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The monoamine neurotransmitter disorders consist of a rapidly expanding heterogeneous group of neurological syndromes characterised by primary and secondary defects in the biosynthesis degradation, or transport of dopamine, norepinephrine, epinephrine, and serotonin. Disease onset can occur any time from infancy onwards. Clinical presentation depends on the pattern and severity of neurotransmitter abnormalities, and is predominated by neurological features (encephalopathy, epilepsy, and pyramidal and extrapyramidal motor disorders) that are primarily attributed to deficiency of cerebral dopamine, serotonin, or both. Many neurotransmitter disorders mimic the phenotype of other neurological disorders (eg, cerebral palsy, hypoxic ischaemic encephalopathy, paroxysmal disorders, inherited metabolic diseases, and genetic dystonic or parkinsonian syndromes) and are, therefore, frequently misdiagnosed. Early clinical suspicion and appropriate investigations, including analysis of neurotransmitters in CSF, are essential for accurate clinical diagnosis. Treatment strategies focus on the correction of monoamine deficiency by replacement of monoamine precursors, the use of monoamine analogues, inhibition of monoamine degradation, and addition of enzyme cofactors to promote monoamine production.

High-fat, low-carbohydrate diets, known as ketogenic diets, have been used as a non-pharmacological treatment for refractory epilepsy. A key mechanism of this treatment is thought to be the generation of ketones, which provide brain cells (neurons and astrocytes) with an energy source that is more efficient than glucose, resulting in beneficial downstream metabolic changes, such as increasing adenosine levels, which might have effects on seizure control. However, some studies have challenged the central role of ketones because medium-chain fatty acids, which are part of a commonly used variation of the diet (the medium-chain triglyceride ketogenic diet), have been shown to directly inhibit AMPA receptors (glutamate receptors), and to change cell energetics through mitochondrial biogenesis. Through these mechanisms, medium-chain fatty acids rather than ketones are likely to block seizure onset and raise seizure threshold. The mechanisms underlying the ketogenic diet might also have roles in other disorders, such as preventing neurodegeneration in Alzheimer's disease, the proliferation and spread of cancer, and insulin resistance in type 2 diabetes. Analysing medium-chain fatty acids in future ketogenic diet studies will provide further insights into their importance in modified forms of the diet. Moreover, the results of these studies could facilitate the development of new pharmacological and dietary therapies for epilepsy and other disorders.

Bioenergetic failure caused by impaired utilisation of glucose and fatty acids contributes to organ dysfunction across multiple tissues in critical illness. Ketone bodies may form an alternative substrate source, but the feasibility and safety of inducing a ketogenic state in physiologically unstable patients is not known. Twenty-nine mechanically ventilated adults with multi-organ failure managed on intensive care units were randomised (Ketogenic n  = 14, Control n  = 15) into a two-centre pilot open-label trial of ketogenic versus standard enteral feeding. The primary endpoints were assessment of feasibility and safety, recruitment and retention rates and achievement of ketosis and glucose control. Ketogenic feeding was feasible, safe, well tolerated and resulted in ketosis in all patients in the intervention group, with a refusal rate of 4.1% and 82.8% retention. Patients who received ketogenic feeding had fewer hypoglycaemic events (0.0% vs. 1.6%), required less exogenous international units of insulin (0 (Interquartile range 0-16) vs.78 (Interquartile range 0-412) but had slightly more daily episodes of diarrhoea (53.5% vs. 42.9%) over the trial period. Ketogenic feeding was feasible and may be an intervention for addressing bioenergetic failure in critically ill patients. Clinical Trials.gov registration: NCT04101071. Critical illness leads to altered metabolic states and bioenergetic failure caused by impaired utilisation of glucose, fatty acids and amino acids. Here the authors show ketogenic diets may provide a safe and acceptable alternative metabolic fuel enabling energy production and maintaining tissue homeostasis.

Dopamine transporter deficiency syndrome is the first identified parkinsonian disorder caused by genetic alterations of the dopamine transporter. We describe a cohort of children with mutations in the gene encoding the dopamine transporter (SLC6A3) with the aim to improve clinical and molecular characterisation, reduce diagnostic delay and misdiagnosis, and provide insights into the pathophysiological mechanisms. 11 children with a biochemical profile suggestive of dopamine transporter deficiency syndrome were enrolled from seven paediatric neurology centres in the UK, Germany, and the USA from February, 2009, and studied until June, 2010. The syndrome was characterised by detailed clinical phenotyping, biochemical and neuroradiological studies, and SLC6A3 mutation analysis. Mutant constructs of human dopamine transporter were used for in-vitro functional analysis of dopamine uptake and cocaine-analogue binding. Children presented in infancy (median age 2·5 months, range 0·5–7) with either hyperkinesia (n=5), parkinsonism (n=4), or a mixed hyperkinetic and hypokinetic movement disorder (n=2). Seven children had been initially misdiagnosed with cerebral palsy. During childhood, patients developed severe parkinsonism-dystonia associated with an eye movement disorder and pyramidal tract features. All children had raised ratios of homovanillic acid to 5-hydroxyindoleacetic acid in cerebrospinal fluid, of range 5·0–13·2 (normal range 1·3–4·0). Homozygous or compound heterozygous SLC6A3 mutations were detected in all cases. Loss of function in all missense variants was recorded from in-vitro functional studies, and was supported by the findings of single photon emission CT DaTSCAN imaging in one patient, which showed complete loss of dopamine transporter activity in the basal nuclei. Dopamine transporter deficiency syndrome is a newly recognised, autosomal recessive disorder related to impaired dopamine transporter function. Careful characterisation of patients with this disorder should provide novel insights into the complex role of dopamine homoeostasis in human disease, and understanding of the pathophysiology could help to drive drug development. Birmingham Children's Hospital Research Foundation, Birth Defects Foundation Newlife, Action Medical Research, US National Institutes of Health, Wellchild, and the Wellcome Trust.

There is growing interest in the use of the ketogenic diet (KD) to treat inherited metabolic diseases including mitochondrial disorders. However, neither the mechanism whereby the diet may be working, nor if it could benefit all patients with mitochondrial disease, is known. This study focusses on decanoic acid (C10), a component of the medium chain triglyceride KD, and a ligand for the nuclear receptor PPAR-γ known to be involved in mitochondrial biogenesis. The effects of C10 were investigated in primary fibroblasts from a cohort of patients with Leigh syndrome (LS) caused by nuclear-encoded defects of respiratory chain complex I, using mitochondrial respiratory chain enzyme assays, gene expression microarray, qPCR and flow cytometry. Treatment with C10 increased citrate synthase activity, a marker of cellular mitochondrial content, in 50 % of fibroblasts obtained from individuals diagnosed with LS in a PPAR-γ-mediated manner. Gene expression analysis and qPCR studies suggested that treating cells with C10 supports fatty acid metabolism, through increasing ACADVL and CPT1 expression, whilst downregulating genes involved in glucose metabolism ( PDK3 , PDK4 ). PCK2 , involved in blocking glucose metabolism, was upregulated, as was CAT , encoding catalase. Moreover, treatment with C10 also decreased oxidative stress in complex I deficient (rotenone treated) cells. However, since not all cells from subjects with LS appeared to respond to C10, prior cellular testing in vitro could be employed as a means for selecting individuals for subsequent clinical studies involving C10 preparations.

Cerebral folate deficiency (CFD) is defined as any neurological syndrome associated with a low cerebrospinal fluid (CSF) concentration of 5-methyltetrahydrofolate (5MTHF) in the presence of normal peripheral folate status. CFD has a wide clinical presentation, with reported signs and symptoms generally beginning at around 4 months of age with irritability and sleep disturbances. These can be followed by psychomotor retardation, dyskinesia, cerebellar ataxia and spastic diplegia. Other signs may include deceleration of head growth, visual disturbances and sensorineural hearing loss. Identification of CFD is achieved by determining 5MTHF concentration in CSF. Once identified, CFD can in many cases be treated by administering oral folinic acid. Supplementation with folic acid is contraindicated and, if used, may exacerbate the CSF 5MTHF deficiency. Generation of autoantibodies against the folate receptor required to transport 5MTHF into CSF and mutations in the folate receptor 1 (FOLR1) gene have been reported to be causes of CFD. However, other mechanisms are probably also involved, as CFD has been reported in Aicardi-Goutiere's and Rett syndromes and in mitochondriopathies. Several metabolic conditions and a number of widely used drugs can also lead to a decrease in the concentration of CSF 5MTHF, and these should be considered in the differential diagnosis if a low concentration of 5MTHF is found following CSF analysis.

Key Points Monoamine neurotransmitter disorders are under-recognized and often misdiagnosed, as many mimic cerebral palsy and other neurological disorders 'Red flag' symptoms of monoamine neurotransmitter disorders include diurnal variation of symptoms, a mixed movement disorder, autonomic disturbance, involvement of the eyes (ptosis, oculogyric crisis) and levodopa responsiveness Many monoamine neurotransmitter disorders are amenable to treatment; appropriate therapy is curative in some disorders Analysis of cerebrospinal fluid neurotransmitter levels aids identification of the specific monoamine pathway defect and is vital for accurate diagnosis of most primary neurotransmitter disorders and selection of appropriate disease-specific pharmacotherapy Research in the past few years has identified novel monoamine neurotransmitter disorders that involve defects in dopamine transport and monoamine vesicle packaging Discoveries of novel genetic defects and biomarkers in monoamine neurotransmitter disorders, together with novel disease models, will improve our understanding of pathophysiological mechanisms and facilitate the development of new treatments Monoamine neurotransmitters are involved in many neurological functions, and defects in their synthesis, metabolism and transport leads to a variety of disorders with subtly different clinical manifestations that make them difficult to diagnose. The authors of this Review outline the clinical features, diagnosis and management of monoamine neurotransmitter disorders, and consider recent and future advances in knowledge and therapeutic options. The monoamine neurotransmitter disorders are important genetic syndromes that cause disturbances in catecholamine (dopamine, noradrenaline and adrenaline) and serotonin homeostasis. These disorders result in aberrant monoamine synthesis, metabolism and transport. The clinical phenotypes are predominantly neurological, and symptoms resemble other childhood neurological disorders, such as dystonic or dyskinetic cerebral palsy, hypoxic ischaemic encephalopathy and movement disorders. As a consequence, monoamine neurotransmitter disorders are under-recognized and often misdiagnosed. The diagnosis of monoamine neurotransmitter disorders requires detailed clinical assessment, cerebrospinal fluid neurotransmitter analysis and further supportive diagnostic investigations. Prompt and accurate diagnosis of neurotransmitter disorders is paramount, as many are responsive to treatment. The treatment is usually mechanism-based, with the aim to reverse disturbances of monoamine synthesis and/or metabolism. Therapeutic intervention can lead to complete resolution of motor symptoms in some conditions, and considerably improve quality of life in others. In this Review, we discuss the clinical features, diagnosis and management of monoamine neurotransmitter disorders, and consider novel concepts, the latest advances in research and future prospects for therapy.

Barth syndrome (BTHS) is an X-linked disorder characterised by cardiac and skeletal myopathy, growth delay, neutropenia and 3-methylglutaconic aciduria (3-MGCA). Patients have TAZ gene mutations which affect metabolism of cardiolipin, resulting in low tetralinoleoyl cardiolipin (CL 4 ), an increase in its precursor, monolysocardiolipin (MLCL), and an increased MLCL/CL 4 ratio. During development of a diagnostic service for BTHS, leukocyte CL 4 was measured in 156 controls and 34 patients with genetically confirmed BTHS. A sub-group of seven subjects from three unrelated families was identified with leukocyte CL 4 concentrations within the control range. This had led to initial false negative disease detection in two of these patients. MLCL/CL 4 in this subgroup was lower than in other BTHS patients but higher than controls, with no overlap between the groups. TAZ gene mutations in these families are all predicted to be pathological. This report describes the clinical histories of these seven individuals with an atypical phenotype: some features were typical of BTHS (five have had cardiomyopathy, one family has a history of male infant deaths, three have growth delay and five have 3-MGCA) but none has persistent neutropenia, five have excellent exercise tolerance and two adults are asymptomatic. This report also emphasises the importance of measurement of MLCL/CL 4 ratio rather than CL 4 alone in the biochemical diagnosis of the BTHS.

Barth syndrome (BTHS) is an X-linked disorder characterised by cardiomyopathy, skeletal myopathy, growth retardation, neutropenia and 3-methylglutaconic aciduria. It is caused by mutations in the TAZ gene which codes for tafazzin, a protein with acyl transferase activity involved in synthesis of cardiolipin. Monolysocardiolipin (MLCL) is an intermediate in this process. Diagnosis of BTHS is difficult, as clinical and biochemical features are variable and numerous TAZ mutations have been described. These factors, together with lack of a straightforward diagnostic test are thought to have contributed to under-diagnosis of the condition. A novel method for cardiolipin analysis by reversed-phase ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) is reported which is less complicated and faster than previously described methods and uses a readily available sample type. The equipment, reagents and expertise required are found in most clinical laboratories performing metabolic investigations. Leukocytes were prepared from whole blood, phospholipids extracted and tetralinoleyl cardiolipin (CL 4 ) and MLCL analysed by UPLC-MS/MS. Reference values were derived from analysis of 76 control and 23 BTHS samples as follows: CL 4 in controls >132 (95 % CI 100–169), BTHS <30.2 (21.3–40.4) pmol/mg protein; MLCL/CL 4 ratio in controls <0.006 (0.004–0.009) and >2.52 (1.51–4.22) in BTHS patients. We describe an improved method for CL 4 and MLCL/CL 4 analysis which can be incorporated into the routine work of a clinical biochemistry laboratory. It shows 100 % sensitivity and specificity for BTHS, making it a suitable diagnostic test.

The neuronal ceroid lipofuscinoses (NCLs or Batten disease) are a group of inherited, fatal neurodegenerative disorders of childhood. In these disorders, glial (microglial and astrocyte) activation typically occurs early in disease progression and predicts where neuron loss subsequently occurs. We have found that in the most common juvenile form of NCL (CLN3 disease or JNCL) this glial response is less pronounced in both mouse models and human autopsy material, with the morphological transformation of both astrocytes and microglia severely attenuated or delayed. To investigate their properties, we isolated glia and neurons from Cln3- deficient mice and studied their basic biology in culture. Upon stimulation, both Cln3 -deficient astrocytes and microglia also showed an attenuated ability to transform morphologically, and an altered protein secretion profile. These defects were more pronounced in astrocytes, including the reduced secretion of a range of neuroprotective factors, mitogens, chemokines and cytokines, in addition to impaired calcium signalling and glutamate clearance. Cln3 -deficient neurons also displayed an abnormal organization of their neurites. Most importantly, using a co-culture system, Cln3- deficient astrocytes and microglia had a negative impact on the survival and morphology of both Cln3- deficient and wildtype neurons, but these effects were largely reversed by growing mutant neurons with healthy glia. These data provide evidence that CLN3 disease astrocytes are functionally compromised. Together with microglia, they may play an active role in neuron loss in this disorder and can be considered as potential targets for therapeutic interventions.