mitochondrial disease involves a group of rare disorders of mitochondrial dysfunction , which is usually the result of gene mutations in either mitochondrial dna ( mtdna ) or nuclear dna ( ndna ) . because mitochondria are the main source of energy production in cells and are present in all tissues except for red blood cells , clinical features manifest typically in tissues with the highest energy requirements including skeletal muscles , brain , myocardium , and endocrine systems . the clinical presentation is highly variable on the disease onset , symptoms , signs , severity , and prognosis . it is common for mitochondrial disease to present with constellations of symptoms , which allows them to be categorized into one of the several syndromes . mitochondrial myopathy , encephalopathy , lactic acidosis , and stroke - like episodes ( melas ) is a progressive neurodegenerative disorder associated with polygenetic , maternally inherited mutations in mtdna . approximately 80% of melas cases are caused by the mutation m.3243a > g of the mitochondrial transfer rna ( trna ) ( leu ( uur ) ) gene ( mt - tl1 ) . thus , the genetic diversity and the unpredictable presentation and clinical course highlights the difficulties in diagnosing melas . promisingly , some new research progresses have been made recently , including the characteristic functional magnetic resonance imaging ( fmri ) imaging , such as magnetic resonance spectroscopy ( mrs ) , which can be used to determine the lactate level of brain , and proton magnetic resonance spectroscopic imaging ( 1h - mrsi ) assessment can be used to predict disease severity . be aware that other diseases may have similar muscle biopsy findings of mitochondrial myopathies ( mm ) , including ragged red fibers ( rrfs ) and cytochrome oxidase ( cox ) negative fibers herein , we review and analyze the research progresses in melas in order to help make better diagnosis of this disease . clinical manifestation of melas is highly variable , which makes it difficult to be timely accurately diagnosed . given that mitochondria are the common pathway for the oxidative decarboxylation of fats , amino acids , and carbohydrates , it is understandable that mitochondrial dysfunction can impair cellular energy metabolism , which can be particularly apparent during periods of superimposed metabolic stress ( e.g. , exercise , infection , or prolonged fasting ) . disease onset can occur at any age although typically the earlier the clinical phenotypes present in life , the severer the disease develops . for example , patients with the melas m.3243a > g gene mutation can present with the later adult - onset deafness and diabetes or fatal encephalomyopathy with seizures and stroke - like episodes ( ses ) in infancy . less than 10% of patients with genetically confirmed melas present before the age of 1 , whereas 6080% are diagnosed by the age of 15 . there are rare case reports in the literatures of patients presenting and being diagnosed as old as 80 years of age . the average observed age of death in melas patients was reported to be 1934.5 years ( range 10.281.8 years ) , with 22% of death occurring younger than 18 years . the predominant feature of melas involves ses , which commonly presents acute cortical blindness , psychiatric disorder , dementia , or paralysis . . a wide range of clinical features can also present , including hearing loss , myopathy , neuropathy , migraine , epilepsy , ataxia , short stature , diabetes mellitus ( dm ) , gastrointestinal disease , and cardiomyopathy . previous studies have reported a large proportion of death attributable to cardiac causes in melas patients . isolated mm severely affecting respiratory muscles is an uncommon clinical spectrum of m.3243a > g mitochondrial disease . reported a prospective cohort study of 85 melas patients ( the largest sample to date ) from 35 families with the m.3243a > g mutation followed for up to 10.6 years . in that study , patients with melas had systemic symptoms including exercise intolerance ( 93% ) , gastrointestinal disturbance ( 90% ) , loss of hearing ( 70% ) , growth failure ( 40% ; the presence of developmental delays and growth failure was associated with an earlier onset of melas ) , diabetes ( 39% ) , hirsutism ( 25% ) , night blindness ( 44% ) , and ptosis ( 41% ) . further , the development symptoms of the patients consist of school difficulties ( 51% ) , child motor delay ( 39% ) , perinatal difficulties ( 34% ) , special education ( 34% ) , and speech delay ( 28% ) . other studies have reported considerable diversity in the clinical phenotypes that result from pathogenic mutations , including nephrotic syndrome and an altered mental status . in 2013 , a chinese case control study of 770 chinese han ethnicity dm patients and 309 healthy control individuals showed 13 ( 1.69% ) individuals carrying the m.3243a > g mutation , which was not found in any of the healthy controls . the data suggested that the m.3243a > g mutation may be a risk for dm , especially in severe cases of the disease in the chinese han population . so far , the biomarkers of melas in blood are rarely identified . as a common marker of injured muscles , plasma creatine kinase levels in patients with melas are typically normal or slightly elevated , no more than five times the upper normal limit . as lactate acidosis is one of the cardinal signs of melas syndrome , the most specific laboratory data for melas include extremely elevated lactic acid and pyruvic acid levels in both plasma and cerebrospinal fluid ( csf ) , and there is evidence that high lactate level is associated with increased mortality . chinnery proposed that fasting blood lactate level above 3 mmol / l , or fasting csf lactate level above 1.5 mmol / l supports a diagnosis of mitochondrial disease . elevated csf lactate level is a reliable marker pointing to mitochondrial origin of disease , even in children who have recently suffered short - lasting seizures . however , normal plasma or csf lactate level does not exclude the presence of a mitochondrial disorder . unlike stroke , computed tomography ( ct ) has limited value in revealing brain lesions in melas , although there may be areas of focal infarction or bilateral basal ganglia calcifications on brain ct . nevertheless , ct is often performed as it is widely available during the acute episodes of seizures and ses in melas patients . mri is the main instrument used for the diagnosis of melas , along with the muscle biopsy and genetic studies . for example , during ses , brain mri t1wi and t2wi show signal changes similar to that of acute cerebral infarction . but serial mri typically reveals lesions that are not restricted to arterial territories , and that migrate over time . these lesions commonly affect the occipital and parietal lobes , although the deep gray matter such as the thalamus may also be affected , likely reflecting its high metabolic demand . further , cortical lesions observed on mri preferentially affect the cortical ribbon , with relative sparing of the deeper white matter , again reflecting the higher metabolic demand of these regions . diffusion - weighted imaging can also reveal a hyperintense lesion in the affected lobes . as there is restricted diffusion , there is also a reduced apparent diffusion coefficient , as seen in ischemic stroke . interestingly , there is evidence that the clinical symptoms and mri / mra findings in some melas patients can improve ( reversible vasoconstriction ) within 4 weeks . in short , the brain lesions of melas in serial mri are quite different from that of acute ischemic stroke . magnetic resonance spectroscopy ( mrs ) is a useful technology to help diagnose melas , as it allows assessment brain metabolites . as a result of mitochondrial dysfunction , aerobic energy transduction is impaired , hence patients with melas have high lactate levels in blood and csf . in melas , mrs can detect a higher than normal level of lactate in parts of the brain not acutely ischemic , thereby confirming a level of metabolic derangement . 1h - mrsi assessment of cerebral metabolism in m.3243a > g mutation carriers is also promising for identification of disease biomarkers and individuals at risk of developing the melas phenotype . elevated central nervous system ( cns ) lactate as estimated by mrsi is associated with shorter survival , confirming previous observations that elevated lactate is associated with increased disease severity . yu et al . reported that mri could also show quantitative changes in oxygen extraction fraction ( oef ) at different phases of ses . the use of oxygen in the brain , especially for those brain regions involved in the disease is usually severely reduced after the episodes of melas . further , oef is significantly reduced in the brain at the acute , subacute phase and interictal phase . increased blood flow can be found in the stroke - like lesions at the acute and subacute phase . ikawa et al . also reported preclinical latent lesions in patients with melas by arterial spin labeling ( asl ) perfusion mri . regional cerebral hyperperfusion is observed on asl perfusion mri in the preclinical phase more than 3 months before the clinical onset of ses in three melas patients . these hyper - perfused areas are not detected by conventional mri in the preclinical phase and developed into acute lesions at the clinical onset of ses , suggesting that asl imaging has the potential for predicting the emergence of ses . positron emission tomography imaging can evaluate regional cerebral blood flow , redox energy states , and glucose metabolism in the living brain , and has been used to further elucidate the pathogenesis of acute stroke - like lesions . the acute melas lesions appear to have an initial increase in blood flow and glucose metabolism in response to oxidative stress . in the subacute phase , there is a decrease in blood flow and glucose use even under the condition of increased oxidative stress . a specific clinical feature of melas is mm , which presents as exercise intolerance and weakness . as the symptoms are the most common presentations of varied myopathy , it is strongly advised for patients who are suspected with melas to accept a muscle biopsy . muscle biopsy is very helpful in confirming the diagnosis of melas , which mainly presents cox negative fibers and rrf . the muscle biopsy is typically performed from a limb muscle , such as the quadriceps femoris or deltoid . the major diagnostic feature is the presence of fibers deficient for cox activity ( mitochondrial cytopathy ) , which represents poor activity of complex iv of the respiratory chain , encoded by both the mtdna and ndna genes . it is important to note that a low frequency of cox - deficient fibers is a normal finding in healthy aged individuals . thus , in general , the detection of any cox - deficient fibers in individuals < 50 years of age , or a higher frequency of cox - deficient fibers ( > 5% ) at any age , is strongly suggestive of a mitochondrial disorder . the identification of cox - deficient fibers is supported by serially staining muscle for cox followed by succinate dehydrogenase ( sdh ) that stains for complex ii , which is encoded entirely by nuclear genes . the demonstration of cox - deficient , sdh - positive muscle fibers exhibits the best sensitivity and specificity for mm . as a general rule , a mosaic appearance of cox negative fibers suggests an mtdna mutation due to the variable degrees of heteroplasmy among muscle cells , whereas uniformly decreased cox activity suggests an ndna mutation which would be equally present in all muscle cells . the sub - sarcolemmal accumulation of mitochondria is a classic feature of melas and mm , and can be demonstrated by sdh histochemistry ( so - called  ragged blue fibers  ) or the modified gomori trichrome stain so - called rrf . again , a low frequency of rrfs ( < 5% ) can be seen in healthy aged individuals . however , the detection of rrfs in individuals < 50 years of age or > 5% rrf at any age is highly suggestive of mm , although even high levels can be secondary to other pathologies such as inclusion body myositis . other characteristic features include the presence of strongly sdh - positive blood vessels seen in muscle of patients with melas harboring m.3243a > g . electron microscopic ( em ) examination of serial frozen sections of these biopsies showed that the smooth muscle cells of the strongly sdh - reactive blood vessels contain marked proliferation of mitochondria , characteristic of patients with melas . em can also be performed on muscle specimens and demonstrate a variety of abnormalities associated with mm , although these are rarely specific for mitochondrial diseases . however , em may provide minor diagnostic criteria for mitochondrial disease in some patients with normal histochemistry , and thus may contribute additional information in selected cases . muscle histochemistry and/or em may be normal even in the context of genetically proven mitochondrial syndromes , particularly in the early disease course or when the biochemical defect does not involve complex iv . rce is technically difficult to perform , even by specialist , and the results should be interpreted in the context of other examinations . demonstrating an rce defect is a crucial diagnostic step in patients particularly children with normal or near - normal muscle histochemistry . mitochondrial dna is a small ( 16,569 bases ) , circular , double - stranded molecule , lacking introns and possessing only one promoter region . mtdna gives rise to two ribosomal rnas , 22 trnas , and 13 protein - encoding messenger rnas that contribute proteins to complexes i , iii , iv , and v of the oxidative phosphorylation system . during fertilization , each cell contains several mitochondria , and each mitochondrion contains numerous genomes , with hundreds of copies of mtdna in every cell . in this setting , the phenomenon of genetic heteroplasmy arises , where a proportion of genomes contain a mutation . consequently , a pathogenic mtdna mutation can only cause a disease phenotype if the level of the mutation within a cell or tissue exceeds a threshold amount , which varies depending on the mutation and the tissue [ figure 1 ] . cell types and tissues within an individual vary in the proportion of abnormal mtdna and a given tissue can also have different levels over time . all of these are considered as the main pathogenesis of mitochondrial diseases caused by the pathogenic mutations of mtdna . myopathy , encephalopathy , lactic acidosis , and stroke - like episodes , if mutational mtdna exceeds the threshold , the mitochondrial function is impaired . if the mutation does not exceed the threshold , the mitochondrial function may be normal . the mitochondrial trna ( leu ( uur ) ) gene ( mt - tl1 ) appears to be a hotspot for pathogenic mtdna mutations . the phenotypic heterogeneity observed with several different mutations spanning only 59 bp of this gene is difficult to interpret . these phenotypes range from myopathy alone ( 3302a > g ) , to proximal and truncal myopathy and sudden death ( 3251a > g ) , to a disorder that often predominantly affects the cns including the melas syndrome ( 3243a > g , 3271t > c , 3252t > c ) . the most common mutation ( bp 3243 ) itself has a very variable phenotype , which has also been found in patients with progressive external ophthalmoparesis , myopathy alone , diabetes , and deafness . approximately 80% of melas cases are caused by the mutation m.3243a > g of the mt - tl1 gene , while 715% of melas patients have the 3271t > c mutation . some case reports show that m. 4322g > a , 3251a > g , 3252t > c , 3256c > t , 3302a > g , 3697g > a , 3946g > a , 3949t > c , 3959g > a , 3995a > g , 12300g > a , and 13513g > a mutations also lead to melas . it is likely that more mutations might be found in patients with melas in the future . genetic tests should be guided based on muscle biopsy findings , and these techniques are performed in combination . mutation load can vary among tissues ( including oocytes ) in an individual and within a family . as blood cells have a high replication rate and selectivity against pathogenic mtdna abnormalities , however , it should not be assumed that mutation load is negligible , as it may be higher in other tissues . in this because melas is a group of rare disorders that presents with varied clinical phenotypes , it is , therefore , easy to misdiagnose . the first symptoms include exercise intolerance , seizures , paralysis , hemianopsia , mental anomaly , gastrointestinal disorders , respiratory muscles weakness , and arrhythmia . patients with melas are usually sent to the emergency room , and are predominantly initially misdiagnosed as myasthenia gravis , epilepsy , cerebral infarction , encephalitis , gastrointestinal diseases , or heart diseases , leading to mistaken treatment and delayed therapeutic opportunity . elevated plasma lactate levels and lesions mainly located in the cortex , with relative sparing of the deeper white matter on ct and mri , indicate suspected melas . further investigation should be performed stepwise to confirm the diagnosis [ figure 2 ] . diagnosis flow chart . * represents the key elements for the right diagnosis of mitochondrial myopathy , encephalopathy , lactic acidosis , and stroke - like episodes . current therapies include supplement of l - arginine , citrulline , atp , vitamins , riboflavin , and coq10 . , melas is a progressive neurodegenerative disorder associated with polygenetic , maternally inherited mutations in mtdna . the predominant feature of melas is ses , and the onset usually occurs in the second decade the main diagnostic feature of muscle biopsy is the presence of rrf and mosaic appearance of cox negative fibers . mitochondrial genetic examination can confirm the diagnosis , the frequent mutations are 3243a > g , 3271t > c , and 3252t > c . new techniques of fmri especially 1h - mrsi and oef enhance the diagnosis of melas . the overall diagnosis should be based on not only the clinical manifestations , but also the laboratory findings from blood and csf biochemistry , neuroimaging , muscle biopsy , and genetic examination . special attention should be paid because of the variability in disease stages and genetic heteroplasmy . for example , muscle biopsy may be normal in the early disease course , and mtdna abnormalities are not detectable in blood . the prognosis of melas patients depends on the location of genetic mutations , mutation load , and the onset of disease and interventions .
objective : mitochondrial myopathy , encephalopathy , lactic acidosis , and stroke - like episodes ( melas ) is a progressive , multisystem affected mitochondrial disease associated with a number of disease - related defective genes . 
 melas has unpredictable presentations and clinical course , and it can be commonly misdiagnosed as encephalitis , cerebral infarction , or brain neoplasms . 
 this review aimed to update the diagnosis progress in melas , which may provide better understanding of the disease nature and help make the right diagnosis as well.data sources : the data used in this review came from published peer review articles from october 1984 to october 2014 , which were obtained from pubmed . 
 the search term is 
 melas.study selection : information selected from those reported studies is mainly based on the progress on clinical features , blood biochemistry , neuroimaging , muscle biopsy , and genetics in diagnosing melas.results:melas has a wide heterogeneity in genetics and clinical manifestations . 
 the relationship between mutations and phenotypes remains unclear . 
 advanced serial functional magnetic resonance imaging ( mri ) can provide directional information on this disease . 
 muscle biopsy has meaningful value in diagnosing melas , which shows the presence of ragged red fibers and mosaic appearance of cytochrome oxidase negative fibers . 
 genetic studies have reported that approximately 80% of melas cases are caused by the mutation m.3243a > g of the mitochondrial transfer rna ( leu ( uur ) ) gene ( mt - tl1).conclusions : melas involves multiple systems with variable clinical symptoms and recurrent episodes . 
 the prognosis of melas patients depends on timely diagnosis . 
 therefore , overall diagnosis of melas should be based on the maternal inheritance family history , clinical manifestation , and findings from serial mri , muscle biopsy , and genetics .