Abstract: Magnetic resonance spectroscopy is a means of non-invasively detecting chemical substances in human tissues and organs using physical phenomena such as magnetic resonance and chemical shifts. Inherited metabolic diseases mostly occur in childhood, and their central nervous system lesions lack specificity, and there is no good diagnostic tool. The use of magnetic resonance spectroscopy to analyze CNS metabolites can provide more valuable information for definitive diagnosis. In this paper, we introduce the characteristics of the maturation of brain tissue with increasing age in children’s CNS magnetic resonance spectra, and focus on the characteristics of magnetic resonance spectra in children with genetic metabolic diseases such as cerebral leukodystrophy, mitochondrial encephalomyopathy, organic acid disease, amino acid disease, metal metabolism disease and lysosomal disease. Magnetic resonance spectroscopy (MRS) is a non-invasive technique for the quantitative or qualitative detection of biochemical changes, energy metabolism and specific compounds in living tissues. A free-sensing decay signal is acquired and this signal is transformed into a wave spectrum by Fourier transformation. In the same homogeneous magnetic field, the same nucleus of different compounds may have a slightly different magnetic field strength due to its different chemical environment, a phenomenon called chemical shift. According to the principle of chemical shift, the same nuclei in different compounds at high field strengths move at different frequencies and produce and release different resonance frequencies. The MRS technique does not require radioactive tracer labeling and does not cause radioactive damage. The atoms used for MRS studies include 31P, 1H, 19F, 13C, etc. This paper mainly summarizes the diagnostic value of 1H-MRS in pediatric genetic metabolic diseases. 2. Major metabolites of MRS and physiopathological significance Five major magnetic resonance wave peaks can be observed in normal brain tissue 1H-MRS. Nitrogen-acetyl aspartate (NAA): chemical shift of 2.0 ppm, present in neurons and axons, as a neuronal marker. The concentration of NAA in brain tissue gradually increases with age, reaching a plateau around 2 y. Decreased NAA reflects neuronal loss or impaired energy metabolism; increased NAA responds to impaired NAA catabolism. Choline complex (Cho): including phosphorylcholine and acetylcholine phosphate, chemical shift 3.2ppm, involved in the composition of the cell membrane, representing a high concentration of substrates required for the formation of cell membrane and myelin sheath. increased Cho is mostly associated with abnormal cell membrane metabolism or demyelinating diseases. Creatine (Cr): including creatine and phosphocreatine, chemical shift 3.0ppm, energy metabolizer and reserve form of high-energy phosphate in the cytoplasm of neurons. Since creatine and phosphocreatine resonance signals cannot be separated, total creatine levels on MRS are not affected and this metabolite is relatively constant in brain tissue. Inositol (mI): chemical shift 3.5 ppm, marker of neuroglial, involved in osmotic pressure regulation. The mI can be elevated in abnormal neuroglial cell metabolism and decreased in hepatic impairment and high blood ammonia. Glutamate (Glx): chemical shift 2.2 or 3.7ppm, glutamine and glutamate complex, glutamate is an excitatory amino acid, which can generate glutamyl ammonia with ammonia and participate in ammonia metabolism in the brain, and has excitotoxic effects. Lactate (Lac): chemical shift 1.3 or 4.1ppm, a product of anaerobic metabolism, increased energy demand and/or impaired cellular oxidative phosphorylation, such as hypoxic-ischemic encephalopathy, mitochondrial disease, and enhanced anaerobic glycolysis. A lactate peak is not detectable under normal conditions. 3 Characteristics of MRS in children As children’s brains continue to mature, the different metabolites of the brain revealed by MRS vary with age and with different parts of the brain. The general principle is that as the brain matures with increasing age during childhood, NAA gradually increases and Cho gradually decreases, and gradually reaches a stable concentration as the brain matures. Generally there is no difference in the concentrations of metabolites in the corresponding parts of the left and right cerebral hemispheres [1]. Second, the diagnostic value of MRS in hereditary cerebral leukodystrophy 1, hereditary cerebral leukodystrophy, also known as cerebral white matter dystrophy, is a group of progressive genetic disorders that mainly involve the white matter of the central nervous system. It is characterized by abnormal development or diffuse damage to the myelin sheath of the central white matter. According to its pathological characteristics, it can be divided into abnormal myelination, which is the formation of abnormal myelin sheaths; hypomyelination, which is the reduction of myelin production; and myelin spongiform degeneration, which is the cystic degeneration of myelin sheaths. 2. MRS features of common hereditary cerebral leukodystrophies 2.1 X-linked adrenoleukodystrophy (X-ALD) [2,3]: peroxisomal disease, X-linked invisible inheritance, male involvement. The disease results from defective adrenoleukodystrophy protein (ALDP) function leading to impaired oxidative metabolism of very long-chain fatty acids in the mitochondria and deposition in neural tissue and adrenal glands. Very long-chain fatty acids (VLCFA) are elevated in plasma or cultured fibroblasts. Clinical symptoms are predominantly neurological, such as progressive mental retardation, motor regression, audiovisual dysfunction, seizures, etc. About 2/3 of patients have adrenocortical insufficiency. The classic site of onset is in the white matter of the brain in the lateral ventricular triangle and the corpus callosum pressure. The typical features of MRS at the lesion site are the decrease or disappearance of the NAA wave peak, significant increase in Cho and mI, and increase in Lac. 2.2 Heterochromatic leukodystrophy (MLD) [4]: lysosomal storage disease, autosomal invisible inheritance. The disease is caused by a decrease in the activity of acylthio-lipase A or acylthio-lipase A cofactor resulting in abnormal accumulation of sulphate lipids in the white matter of the brain. It is characterized clinically by ataxia, motor regression, decreased intelligence, epilepsy and psychiatric symptoms. The lesions are located in the anterior horn of the lateral ventricles, the soma and the deep white matter of the brain. The MRS of the lesion area shows decreased NAA, increased mI due to myelin loss and glial cell proliferation, and increased Lac. At the same time, similar metabolite abnormalities were seen in the gray matter, thalamus and striatum, but were less pronounced than in the white matter. 2.3 Globocellular leukodystrophy (Krabbe disease, GLD)[5] :Lysosomal storage disease, autosomal invisible inheritance. The disease is due to a defect in galactocephaloside-β-galactosidase, which prevents the degradation of galactocephalosides into ceramide and galactose. The clinical onset is mostly in infancy with progressive feeding difficulties, visual and auditory impairment, and later on, denervation of the cerebral tonicity. The onset of the disease is mainly located in the cerebellum, deep gray matter nuclei (thalamus and caudate nucleus), and brainstem. The MRS of the lesion area shows a significant increase in Cho and mI; the NAA waveform is reduced. 2.4 Alexander’s disease (AD)[6] :Autosomal dominant, it is now mostly believed that the defective glial fibronectin gene leads to the deposition of glassy eosinophilic material, causing extensive demyelination changes and macrocephaly formation in the white matter region of the central nervous system. The clinical picture is characterized by regression of motor intelligence, large head with forehead prominence, seizures, and ataxia. The site of the disease is mainly the frontal white matter abnormalities, but the basal ganglia and cerebellum may also be involved. The white matter mI is abnormally elevated (suggesting glial cell hyperplasia), NAA is decreased, Lac is elevated, and Cho is normal in the white matter but significantly elevated in the gray matter. 2.5 Spongiform cerebral leukomalacia (CD) [6]: autosomal invisible inheritance. Defective aspartate acyltransferase causes elevated N-acetylaspartate. The main clinical symptoms are hypotonia, large head, and difficulty in erecting the neck. The lesion begins in the bowed fibers of the subcortical white matter and gradually involves the deeper white matter. The disease is characterized by abnormal elevation of NAA in the white matter of the lesion. Other symptoms include decreased Cho concentration and increased mI concentration. 2.6 Pey-May disease (PMD)[4] :X-linked invisible inheritance. It is caused by a defect in proteolipid protein 1 (PLP1), a gene that regulates this protein that causes overexpression or decreased expression of this protein, resulting in abnormal myelin formation and oligodendrocyte death. The main clinical symptoms are hypotonia, nystagmus and delayed motor development. All white matter of the brain is involved in the lesion. The MRS of the lesion area is significantly reduced or disappeared by the Cho wave, suggesting a serious impairment of myelin formation; the mI and Cr are elevated, and the NAA wave may be normal or slightly decreased. However, some scholars have also studied[7] that the absolute concentrations of NAA , Cr and mI are significantly elevated in the white matter area of the disease. 2.7 Leukoencephalopathy with brainstem and spinal cord damage with hyperlactate (LBSL) [8]: a new type of cerebral white matter disease, namely leukoencephalopathy with brainstem and spinal cord damage with hyperlactate (LBSL). The disease mostly develops at the age of 3-16 years and is characterized clinically by sensory ataxia and tremor, and in adolescence there may be distal spasticity and the symptoms can be asymmetric on both sides. The lesions are mainly in the cone tract, posterior spinal cord and corticospinal tract. MRS examination of the affected white matter areas reveals decreased NAA and elevated Lac, and some patients have elevated Cho. The above studies of common cerebral white matter disorders have shown that different cerebral white matter sites are damaged by different cerebral white matter disorders. With the exception of CD, most leukoencephalopathy lesions show reduced NAA levels and elevated Cho and MI levels, with significant Lac peaks in some lesions. As the lesion progresses, the NAA decreases more significantly. MRS characteristics of mitochondrial encephalomyopathy 1. Mitochondrial encephalomyopathy (ME) is a multisystem damage caused by mutations in mitochondrial genetic genes that result in defective mitochondrial enzyme function and impaired ATP production. The clinical symptoms of mitochondrial encephalomyopathy include motor sensory impairment, headache, altered muscle tone, epilepsy and other central nervous system encephalopathies and myopathic damage such as muscle weakness and skeletal muscle lysis. Mitochondrial encephalomyopathy can be accompanied by myocardial damage, hearing damage, pigmentary retinopathy, diabetes, short stature and other symptoms of multi-system damage. 2, MRS and lactic acid in mitochondrial encephalomyopathy Mitochondrial encephalomyopathy is a defective mitochondrial respiratory chain resulting in abnormal metabolism, impaired ATP formation, and anaerobic glycolysis in tissues, producing large amounts of lactic acid, especially in high energy-consuming tissues and organs such as the brain and myocardium. At the same time, the lactic acid produced cannot be rapidly utilized, resulting in the accumulation of lactic acid in brain tissues leading to hyperlactic acid. the presence of lactic acid peak in MRS examination can be a characteristic manifestation of mitochondrial encephalomyopathy [9,10]. The current study concluded [11] that metabolic changes in mitochondrial encephalomyopathy precede morphological changes. The detection of hyperlactate bimodal peaks using MRS is about 2 weeks earlier than the appearance of abnormally high signal on DWI, thus MRS contributes to the early diagnosis of mitochondrial encephalomyopathy. When there is a high clinical suspicion of mitochondrial encephalomyopathy and no significant abnormal signal is seen on conventional and DWI, MRS can help in the diagnosis of mitochondrial encephalomyopathy if it detects abnormal lactate peaks. MRS can also be used to detect lactate levels in the cerebrospinal fluid, allowing noninvasive monitoring of metabolic changes in the brain of patients with mitochondrial encephalomyopathy. It can avoid the invasive nature of repeated cerebrospinal fluid punctures and the presence of possible complications. The presence of a lactate peak in the cerebrospinal fluid has significant value in the differential diagnosis of mitochondrial encephalomyopathy from other diseases [12]. 3. MRS findings in major mitochondrial encephalomyopathies 3.1 MRS features in mitochondrial encephalomyopathy with hyperlactatemia and stroke-like episodes syndrome (MELAS): MELAS is more common with childhood onset and clinical manifestations include sudden stroke, hemiparesis, hemianopia and cortical blindness, recurrent seizures, migraine and vomiting. Feng Feng et al [13] investigated the effect of different echo times on the detection results of the lesion area. A total of 7 patients with MELAS, 1 case used long echo time (TE=144ms) MRS, and no lactate was detected in the lesion area. The other 6 cases used short echo time (TE=35ms) and all detected lactate peaks in the significant lesion area. 7 cases showed a decreasing trend of NAA/Cr in the significant lesion area. In three of the cases, a significant lactate wave peak was also seen in the cerebrospinal fluid MRS. Moller et al [14] found differences in metabolites in abnormal and non-abnormal brain areas on MRI. In MELAS patients, Lac was significantly elevated but NAA, Glu, Ins and Cr were significantly decreased in abnormal MRI areas; in brain areas without abnormalities on MRI, Lac was slightly elevated and NAA and Cr were slightly decreased. He Dan et al [15] performed a follow-up study on the application of MRS in patients with MELAS. 5 patients underwent MRS in the area of the lesion shown on MRI and found a slightly decreased NAA peak and an abnormally high Lac peak, with no significant increase in Cho peak. 4 patients showed normal areas on MRI and 5 sites were selected for MRS, and Lac peaks were found in 3 sites. Three of these patients underwent follow-up studies, and four new areas of lesions were identified, all of which showed abnormally high Lac peaks. The Lac peaks were still not completely disappeared in areas where the original 4 lesions had returned to normal on conventional MRI, showing mild-moderate elevation. the NAA/Cr ratio was slightly lower in the MRI lesion-positive area compared to the contralateral MRI-negative area, while the Lac/Cr ratio was significantly higher and the Cho/Cr ratio was unchanged Yan Fengshan et al [16] studied 8 patients with MELAS in whom lactate peaks were seen on MRS, with 4 cases The alanine peak was observed in four cases. 3.2 Characteristics of MRS in Leigh’s disease: It is also known as subacute necrotizing encephalomyelopathy. It is most often seen in the neonatal period, with clinical symptoms such as respiratory distress, convulsions, and severe motor developmental delay, and often dies in infancy. Xiao Jiangxi et al [17] studied the neurological damage in patients with Leigh’s and provided evidence for clinical control and early intervention in familial cases. The authors mainly found that (i) in the thalamus and pallidum abnormal signal areas, NAA/Cr was reduced and Lac/Cr was increased, suggesting the presence of pathological changes of neuronal reduction and glial cell hyperplasia in Leigh’s patients in the pallidum and thalamus; (ii) in the pallidum, the values of Cho/Cr in the MRI-negative group were higher than those in normal controls; in the thalamus, the MRI-negative group had NAA/Cr values were lower in the MRI-negative group than in the normal control group, indicating that elevated Cho/Cr in the pallidum and decreased NAA/Cr in the thalamus in the absence of abnormalities on conventional MRI suggest the presence of metabolic abnormalities and allow for an early diagnosis of Leigh’s disease.Sijens et al [18] studied two Leigh’s patients and found Lac bimodal peaks in both white and gray matter areas of the brain, suggesting metabolic involvement of the entire brain. Further study revealed that the Cho peaks in the white matter region of Leigh’s patients were higher than the Cho peaks in the gray matter region, suggesting significant myelin loss in the white matter region. In conclusion, the lactate peak is evident in the MRS test for mitochondrial encephalomyopathy with elevated lactate in brain tissue. Although lactate peaks can also appear in other CNS diseases such as early cerebral infarction, demyelinating lesions, and brain tumors, lactate peaks in mitochondrial encephalomyopathy can be detected in brain areas without lesions on MRI, especially in cerebrospinal fluid areas, in addition to brain areas with abnormal MRI signals, whereas lactate peaks in general cerebral infarction, brain tumors, and demyelinating lesions can be detected only in lesion areas, and this point This is an important differentiation value. Organic acids are carboxy acids produced during the intermediate metabolism of amino acids, fats and sugars. Organic acid metabolism disorder is due to the deficiency of certain enzymes, resulting in the accumulation of related carboxy acids and their metabolites. Clinically, some patients have an acute onset in the form of vomiting, metabolic acidosis, hypoglycemia, and coma, and some patients present with progressive neurological impairment such as intellectual motor deficits and seizures. Spanish scholars [19] studied seven patients with glutaric aciduria, four of whom presented with acute encephalopathy. three were examined by MRS, and in all MRS studies of the basal ganglia, a decrease in NAA and NAA/Cr ratios was found, suggesting necrosis of neurons. Turkish scholars [20] studied MRS in 19 patients with metabolic diseases, including 3 cases of organic acid metabolic disease. 1 case of maple glycosuria, MRS found decreased NAA/Cr and increased Cho/Cr, ml/Cr and Glx/Cr in the central nervous system, and Lac peak and abnormal methyl peak were found. 1 case of glutaric aciduria type I, MRS found decreased NAA/Cr, Cho /Cr, ml/Cr and Glx/Cr were elevated, and no Lac peak was detected. 1 case of type 2 hydroxyglutaric aciduria, MRS found elevated ml/Cr and Glx/Cr, no significant abnormalities in NAA/Cr and Cho/Cr, and no Lac peak was detected. All of the above MRS of organic acid metabolic disorders suggest evidence of neuronal damage. V. Impaired amino acid metabolism Defective enzymes in the process of amino acid metabolism can cause abnormal accumulation of related amino acids and their metabolites and organ damage, with liver, brain and kidney involvement predominant. Kunti Wang et al [21] studied 32 untreated children with hyperphenylalaninemia (HPA), 18 males and 14 females, aged 33 days-14 years. The results revealed that (i) a phenylalanine (Phe) wave peak was seen at 7.36 ppm on MRS, and the peak was low on MRS because Phe is very low in brain tissue. the presence of the Phe wave peak indicates abnormal accumulation of Phe in the brain and can assist in the diagnosis of HPA. ②The blood-brain interstitial Phe concentration in children with HPA is positively correlated. Since the blood-brain Phe concentration is positively correlated, it suggests that most patients can have better and effective control of Phe in the brain as long as the blood Phe concentration can be controlled. (iii) Of the 32 cases, 22 cases were greater than 4 months. There was a negative correlation between blood and brain Phe concentrations and IQ in all 22 cases of children older than 4 months. Multiple linear regression revealed a closer relationship between brain Phe concentration and IQ. The authors concluded that the application of MRS allows non-invasive and quantitative detection of brain Phe concentration in HPA patients to understand the extent of brain damage in children with HPA. Turkish scholars [22] studied a PKU patient and MRS found a slight increase in Cho/Cr and a normal range of NAA/Cr and did not find a phenylalanine peak. German scholar Ethofer et al [23] studied a patient with succinic semialdehyde dehydrogenase deficiency (SSADH), a disorder of γ-aminobutyric acid (GABA) catabolism in the central nervous system, leading to the accumulation of 4-hydroxybutyrate (GHB) metabolites in the body. The authors applied MRS technique and found significantly elevated GABA concentration and trace GHB in the white matter and gray matter of the patient’s brain. VI. Metal metabolism disorders Metal metabolism disorders are mostly due to copper and iron metabolism disorders resulting in copper and iron not being excreted from the body and accumulating in the central nervous system and other organs leading to brain damage and multiple organ dysfunction. H. Lou et al [24] studied 12 clinically diagnosed children with hepatomegaly (WD), and performed MRS examinations. The NAA/Cr in the nucleus accumbens and caudate nucleus, which are susceptible to hepatomegaly, were significantly lower than those in the thalamus, suggesting that the neurons in the nucleus accumbens and caudate nucleus were severely damaged. (2) The decrease in NAA/Cr ratio was most pronounced in lesions with low signal on DWI in patients with hepatomegaly, and was accompanied by an increase in Cho/Cr ratio, an alteration consistent with the loss of neuronal cells and extensive proliferation of astrocytes seen on pathology. This study found that the combination of magnetic resonance DWI and wave spectral analysis can effectively evaluate the microstructural and metabolic changes during copper deposition in hepatomegaly, thus providing a feasible observation method for clinical monitoring of the effect of copper expulsion therapy and the prognosis of the disease course. The Polish scholar Tarnacka et al [25] studied 37 newly diagnosed WD patients and applied MRS to evaluate their metabolic changes. This author divided WD into two groups according to clinical presentation, namely hepatic (hWD) and neurological (nWD), and had a normal control study group. Their study found that all WD patients with CNS neurons, either hWD or nWD, had neuronal degenerative lesions resulting in reduced NAA/Cr. The clinical relevance of pallidum lesions was further investigated. nWD patients were evaluated for neurological function by WDNRS (Neurological Function Score in WD patients); hWD patients were evaluated for hepatic function by WDHRS (Liver Function Score in WD patients). The authors found that in patients with nWD and hWD, the clinical performance function score was negatively correlated with the pallidocyte NAA/Cr ratio. Xiao Li et al [26] reported the MRS features of Hallervorden-Spatz syndrome (HSS), an autosomal recessive disease with abnormal iron salt deposition in the brain (mainly in the nigrostriatal red nucleus of the pallidum), which is a neurodegenerative disease with iron deposition in the brain. The authors found that MRS showed reduced NAA in the pale bulb region and a significant decrease in the right NAA/Cr ratio, suggesting significant neuronal lesions in this region. VII. Lysosomal deposition disease Lysosomes are cellular organelles that contain a variety of hydrolytic enzymes inside. When lysosomal enzymes are defective, sphingolipids, glycoproteins, and aminoglucan cannot be degraded normally, causing cytotoxicity and dysfunction of brain and other organs. Qin Chengwei et al [27] studied the MRS in the head of a child with mucopolysaccharide accumulation disease type II (MPS-II), and the authors found a mild to moderate increase in the ml peak and a mild increase in the Cho peak in the MRS region of interest, with normal NAA and Cr peaks and no Lac peak, and no abnormal wave spectrum in the adjacent cingulate region. The child’s MRS was re-examined six months after enzyme replacement therapy and showed no signs of disease progression. The authors concluded that the ml peak in the MRS of this disease can evaluate the degree of MPS storage in the brain. Ren Aijun et al [28] studied the MRS manifestations of infantile and late infantile neuronal waxy lipofuscin deposition disease (NCL). The authors performed MES in a patient with NCL 6 years after the onset of the disease and did not detect a NAA peak, a significantly lower Cho/Cr peak, and a markedly elevated ml peak. four children with late-infantile type NCL who underwent MRS with a history of 2, 3, 4, and 5 years had progressively lower corresponding NAA/Cr values and insignificant changes in Cho/Cr. The authors concluded that in the CNS of NCL patients, there is a large amount of neuronal cell loss and a decrease in NAA levels. Even in the late stage of NCL, NAA is undetectable, indicating severe neuronal cell loss. The significantly elevated ml in this child indicates severe glial cell proliferation in the brain. The authors concluded that the NAA level gradually decreased as the disease progressed, Cho and Cr first increased and later also decreased, and a ml peak and a lactate peak gradually appeared. Mochel et al [29] studied six patients with free sialic acid storage disease. This patient had significantly elevated free sialic acid in urine and cerebrospinal fluid, MRI showed low white matter myelin production, and MRS showed a significant elevation of NAA in the cerebrospinal fluid. VIII. Conclusion In conclusion, MRS technique has been widely used in the study of metabolic diseases in children. Since the spectrum of inherited metabolic diseases in children is broad and the clinical manifestations are complex and diverse, the application of MRS must be combined with the history, signs and central nervous system imaging of the disease to provide more valuable information. With the development of technology, the improvement of magnetic resonance equipment and the increase of magnetic field strength, we believe that MRS can provide more valuable information for the diagnosis of pediatric genetic metabolic diseases in the future.