MR evaluation of cerebral oxygen metabolism and blood flow in stroke-like episodes of MELAS

The metabolic hypothesis is more likely than the epileptogenic hypothesis to explain stroke-like lesions

Stroke-like episodes (SLEs) are a hallmark of mitochondrial encephalopathy, lactic acidosis, and stroke-like episode (MELAS) syndrome but occur in other mitochondrial disorders (MIDs) as well. The morphological equivalent of the SLE is the stroke-like lesion (SLL) on magnetic resonance imaging (MRI). The pathophysiology of SLLs is under debate, but several hypotheses have been raised to explain the phenomenon. Of these, the metabolic, epileptogenic, and vascular hypotheses are the most frequently discussed. There are several arguments for and against these hypotheses, but a consensus has not been reached which of them provides the correct explanation. A recent consensus statement generated by a panel of experts applying the Delphi method, favoured the epileptogenic hypothesis and recommended treatment of SLEs with antiepileptic drugs, irrespective if the patient presented with a seizure or epileptiform discharges on electroencephalography (EEG) or not. We disagree with this general procedure and provide the following arguments against the epileptogenic hypothesis: 1. not each SLE is associated with seizures. 2. epileptiform discharges may be absent on EEG during a SLE. 3. SLLs are not restricted to the cortex. 4. antiseizure-drugs (ASDs) may not prevent the progression or recurrence of a SLL. 5. ASDs may terminate seizures but no other phenotypic feature of a SLE. 6. patients already under ASDs are not immune from developing a SLL. 7. SLLs usually last longer than seizures. 8. no animal model supports the epileptogenic hypothesis. The strongest arguments for the metabolic hypothesis are that SLLs are not confined to a vascular territory, that the oxygen-extraction fraction within a SLL is reduced, and that there is hypometabolism within a SLL on FDG-PET. SLLs may respond to antioxidants, NO-precursors, steroids, or the ketogenic diet. ASDs should be applied only if there is clinical or electrophysiological evidence of seizure-activity.

Arterial Spin Labeling in Pediatric Neuroimaging

Perfusion imaging using arterial spin labeling noninvasively evaluates cerebral blood flow utilizing arterial blood water as endogenous tracer. It does not require the need of radiotracer or intravenous contrast and offers unique complimentary information in the imaging of pediatric brain. Common clinical applications include neonatal hypoxic ischemic encephalopathy, pediatric stroke and vascular malformations, epilepsy and brain tumors. Future applications may include evaluation of silent ischemia in sickle cell patients, monitor changes in intracranial pressure in hydrocephalus, provide additional insights in nonaccidental trauma and chronic traumatic brain injury (TBI) and in functional Magnetic resonance imaging (MRI). The purpose of this review article is to evaluate the technical considerations including pitfalls, physiological variations, clinical applications and future directions of arterial spin labeling imaging.

The metabolic hypothesis is more likely than the epileptogenic hypothesis to explain stroke-like lesions

Stroke-like episodes (SLEs) are a hallmark of mitochondrial encephalopathy, lactic acidosis, and stroke-like episode (MELAS) syndrome but occur in other mitochondrial disorders (MIDs) as well. The morphological equivalent of the SLE is the stroke-like lesion (SLL) on magnetic resonance imaging (MRI). The pathophysiology of SLLs is under debate, but several hypotheses have been raised to explain the phenomenon. Of these, the metabolic, epileptogenic, and vascular hypotheses are the most frequently discussed. There are several arguments for and against these hypotheses, but a consensus has not been reached which of them provides the correct explanation. A recent consensus statement generated by a panel of experts applying the Delphi method, favoured the epileptogenic hypothesis and recommended treatment of SLEs with antiepileptic drugs, irrespective if the patient presented with a seizure or epileptiform discharges on electroencephalography (EEG) or not. We disagree with this general procedure and provide the following arguments against the epileptogenic hypothesis: 1. not each SLE is associated with seizures. 2. epileptiform discharges may be absent on EEG during a SLE. 3. SLLs are not restricted to the cortex. 4. antiseizure-drugs (ASDs) may not prevent the progression or recurrence of a SLL. 5. ASDs may terminate seizures but no other phenotypic feature of a SLE. 6. patients already under ASDs are not immune from developing a SLL. 7. SLLs usually last longer than seizures. 8. no animal model supports the epileptogenic hypothesis. The strongest arguments for the metabolic hypothesis are that SLLs are not confined to a vascular territory, that the oxygen-extraction fraction within a SLL is reduced, and that there is hypometabolism within a SLL on FDG-PET. SLLs may respond to antioxidants, NO-precursors, steroids, or the ketogenic diet. ASDs should be applied only if there is clinical or electrophysiological evidence of seizure-activity.

Metabolic stroke or stroke-like lesion: Peculiarities of a phenomenon

OBJECTIVES

One of the most frequent cerebral lesions in mitochondrial disorders(MIDs) on imaging is the stroke-like lesion(SLL) clinically manifesting as stroke-like episode (SLE, metabolic stroke). This review aims at discussing recent advances concerning the presentation, diagnosis, and treatment of SLLs.

METHODS

Systematic literature review using appropriate search terms.

RESULTS

SLLs are the hallmark of MELAS but occasionally occur in other MIDs. SLLs are best identified on multimodal, cerebral MRI. SLLs may present as uni-/multilocular, symmetric/asymmetric, cortical/subcortical, supra-/infratentorial condition, initially resembling a cytotoxic edema and later a vasogenic edema, or a variable mix between them. SLLs run through an acute and a chronic stage. The acute stage is characterised by a progressively expanding lesion over days, weeks, or months, showing up as increasing hyperintensity on T2/FLAIR, DWI, and PWI and by hyperperfusion, that does not conform to a vascular territory. ADC maps are initially hypointens to become hyperintens during the course. More rarely, a variable mixture of hyper- and hypointensities may be found. The chronic stage is characterised by hypoperfusion, gadolinium enhancement, and regression of hyperintensities to various endpoints. SLLs originate from an initial cortical lesion due to focal metabolic breakdown, which either remains stable or expands within the cortex or to subcortical areas. Some SLLs show spontaneous reversibility (fleeing cortical lesions) suggesting that neuronal/glial damage does not reach the threshold of irreversible cell death.

CONCLUSIONS

SLLs are a unique feature of various MIDs in particular MELAS. SLLs are dynamic and change their appearance over time. SLLs are accessible to treatment.

Levetiracetam administration is correlated with lower mortality in patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes: a retrospective study

Background:

Studies on the relationship between antiepileptic drug (AED) administration and clinical outcomes in patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) remain scarce. Levetiracetam (LEV) is an AED that is neuroprotective in various neurologic disorders. This study aimed to determine the impact of LEV on the outcome of MELAS.

Methods:

A retrospective, single-center study was performed based on a large cohort of patients with MELAS with a history of seizures (n = 102). Decisions on antiepileptic therapies were made empirically. Patients were followed up for 1 to 8 years (median, 4 years) and divided into 2 groups based on whether LEV was administered (LEV or non-LEV). The modified Rankin scale (mRS) scores and mortality risks were analyzed in all patients.

Results:

LEV, carbamazepine, benzodiazepines, topiramate, oxcarbazepine, valproate, and lamotrigine were administered in 48, 37, 18, 13, 11, 9, and 9 patients, singly or in combination, respectively. The mean mRS score of the LEV group (n = 48) was lower than that of the non-LEV group (n = 54; mean ± standard deviation, 2.79 ± 1.47 vs. 3.83 ± 1.93, P = 0.006) up to the end of the study. Nevertheless, there was no difference in the proportion of subjects without disability (mRS ranging 0–1) between the groups (P = 0.37). The multivariate regressions revealed that LEV treatment was associated with lower mRS scores (odds ratio 0.32, 95% confidence interval [CI] 0.15–0.68, P = 0.003) and mortality rates (hazard ratio 0.24, 95% CI 0.08–0.74, P = 0.013). There was a significant difference in the Kaplan-Meier survival curves between the groups (χ² = 4.29, P = 0.04).

Conclusions:

The LEV administration is associated with lower mortality in patients with MELAS in this retrospective study. Further laboratory research and prospective cohort studies are needed to confirm whether LEV has neuroprotective effects on patients with mitochondrial diseases.

Simultaneous R2, R2' and R2* measurement of skeletal muscle in a rabbit model of unilateral artery embolization

PURPOSE

To demonstrate the feasibility of using a susceptibility-based MRI technique with multi-echo gradient and spin echo (MEGSE) sequence to achieve simultaneous R2, R2' and R2* measurement and assess skeletal muscle oxygenation alternations in a rabbit model of unilateral artery embolization.

MATERIALS AND METHODS

Approved by the local institutional review board for experimental animal studies, nine New Zealand White rabbits were included in this study. The MEGSE sequence consists of embedding a set of gradient echoes around the echo of a single spin-echo sequence using several gradient echoes to collect the magnetization intensity during the formation and attenuation of spin-echo simultaneously after 180° radio frequency pulse. Within-session and between-day tests were conducted to evaluate the reproducibility of this skeletal muscle oxygenation alternations measurement. Furthermore, all the MEGSE scans of skeletal muscle were conducted using a 3-T clinical MRI scanner during resting state (before unilateral artery embolization operation, pre), 1 h after unilateral artery embolization operation (post1) and 2 h after unilateral artery embolization operation (post2) model to verify the feasibility and sensitivity of this method.

RESULTS

The within-session coefficient of variations (CVs) of R2, R2' and R2* measurements were 1.57%, 3.33% and 2.57%, while the between-day CVs of were 1.42%, 5.85% and 2.85%. In all rabbits, the mean R2 decreased significantly from 36.46 ± 1.03 s^-1 (pre) to 30.58 ± 2.11 s^-1 (post1,**P < 0.01, relative to pre) and 28.62 ± 1.53 s^-1 (post2, **P < 0.01, relative to post1), and the mean R2' went up markedly from 9.88 ± 2.14 s^-1 (pre) to 16.10 ± 2.74 s^-1 (post1, **P < 0.01) and 17.33 ± 2.25 s^-1 (post2, **P < 0.05). The mean R2* increased from 43.27 ± 3.75 s^-1 (pre) to 47.90 ± 5.08 s^-1 (post1, *P < 0.05) and to 48.04 ± 4.42 s^-1 (post2, NS, P > 0.05).

CONCLUSION

This study demonstrates the feasibility of simultaneous R2, R2' and R2* measurement method for the evaluation of skeletal muscle ischemia. Besides, this study indicates the sensitivity of the R2 and R2' compared with R2* and especially the necessity of R2 and R2' measurement for the further evaluation of skeletal muscle ischemia which always causes both edema and hypoxia in a rabbit model of unilateral artery embolization.

Mitochondrial metabolic stroke: Phenotype and genetics of stroke-like episodes

Stroke-like episodes (SLEs) are the hallmark of mitochondrial encephalopathy with lactic acidosis and stroke-like episode (MELAS) syndrome but rarely occur also in other specific or nonspecific mitochondrial disorders. Pathophysiologically, SLLs are most likely due to a regional disruption of the blood-brain barrier triggered by the underlying metabolic defect, epileptic activity, drugs, or other factors. SLEs manifest clinically with a plethora of cerebral manifestations, which not only include features typically seen in ischemic stroke, but also headache, epilepsy, ataxia, visual impairment, vomiting, and psychiatric abnormalities. The morphological correlate of a SLE is the stroke-like lesion (SLL), best visualised on multimodal MRI. In the acute stages, a SLL presents as vasogenic edema but may be mixed up with cytotoxic components. Additionally, SLLs are characterized by hyperperfusion on perfusion studies. In the chronic stage, SLLs present with a colorful picture before they completely disappear, or end up as white matter lesion, cyst, laminar cortical necrosis, focal atrophy, or as toenail sign. Treatment of SLLs is symptomatic and relies on recommendations by experts. Beneficial effects have been reported with nitric-oxide precursors, antiepileptic drugs, antioxidants, the ketogenic diet, and steroids. Lot of research is still needed to uncover the enigma SLE/SLL.

Cerebral imaging in paediatric mitochondrial disorders

OBJECTIVES

Because the central nervous system (CNS) is the second most frequently affected organ in mitochondrial disorders (MIDs) and since paediatric MIDs are increasingly recognised, it is important to know about the morphological CNS abnormalities on imaging in these patients. This review aims at summarising and discussing current knowledge and recent advances concerning CNS imaging abnormalities in paediatric MIDs.

METHODS

A systematic literature review was conducted.

RESULTS

The most relevant CNS abnormalities in paediatric MIDs on imaging include white and grey matter lesions, stroke-like lesions as the morphological equivalent of stroke-like episodes, cerebral atrophy, calcifications, optic atrophy, and lactacidosis. Because these CNS lesions may be seen with or without clinical manifestations, it is important to screen all MID patients for cerebral involvement. Some of these lesions may remain unchanged for years whereas others may be dynamic, either in the sense of progression or regression. Typical dynamic lesions are stroke-like lesions and grey matter lesions. Clinically relevant imaging techniques for visualisation of CNS abnormalities in paediatric MIDs are computed tomography, magnetic resonance (MR) imaging, MR spectroscopy, single-photon emission computed tomography, positron-emission tomography, and angiography.

CONCLUSIONS

CNS imaging in paediatric MIDs is important for diagnosing and monitoring CNS involvement. It also contributes to the understanding of the underlying pathomechanisms that lead to CNS involvement in MIDs.

Black Toenail Sign in MELAS Syndrome

BACKGROUND

Mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome is a mitochondrial disorder often causing progressive brain injury that is not confined to large arterial territories. Severe insults ultimately lead to gyral necrosis affecting the cortex and juxtacortical white matter; the neuroimaging correlate is partial gyral signal suppression on T2/FLAIR sequences that resemble black toenails. We aimed to characterize the imaging features and the natural history of MELAS-related gyral necrosis.

MATERIALS AND METHODS

Databases at two children's hospitals were searched for brain magnetic resonance imaging studies of individuals with MELAS. Examinations with motion artifact and those lacking T2/FLAIR sequences were excluded. The location, the cumulative number, and the maximum transverse diameter of necrotic gyral lesions were assessed using T2-weighted images and T2/FLAIR sequences. Wilcoxon signed-rank test was employed to evaluate the relationship between disease duration and the number of necrotic lesions.

RESULTS

One hundred twenty-four examinations from patients with 14 unique MELAS patients (16 ± 3 years) were evaluated. Six of the eight patients who developed brain lesions also developed gyral necroses (mean 13, range 0 to 44). Necrotic lesions varied in maximal diameter from 4 to 25 mm. Cumulative necrotic lesions correlated with disease duration (P < 0.001).

CONCLUSIONS

The black toenail sign signifying gyral necrosis is a common imaging feature in individuals with MELAS syndrome. The extent of gyral necrosis correlates with disease duration.

Cerebral Manifestations of Mitochondrial Disorders

This review aims at summarizing and discussing previous and recent findings concerning the cerebral manifestations of mitochondrial disorders (MIDs). MIDs frequently present as mitochondrial multiorgan disorder syndrome (MIMODS) either already at onset or later in the course. After the muscle, the brain is the organ second most frequently affected in MIMODS. Cerebral manifestations of MIDs are variable and may present with or without a lesion on imaging or functional studies, but there can be imaging/functional lesions without clinical manifestations. The most well-known cerebral manifestations of MIDs include stroke-like episodes, epilepsy, headache, ataxia, movement disorders, hypopituitarism, muscle weakness, psychiatric abnormalities, nystagmus, white and gray matter lesions, atrophy, basal ganglia calcification, and hypometabolism on 2-deoxy-2-[fluorine-18]fluoro-D-glucose positron-emission tomography. For most MIDs, only symptomatic therapy is currently available. Symptomatic treatment should be supplemented by vitamins, cofactors, and antioxidants. In conclusion, cerebral manifestations of MIDs need to be recognized and appropriately managed because they strongly determine the outcome of MID patients.

Cortical venous disease severity in MELAS syndrome correlates with brain lesion development

PURPOSE

MELAS syndrome is a mitochondrial disorder typified by recurrent stroke-like episodes, seizures, and progressive brain injury. Abnormal mitochondria have been found in arterial walls implicating a vasculogenic etiology. We have observed abnormal cortical vein T2/FLAIR signal in MELAS patients, potentially representing wall thickening and sluggish flow. We sought to examine the relationship of hyperintense veins and brain lesions in MELAS.

METHODS

Imaging databases at two children's hospitals were searched for brain MRIs from MELAS patients. Artifact, sedated exams, and lack of 2D-T2/FLAIR sequences were exclusion criteria. Each exam was assigned a venous score based on number of T2/FLAIR hyperintense veins: 1 = <10, 2 = 10 to 20, 3 = >20. Cumulative brain lesions and venous score in MELAS and aged-matched normal exams were compared by Mann-Whitney test.

RESULTS

A total of 106 exams from 14 unique MELAS patients (mean 16 ± 3 years) and 30 exams from normal aged-matched patients (mean 15 ± 3 years) were evaluated. Median venous score between MELAS and control patients significantly differed (3 versus 1; p < 0.001). In the MELAS group, venous score correlated with presence (median = 3) or absence (median = 1) of cumulative brain lesions. In all 8 MELAS patients who developed lesions, venous hyperintensity was present prior to, during, and after lesion onset. Venous score did not correlate with brain lesion acuity.

CONCLUSION

Abnormal venous signal correlates with cumulative brain lesion severity in MELAS syndrome. Cortical venous stenosis, congestion, and venous ischemia may be mechanisms of brain injury. Identification of cortical venous pathology may aid in diagnosis and could be predictive of lesion development.

Cerebral small vessel disease: Capillary pathways to stroke and cognitive decline

Cerebral small vessel disease (SVD) gives rise to one in five strokes worldwide and constitutes a major source of cognitive decline in the elderly. SVD is known to occur in relation to hypertension, diabetes, smoking, radiation therapy and in a range of inherited and genetic disorders, autoimmune disorders, connective tissue disorders, and infections. Until recently, changes in capillary patency and blood viscosity have received little attention in the aetiopathogenesis of SVD and the high risk of subsequent stroke and cognitive decline. Capillary flow patterns were, however, recently shown to limit the extraction efficacy of oxygen in tissue and capillary dysfunction therefore proposed as a source of stroke-like symptoms and neurodegeneration, even in the absence of physical flow-limiting vascular pathology. In this review, we examine whether capillary flow disturbances may be a shared feature of conditions that represent risk factors for SVD. We then discuss aspects of capillary dysfunction that could be prevented or alleviated and therefore might be of general benefit to patients at risk of SVD, stroke or cognitive decline.

Arterial spin labeling in clinical pediatric imaging

Arterial spin labeling (ASL) perfusion-weighted magnetic resonance imaging is the only approach that enables direct and non-invasive quantitative measurement of cerebral blood flow in the brain regions without administration of contrast material and without radiation. ASL is thus a promising perfusion imaging method for assessing cerebral blood flow in the pediatric population. Concerning newborns, there are current limitations because of their smaller brain size and lower brain perfusion. This article reviews and illustrates the use of ASL in pediatric clinical practice and discusses emerging cerebral perfusion imaging applications for children due to the highly convenient implementation of the ASL sequence.

A neuroradiologist's guide to arterial spin labeling MRI in clinical practice

Arterial spin labeling (ASL) is a non-invasive MRI technique to measure cerebral blood flow (CBF). This review provides a practical guide and overview of the clinical applications of ASL of the brain, as well its potential pitfalls. The technical and physiological background is also addressed. At present, main areas of interest are cerebrovascular disease, dementia and neuro-oncology. In cerebrovascular disease, ASL is of particular interest owing to its quantitative nature and its capability to determine cerebral arterial territories. In acute stroke, the source of the collateral blood supply in the penumbra may be visualised. In chronic cerebrovascular disease, the extent and severity of compromised cerebral perfusion can be visualised, which may be used to guide therapeutic or preventative intervention. ASL has potential for the detection and follow-up of arteriovenous malformations. In the workup of dementia patients, ASL is proposed as a diagnostic alternative to PET. It can easily be added to the routinely performed structural MRI examination. In patients with established Alzheimer’s disease and frontotemporal dementia, hypoperfusion patterns are seen that are similar to hypometabolism patterns seen with PET. Studies on ASL in brain tumour imaging indicate a high correlation between areas of increased CBF as measured with ASL and increased cerebral blood volume as measured with dynamic susceptibility contrast-enhanced perfusion imaging. Major advantages of ASL for brain tumour imaging are the fact that CBF measurements are not influenced by breakdown of the blood–brain barrier, as well as its quantitative nature, facilitating multicentre and longitudinal studies.

Quantitative measurement of cerebral oxygen extraction fraction using MRI in patients with MELAS

OBJECTIVE

To quantify the cerebral OEF at different phases of stroke-like episodes in patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) by using MRI.

METHODS

We recruited 32 patients with MELAS confirmed by gene analysis. Conventional MRI scanning, as well as functional MRI including arterial spin labeling and oxygen extraction fraction imaging, was undertaken to obtain the pathological and metabolic information of the brains at different stages of stroke-like episodes in patients. A total of 16 MRI examinations at the acute and subacute phase and 19 examinations at the interictal phase were performed. In addition, 24 healthy volunteers were recruited for control subjects. Six regions of interest were placed in the anterior, middle, and posterior parts of the bilateral hemispheres to measure the OEF of the brain or the lesions.

RESULTS

OEF was reduced significantly in brains of patients at both the acute and subacute phase (0.266 ± 0.026) and at the interictal phase (0.295 ± 0.009), compared with normal controls (0.316 ± 0.025). In the brains at the acute and subacute phase of the episode, 13 ROIs were prescribed on the stroke-like lesions, which showed decreased OEF compared with the contralateral spared brain regions. Increased blood flow was revealed in the stroke-like lesions at the acute and subacute phase, which was confined to the lesions.

CONCLUSION

MRI can quantitatively show changes in OEF at different phases of stroke-like episodes. The utilization of oxygen in the brain seems to be reduced more severely after the onset of episodes in MELAS, especially for those brain tissues involved in the episodes.

Mutations that affect mitochondrial functions and their association with neurodegenerative diseases

Mitochondria are essential for mammalian and human cell function as they generate ATP via aerobic respiration. The proteins required in the electron transport chain are mainly encoded by the circular mitochondrial genome but other essential mitochondrial proteins such as DNA repair genes, are coded in the nuclear genome and require transport into the mitochondria. In this review we summarize current knowledge on the association of point mutations and deletions in the mitochondrial genome that are detrimental to mitochondrial function and are associated with accelerated ageing and neurological disorders including Alzheimer's, Parkinson's, Huntington's and Amyotrophic lateral sclerosis (ALS). Mutations in the nuclear encoded genes that disrupt mitochondrial functions are also discussed. It is evident that a greater understanding of the causes of mutations that adversely affect mitochondrial metabolism is required to develop preventive measures against accelerated ageing and neurological disorders caused by mitochondrial dysfunction.