Relation of interictal spike frequency to 1H-MRSI-measured NAA/Cr

When are metabolic ratios superior to absolute quantification? A statistical analysis

Metabolite levels measured using magnetic resonance spectroscopy (MRS) are often expressed as ratios rather than absolute concentrations. However, the inter-subject variability of the denominator metabolite can introduce uncertainty into a metabolite ratio. In a clinical setting, there are no guidelines on whether ratios or absolute quantification should be used for a more accurate classification of normal versus abnormal results based on their statistical properties. In a research setting, the choice of one over the other can have significant implications on sample size, which must be factored in at the study design stage. Herein, we derive the probability distribution function for the ratio of two normally distributed random variables, and present analytical expressions for the comparison of ratios with absolute quantification in terms of both sample size and area under the receiver operator characteristic curve. The two approaches are compared for typical metabolite values found in the literature, and their respective merits are illustrated using previously acquired clinical MRS data in two pathologies: mild traumatic brain injury and multiple sclerosis. Our analysis shows that the decision between ratios and absolute quantification is non-trivial: in some cases, ratios might offer a reduction in sample size, whereas, in others, absolute quantification might prove more desirable for individual (i.e. clinical) use. The decision is straightforward and exact guidelines are provided in the text, given that population means and standard deviations of numerator and denominator can be reliably estimated.

Proton MR Spectroscopy in Patients with Nonlesional Insular Cortex Epilepsy Confirmed by Invasive EEG Recordings

BACKGROUND AND PURPOSE

Recent studies suggest that a nonnegligible proportion of drug-resistant epilepsy surgery candidates have an epileptogenic zone that involves the insula. We aimed to examine the value of proton magnetic resonance spectroscopy (¹ H-MRS) in identifying patients with insular cortex epilepsy.

METHODS

Patients with possible nonlesional drug-refractory insular epilepsy underwent a voxel-based ¹ H-MRS study prior to an intracranial electroencephalographic (EEG) study. Patients were then divided into two groups based on invasive EEG findings: the insular group with evidence of insular seizures and the noninsular group with no evidence of insular seizures. Sixteen age-matched healthy controls were also scanned for normative data.

RESULTS

Twenty-two epileptic patients were recruited, 12 with insular seizures and 10 with extra-insular seizures. Ipsilateral and contralateral insular N-acetyl-aspartate concentrations ([NAA]) and NAA/Cr ratios were found to be similar in both patient groups. No significant differences in [NAA] or NAA/Cr ratios were found between the insular group, noninsular group, and healthy controls. [NAA] and NAA/Cr asymmetry indices correctly lateralized the seizure focus in only 16.7% and 0% of patients, respectively.

CONCLUSIONS

Our preliminary findings suggest that ¹ H-MRS fares poorly in identifying patients with nonlesional insular epilepsy.

Childhood temporal lobe epilepsy: correlation between electroencephalography and magnetic resonance spectroscopy: a case-control study

Background

The diagnosis of epilepsy should be made as early as possible to give a child the best chance for treatment success and also to decrease complications such as learning difficulties and social and behavioral problems. In this study, we aimed to assess the ability of magnetic resonance spectroscopy (MRS) in detecting the lateralization side in patients with Temporal lobe epilepsy (TLE) in correlation with EEG and MRI findings.

Methods

This was a case–control study including 40 patients diagnosed (clinically and by EEG) as having temporal lobe epilepsy aged 8 to 14 years (mean, 10.4 years) and 20 healthy children with comparable age and gender as the control group. All patients were subjected to clinical examination, interictal electroencephalography and magnetic resonance imaging (MRI). Proton magnetic resonance spectroscopic examination (MRS) was performed to the patients and the controls.

Results

According to the findings of electroencephalography, our patients were classified to three groups: Group 1 included 20 patients with unitemporal (lateralized) epileptic focus, group 2 included 12 patients with bitemporal (non-lateralized) epileptic focus and group 3 included 8 patients with normal electroencephalography. Magnetic resonance spectroscopy could lateralize the epileptic focus in 19 patients in group 1, nine patients in group2 and five patients in group 3 with overall lateralization of (82.5%), while electroencephalography was able to lateralize the focus in (50%) of patients and magnetic resonance imaging detected lateralization of mesial temporal sclerosis in (57.5%) of patients.

Conclusion

Magnetic resonance spectroscopy is a promising tool in evaluating patients with epilepsy and offers increased sensitivity to detect temporal pathology that is not obvious on structural MRI imaging.

Depression and epilepsy: epidemiologic and neurobiologic perspectives that may explain their high comorbid occurrence

Depression is the most frequent psychiatric comorbidity in people with epilepsy (PWE) with lifetime prevalence rates ranging between 30 and 35%. Multifactorial variables play a pathogenic role in the high comorbid occurrence of these two disorders. These variables were critically examined during an international symposium held in Chicago in September 2010, the results of which are presented in two companion manuscripts. The first manuscript summarizes new epidemiologic data highlighting the bidirectional relation between depression and epilepsy and related methodological issues in studying this relationship. An examination of the neurobiologic aspects of primary mood disorders, mood disorders in PWE and pathogenic mechanisms of epilepsy derived from studies in animal models and humans is allowing a better understanding of the complex relation between the two conditions. In the first manuscript, we review data from animal models of epilepsy in which equivalent symptoms of depression and anxiety disorders develop and, conversely, animal models of depression in which the kindling process is facilitated. Data from structural and functional neuroimaging studies in humans provide a further understanding of potential common pathogenic mechanisms operant in depression and epilepsy that may explain their high comorbidity. The negative impact of depression on the control of seizure disorders has been documented in various studies. In this manuscript, these data are reviewed and potential mechanisms explaining this phenomenon are proposed.

Temporal lobe epilepsy in children

The temporal lobe is a common focus for epilepsy. Temporal lobe epilepsy in infants and children differs from the relatively homogeneous syndrome seen in adults in several important clinical and pathological ways. Seizure semiology varies by age, and the ictal EEG pattern may be less clear cut than what is seen in adults. Additionally, the occurrence of intractable seizures in the developing brain may impact neurocognitive function remote from the temporal area. While many children will respond favorably to medical therapy, those with focal imaging abnormalities including cortical dysplasia, hippocampal sclerosis, or low-grade tumors are likely to be intractable. Expedient workup and surgical intervention in these medically intractable cases are needed to maximize long-term developmental outcome.

Correlation between memory, proton magnetic resonance spectroscopy, and interictal epileptiform discharges in temporal lobe epilepsy related to mesial temporal sclerosis

OBJECTIVE

The aim of the study described here was to examine the relationship between memory function, proton magnetic resonance spectroscopy ((1)H-MRS) abnormalities, and interictal epileptiform discharge (IED) lateralization in patients with temporal lobe epilepsy (TLE) related to unilateral mesial temporal sclerosis.

METHODS

We assessed performance on tests of memory function and intelligence quotient (IQ) in 29 right-handed outpatients and 24 controls. IEDs were assessed on 30-minute-awake and 30-minute-sleep EEG samples. Patients had (1)H-MRS at 1.5 T.

RESULTS

There was a negative correlation between IQ (P=0.031) and Rey Auditory Verbal Learning Test results (P=0.022) and epilepsy duration; between(1)H-MRS findings and epilepsy duration (P=0.027); and between N-acetylaspartate (NAA) levels and IEDs (P=0.006) in contralateral mesial temporal structures in the left MTS group. (1)H-MRS findings, IEDs, and verbal function were correlated.

CONCLUSIONS

These findings suggest that IEDs and NAA/(Cho+Cr) ratios reflecting neural metabolism are closely related to verbal memory function in mesial temporal sclerosis. Higher interictal activity on the EEG was associated with a decline in total NAA in contralateral mesial temporal structures.

Single-voxel magnetic resonance spectroscopy of brain tissue adjacent to arachnoid cysts of epileptic patients

Intracranial arachnoid cysts (ACs) are usually asymptomatic, benign developmental anomalies. The most frequent clinical manifestations are cranial expansion, hydrocephaly, headache, epileptic seizures, psychomotor retardation, and aphasia. It is unknown whether there is a correlation between intracranial AC and epileptic seizures without obvious intracranial pressure signs. In vivo magnetic resonance spectroscopy is a technique used for the noninvasive investigation of the various metabolites of cerebral biochemical reactions. Magnetic resonance spectroscopy is also being used increasingly commonly in epileptogenic situations as a noninvasive technique. The purpose of this study was to evaluate the proton magnetic resonance spectroscopic pattern of the contents of tissue adjacent to AC and to determine whether there are any characteristic spectral patterns that may be helpful in evaluating whether these lesions are epileptogenic foci. In conclusion, although the number of cases was limited, this finding may be seen as indicating that there is no association between AC and epilepsy.

Recent advances in magnetic resonance neurospectroscopy

Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.

1H-MR spectroscopy indicates severity markers in temporal lobe epilepsy: correlations between metabolic alterations, seizures, and epileptic discharges in EEG

PURPOSE

In this study, hippocampal metabolite alterations in (1)H-MR spectroscopy ((1)H-MRS) were correlated to the findings of intensive video-EEG monitoring and duration of seizure symptoms in patients with temporal lobe epilepsy (TLE).

METHODS

The 14 patients with mesial TLE and no pathological findings in imaging were investigated by (1)H-MRS. Seizures were analyzed by: number of clinical seizures in 24 h, exact duration of clinical symptoms in 24 h, frequency of interictal epileptiform discharges (IEDs) and ictal activity, duration of ictal activity, and IEDs occurring within 24 h in intensive EEG monitoring. Pearson Correlation Coefficient (PCC) was calculated between spectral metabolite alterations and the parameters mentioned above.

RESULTS

In the analysis, a negative correlation was found between total (t) NAA values and degree of IEDs in EEG (p = 0.04); a positive correlation was found between Cr levels and duration of seizure symptoms registered by video monitoring (p = 0.01).

CONCLUSIONS

Our study shows that, in some patients, (1)H-MRS is able to refer the severity of TLE. The degree of tNAA reduction in (1)H-MRS, probably indicating neuronal dysfunction, is associated with interictal spiking in intensive EEG monitoring. Duration of seizure symptoms associated with increased Cr peaks probably reflects increased gliosis.

Identifying the affected hemisphere by (1)H-MR spectroscopy in patients with temporal lobe epilepsy and no pathological findings in high resolution MRI

Up to 30% of patients with temporal lobe epilepsy (TLE) remain without remarkable changes in MRI. In this study we investigated the role of (1)H-MR spectroscopy ((1)H-MRS) in lateralizing the affected hemisphere in the mentioned patient group. Twenty-two consecutive patients diagnosed with TLE were investigated by high resolution MRI and (1)H-MRS. We examined the incidence and diagnostic accuracy of temporal metabolite alterations determined by Linear Combination of Model Spectra (L C Model) via water reference. Metabolite values of each hemisphere of TLE patients were compared with healthy controls. Results of metabolite alterations were related to intensive video EEG focus localization. Reduction of N-acetylaspartate + N-acetylaspartyl-glutamate (tNAA) in the affected hemisphere revealed identification in six of nine patients (66%) with unilateral TLE. Group comparison revealed a significant reduction of tNAA (6.1+/-0.8*) in the involved temporal lobe compared with controls (6.67+/-0.4*, P=0.026). Choline levels were significantly increased in the affected hemisphere (1.42+/-0.17*) compared with healthy controls (1.22+/-0.17*, P=0.035). The results of our study show that (1)H-MRS is able to identify the affected hemisphere of MRI negative TLE patients and can be used as an additive tool in multimodal focus localization.

Proven startle-provoked epileptic seizures in childhood: semiologic and electrophysiologic variability

PURPOSE

To delineate further the clinical and electrophysiologic features of proven startle-provoked epileptic seizures (SPESs) in children.

METHODS

Clinical, neuroradiologic, and neurophysiologic data of 22 consecutive patients with SPESs were analyzed. Eighty-nine SPESs were documented by video-EEG and evaluated with respect to semiology and ictal and interictal EEG findings.

RESULTS

Mean age was 68 months (10-178 months). Most children had severe mental retardation (86%). Neuroimaging demonstrated diffuse cerebral abnormalities in 15 of 19. Somatosensory evoked potentials revealed cortical abnormalities in 10 of 13 children. The underlying causes were heterogeneous. Only two patients were normally developed. Seizure frequency was usually high (>10/day). Two children had less frequent SPESs (two per month; two per week). Seizures were easily precipitated by sudden sound (n=15), unexpected touch (n=3), or both (n=4). The most common semiologic findings (50%) were generalized tonic seizures or those characterized by a predominant tonic phase, followed in frequency by myoclonic seizures (36%), which were generalized in seven, and unilateral in one. Generalized clonic seizures were observed in one. A complex seizure spread was documented in two children. The most common ictal EEG finding (60%) was a diffuse electrodecremental pattern (DEP). Generalized spike/polyspike waves were found in five and focal discharges in four.

CONCLUSIONS

Our results imply that startle epilepsy is not a uniform epileptic entity. We were able to demonstrate a number of distinct patterns of SPESs, characterized by clinical, semiologic, and electrophysiologic features. Considering the high diversity of SPES patients, a common underlying pathophysiologic mechanism seems unlikely.

Interictal diffusion MRI in partial epilepsies explored with intracerebral electrodes

Patients with refractory partial seizures may benefit from epilepsy surgery. However, invasive investigations are often needed to define the precise location and limits of the epileptogenic zone (EZ). In this study, we asked whether diffusion tensor imaging (DTI) might provide a non-invasive alternative to locate the EZ or at least provide insights to help place intracerebral electrodes for stereo-electroencephalography (SEEG). Whole brain DTI and voxel-based analysis (SPM99) was used to assess diffusion properties objectively in 16 epilepsy patients investigated with SEEG. Epilepsy was symptomatic in two patients and cryptogenic in the 14 remaining patients. The suspected onset of seizures was temporal in 10 patients, frontal in 2 and occipital in 4. Individual maps of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were calculated and compared to a database of 40 healthy volunteers. Thirteen of 16 patients exhibited diffusion abnormalities. ADC abnormalities were better correlated with SEEG data than FA abnormalities which were usually located at a distance or in the white matter. A significant increase in ADC (P < 0.01) was found in 11 patients and was located in the regions explored with depth electrodes in 7 of them. Surgery outcome was available in 3 of these 7 patients (2 were seizure free and 1 not). DTI specificity was better in extratemporal lobe epilepsy (83%) than in temporal lobe epilepsy (20%). When abnormalities concurred with the SEEG data, the concordance was optimal between the localization of the diffusion abnormalities and the irritative zone defined by SEEG. These encouraging, preliminary results, suggest that DTI examinations may provide accurate spatial data on the location and extent of the epileptogenic network in extratemporal lobe epilepsies.

Areas of Interictal Spiking Are Associated with Metabolic Dysfunction in MRI‐negative Temporal Lobe Epilepsy

PURPOSE

The objective of our study was to determine noninvasively whether metabolic dysfunction is present in focal areas of interictal electrophysiologic abnormality and whether metabolic dysfunction correlates with frequency of spiking.

METHODS

We used a prospective, power analysis-driven, age-matched design to study 20 subjects with nonlesional temporal lobe epilepsy by using magnetoencephalography (MEG) and proton magnetic resonance spectroscopy (1H-MRS). MEG was used to localize the source area of interictal spikes. 1H-MRS measured integrated peak areas for N-acetyl compounds (NAA) and choline-containing compounds (Cho) in both hippocampi, the MEG spike zone, and the region contralateral to the MEG spike zone in all subjects. 1H-MRS was performed in seven controls.

RESULTS

Fifteen of 20 subjects had a lower NAA/Cho ratio in the MEG spike zone compared with the contralateral homologous region. NAA/Cho was significantly decreased in the MEG spike zone (p < 0.01). NAA/Cho ratios were not significantly different between the hippocampus ipsilateral and contralateral to the spike activity, or from control hippocampi. NAA/Cho ratios did not correlate with spike frequency.

CONCLUSIONS

Metabolic dysfunction is present in focal areas of interictal spiking in nonlesional temporal lobe epilepsy. These findings confirm that functional abnormalities can be detected in vivo in radiographically normal-appearing cortex exhibiting abnormal excitability.

Interictal epileptiform discharges relate to 1H-MRS-detected metabolic abnormalities in mesial temporal lobe epilepsy

PURPOSE

To examine whether the distribution of interictal epileptiform discharges (IEDs) recorded from foramen ovale electrodes (FOEs) correlates with metabolite levels in medial structures of the temporal lobes in patients with mesial temporal lobe epilepsy (MTLE).

METHODS

We studied 34 patients with MTLE. The lateralization ratio of IEDs was calculated after counting IEDs recorded from FOEs during prolonged video-EEG monitoring. Metabolite ratio between N-acetylaspartate (NAA) and creatine (Cr) was calculated in each medial temporal structure. The correlation between the lateralization ratio of IEDs and metabolic ratio was evaluated.

RESULTS

The asymmetry indexes of IEDs correlated with the asymmetry index of NAA/Cr ratio in the medial temporal structure (rho = -0.380; p = 0.027). Analysis of IEDs and metabolite ratio revealed a significant inverse relation in the contralateral side to the major epileptogenic focus (rho = -0.360; p = 0.037); however, this significance was not present in the ipsilateral side (rho = -0.018; p = 0.921).

CONCLUSIONS

There is a correlation between the neuronal dysfunction or damage detected by MRS and the epileptic activity in the contralateral medial temporal structure. Our results suggest that the pathomechanism underlying the contralateral reduction of NAA/Cr ratio, frequently observed in patients with MTLE, might be related to the process of epileptogenesis generating the independent contralateral IEDs.

Metabolic and electrophysiological alterations in subtypes of temporal lobe epilepsy: a combined proton magnetic resonance spectroscopic imaging and depth electrodes study

PURPOSE

This study compared the metabolic regional alterations, characterized by proton magnetic spectroscopic imaging ((1)H-MRSI), with electrophysiological abnormalities recorded by using depth electrodes and with structural lesions, in patients with several subtypes of temporal lobe epilepsy (TLE).

METHODS

Twenty-five subjects were investigated, including 15 controls and 10 patients with drug-resistant unilateral TLE, nine of whom had structural abnormalities identified by MRI. All patients underwent noninvasive presurgical evaluation and then stereoelectroencephalography (SEEG). We performed an original metabolic exploration combining two (1)H-MRS imaging acquisitions associated with two single-voxel acquisitions (temporal poles) to map the most informative regions of interest (ROIs) including mesial and neocortical localizations. The N-acetyl aspartate/(choline+creatine) ratio was chosen as a metabolic index. SEEG analysis allowed the classification of each ROI as electrically normal or abnormal (i.e., involved in ictal and/or interictal discharges). Groups were compared by using a nonparametric Mann-Whitney U test.

RESULTS

N-Acetyl aspartate/(choline+creatine) was significantly lower in all regions involved in SEEG electrophysiological epileptic abnormalities than in controls (p < 0.05). In contrast, the regions without any electrophysiological abnormalities were not metabolically different from those in controls (p > 0.05) except in one ROI. No differences between the metabolic profiles of epileptogenic and irritative zones were found. The metabolic alterations included, but also extended beyond, the lesions. The presence of metabolic abnormalities in mesial structures was not specific for the mesial subtype and generally extended outside the mesial structures.

CONCLUSIONS

These results indicate that metabolic abnormalities are linked to ictal and interictal epileptiform activities rather than to structural alterations in TLE.

Time course of postoperative recovery of N-acetyl-aspartate in temporal lobe epilepsy

PURPOSE

To assess the time course of increases in N-acetyl-aspartate/creatine (NAA/Cr), which can be measured using proton MR spectroscopic imaging (1H-MRSI), in patients with intractable nonlesional temporal lobe epilepsy (TLE) after successful epilepsy surgery.

METHODS

We performed pre- and postoperative 1H-MRSI in 16 seizure-free (SF) patients and 16 not seizure-free (NSF) TLE patients. We calculated a mixed-design analysis of variance (ANOVA) between SF and NSF groups, ipsi- and contralateral to the side of operation, and pre- and postoperative NAA/Cr measurements. We applied nonlinear regression between pre- and postoperative NAA/Cr differences and the time interval between 1H-MRSI scans to fit a negative exponential model to NAA recovery.

RESULTS

Mixed-design ANOVA revealed that (a) postoperative NAA/Cr was significantly higher in SF than in NSF patients (p = 0.02) and that (b) in the SF group, postoperative NAA/Cr values were significantly higher than preoperative values (p < 0.05) and returned to the normal range in most patients. According to our nonlinear regression model, in SF patients, there was a 50% increase relative to preoperative NAA/Cr values after 5.8 months, whereas an improvement of 95% was reached after 25 months.

CONCLUSIONS

Our results extend preliminary observations of postoperative NAA recovery of SF patients by characterizing the time course of recovery as an exponential function with a half-time of approximately 6 months. The reversal of neuronal metabolic dysfunction remote from the epileptic focus may underlie the clinical observation of improvement of cognitive dysfunction after successful epilepsy surgery.