Magnetic Resonance Findings in a Neonate With Nonketotic Hyperglycinemia: Case Report

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.

Neurometabolic diseases of childhood

Metabolic diseases affecting the pediatric brain are complex conditions, the underlying mechanisms leading to structural damage are diverse and the diagnostic imaging manifestations are often non-specific; hence early, sensitive and specific diagnosis can be challenging for the radiologist. However, misdiagnosis or a delayed diagnosis can result in a devastating, irreversible injury to the developing brain. Based upon the inborn error, neurometabolic diseases can be subdivided in various groups depending on the predominantly involved tissue (e.g., white matter in leukodystrophies or leukoencephalopathies), the involved metabolic processes (e.g., organic acidurias and aminoacidopathies) and primary age of the child at presentation (e.g., neurometabolic disorders of the newborn). This manuscript summarizes these topics.

Neonatal neuroimaging findings in inborn errors of metabolism

Individually, metabolic disorders are rare, but overall they account for a significant number of neonatal disorders affecting the central nervous system. The neonatal clinical manifestations of inborn errors of metabolism (IEMs) are characterized by nonspecific systemic symptoms that may mimic more common acute neonatal disorders like sepsis, severe heart insufficiency, or neonatal hypoxic-ischemic encephalopathy. Certain IEMs presenting in the neonatal period may also be complicated by sepsis and cardiomyopathy. Early diagnosis is mandatory to prevent death and permanent long-term neurological impairments. Although neuroimaging findings are rarely specific, they play a key role in suggesting the correct diagnosis, limiting the differential diagnosis, and may consequently allow early initiation of targeted metabolic and genetic laboratory investigations and treatment. Neuroimaging may be especially helpful to distinguish metabolic disorders from other more common causes of neonatal encephalopathy, as a newborn may present with an IEM prior to the availability of the newborn screening results. It is therefore important that neonatologists, pediatric neurologists, and pediatric neuroradiologists are familiar with the neuroimaging findings of metabolic disorders presenting in the neonatal time period.