Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is a rare neurodegenerative disorder caused by the impairment of the mitochondrial respiratory chain. Approximately 80% of cases have an A to G mutation at nucleotide 3243 (A3243G) in the mitochondrial DNA molecule (MT-TL1), and heteroplasmy causes different mutational burdens within tissues, resulting in diverse clinical phenotypes. Patients usually present with stroke-like episodes (hemianopia, cortical blindness, and dysarthria), seizures, recurrent headaches, or muscle weakness. Elevated lactate levels in blood and cerebrospinal fluid (CSF) and a change in the lactate-to-pyruvate ratio indicate respiratory chain dysfunction [1-3]. Typical magnetic resonance imaging (MRI) findings in an acute stroke-like episode of MELAS include cortical lesions. Magnetic resonance spectroscopy (MRS) is a noninvasive method that reveals metabolic information regarding brain tissue. Increased lactate levels reflect anaerobic glycolysis, and N-acetylaspartate (NAA) is a marker of neuronal integrity that represents neuronal injury associated with infarction. MRS has been used to diagnose stroke‐like episodes in MELAS, with elevated lactate peaks and low NAA. Several researchers have previously reported that MRS is a useful tool for monitoring the progress of disease in patients with mitochondrial disorders [4-7]. This study aimed to evaluate the role of MRS in the initial assessment of MELAS.

The data of MELAS patients who were admitted to the pediatric neurology department at Gangnam Severance Hospital (Seoul, Korea) between January 2006 and December 2020 were reviewed. In total, 34 patients had findings compatible with the MELAS clinical diagnostic criteria, as enumerated by Bernier et al. [8]. This was a retrospective study of patients with MELAS who met the following inclusion criteria: genetically confirmed diagnosis of a pathogenic variant associated with MELAS (MT-TL1, m.3243A>G) and MRS performed at our center. Mutation carriers without neurological symptoms were excluded.

To identify the A3243G mutation, polymerase chain reaction amplification and restriction fragment length polymorphism analysis of total DNA extracted from leukocytes were performed. Based on this analysis, 25 patients with pathogenic mutations were included among the 34 symptomatic patients in this study. Demographic, clinical, laboratory, and MRI data were reviewed. This study was approved by the Institutional Review Board of the Gangnam Severance Hospital, Yonsei University College of Medicine(3-2017-0168). Informed consent for this retrospective study was waived by the board.

In this study, we evaluated the role of MRS in the initial diagnosis of MELAS. Most initial MELAS evaluations begin with a series of biochemical studies of blood, urine, and CSF. Elevated lactate levels have long been associated with mitochondrial diseases. False elevations of plasma lactate are common because of improper specimen collection and handling. An elevated lactate-to-pyruvate ratio provides a more useful tool than serum lactate values for indicating respiratory chain dysfunction. CSF lactate levels are not influenced by the collection technique and are elevated in mitochondrial disorder patients with predominant brain manifestations.

Suomalainen et al. [11] reviewed the sensitivity (52%) and specificity (92%) of lactate measurements in patients with genetically confirmed mitochondrial disease. Blood lactate levels can be normal in MELAS patients; however, even in the presence of normal serum lactate, the CSF lactate level may be a more reliable diagnostic marker for MELAS. Tsujikawa et al. [12] reported that the concentration of CSF lactate was closely correlated with that of brain tissue lactate obtained via MRS. Elevations of brain lactate levels are generally indicative of increased anaerobic glycolytic rates. NAA reduction reflects neuronal injury and impaired mitochondrial dysfunction after recurrent stroke-like episodes.

Previous studies have reported that brain metabolites could be evaluated using MRS to determine disease progression and perform treatment monitoring in patients with mitochondrial disorders [5,13]. Heteroplasmy results in a variety of clinical phenotypes, from asymptomatic carriers to fully symptomatic patients, as well as different mutational burdens between and within tissues. Weiduschat et al. [7] recently reported that metabolic abnormalities detected by MRS in asymptomatic A3243G carriers have potential clinical value as biomarkers of disease progression and therapeutic response. A recent study comparing the efficacy of MRS after acute neurologic symptoms in patients with acute ischemic stroke (AIS) and patients with MELAS found a lactic acid peak in only 69.2% of AIS patients, whereas a lactic acid peak was found in all patients with MELAS [14,15]. Therefore, MRS findings can reflect a lack of energy production due to impaired oxidative phosphorylation caused by nonspecific ischemia and hypoxia in the acute phase, and the relatively high sensitivity of MRS makes it a useful tool for the diagnosis of MELAS patients.

This study aimed to evaluate the clinical value of non-invasive MRS to detect brain metabolites in the initial diagnosis of genetically confirmed A3243G MELAS patients. Among 34 MELAS cases, no significant difference was found for increased lactate (P>0.05) or decreased NAA peaks (P>0.05) in MRS between the groups with and without genetic confirmation. Twenty-five of the 34 patients were diagnosed with genetically confirmed MELAS with the A3243G pathogenic mutation. Only 18 (72%) of those 25 patients had a lactate peak on initial MRS.

The detection of a lactate peak on MRS is dependent on the timing or severity of the disease. In the 25 genetically confirmed MELAS patients, the timing of the initial MRS test was divided into groups according to whether MRS was performed in the acute stage, despite the limited number of samples, and lactic acid peaks were found in all MRS data performed within 2 weeks after the first neurologic symptoms. In this study, the neurological symptoms of MELAS most commonly presented as stroke-like episodes, and elevated lactate peaks in MRS were identified in all MELAS patients in the acute stage with or without serum lactic acidosis. Elevated lactate peaks in initial MRS were not identified in patients with resolving first stroke-like symptoms, who showed symptoms at least 28 days before MRS. However, elevated lactate peaks on MRS and severe diffuse cerebral atrophy on MRI were shown in eight of 15 patients who underwent MRS more than 2 weeks after initial onset, and those patients had at least two recurrent stroke-like episodes despite the current absence of acute symptoms. Decreased NAA peaks were found in 17 of the 25 MELAS patients. The decreasing trend in NAA concentration reflects brain damage due to recurrent stroke-like attacks during prolonged periods with or without acute symptoms.

In conclusion, although initial MRS alone is not enough to definitely diagnose MELAS, it is worthwhile as a non-invasive complementary diagnostic tool along with genetic testing, as well as for monitoring the disease progress of MELAS.