Urea cycle disorders: brain MRI and neurological outcome

Phenotypes of undiagnosed adults with actionable OTC and GLA variants

Inherited metabolic disorders (IMDs) are variably expressive, complicating identification of affected individuals. A genotype-first approach can identify individuals at risk for morbidity and mortality from undiagnosed IMDs and can lead to protocols that improve clinical detection, counseling, and management. Using data from 57,340 participants in two hospital biobanks, we assessed the frequency and phenotypes of individuals with pathogenic/likely pathogenic variants (PLPVs) in two IMD genes: GLA, associated with Fabry disease, and OTC, associated with ornithine transcarbamylase deficiency. Approximately 1 in 19,100 participants harbored an undiagnosed PLPV in GLA or OTC. We identified three individuals (2 male, 1 female) with PLPVs in GLA, all of whom were undiagnosed, and three individuals (3 female) with PLPVs in OTC, two of whom were undiagnosed. All three individuals with PLPVs in GLA (100%) had symptoms suggestive of mild Fabry disease, and one individual (14.2%) had an ischemic stroke at age 33, likely indicating the presence of classic disease. No individuals with PLPVs in OTC had documented hyperammonemia despite exposure to catabolic states, but all (100%) had chronic symptoms suggestive of attenuated disease, including mood disorders and migraines. Our findings suggest that GLA and OTC variants identified via a genotype-first approach are of high penetrance and that population screening of these genes can be used to facilitate stepwise phenotyping and appropriate care.

Fifteen years of urea cycle disorders brain research: Looking back, looking forward

Urea cycle disorders (UCD) are inherited diseases resulting from deficiency in one of six enzymes or two carriers that are required to remove ammonia from the body. UCD may be associated with neurological damage encompassing a spectrum from asymptomatic/mild to severe encephalopathy, which results in most cases from Hyperammonemia (HA) and elevation of other neurotoxic intermediates of metabolism. Electroencephalography (EEG), Magnetic resonance imaging (MRI) and Proton Magnetic resonance spectroscopy (MRS) are noninvasive measures of brain function and structure that can be used during HA to guide management and provide prognostic information, in addition to being research tools to understand the pathophysiology of UCD associated brain injury. The Urea Cycle Rare disorders Consortium (UCDC) has been invested in research to understand the immediate and downstream effects of hyperammonemia (HA) on brain using electroencephalogram (EEG) and multimodal brain MRI to establish early patterns of brain injury and to track recovery and prognosis. This review highlights the evolving knowledge about the impact of UCD and HA in particular on neurological injury and recovery and use of EEG and MRI to study and evaluate prognostic factors for risk and recovery. It recognizes the work of others and discusses the UCDC's prior work and future research priorities.

Transient Cortical Blindness in a Toddler With Heterozygous Ornithine Transcarbamylase Deficiency

Ornithine transcarbamylase (OTC) deficiency is an incredibly rare disease in the subgroup of urea cycle disorders. Although typically seen in the neonate shortly after initiating high protein feeds (human breastmilk or infant formula), patients with partial/heterozygous deficiencies can often be diagnosed later in life with unique sequelae. One such manifestation is sudden, transient vision loss shortly after an initial episode of hyperammonemia in a patient without a known diagnosis of OTC deficiency. Only two such occurrences are documented in academic literature and both share many similar presenting features hinting that a hidden, but the consistent pathophysiologic mechanism of this disease is at play. Scarce research is available to propose a concise explanation; however, recent advancements in the literature point toward the brain’s inability to uptake glucose and convert it into glutamate in patients with partial OTC deficiency as a likely explanation.

Intracranial Abnormalities with Diffusion Restriction

Multiple pathologic conditions can cause changes in the random movement of water, which can be detected with diffusion-weighted imaging (DWI). DWI plays a powerful clinical role in detecting restricted diffusion associated with acute brain infarction. Other disorders can also result in restricted diffusion. This article focuses on showing examples of common and uncommon disorders that have restricted diffusion secondary to cytotoxic and/or intramyelinic edema. These disorders include ischemia, infection, noninfectious demyelinating diseases, genetic mutations affecting metabolism, acquired metabolic disorders, toxic or drug exposures, neoplasms and tumorlike lesions, radiation treatment, trauma, and denervation.

Review of Multi-Modal Imaging in Urea Cycle Disorders: The Old, the New, the Borrowed, and the Blue

The urea cycle disorders (UCD) are rare genetic disorder due to a deficiency of one of six enzymes or two transport proteins that act to remove waste nitrogen in form of ammonia from the body. In this review, we focus on neuroimaging studies in OTCD and Arginase deficiency, two of the UCD we have extensively studied. Ornithine transcarbamylase deficiency (OTCD) is the most common of these, and X-linked. Hyperammonemia (HA) in OTCD is due to deficient protein handling. Cognitive impairments and neurobehavioral disorders have emerged as the major sequelae in Arginase deficiency and OTCD, especially in relation to executive function and working memory, impacting pre-frontal cortex (PFC). Clinical management focuses on neuroprotection from HA, as well as neurotoxicity from other known and yet unclassified metabolites. Prevention and mitigation of neurological injury is a major challenge and research focus. Given the impact of HA on neurocognitive function of UCD, neuroimaging modalities, especially multi-modality imaging platforms, can bring a wealth of information to understand the neurocognitive function and biomarkers. Such information can further improve clinical decision making, and result in better therapeutic interventions. In vivo investigations of the affected brain using multimodal neuroimaging combined with clinical and behavioral phenotyping hold promise. MR Spectroscopy has already proven as a tool to study biochemical aberrations such as elevated glutamine surrounding HA as well as to diagnose partial UCD. Functional Near Infrared Spectroscopy (fNIRS), which assesses local changes in cerebral hemodynamic levels of cortical regions, is emerging as a non-invasive technique and will serve as a surrogate to fMRI with better portability. Here we review two decades of our research using non-invasive imaging and how it has contributed to an understanding of the cognitive effects of this group of genetic conditions.

The Application of Neurodiagnostic Studies to Inform the Acute Management of a Newborn Presenting With Sarbamoyl Shosphate Synthetase 1 Deficiency

Neonatal-onset urea cycle disorders (UCDs) may result in hyperammonemic (HA) encephalopathy presenting with several neurologic sequelae including seizures, coma, and death. However, no recommendations are given in how and when neurodiagnostic studies should be used to screen or assess for these neurologic complications. We present a case of carbamoyl phosphate synthetase 1 (CPS1) deficiency in a newborn female in which electroencephalogram monitoring to assess encephalopathy and seizures, and magnetic resonance imaging measurements of brain metabolites were used to guide care during her hyperammonemic crisis. Her neurologic course and response to treatment characterizes the significant neurologic impact of HA encephalopathy. Our group herein proposes a clinical neurodiagnostic pathway for managing acute HA encephalopathy.

Urea Cycle Disorders: A Neuroimaging Pattern Approach Using Diffusion and FLAIR MRI

BACKGROUND AND PURPOSE

This study aimed to assess characteristic regions of MRI involvement utilizing diffusion weighted imaging (DWI) and fluid-attenuated inversion recovery (FLAIR) at urea cycle disorder (UCD) diagnosis to determine the possible association between initial MRI patterns within 10 days of the first hyperammonemia episode, serum ammonia levels, and severity of neurological outcome based on clinical follow-up of >30 days.

METHODS

Ten patients with UCDs (4 females; median age: 5.4 years, age range: 6 days-54 years) were included who underwent MRI during a first episode of hyperammonemia. The topographical distribution of the DWI and FLAIR abnormalities in the cerebral cortex, deep gray matter, white matter, posterior limb of internal capsule, cerebral peduncle, and cerebellum was evaluated. Possible correlations between the brain injury patterns on DWI/FLAIR images, serum ammonia levels, and severity of neurological outcome were investigated by a trend correlation.

RESULTS

The UCD cohort (n = 10) involved four ornithine transcarbamoylase deficiencies, four argininosuccinic aciduria, one carbomoylphosphate synthetase deficiency, and one citrullinemia type-1. The observed trend in the distribution of DWI abnormalities as the severity of neurological sequela outcome increased was with diffuse cerebral cortex or corpus striatum involvement. Patients with initial peak serum ammonia ≥450 µmol/L had a grade 2 to 4 outcome, and those with peak ammonia <450 µmol/L had a grade 0 or 1 outcome.

CONCLUSIONS

The presence of more severe neurological outcome could be associated with diffuse cerebral cortex or corpus striatum involvement on DWI and high serum ammonia levels in patients with UCD.

Multimodal imaging in urea cycle-related neurological disease - What can imaging after hyperammonemia teach us?

BACKGROUND

Urea cycle-related brain disease may take on variable neuroimaging manifestations, ranging from normal to abnormal with or without a signature appearance. In the past, we have described the usefulness of multimodal imaging in identifying biomarkers of neuronal injury in UCD patients. In this study, we report unique findings in an adolescent male with neonatal-onset OTC deficiency after an episode of hyperammonemia.

MATERIALS AND METHODS

Multiplanar, multisequence MR imaging (T1WI, T2WI, T2 FLAIR, diffusion weighted images and gradient echo) of the brain was performed on seven separate occasions over the course following the acute illness; first five exams were performed within 28 days of admission and the final two exams were performed approximately 3 and 5 months later.

RESULTS

1.The initial MR revealed increased signal on T2WI in the basal ganglia, claustrum and frontoparietal white matter; which remained stable over time. By the 5th exam, signal changes had developed in frontal cortex; reflecting permanent injury. 2. DTI tractography of the corticospinal tracts displayed revealed diminution of the number of projectional and commissural fibers over time. 3. Blood flow measurements demonstrated hypoperfusion on the fifth exams followed by hyperperfusion on the final two studies. 4. MR spectroscopy demonstrated that glutamine was elevated during hyperammonemia with myoinositol reduction, reflecting osmotic buffering.

CONCLUSION

This particular multimodal magnetic resonance neuroimaging showed novel, temporally specific manifestations over the disease course in OTC deficiency. This prospective imaging study expands our understanding of the effect of hyperammonemia on the structure and biochemistry of the nervous system.

Investigating ataxia in childhood

Ataxia is a common presentation to an acute paediatric unit and it can often be difficult to determine the cause. It is important to distinguish between serious causes, for example, brain tumours and encephalitis, and more benign causes in order to guide investigations and treatment. In this review, we describe the different types of ataxia, the causes associated with them, the examination findings and what investigations to perform in order to make a diagnosis.

The neuroimaging findings of monocarboxylate transporter 1 deficiency

Monocarboxylate transporter 1 (MCT1) deficiency was first described in 2014 by Hasselt et al. as a novel genetic cause of recurrent ketoacidosis. Patients present in the first year of life with acute episodes of ketoacidosis triggered by fasting or infections. Patients with homozygous mutations are known to have a more severe phenotype with mild to moderate developmental delay and an increased prevalence of epilepsy. There is only one recent report of the neuroimaging findings of this disorder as reported by Al-Khawaga et al. (Front Pediatr. 7:299, 2019). We report the neuroimaging abnormalities in two siblings with similar clinical presentation of recurrent ketoacidosis, seizures, and developmental delay. Whole exome sequencing in the younger sibling confirmed a known pathogenic homozygous mutation in MCT1, also known as SLC16A1 gene. Brain MRI showed a similar very distinctive pattern of signal abnormality at the gray-white matter junction, basal ganglia, and thalami in both patients. Both siblings had agenesis of the corpus callosum. Knowledge of this pattern of brain involvement might contribute to an earlier diagnosis and timely management of this rare and under recognized disorder.

Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision

In 2012, we published guidelines summarizing and evaluating late 2011 evidence for diagnosis and therapy of urea cycle disorders (UCDs). With 1:35 000 estimated incidence, UCDs cause hyperammonemia of neonatal (~50%) or late onset that can lead to intellectual disability or death, even while effective therapies do exist. In the 7 years that have elapsed since the first guideline was published, abundant novel information has accumulated, experience on newborn screening for some UCDs has widened, a novel hyperammonemia-causing genetic disorder has been reported, glycerol phenylbutyrate has been introduced as a treatment, and novel promising therapeutic avenues (including gene therapy) have been opened. Several factors including the impact of the first edition of these guidelines (frequently read and quoted) may have increased awareness among health professionals and patient families. However, under-recognition and delayed diagnosis of UCDs still appear widespread. It was therefore necessary to revise the original guidelines to ensure an up-to-date frame of reference for professionals and patients as well as for awareness campaigns. This was accomplished by keeping the original spirit of providing a trans-European consensus based on robust evidence (scored with GRADE methodology), involving professionals on UCDs from nine countries in preparing this consensus. We believe this revised guideline, which has been reviewed by several societies that are involved in the management of UCDs, will have a positive impact on the outcomes of patients by establishing common standards, and spreading and harmonizing good practices. It may also promote the identification of knowledge voids to be filled by future research.

Metabolic, endocrine, and other genetic disorders

Metabolic, endocrine, and genetic diseases of the brain include a very large array of disorders caused by a wide range of underlying abnormalities and involving a variety of brain structures. Often these disorders manifest as recognizable, though sometimes overlapping, patterns on neuroimaging studies that may enable a diagnosis based on imaging or may alternatively provide enough clues to direct further diagnostic evaluation. The diagnostic workup can include various biochemical laboratory or genetic studies. In this chapter, after a brief review of normal white-matter development, we will describe a variety of leukodystrophies resulting from metabolic disorders involving the brain, including mitochondrial and respiratory chain diseases. We will then describe various acidurias, urea cycle disorders, disorders related to copper and iron metabolism, and disorders of ganglioside and mucopolysaccharide metabolism. Lastly, various other hypomyelinating and dysmyelinating leukodystrophies, including vanishing white-matter disease, megalencephalic leukoencephalopathy with subcortical cysts, and oculocerebrorenal syndrome will be presented. In the following section on endocrine disorders, we will examine various disorders of the hypothalamic-pituitary axis, including developmental, inflammatory, and neoplastic diseases. Neonatal hypoglycemia will also be briefly reviewed. In the final section, we will review a few of the common genetic phakomatoses. Throughout the text, both imaging and brief clinical features of the various disorders will be discussed.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

Posterior fossa syndrome in a patient with an ornithine transcarbamylase deficiency

The posterior fossa syndrome (PFS) is a well-known clinical entity and mainly occurs in children. Ornithine transcarbamylase deficiency (OTC) is the most common urea cycle disorder, which occurs in an estimated 1 per 50.000 live births in Japan. Symptoms are mostly due to hyperammonemia and include nausea, vomiting, lethargia and even convulsions and coma. Common neurological symptoms at presentation of a hyperammonemia are a decreased level of consciousness, abnormal motor function or seizures. In this case we describe a girl with late onset OCT deficiency presenting with transient mutism and subsequent dysarthria, ataxia and behavioural changes. This is an exceptional report of a not yet described neurologic syndrome in OTC.

SYNOPSIS

Neurologic symptoms in ornithine transcarbamylase deficiency do not only occur during an episode of hyperammonemia and may present as a transient neurologic symptoms compatible with the posterior fossa syndrome.

MRI for neurodevelopmental prognostication in the high-risk term infant

MRI performed in the neonatal period has become a tool widely used by clinicians and researchers to evaluate the developing brain. MRI can provide detailed anatomical resolution, enabling identification of brain injuries due to various perinatal insults. This review will focus on the link between neonatal MRI findings and later neurodevelopmental outcomes in high-risk term infants. In particular, the role of conventional and advanced MR imaging in prognosticating outcomes in neonates with hypoxic-ischemic encephalopathy, ischemic perinatal stroke, need for extracorporeal membrane oxygenation life support, congenital heart disease, and other neonatal neurological conditions will be discussed.

Inborn errors of metabolism: combining clinical and radiologic clues to solve the mystery

OBJECTIVE

Inborn errors of metabolism in children can be challenging to interpret because of the similarity of their appearances on imaging. There are important clues to the diagnosis based on clinical history, head circumference (e.g., macrocephaly), geographic distribution of lesions (e.g., subcortical vs deep white matter or frontal vs parietooccipital), and other imaging features (e.g., contrast enhancement, calcification, cysts, and cortical dysplasia).

CONCLUSION

In this article, we present an algorithm-based approach to diagnosing pediatric metabolic disease with a discussion of key imaging features.

Neonatal citrullinemia: novel, reversible neuroimaging findings correlated with ammonia level changes

BACKGROUND

Citrullinemia type I is an autosomal recessive disorder of the urea cycle in which a patient lacks the cytosolic enzyme, argininosuccinic acid synthetase. This enzyme deficiency results in elevated levels of ammonia, glutamine, and citrulline. The accumulation of ammonia and glutamine causes neurodegenerative changes that are detectible on magnetic resonance imaging. This is the first case report of citrullinemia with repeat magnetic resonance images and electroencephalographs in the acute phase of hyperammonemia.

CASE

This 3800 g white boy was born at 40 weeks 4 days gestation to a 25-year-old mother. He was delivered at home to a certified midwife with no reported complications. He was doing well until day of life 4, when the mother reported he would no longer latch to feed. He was observed to have markedly elevated ammonia levels and ultimately diagnosed with citrullinemia type I. The initial magnetic resonance image was markedly abnormal. After aggressive medical management, his repeat magnetic resonance image revealed marked improvement in the acute setting.

CONCLUSION

Early and aggressive management of hyperammonemia can result in improved magnetic resonance imaging findings in the acute setting. It is too early to know if this will translate to an improved clinical outcome. Clinical suspicion must remain high for urea cycle disorders in neonates with magnetic resonance image changes similar to those resulting from hypoxic-ischemic injury.

Advances in urea cycle neuroimaging: Proceedings from the 4th International Symposium on urea cycle disorders, Barcelona, Spain, September 2013

Our previous imaging research performed as part of a Urea Cycle Rare Disorders Consortium (UCRDC) grant, has identified specific biomarkers of neurologic injury in ornithine transcarbamylase deficiency, OTCD. While characterization of mutations can be achieved in most cases, this information does not necessarily predict the severity of the underlying neurological syndrome. The biochemical consequences of any mutation may be modified additionally by a large number of factors, including contributions of other enzymes and transport systems that mediate flux through the urea cycle, diet and other environmental factors. These factors likely vary from one patient to another, and they give rise to heterogeneity of clinical severity. Affected cognitive domains include non-verbal learning, fine motor processing, reaction time, visual memory, attention, and executive function. Deficits in these capacities may be seen in symptomatic patients, as well as asymptomatic carriers with normal IQ and correlate with variances in brain structure and function in these patients. Using neuroimaging we can identify biomarkers that reflect the downstream impact of UCDs on cognition. This manuscript is a summary of the presentation from the 4th International Consortium on urea cycle disorders held in, Barcelona, Spain, September 2, 2014.

A female carrier of ornithine carbamoyltransferase deficiency masquerading as attention deficit-hyperactivity disorder

Many females who are heterozygous for ornithine carbamoyltransferase (OTC) deficiency are asymptomatic or intermittently symptomatic with great phenotypic variability. Therefore, the diagnosis of this condition is occasionally a challenge and is often delayed. A 12-year-old girl who was initially diagnosed as having attention deficit-hyperactivity disorder (ADHD) became comatose and developed right-sided hemiparesis during her psychiatric admission. Brain magnetic resonance imaging indicated diffuse but extensive swelling in the left hemisphere with multiple lesions suggestive of an old infarction. Repeated evaluations revealed hyperammonemia and orotic aciduria, and she was diagnosed as having an OTC deficiency. Genetic analysis revealed a heterozygous mutation of N47I in the X-linked OTC gene. Her mental status and hemiparesis improved after hyperammonemia treatment. Here, we report a rare case of a manifestating female carrier with severe symptoms of OTC deficiency masquerading as ADHD.

Magnetic resonance imaging findings and neurodevelopmental outcomes in neonates with urea-cycle defects

The urea-cycle functions to facilitate ammonia excretion, a disruption of which results in the accumulation of toxic metabolites. The neurological outcome of neonatal-onset urea-cycle defects (UCDs) is poor, and there are no good predictors of prognosis beyond ammonia levels at presentation. The role of neuroimaging in the prognosis of neonatal-onset UCDs is unclear. We describe the magnetic resonance imaging (MRI) findings of two patients with neonatal-onset UCDs (argininosuccinic aciduria and citrullinemia) at presentation and at 2-year follow-up, and present a review of the literature on neuroimaging in this age-group. We observed two potentially significant distinct patterns of cerebral involvement on MRI: (1) a central and focal pattern of involvement limited to the basal ganglia, perirolandic regions, and internal capsule; and (2) diffuse involvement of the cerebral cortex, internal capsule, basal ganglia, and variably thalami and brain stem. Patients with more diffuse findings tended to have higher serum glutamine peaks and worse neurological outcomes, while those with central involvement, aggressive acute management, and early liver transplantation tended to have better outcomes. We propose that MRI imaging of the brain may have prognostic value following presentation with neonatal UCDs, particularly in identifying patients at risk for poor outcome. The role and timing of follow-up neuroimaging is currently unclear. Further collaborative studies are necessary to evaluate whether patterns of MRI findings vary with specific UCD subtypes, and are predictive of clinical outcomes in neonatal UCDs.

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.