A 9-month-old boy with a history of seizures underwent a neurologic and biochemical-genetic evaluation. The brain MRI results were compatible with a diagnosis of Leigh disease, also known as subacute necrotizing encephalomyelopathy, a rare neurometabolic disorder that affects the central nervous system. The patient had been prescribed several antiepileptic medications, including levetiracetam, lamotrigine, phenobarbital, vigabatrin, and topiramate.
Student Discussion Document (pdf)
Adam S. Ptolemy,1* Yijun Li,2 Tamara Sanderson,1 Omar Khwaja,3 Gerard T. Berry,2 and Mark Kellogg1
1Department of Laboratory Medicine; 2Manton Center for Orphan Disease Research, Division of Genetics, Department of Pediatrics; and 3 Department of Neurology, Children’s Hospital Boston, Harvard Medical School, Boston, MA.
*Address correspondence to this author at: Department of Laboratory Science, Covance Central Laboratory Services Inc., 8211 SciCor Dr., Indianapolis, IN 46214. Fax 317-273-7990; e-mail email@example.com.
A 9-month-old boy with a history of seizures underwent a neurologic and biochemical-genetic evaluation. The brain MRI results were compatible with a diagnosis of Leigh disease, also known as subacute necrotizing encephalomyelopathy, a rare neurometabolic disorder that affects the central nervous system. The patient had been prescribed several antiepileptic medications, including levetiracetam, lamotrigine, phenobarbital, vigabatrin,
and topiramate. Metabolic screening for free amino acids was performed on the child’s urine, with concentrations quantified with an automated amino acid analyzer (Hitachi L-8800). This commercially available system couples ion-exchange liquid chromatography
with postcolumn ninhydrin derivatization before UV detection. This analysis revealed a very large peak with a retention time consistent with the elution of tryptophan (Fig. 1). The calculated urinary excretion was 125 204 μmol/g creatinine. In addition, the urinary
concentrations of γ-aminobutyric acid, β-alanine, β-aminoisobutyric acid, and glutamine were also increased substantially. The concentrations of the remaining amino acids were within their respective reference intervals. For confirmation, we submitted a urine aliquot to an external reference laboratory for analysis by liquid chromatography–tandem mass spectrometry. This analysis revealed a tryptophan excretion of 71 μmol/g creatinine (reference interval, 15–302 μmol/g creatinine).
Questions to Consider
- What are 2 pathologic conditions that produce large increases in urine tryptophan?
- What could potentially induce false increases in tryptophan concentrations when urine amino acids are quantified by ion-exchange liquid chromatographic separations and postcolumn ninhydrin derivatization before detection?
- On the basis their respective chemical structures, do any of the prescribed antiepileptic drugs (AEDs) react with ninhydrin and thus interfere with tryptophan measurement?
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