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HHH syndrome

HHH syndrome Causes

Hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome is a genetic/metabolic disorder caused by a defect in the ORNT1, a mitochondrial ornithine transporter.

HHH syndrome Definition

Hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome is a very rare inborn error of metabolism; the age at presentation and long-term prognosis vary widely among affected individuals. Growth and developmental delays, periodic confusion and ataxia, and learning disabilities (especially speech delay), are typical presenting symptoms. In this syndrome, a defect in the transport of ornithine into the mitochondrial matrix significantly hinders the urea cycle, thereby impeding nitrogen disposal.

HHH syndrome Diagnosis

Newborn screening of dried blood spots using tandem mass spectrometry (MS/MS) is capable of identifying Ornithine. HHH Syndrome patients have elevated Ornithine levels of five to ten times the normal amount. Alanine may be heightened. Hyperammonemia arises postprandially and is chronically elevated on a high protein diet, but may be normal when fasting.

HHH syndrome Symptoms and Signs

Most affected patients display some symptoms, such as lethargy, vomiting, ataxia or chroeoathetosis, impaired growth and delayed development. Mild to profound mental retardation is usually evident by childhood. Over time, patients will gravitate to a diet low in milk and meat during their childhood.

HHH syndrome Treatment

Few patients with HHH Syndrome have been treated from an early age, before the onset of disabling symptoms. Dietary restriction of protein is the basic treatment, with supporting therapy for the prevention and control the hyperammonemia. A trial of Ornithine, Arginine, or Citrulline supplementation may decrease plasma ammonia.

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