Nonketotic hyperglycinemia: diffusion magnetic resonance imaging findings

Brain imaging in classic nonketotic hyperglycinemia: Quantitative analysis and relation to phenotype

Patients with severe nonketotic hyperglycinemia (NKH) have absent psychomotor development and intractable epilepsy, whereas attenuated patients have variable psychomotor development and absent or treatable epilepsy; differences in brain magnetic resonance imaging (MRI) between phenotypes have not been reported. In a retrospective cross-sectional study, we reviewed 38 MRI studies from 24 molecularly proven NKH patients, and 2 transient NKH patients. Quantitative analyses included corpus callosum size, apparent diffusion coefficient, automated brain volumetric analysis, and glycine/creatine ratio by spectroscopy. All patients age <3 months had restricted diffusion in the posterior limb of the internal capsule, anterior brainstem, posterior tegmental tracts, and cerebellum, not present in transient NKH. In older infants, the pattern evolved and included generalized diffusion restriction in the supratentorial white matter, which quantitatively peaked between 3 and 12 months. No patient had absent corpus callosum or gyral malformation. The corpus callosum was relatively short in severe compared to attenuated phenotypes, and thin in severe cases only. The corpus callosum growth rate differed by severity; age-matched Z-scores of thickness worsened in severe cases only. Cerebral volume was decreased in the hippocampus, globus pallidus, cerebral cortex, thalamus, and cerebellum. Severe patients had greatest glycine/creatine ratios. In this study, no brain malformations were identified. The growth failure of the corpus callosum is worse in severe NKH, whereas the diffusion restriction pattern, reflecting microspongiosis, does not discriminate by phenotypic severity. NKH is therefore a disorder of brain growth best recognized in the corpus callosum, whereas spongiosis is not prognostic.

Nonketotic hyperglycinemia: spectrum of imaging findings with emphasis on diffusion-weighted imaging

PURPOSE

The purpose of this study was to explore brain abnormalities in nonketotic hyperglycinemia (NKH) using diffusion-weighted imaging (DWI) and when feasible, diffusion tensor imaging (DTI) and tractography.

METHODS

Seven patients with confirmed diagnosis of NKH (8 days-2 years) underwent brain MRI. Conventional T1 and T2WI were acquired in all patients, DWI in six and DTI and tractography in two (4 months and 2 years). Measurements of fractional anisotropy (FA), radial diffusivity (RD), axial diffusivity (AD) and Trace from eight white matter regions were compared between the two patients and age-matched controls. Tractography of corpus callosum, superior longitudinal fasciculus and corticospinal tracts was performed with extraction of their FA and diffusivity indices.

RESULTS

MRI showed nonspecific brain atrophy in three children. Corpus callosum atrophy was found as a part of these atrophic changes. Cerebellar vermian hypoplasia and supratentorial hydrocephalus were seen in one patient. The topographic distribution of diffusion restriction was different among patients. The affected white matter regions were not predominantly following the expected areas of myelination according to patients' age. Deep grey matter nuclei were affected in one patient. DTI revealed lower FA with higher RD in most of the measured white matter regions and tracts. These changes were more appreciated in the 2-year-old patient. However, Trace was higher in the 2-year-old patient and lower in the 4-month-old one. The extracted tracts were decreased in volume.

CONCLUSION

DWI, DTI and tractography with FA and diffusivity measurements can give insights into white matter microstructural alterations that can occur in NKH.

Detection of nonketotic hyperglycinemia in a neonate using proton magnetic resonance spectroscopy

Nonketotic hyperglycinemia (NKH) is a rare metabolic disorder caused by a defect in the glycine cleavage enzyme system, resulting in high glycine concentrations in the brain. We report a neonate in which proton magnetic resonance spectroscopy provided biochemical evidence of elevated brain glycine levels and facilitated early diagnosis of NKH and guided clinical management.

Biochemical and molecular predictors for prognosis in nonketotic hyperglycinemia

Objective

Nonketotic hyperglycinemia is a neurometabolic disorder characterized by intellectual disability, seizures, and spasticity. Patients with attenuated nonketotic hyperglycinemia make variable developmental progress. Predictive factors have not been systematically assessed.

Methods

We reviewed 124 patients stratified by developmental outcome for biochemical and molecular predictive factors. Missense mutations were expressed to quantify residual activity using a new assay.

Results

Patients with severe nonketotic hyperglycinemia required multiple anticonvulsants, whereas patients with developmental quotient (DQ) > 30 did not require anticonvulsants. Brain malformations occurred mainly in patients with severe nonketotic hyperglycinemia (71%) but rarely in patients with attenuated nonketotic hyperglycinemia (7.5%). Neonatal presentation did not correlate with outcome, but age at onset ≥ 4 months was associated with attenuated nonketotic hyperglycinemia. Cerebrospinal fluid (CSF) glycine levels and CSF:plasma glycine ratio correlated inversely with DQ; CSF glycine > 230 μM indicated severe outcome and CSF:plasma glycine ratio ≤ 0.08 predicted attenuated outcome. The glycine index correlated strongly with outcome. Molecular analysis identified 99% of mutant alleles, including 96 novel mutations. Mutations near the active cleft of the P‐protein maintained stable protein levels. Presence of 1 mutation with residual activity was necessary but not sufficient for attenuated outcome; 2 such mutations conferred best outcome. Divergent outcomes for the same genotype indicate a contribution of other genetic or nongenetic factors.

Interpretation

Accurate prediction of outcome is possible in most patients. A combination of 4 factors available neonatally predicted 78% of severe and 49% of attenuated patients, and a score based on mutation severity predicted outcome with 70% sensitivity and 97% specificity. Ann Neurol 2015;78:606–618

Evidence that glycine induces lipid peroxidation and decreases glutathione concentrations in rat cerebellum

Patients with non-ketotic hyperglycinemia (NKH) present severe neurological symptoms and brain abnormalities involving cerebellum. Although the pathomechanisms underlying the cerebellum damage have not been studied, high tissue levels of glycine (GLY), the biochemical hallmark of this disorder have been suggested to contribute to the neuropathology of this disease. We investigated the in vitro effects of GLY on important parameters of oxidative stress and energy metabolism in cerebellum of 30-day-old rats. Our results show that GLY increased 2',7'-dichlorofluorescin oxidation, suggesting that reactive species production are augmented by GLY in the cerebellum. However, hydrogen peroxide generation was not altered by GLY. GLY also increased thiobarbituric acid-reactive substances (TBA-RS) levels and reduced the glutathione (GSH) content, indicating that this amino acid provokes lipid oxidative damage and compromises the non-enzymatic antioxidant defenses, respectively, in cerebellum. The antioxidants melatonin and trolox (the hydrosoluble analog of vitamin E) prevented the GLY-induced increase of TBA-RS and decrease of GSH in cerebellum, indicating the involvement of hydroxyl and peroxyl radicals in these effects. The NMDA receptor antagonist MK-801 also attenuated GLY-induced decrease of GSH, suggesting that this effect is mediated through NMDA receptor. In contrast, GLY did not alter the protein carbonyl formation and total and protein-bound sulfhydryl group content, as well as catalase and superoxide dismutase activities. Furthermore, GLY did not alter the activities of the respiratory chain complexes and creatine kinase. Our present data indicate that oxidative stress elicited by GLY in vitro may be a potential pathomechanism involved in the cerebellar dysfunction observed in NKH.

Characteristic MRI findings in neonatal nonketotic hyperglycinemia due to sequence changes in GLDC gene encoding the enzyme glycine decarboxylase

Neonatal metabolic encephalopathy may be related to electrolyte imbalances, endocrine dysfunction, or inborn errors of metabolism. The metabolic encephalopathies are always a diagnostic challenge to the neonatologist and pediatricians. This is more so because the signs and symptoms of are nonspecific and are often similar to those with other neonatal emergencies. Clinical suspicion of neonatal encephalopathy should be considered in any infant exhibiting an abnormal level of consciousness, seizures, tone and reflex abnormalities, apnea, aspiration, and feeding difficulties. A definitive diagnosis is frequently not possible during the neonatal care unit or emergency department. But the proper initial management based on the probable diagnosis can be life-saving or reduce neurologic sequelae. In the case of inborn errors of metabolism, imaging may play a vital role either in limiting the differential diagnosis or yet times pointing towards the specific diagnosis or error of metabolism. We report DWI-ADC changes on MRI in the acute stages of neonatal nonketotic hyperglycinemia (NKH) due to sequence changes in GLDC gene.

Reversible diffusion weighted imaging changes in propionic acidemia

Propionic acidemia is an inborn error of metabolism with neurologic manifestations. We describe a 3-year-old boy with propionic acidemia presenting with a metabolic crisis including headache, vomiting, and altered mental status with metabolic acidosis. Electroencephalography showed focal slowing in right temporal region. Magnetic resonance imaging (MRI) of the brain showed restricted diffusion with apparent diffusion coefficient correlate in the right parietooccipital region. Correction of metabolic acidosis led to clinical improvement and normalization of MRI diffusion weighted imaging/apparent diffusion coefficient changes. This article suggests that restricted diffusion resulting from metabolic crises in propionic acidemia may be reversible in some cases.

MRI of childhood epilepsy due to inborn errors of metabolism

OBJECTIVE

The purpose of this pictorial essay is to classify epilepsy syndromes due to inborn errors of metabolism according to age at onset and type of seizure and to show the MRI features of many of the syndromes.

CONCLUSION

Epilepsy syndromes due to inborn errors of metabolism are rare, but it is important to detect them at an early stage because some are treatable. MRI is an important tool in the assessment of these patients.

The efficacy of vagus nerve stimulation in intractable epilepsy associated with nonketotic hyperglycinemia in two children

Nonketotic hyperglycinemia is an inborn error of glycine metabolism and these patients frequently suffer from intractable epilepsy despite treatment with sodium benzoate, dextromethophan, and multiple anticonvulsants. We encountered 2 infants with nonketotic hyperglycinemia whose intractable generalized convulsive seizures were difficult to control with sodium benzoate, dextromethophan, and multiple anticonvulsants. However, after the addition of vagus nerve stimulation, their intractable generalized seizures were >75% reduced in frequency, the numbers of multiple anticonvulsants were reduced, and the quality of life significantly improved. The efficacy in seizure reduction persists for at least 3 years in both children.

Agenesis of the corpus callosum and cerebral anomalies in inborn errors of metabolism

Dysgenesis of the corpus callosum has been recognized as a marker for aberrant development of the central nervous system. It has been suggested that developmental defects of the corpus callosum may be more frequently encountered in patients with inborn errors of metabolism. The objectives of the present study were to determine the prevalence of developmental defects of the corpus callosum in patients attending a genetics-metabolic disorders clinic, to describe the spectrum of abnormalities in brain development in patients with confirmed inborn errors of metabolism and abnormalities of the corpus callosum as ascertained by neuroimaging and/or postmortem studies. Nineteen patients (10 males, 9 females) with confirmed metabolic diagnoses were identified by systematic search of the genetics clinic database. All 19 (100%) expressed variable degrees of hypoplasia, complete or partial agenesis (ACC). Abnormalities of head size were noted in 17/19 (89.5%). The majority 12/17 (70.5%) were associated with microcephaly, while macrocrania was noted in 5/17 (29.5%). Associated central nervous system (CNS) anomalies included abnormalities in ventricular morphology in 18/19 (94.7%), ventriculomegaly in 11/19 (63.1%), increased extraxial cerebrospinal fluid space in 11/19 (57.9%), changes in the gray matter (neuronal migration defects, porencephaly) in 9/19 (47.3%), white matter changes in 12/19 (63.1%) and abnormalities of the posterior fossa and hindbrain in 12/19 (63.1%). In patients with inborn errors of metabolism, dysgenesis of the corpus callosum serves as a marker for other developmental defects within the nervous system. We discuss here potential mechanisms by which metabolic defects affect diverse biochemical pathways, altering key neurobiological processes (e.g. defective cell membrane formation, cellular bioenergetics and cell-to-cell signaling), that eventually lead to structural abnormalities in the developing nervous system.

Clinical variability in glycine encephalopathy

Glycine encephalopathy (GCE) is an autosomal recessive error of glycine degradation, resulting in a poor outcome with severe mental retardation, intractable seizures and spasticity. Milder variants with a significantly better outcome have been reported, but an early prediction of the long-term outcome is not yet possible. With regard to the long-term outcome, the data reported in the literature of children with different GCE forms were compared. Determination of cerebrospinal fluid and plasma glycine concentrations at the time of diagnosis were not useful in differentiating mild and severe outcomes. By contrast, several clinical parameters correlate with a poor outcome: spastic quadriparesis, truncal hypotonia, typical electroencephalography patterns, congenital and cerebral malformations (e.g., corpus callosum hypoplasia). Hyperactivity, behavioral problems and choreiform movement disorders are associated with a milder outcome. Thus, prediction of the outcome of GCE may be facilitated by searching for selected clinical parameters. In addition, early neuroimaging may be a valuable tool in predicting the outcome of GCE.

Diffusion-weighted MR imaging in leukodystrophies

Leukodystrophies are genetically determined metabolic diseases, in which the underlying biochemical abnormality interferes with the normal build-up and/or maintenance of myelin, which leads to hypo- (or arrested) myelination, or dysmyelination with resultant demyelination. Although conventional magnetic resonance imaging has significantly contributed to recent progress in the diagnostic work-up of these diseases, diffusion-weighted imaging has the potential to further improve our understanding of underlying pathological processes and their dynamics through the assessment of normal and abnormal diffusion properties of cerebral white matter. Evaluation of conventional diffusion-weighted and ADC map images allows the detection of major diffusion abnormalities and the identification of various edema types, of which the so-called myelin edema is particularly relevant to leukodystrophies. Depending on the nature of histopathological changes, stage and progression gradient of diseases, various diffusion-weighted imaging patterns may be seen in leukodystrophies. Absent or low-grade myelin edema is found in mucopolysaccharidoses, GM gangliosidoses, Zellweger disease, adrenomyeloneuropathy, L-2-hydroxyglutaric aciduria, non-ketotic hyperglycinemia, classical phenylketonuria, Van der Knaap disease and the vanishing white matter, medium grade myelin edema in metachromatic leukodystrophy, X-linked adrenoleukodystrophy and HMG coenzyme lyase deficiency and high grade edema in Krabbe disease, Canavan disease, hyperhomocystinemias, maple syrup urine disease and leukodystrophy with brainstem and spinal cord involvement and high lactate.

Magnetic resonance imaging in neonatal nonketotic hyperglycinemia

This report presents two neonates with nonketotic hyperglycinemia in whom conventional magnetic resonance imaging revealed structural cerebral abnormalities, diffusion-weighted imaging indicated abnormalities of myelinated white matter, and magnetic resonance spectroscopy provided biochemical evidence of elevated cerebral glycine levels. The early use of combined magnetic resonance modalities in these severely affected infants helped in prognostication and clinical management.

Four cases of neonatal non-ketotic hyperglycinaemia

Non-ketotic hyperglycinaemia is an autosomal recessive disorder of glycine metabolism caused by a defect in the glycine cleavage system. Affected neonates present with lethargy, feeding difficulty, hypotonia, apnoea, poorly controlled convulsions and coma. Four cases are reported, three of whom died in the neonatal period. The fourth case was treated with dextromethorphan and sodium benzoate. He survived with neurodevelopmental delay but is now almost seizure-free.