surveillance. Liver function tests are checked at intervals. In the teens, hepatic fibrosis is identified on imaging in some cases. The hepatology team joins the metabolic team. Hypertension surveillance and management become part of the plan. The conversation about whether liver transplantation will alter the hepatic and metabolic trajectory is on the table at intervals across the third decade.

That is what ASA care looks like in practice. The screen prevents the neonatal death. The metabolic management controls the urea cycle. The liver and vascular complications are the long-term clinical anchors that require their own surveillance infrastructure.