In vivo 1H magnetic resonance spectroscopic measurement of brain glycine levels in nonketotic hyperglycinemia

MR spectroscopy in pediatric neuroradiology

Magnetic resonance spectroscopy (MRS), being able to identify and measure some brain components (metabolites) in pathologic lesions and in normal-appearing tissue, offers a valuable additional diagnostic tool to assess several pediatric neurological diseases. In this review we will illustrate the basic principles and clinical applications of brain proton (H¹; hydrogen) MRS (H¹MRS), by now the only MRS method widely available in clinical practice. Performing H¹MRS in the brain is inherently less complicated than in other tissues (e.g., liver, muscle), in which spectra are heavily affected by magnetic field inhomogeneities, respiration artifacts, and dominating signals from the surrounding adipose tissues. H¹MRS in pediatric neuroradiology has some advantages over acquisitions in adults (lack of motion due to children sedation and lack of brain iron deposition allow optimal results), but it requires a deep knowledge of pediatric pathologies and familiarity with the developmental changes in spectral patterns, particularly occurring in the first two years of life. Examples from our database, obtained mainly from a 1.5 Tesla clinical scanner in a time span of 15 years, will demonstrate the efficacy of H¹MRS in the diagnosis of a wide range of selected pediatric pathologies, like brain tumors, infections, neonatal hypoxic-ischemic encephalopathy, metabolic and white matter disorders.

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.

Genetics of neonatal-onset epilepsies

Although the majority of seizures in neonates are related to acute brain injury, a substantial minority are the first symptom of a neonatal-onset epilepsy often linked to a pathogenic genetic variant. Historically, studies on neonatal seizures including treatment response and long-term consequences have lumped all etiologies together. However, etiology has been consistently shown to be the most important determinant of outcome. In the past few years, an increasing number of monogenic disorders have been described and might explain up to a third of neonatal-onset epilepsy syndromes previously included under the umbrella of Ohtahara syndrome and early myoclonic encephalopathy. In this chapter, we define the concept of genetic epilepsy and review the classification. Then, we review the most relevant monogenic neonatal-onset epilepsies, detail their underlying pathophysiologic mechanisms, and present their electroclinical phenotypes. We highlight that, in some cases, such as neonates with KCNQ2 or KCNT1 gene mutations, the early recognition of the electroclinical phenotype can lead to targeted diagnostic testing and precision medicine treatment, enabling the possibility of improved outcome.

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.

Clinical magnetic resonance spectroscopy of the central nervous system

Proton magnetic resonance spectroscopy (1H MRS) is a noninvasive imaging technique that can easily be added to the conventional magnetic resonance (MR) imaging sequences. Using MRS one can directly compare spectra from pathologic or abnormal tissue and normal tissue. Metabolic changes arising from pathology that can be visualized by MRS may not be apparent from anatomy that can be visualized by conventional MR imaging. In addition, metabolic changes may precede anatomic changes. Thus, MRS is used for diagnostics, to observe disease progression, monitor therapeutic treatments, and to understand the pathogenesis of diseases. MRS may have an important impact on patient management. The purpose of this chapter is to provide practical guidance in the clinical application of MRS of the brain. This chapter provides an overview of MRS-detectable metabolites and their significance. In addition some specific current clinical applications of MRS will be discussed, including brain tumors, inborn errors of metabolism, leukodystrophies, ischemia, epilepsy, and neurodegenerative diseases. The chapter concludes with technical considerations and challenges of clinical MRS.

Imaging manifestations of the leukodystrophies, inherited disorders of white matter

The leukodystrophies are a diverse set of inherited white matter disorders and are uncommonly encountered by radiologists in everyday practice. As a result, it is challenging to recognize these disorders and to provide a useful differential for the referring physician. In this article, leukodystrophies are reviewed from the perspective of 4 imaging patterns: global myelination delay, periventricular/deep white matter predominant, subcortical white matter predominant, and mixed white/gray matter involvement patterns. Special emphasis is placed on pattern recognition and unusual combinations of findings that may suggest a specific diagnosis.

The evaluation of treatment-resistant epilepsy

An estimated 10% to 40% of children with epilepsy have treatment-resistant epilepsy. Persistent seizures have negative psychosocial, behavioral, cognitive, and financial consequences and are associated with an increased mortality rate. Accurate syndromic and etiologic diagnoses are of vital importance because they may guide medical and/or surgical decision making. Revisitation of the history to confirm the diagnosis of epilepsy and the appropriateness of medication trials to date is vital. Routine imaging should include structural magnetic resonance imaging (MRI) with an established epilepsy protocol. In the setting of a normal previous MRI, repeat imaging may be indicated and may be supplemented with other imaging modalities. The admission for prolonged inpatient video-encephalographic monitoring may lead to a revision of a pre-existing diagnosis. Laboratory evaluations should include genetic, metabolic, and infectious/inflammatory studies when indicated. In this review, we discuss the implication of seizure semiology and syndrome classification when searching for an underlying diagnosis in treatment-resistant epilepsy, and will review both basic and more advanced procedures/studies that may aid diagnosis.

Neonatal non-ketotic hyperglycinemia

The typical imaging findings of neonatal non-ketotic hyperglycinemia have rarely been described in the radiologic literature with only few individual cases or small series reported. In this article, we present a case of neonatal onset non-ketotic hyperglycinemia, imaged at 6 days of age, and discuss characteristic MRI and MR spectroscopic findings.

Early onset epilepsy and inherited metabolic disorders: diagnosis and management

Epileptic encephalopathies presenting in early life present a diagnostic and therapeutic challenge. These disorders present with multiple seizure types that are treatment resistant and associated with significant abnormalities on electroencephalographic studies. The underlying etiology in many cases may be related to an inborn error of metabolism. Efforts to establish the specific diagnosis of a genetic defect or an inborn error of metabolism often results in requests for a vast array of biochemical and molecular tests leading to an expensive workup. In this review, we present the clinician with information that provides a rationale for a selective and nuanced approach to biochemical assays, and initial treatment strategies while waiting for a specific diagnosis to be established. A careful consideration of the presentation, identification of potentially treatable conditions, and consultation with the biochemical genetics laboratory can lead to a greater measure of success while limiting cost overruns. Such a targeted approach is hoped will lead to an early diagnosis and appropriate interventions.

Intracerebral blood and MRS in neonatal nonketotic hyperglycinemia

Nonketotic hyperglycinemia is an inborn error of glycine metabolism leading to the accumulation of glycine in the brain. The neonatal form presents in the first days after birth with encephalopathy, seizures. and characteristic "hiccups." Rapid progression can lead to intractable seizures, coma, and respiratory failure. The outcome is invariably poor, and many die before age 1 year. The diagnosis of nonketotic hyperglycinemia is traditionally based on the finding of a raised cerebrospinal fluid-to-plasma-glycine ratio. An elevated glycine peak, using long echo time proton magnetic resonance spectroscopy, is also specific to nonketotic hyperglycinemia. The presence of blood in a cerebrospinal fluid sample is known to render the glycine level uninterpretable. However, there have been no reports of intracerebral blood confounding the cerebral glycine detected on long echo time proton magnetic resonance spectroscopy. We report on an infant who presented with neonatal encephalopathy and imaging findings consistent with hypoxic-ischemic encephalopathy, as well as intracerebral hemorrhage and an abnormal glycine peak on spectroscopy. The source of the abnormal glycine on long echo time proton magnetic resonance spectroscopy was intracerebral blood, highlighting an important confounder in the interpretation of long echo time proton magnetic resonance spectroscopy in suspected nonketotic hyperglycinemia.

Which pulse sequence is optimal for myo-inositol detection at 3T?

Optimized myo-inositol (mI) detection is important for diagnosing and monitoring a multitude of pathological conditions of the brain. Simulations are presented in this work, performed to decide which pulse sequence has the most significant advantage in terms of improving repeatability and accuracy of mI measurements at 3T over the pulse sequence used typically in the clinic, a TE = 35 ms PRESS sequence. Five classes of pulse sequences, four previously suggested for optimized mI detection (a short TE PRESS, a Carr-Purcell PRESS sequence, an optimized STEAM sequence, an optimized zero quantum filter), and one optimized for mI detection in this work (a single quantum filter) were compared to a standard, TE = 35 ms pulse sequence. While limiting the SNR of an acquisition to the equivalent SNR of a spectrum acquired in 5 min from an 8 cc voxel, it was found through simulations that the most repeatable mI measurements would be obtained with a Carr-Purcell sequence. This sequence was implemented in a clinical scanner, and improved mI measurements were demonstrated in vivo.

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.

MR Spectroscopy of Metabolic Disorders

The application of MR spectroscopy (MRS) in pediatric brain disorders yields valued information on pathologic processes, such as ischemia, demyelination, gliosis, and neurodegeneration. Because these processes manifest in inborn errors of metabolism, the purposes of this article are to (1) describe the spectral changes that are associated with the relatively common metabolic disorders, with summaries of known spectroscopic features of these disorders; (2) offer suggestions for recognition and distinction of disorders; and (3) provide general guidelines for MRS implementation. Although many conditions have a similar presentation, MRS offers valuable information for the individual patient in diagnosis and therapy when integrated fully into the clinical setting.

Persistent NKH with transient or absent symptoms and a homozygous GLDC mutation

Three of four nonketotic hyperglycinemia patients homozygous for a novel GLDC mutation (A802V) were treated by assisted respiration and/or sodium benzoate with or without ketamine and had transient neonatal or absent symptoms and normal developmental outcome, despite persisting biochemical evidence of nonketotic hyperglycinemia. This exceptional outcome may be related to the high residual activity of the mutant protein (32% of wild type) and therapeutic intervention during a critical period of heightened brain exposure and sensitivity to glycine.

Imaging of neurogenetic and neurometabolic disorders of childhood

Magnetic resonance imaging (MRI) has emerged as a powerful tool in the study of normal and abnormal brain structure, function, and biochemistry. In particular, functional MRI has come into its own as a tool to study normal and abnormal brain functions such as learning, memory, and motor learning, as well as delineation of neurogenetic cognitive phenotypes. White matter microstructure can be studied using diffusion tensor imaging, which may allow abnormal white matter to be visualized prior to abnormalities on anatomic MRI. Magnetic resonance spectroscopy, a noninvasive method to study brain biochemistry, may allow for the delineation of regional metabolic changes as a result of disease progression and/or therapeutic intervention. With MRI techniques, one can investigate the relationship between structure, function, genes, and behavior. This report discusses the research applications of MRI to the study of neurogenetic disorders of childhood.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.

Magnetic resonance spectroscopy study of glycine pathways in nonketotic hyperglycinemia

Nonketotic hyperglycinemia is a life-threatening disorder in neonates characterized by a deficiency of the glycine cleavage system. We report on four cases of the neonatal form of the disease, which were investigated by in vitro(1)H magnetic resonance spectroscopy of blood and cerebrospinal fluid, and in vivo(1)H magnetic resonance spectroscopy of brain. The existence of glycine disposal pathways leading to an increase in lactate in fluids and creatine in fluids and brain was demonstrated. This is the first observation of elevated creatine in brain in nonketotic hyperglycinemia. A recurrent decrease of glutamine and citrate was observed in cerebrospinal fluid, which might be related to abnormal glutamine metabolism in brain. Finally, the cerebral N-acetylaspartate to myo-inositol-glycine ratio was identified as a prognostic indicator of the disease.

Localized proton MR spectroscopic detection of nonketotic hyperglycinemia in an infant

Nonketotic hyperglycinemia (NKH) is a rare metabolic brain disease caused by deficient activity of the glycine cleavage system. Localized proton MR spectroscopy (echo-time 166 msec), performed in an infant with the typical clinical and biochemical features of neonatal NKH, showed a markedly increased peak intensity at 3.55 ppm, which was assigned to glycine. Serial proton MR spectroscopic studies indicated that glycine/choline and glycine/total creatine ratios correlated closely with the patient's clinical course. Proton MR spectroscopy was useful for the non-invasive detection and monitoring of cerebral glycine levels in this infant with NKH.