Phosphorus magnetic resonance spectroscopic imaging in patients with frontal lobe epilepsy

Topiramate Decelerates Bicarbonate-Driven Acid-Elimination of Human Neocortical Neurons: Strategic Significance for its Antiepileptic, Antimigraine and Neuroprotective Properties

BACKGROUND

Mammalian central neurons regulate their intracellular pH (pHi) strongly and even slight pHi-fluctuations can influence inter-/intracellular signaling, synaptic plasticity and excitability.

OBJECTIVE

For the first time, we investigated topiramate´s (TPM) influence on pHi-behavior of human central neurons representing a promising target for anticonvulsants and antimigraine drugs.

METHODS

In slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal lobe epilepsy, BCECF-AM-loaded neocortical pyramidal-cells were investigated by fluorometry. The pHi-regulation was estimated by using the recovery-slope from intracellular acidification after an Ammonium-Prepulse (APP).

RESULTS

Among 17 pyramidal neurons exposed to 50 μM TPM, seven (41.24%) responded with an altered resting-pHi (7.02±0.12), i.e., acidification of 0.01-0.03 pH-units. The more alkaline the neurons, the greater the TPM-related acidifications (r=0.7, p=0.001, n=17). The recovery from APPacidification was significantly slowed under TPM (p<0.001, n=5). Further experiments using nominal bicarbonate-free (n=2) and chloride-free (n=2) conditions pointed to a modulation of the HCO3 -- driven pHi-regulation by TPM, favoring a stimulation of the passive Cl-/HCO3 --antiporter (CBT) - an acid-loader predominantly in more alkaline neurons.

CONCLUSION

TPM modulated the bicarbonate-driven pHi-regulation, just as previously described in adult guinea-pig hippocampal neurons. We discussed the significance of the resulting subtle acidifications for beneficial antiepileptic, antimigraine and neuroprotective effects as well as for unwanted cognitive deficits.

Levetiracetam mediates subtle pH-shifts in adult human neocortical pyramidal cells via an inhibition of the bicarbonate-driven neuronal pH-regulation - Implications for excitability and plasticity modulation

The intracellular pH (pHi) of mammalian central neurons is tightly regulated and small pHi-fluctuations can fine-tune inter-/intracellular signaling, excitability, and synaptic plasticity. The research-gap about the pHi-regulation of human brain neurons is addressed here by testing possible influences of the anticonvulsant levetiracetam (LEV). BCECF-AM-loaded neocortical pyramidal cells were fluorometrically investigated in slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal-lobe epilepsy. Recovery-slope from intracellular acidification following an ammonium prepulse (APP) was used to measure the pHi-regulation. Among twenty pyramidal cells exposed to 50 μM LEV, the resting pHi (7.09 ± 0.14) was lowered in eight (40%) neurons, on average by 0.02 ± 0.011 pH-units. In three (15%) and nine (45%) neurons, a minimal alkaline shift (0.017 ± 0.004 pH-units) and no pHi-shift occurred, respectively. The LEV-induced pHi-shifts were positively correlated with the resting pHi (r = 0.6, p = 0.006, n = 20). In five neurons, which all had responded on LEV with an acidification before, the recovery from APP-acidification was significantly delayed during LEV (p < 0.001). This inhibitory LEV-effect on pHi-regulation i) was similar to that of 200 μM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (n = 2) and ii) did not occur under nominal bicarbonate-free conditions (n = 2). Thus, LEV lowered the pHi of human neocortical pyramidal cells most likely by a weakening of the transmembrane HCO3(-)-mediated acid-extrusion. This might contribute to LEV's anticonvulsive potency. Neurons with more acidic resting pHi-values showed a minimal alkalization upon LEV providing a mechanism for paradoxical proconvulsive LEV-effects rarely observed in epilepsy patients. The significance of these subtle pHi-shifts for cortical excitability and plasticity is discussed.

Modulatory effects of neuropsychopharmaca on intracellular pH of hippocampal neurones in vitro

BACKGROUND AND PURPOSE

The intracellular pH (pHi) of neurones is tightly regulated by, for example, membrane-bound acid-exchangers and loaders. Nevertheless, excessive bioelectric activity lowers steady-state pHi. In turn, even a moderate acidification can inhibit neuronal activity, a process believed to be part of a negative feedback loop controlling neuronal excitation. As moclobemide, an antidepressant, and also some antiepileptic drugs can reduce neuronal pHi in hippocampus slices in vitro, we screened a panel of currently used neuropsychopharmaca for comparable effects.

EXPERIMENTAL APPROACH

BCECF-AM loaded hippocampal slices were superfused with 16 different neuroleptics, antidepressants and antiepileptics under bicarbonate-buffered conditions. Changes in steady-state pHi of CA3 neurones were measured fluorometrically.

KEY RESULTS

The antipsychotics haloperidol, clozapine, ziprasidone, and the antidepressants amitriptyline, doxepin, trimipramine, citalopram, mirtazapine, as well as the anticonvulsive drug tiagabine reversibly reduced the steady-state pHi by up to 0.35 pH-units in concentrations of 5-50 microM. In contrast, venlafaxine, the anticonvulsants carbamazepine, clonazepam, gabapentin, lamotrigine, zonisamide, and the mood stabilizer lithium had no effect on neuronal pHi.

CONCLUSION AND IMPLICATIONS

These data substantiate the view that clinically relevant concentrations of neuroleptics and antidepressants can mediate changes in neuronal pHi, which may contribute to their pharmacological mode of action. Effects on pHi should be taken into account when therapeutic or even harmful effects of these drugs are evaluated.

Novel approaches to imaging epilepsy by MRI

As the concept of a network of injury has emerged in the treatment of epilepsy, the importance of evaluating that network noninvasively has also grown. Recently, studies utilizing magnetic resonance spectroscopic imaging, manganese-enhanced MRI and functional (f)MRI measures of resting state connectivity have demonstrated their ability to detect injury and dysfunction in cerebral networks involved in the propagation of seizures. The ability to noninvasively detect neuronal injury and dysfunction throughout cerebral networks should improve surgical planning, provide guidance for placement of devices that target network propagation and provide insights into the mechanisms of recurrence following resective surgery.

In vitro1H NMR spectroscopy shows an increase in N-acetylaspartylglutamate and glutamine content in the hippocampus of amygdaloid-kindled rats

We examined energy metabolism and amino acid content in the hippocampus of amygdaloid-kindled rats using (1)H NMR spectroscopy. Three weeks after the last stage 5 seizure, kindled rats were killed by microwave irradiation. The hippocampus was dissected out and subjected to MeOH/CHCl(3) extraction. All (1)H spectra were analyzed to quantify absolute concentrations using a non-linear least squares method, combined with a prior knowledge of chemical shifts. Saturation effects were compensated for by the T1 measurement of each component. Levels of energy metabolism-related compounds, phosphocreatine, creatine, glucose and succinate were the same in both kindled rats and sham controls. Lactate concentration had a tendency to increase, although this was not statistically significant. When compared with sham controls, levels of aspartate, glutamate, glycine and glutamine, as well as GABA and inositol, were increased in the ipsilateral but not the contralateral hippocampus. In contrast, levels of taurine, alanine and threonine were unchanged. Finally, N-acetylaspartylglutamate content was elevated, whereas N-acetyl-l-aspartate content was unaltered in the ipsilateral hippocampus of kindled animals. Our results suggest that amygdala kindling may affects amino acid metabolism, but not energy metabolism.

Levetiracetam inhibits Na+-dependent Cl-/HCO3- exchange of adult hippocampal CA3 neurons from guinea-pigs

The novel anticonvulsant levetiracetam (LEV) was tested for effects on bioelectric activity and intracellular pH (pHi) regulation of hippocampal CA3 neurons from adult guinea-pigs. In 4-aminopyridine-treated slices, LEV (10-100 microm) reduced the frequency of action potentials and epileptiform bursts of CA3 neurons by 30-55%, while the shape of these potentials remained largely unchanged. Suppressive effects were reversed by an increase of pHi with trimethylamine (TMA). Using BCECF-AM-loaded slices, we found that LEV (10-50 microm) reversibly lowered neuronal steady-state pHi by 0.19 +/- 0.07 pH units in the presence of extracellular CO2/HCO3- buffer. In the nominal absence of extracellular CO2/HCO3- or in Na+-free CO2/HCO3(-)-buffered solution, LEV had no effect on steady-state pHi. Recovery of pHi subsequent to ammonium prepulses remained unchanged in the absence of CO2/HCO3- buffer, but was significantly reduced by LEV in the presence of CO2/HCO3- buffer. These findings show that LEV inhibits HCO3(-)-dependent acid extrusion, but has no effect on Na+/H+ exchange. LEV did not affect Na+-independent Cl-/HCO3- exchange because intracellular alkalosis upon withdrawal of extracellular Cl- remained unchanged. These data show that LEV at clinically relevant concentrations inhibits Na+-dependent Cl-/HCO3- exchange, lowers neuronal pHi, and thereby may contribute to its anticonvulsive activity.

1H and 31P Spectroscopic Imaging of Epilepsy: Spectroscopic and Histologic Correlations

Although MRS measurements are useful in assessing the biochemical alterations underlying human epilepsy, to date their use has been limited primarily by three factors: (a) the lack of widespread methods and appropriate hardware for acquiring high-resolution spectroscopic imaging data, (b) difficulties in spectral interpretation associated with metabolic heterogeneity, and (c) difficulties in biological interpretation due to a lack of correlative histologic studies. In this work, we (a) describe approaches to overcome these hurdles, and (b) discuss the biological interpretation of the spectroscopic findings in TLE.

Topiramate modulates pH of hippocampal CA3 neurons by combined effects on carbonic anhydrase and Cl-/HCO3- exchange

Topiramate (TPM) is an anticonvulsant whose impact on firing activity and intracellular pH (pHi) regulation of CA3 neurons was investigated. Using the 4-aminopyridine-treated hippocampal slice model bathed in bicarbonate-buffered solution, TPM (25-50 microm) reduced the frequency of epileptiform bursts and action potentials without affecting membrane potential or input resistance. Inhibitory effects of TPM were reversed by trimethylamine-induced alkalinization. TPM also lowered the steady-state pHi of BCECF-AM-loaded neuronal somata by 0.18+/-0.07 pH units in CO(2)/HCO(3)(-)-buffered solution. Subsequent to an ammonium prepulse, TPM reduced the acidotic peak but clearly slowed pHi recovery. These complex changes were mimicked by the protein phosphatase inhibitor okadaic acid. Alkalosis upon withdrawal of extracellular Cl(-) was augmented by TPM. Furthermore, at decreased pHi due to the absence of extracellular Na(+), TPM reversibly increased pHi. These findings demonstrate that TPM modulates Na(+)-independent Cl(-)/HCO(3)(-) exchange. In the nominal absence of extracellular CO(2)/HCO(3)(-) buffer, both steady-state pHi and firing of epileptiform bursts remained unchanged upon adding TPM. However, pHi recovery subsequent to an ammonium prepulse was slightly increased, as was the case in the presence of the carbonic anhydrase (CA) inhibitor acetazolamide. Thus, a slight reduction of intracellular buffer capacity by TPM may be due to an inhibitory effect on intracellular CA. Together, these findings show that TPM lowers neuronal pHi most likely due to a combined effect on Na(+)-independent Cl(-)/HCO(3)(-) exchange and CA. The apparent decrease of steady-state pHi may contribute to the anticonvulsive property of TPM.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.

1H MR spectroscopy in patients with mesial temporal epilepsy

The study provides a review of the basic examination procedures and results of proton magnetic resonance spectroscopy (1H MRS) in patients suffering from mesial temporal lobe epilepsy (MTLE). The source of seizures in MTLE is most often an epileptogenic focus secondary to hippocampal sclerosis. 1H MRS currently plays an important role in the non-invasive diagnosis of this type of epileptogenic lesion. The decisive 1H MRS parameter characterizing an epileptogenic lesion is a statistically significantly decreased value of N-acetylaspartate levels compared with control values, most often associated with a decrease in the ratios of the intensities of NAA/Cr, NAA/Cho and NAA/(Cr + Cho) signals. Moreover, MRS makes it possible to distinguish bilateral involvement of mesial temporal structures typically associated with a bilateral decrease in the levels of metabolites and/or their ratios. As regards other metabolic compounds which play an important role in the pathobiochemistry of epilepsy, MRS is employed to study the action of gamma-aminobutyric acid (GABA), inositol, lactate, glutamine, and glutamate, the clinical function of which has not been fully clarified as yet. It is in this context that one should consider the application of 1H MRS in evaluating the action of some new anti-epileptic agents affecting excitatory and inhibitory amino acids. There is no doubt that in vivo 1H MRS, along with other imaging methods, has made a significant contribution to the clinical and biochemical description of epileptic seizures and has assumed a prominent position among the techniques of pre-operative examination in epileptic surgery.

Mesial frontal epilepsy

The mesiofrontal cortex comprises a number of distinct anatomic and functional areas. Structural lesions and cortical dysgenesis are recognized causes of mesial frontal epilepsy, but a specific gene defect may also be important, as seen in some forms of familial frontal lobe epilepsy. The predominant seizure manifestations, which are not necessarily strictly correlated with a specific ictal onset zone, are absence, hypermotor, and postural tonic seizures. Other seizure types also occur. The task of localization of the epileptogenic zone can be challenging, whether EEG or imaging methods are used. Successful localization can lead to a rewarding outcome after epilepsy surgery, particularly in those with an imaged lesion.

Regional distribution of interictal 31P metabolic changes in patients with temporal lobe epilepsy

PURPOSE

We compared the 31P metabolites in different brain regions of patients with temporal lobe epilepsy (TLE) with those from controls.

METHODS

Ten control subjects and 11 patients with TLE were investigated with magnetic resonance imaging (MRI) and [31P]MR spectroscopic imaging (MRSI). [31P]MR spectra were selected from a variety of brain regions inside and outside the temporal lobe.

RESULTS

There were no asymmetries of inorganic phosphate (Pi), pH, or phosphomonoesters (PME) between regions in the left and right hemispheres of controls. In patients with TLE, Pi and pH were higher and PME was lower throughout the entire ipsilateral temporal lobe as compared with the contralateral side and there were no significant asymmetries outside the temporal lobe. The degree of ipsilateral/contralateral asymmetry for all three metabolites was substantially greater for the temporal lobe than for the frontal, occipital, and parietal lobes, and these asymmetries provided additional data for seizure localization. As compared with levels in controls, Pi and pH were increased and PME were decreased on the ipsilateral side in patients with TLE. There were changes in Pi, pH, and PME on the contralateral side in persons with epilepsy as compared with controls, contrary to changes on the ipsilateral side.

CONCLUSIONS

Our findings provide some insight into the metabolic changes that occur in TLE and may prove useful adjuncts for seizure focus lateralization or localization.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

Surgical outcomes in pure frontal lobe epilepsy and foci that mimic them

In this study we examined 37 subjects with a diagnosis of intractable frontal lobe epilepsy (FLE) based on non-invasive pre-surgical evaluation. Twenty-six underwent chronic intracranial ictal recordings (CIR) with video monitoring; 20 of these went on to surgical resection. Eleven underwent surgery without CIR. Retrospectively, we determined that 19 had pure FLE, 12 had frontal plus extrafrontal epileptogenic zones, and six others did not have FLE. We analysed the whole group and individual categories to evaluate the determinants of surgical outcome. Sixty percent of the pure frontal group is seizure free with all having > or = 75% reduction. The frontal-plus group had only 10% seizure free with 70% having > or = 75% reduction. Being in the pure frontal group was associated with better outcomes than the 'frontal-plus' group (P < 0.05; chi-square). Subjects with FSIQ > or = 85, focal pathologies and 18FDG-PET scans which were normal or had focal abnormalities (P < or = 0.05, all, chi-square) were more likely to have excellent outcomes. MRI abnormalities, surface EEG, and location and size of resection were not predictive of surgical outcomes. Rasmussen's encephalitis, incomplete surgical strategies and bilateral foci were apparent in those with poor outcomes, and surgical size predicted post-operative deficits (chi-square; P < 0.001). We conclude that careful, hypothesis-driven implants and operating procedures can result in good surgical outcomes for frontal lobe epilepsy subjects even when lesions are not apparent on routine neuroimaging.

High spatial resolution and speed in MRSI

The in vivo applications of magnetic resonance spectroscopic imaging (MRSI) have expanded significantly over the past 10 years and have reached the point where clinical trials are underway for a number of different diseases. One of the limiting factors in the widespread use of this technology has been the lack of widely available tools for obtaining data which are localized to sufficiently small tissue volumes to make an impact upon diagnosis and treatment planning. This is especially difficult within the timeframe of a clinical MR examination, which requires that both anatomic and metabolic data are acquired and processed. Recent advances in the hardware and software associated with clinical scanners have provided the potential for improvements in the spatial and time resolution of imaging and spectral data. The two areas which hold the most promise in terms of MRSI data are the use of phased array coils and the implementation of echo planar k-space sampling techniques. These could have immediate impact for 1H MRSI and may prove valuable for future applications of 31P MRSI.

Identification of frontal lobe epileptic foci in children using positron emission tomography

PURPOSE

Presurgical evaluation for intractable frontal lobe epilepsy (FLE) is difficult and invasive, partly because anatomic neuroimaging studies with computed tomography (CT) and magnetic resonance imaging (MRI) typically do not show a discrete lesion. In adult patients with FLE, functional neuroimaging of glucose metabolism with positron emission tomography (PET) is less sensitive in detecting focal metabolic abnormalities than in temporal lobe epilepsy (TLE). Comparable data on children with FLE are not available.

METHODS

We used high-resolution PET scanning of glucose metabolism to evaluate 13 children (age 17 months to 17 years; mean age 9.5 years) with intractable FLE being considered for surgical treatment. Only children with normal CT and MRI scans were included.

RESULTS

Hypometabolism including the frontal lobe was evident in 12 of the 13 children, was unilateral in 11 of 13, and was restricted to the frontal lobe in 8 of 13. One child showed bilateral frontal cortex hypometabolism and another had an ictal PET scan demonstrating unilateral frontal cortex hypermetabolism surrounded by hypometabolism. Additional hypometabolic areas outside the frontal cortex were observed in 5 children in parietal and/or temporal cortex. Localization of seizure onset on scalp EEG was available in 10 children and corresponded to the location of frontal lobe PET abnormality in 8. However, in 4 of the 10 children, the extent of hypometabolism exceeded the epileptogenic region indicated by ictal EEG. In 2 of the 13 children, the abnormality evident on EEG was more extensive than that evident on PET. In the remaining 3 children for whom only interictal EEG data were available, the PET foci did not correspond in location to the interictal EEG abnormalities. In 11 of the 13 children, the presumed region of seizure onset in the frontal lobe, as based on analysis of seizure semiology, corresponded to the locations of frontal lobe glucose metabolism abnormalities.

CONCLUSIONS

Although high-resolution PET appears to be very sensitive in localizing frontal lobe glucose metabolic abnormalities in children with intractable FLE and normal CT/MRI scans, the significance of extrafrontal metabolic disturbances requires further study; these may represent additional epileptogenic areas, effects of diaschisis, seizure propagation sites, or secondary epileptogenic foci.

Clinical applications of magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is a new tool for evaluation of patients with epilepsy, demonstrating abnormalities of energy and lipid metabolism ictally and, more recently, interictally. These metabolic abnormalities include increased inorganic phosphate, pH, and decreased phosphomonoesters as determined by 31P MRS, as well as decreased N-acetylaspartate determined by 1H MRS. Furthermore, increased lactic acid has been detected postictally. These metabolic changes appear to be confined to the region of seizure origination and can be detected interictally. Therefore, they can be used for lateralization of the epileptogenic focus. Ongoing research suggests that these abnormalities may also be useful in localization of the focus, demonstrating metabolic alterations in temporal lobe epilepsy (TLE) similar to those in neocortical epilepsy. However, further technical development will be required before the goal of using these techniques for localization of the epileptogenic focus can be realized. For TLE lobe epilepsy at least, the clinical utility of 1H MRS to lateralize the seizure focus has clearly been demonstrated by several centers. The consistent findings in TLE suggest that 1H MRS is ready to become part of the evaluation process of patients with medically refractory epilepsy being evaluated for seizure surgery.

Neuroimaging of focal malformations of cortical development

Neuroimaging is playing an increasingly important role in the evaluation of patients with malformations of cerebral cortical development. In this review, the authors address optimal neuroimaging of cortical malformations using x-ray computed tomography, single-photon-emission computed tomography, positron emission tomography, magnetic resonance imaging, and magnetic resonance spectroscopy. Initially, the authors discuss the strengths and weaknesses of the various imaging techniques. This is followed by a discussion of the clinical and neuroimaging characteristics of several different imaging manifestations of focal malformations of cortical development, including polymicrogyria, focal subcortical heterotopia, schizencephaly, focally thickened gyri, focally irregular gyri, hemimegalencephaly, and transmural dysplasia. The authors intend that, after reading this review, the reader will have a better understanding of the optimal neuroimaging techniques for evaluating these malformations and their many neuroimaging appearances.

High-energy phosphate metabolism in the frontal lobes of patients with panic disorder detected by phase-encoded 31P-MRS

Using phosphorus-31 magnetic resonance spectroscopy (31P-MRS), we analyzed the brain phosphorous metabolism in 18 patients with panic disorder (PD) and in 18 sex-, age-, and handedness-matched normal controls (NC). All patients were receiving ongoing drug treatments for PD. The evaluation of phosphorous metabolism in the whole frontal lobes revealed no significant differences between the patients and controls in 31P metabolite levels, although the PD patients showed slightly decreased inorganic phosphate (Pi) concentration of the frontal lobes. Moreover, we found a significant asymmetry (left > right) of phosphocreatine (PCr) concentration in the frontal lobes in the patients with PD, suggesting that abnormalities of phosphorous metabolism are present in the frontal lobes of PD patients. Two patients in whom a limited panic episode occurred during measurements showed frontal lobe intracellular pH higher than that in the other patients and that in the NC, suggesting respiratory gaseous alkalosis due to hyperventilation in the anxiety state. 31P-MRS has potential for application in the assessment of brain abnormalities and anxiety state, such as that accompanied by hyperventilation, in PD patients.

Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is noninvasive and may be readily combined with magnetic resonance imaging (MRI). Attention has focussed on proton (1H) and phosphorus (31P) MRS, and studies have been undertaken by using single voxels or many voxels simultaneously (chemical-shift imaging, magnetic resonance spectroscopic imaging). The latter is more difficult and prone to artefact but potentially yields significantly more information. 1H MRS has principally yielded data on concentrations of N-acetyl aspartate (NAA), choline, creatine, and phosphocreatine. NAA is located primarily within neurons, and reduction of the ratio of NAA to choline, creatine, and phosphocreatine is a marker of neuronal loss and dysfunction. This technique may be useful as a noninvasive tool for localizing epileptogenic foci, but its role requires further evaluation. As with all functional imaging methods, coregistration with high-quality MRI is essential for interpreting data. 1H MRS can be used also to estimate cerebral concentrations of several neurotransmitters: glutamate, glutamine, and gamma-aminobutyric acid (GABA). This may prove useful for characterizing the neurometabolic profiles of patients with different epilepsy syndromes and for evaluating the effects of medical and surgical treatments. 31P MRS can detect adenosine triphosphate, phosphodiesters, phosphomonoesters, phosphocreatine, and inorganic phosphate, and estimate intracerebral pH. Abnormalities that have been associated with epileptogenic brain areas include increased inorganic phosphate, reduced phosphomonoesters, and increased pH. Only small numbers of patients have been studied, however, so that conclusions are not definitive, and the clinical role of this technique is not yet established.

Evaluation of 1H magnetic resonance spectroscopic imaging as a diagnostic tool for the lateralization of epileptogenic seizure foci

The purpose of this study was to assess whether a visual examination of 1H spectroscopic images could correctly lateralize patients with intractable temporal lobe epilepsy. 20 patients with intractable temporal lobe epilepsy and 10 volunteers were included in this study. Spectroscopic images were analysed using a protocol based on visual inspection. Images of the metabolites N-acetyl aspartate (NAA), choline (Cho), creatine (Cr) and lactate were obtained from a transverse plane oriented along the sylvian fissure. Images from each individual were evaluated independently by six reviewers. Results of the lateralization procedure obtained from the visual examinations were compared with those obtained from quantitative analysis of the spectra and with those obtained by magnetic resonance imaging (MRI), positron emission tomography (PET), neuropsychological examinations, and electroencephalographic (EEG) recordings. NAA images were found to be the most effective, amongst metabolite images, in lateralizing the epileptogenic lobe. Using the site selected for resection as the definition of the correct lateralization, 70% of the patients who underwent temporal lobectomy were correctly lateralized by the majority of the examiners using the visual inspection protocol. Based on the results of this study it is concluded that visual examination of 1H spectroscopic images is potentially valid in lateralizing patients with intractable temporal lobe seizures. Confidence in the visual interpretation increased as the difference in NAA signal intensity between the temporal lobes increased. The threshold above which the majority of the examiners correctly lateralized the patients was approximately 15% in NAA signal loss in the ipsilateral lobe.

Initial observations on effect of vigabatrin on in vivo 1H spectroscopic measurements of gamma-aminobutyric acid, glutamate, and glutamine in human brain

Recent developments involving 1H nuclear magnetic resonance (NMR) spectroscopic editing techniques have allowed noninvasive measurements of gamma-aminobutyric acid (GABA) in human cerebrum. The additional information gained from GABA and macromolecule measurements permitted more precise glutamate (Glu) and glutamine (Gln) measurements. Occipital lobe GABA in 10 nonepileptic, healthy subjects was 1.0 mumol/g brain [95% confidence interval (CI) 0.9-1.1]. Vigabatrin (VGB) is a safe and effective antiepileptic drug (AED) that irreversibly inhibits neuronal and glial GABA-transaminase. GABA levels were increased in all patients treated with VGB. With a standard dose of 3-6 g/day, GABA levels were 2.6 mumol/g (95% CI 2.3-2.8). Mean occipital GABA level measured in epileptic patients not receiving VGB was 0.9 mumol/g (95% CI 0.7-1.1). Gln was increased by 1.9 mumol/g and Glu was decreased by 0.8 mumol/g in patients receiving VGB as compared with patients receiving standard medications alone.

Proton magnetic resonance spectroscopic imaging in patients with frontal lobe epilepsy

Proton magnetic resonance spectroscopic imaging (1H MRSI) has demonstrated decreased N-acetyl compounds (NA) in the epileptogenic hippocampus in patients with temporal lobe epilepsy. We studied 8 patients with frontal lobe epilepsy and found mean NA/creatine (Cr) in the epileptogenic frontal lobe decreased by 27% compared with that of the contralateral homologous region (1.81 +/- 0.36 vs 2.49 +/- 0.60, p < 0.008). In every patient, NA/Cr was decreased in the epileptogenic region by at least 5%. These findings suggest that 1H MRSI may be useful in the presurgical evaluation of patients with frontal lobe epilepsy.

Application of spectroscopic imaging in epilepsy

Functional and anatomical neuroimaging has had a dramatic effect on the evaluation of patients for seizure surgery. The demonstration by PET that the epileptogenic focus has interictal metabolic abnormalities has allowed a greater number of patients to come to seizure surgery, with fewer of these patients requiring intracranial electrode evaluations. Metabolic changes have also been demonstrated utilizing single voxel and whole brain 1H and 31P MRS imaging techniques with the interictal focus characterized by increased Pi, pH, and decreased PME and NAA. These findings can be used to accurately lateralize temporal lobe as well as frontal lobe epilepsy. Furthermore, there is evidence that these findings can be used to localize the seizure focus with the changes specific for the epileptogenic region; although, more diffuse changes both ipsilaterally and contralaterally have been seen. In patients with anterior hippocampal seizure foci the pH is significantly alkaline only in the ipsilateral hippocampus, whereas the increased Pi and decreased PME can be seen throughout the ipsilateral temporal lobe. When compared to controls the contralateral hemisphere is acidotic. Decreased NAA concentrations as well as NAA/Cr ratios have been demonstrated in the epileptogenic region in temporal and frontal lobe epilepsy. The decreased NAA has been correlated with the severity of cell loss, and may be a more sensitive measure than qualitative or quantitative measures of the hippocampal atrophy; however, the NAA decrease is more widespread than just the epileptogenic focus but may be maximal at the site of seizure initiation. In preliminary work, NAA maps of deviation from normality have suggested that the maximal change to coincide with the epileptogenic region. These results suggest that in focal epilepsy there is abnormal metabolic activity throughout the brain detectable by MRS, with patterns of metabolic asymmetry that are useful for seizure localization.

Magnetic resonance imaging and epilepsy: neurosurgical decision making

Advances in magnetic resonance imaging (MRI) techniques have had an important impact on the decision-making process leading to surgical resection for chronic seizures. The MRI is now obtained relatively early in the work-up, and, when it shows abnormality, it assumes a crucial role in the detection of specific surgically remediable syndromes. These syndromes, when diagnosed by MR and other confirmatory studies such as electroencephalography (EEG), positron emission tomography (PET), magnetoencephalography (MEG), and neuropsychological testing, define the essential part of the surgical plan; that is, removal of the disease substrate. The availability of a host of MR techniques enable us to investigate epilepsy not only as a structural pathology but as physiological pathology reflected in abnormal blood flow, metabolism, and synaptic transmission. The mainstay of surgical treatment is the removal of the anatomic pathology, but other MR techniques may be helpful in the delineation of dual pathology in lesional cases, in appreciation of the full extent of microscopic pathology in developmental lesions, and in the imposition of restrictions on the resection based upon functional mapping. Finally, functional and anatomic maps obtained preoperatively can be related directly to the spatial coordinates of the exposed brain in the operating room using MRI-based frameless stereotactic methods. The final outcome, then, is the removal of the disease substrate without injury to adjacent, functionally salient cortical regions.

Symbiosis between in vivo and in vitro NMR spectroscopy: the creatine, N-acetylaspartate, glutamate, and GABA content of the epileptic human brain

High resolution 1H NMR spectroscopy was used to analyze temporal lobe biopsies obtained from patients with epilepsy. Heat-stabilized cerebrum, dialyzed cytosolic macromolecules, and perchloric acid extracts were studied using one- and two-dimensional spectroscopy. Anterior temporal lobe neocortex was enriched in GABA, glutamate, alanine, N-acetylaspartate, and creatine. Subjacent white matter was enriched in aspartate, glutamine, and inositol. The N-acetylaspartate/creatine mole ratio was lower in anterior temporal neocortex with mesial (0.66) than neocortical (0.80) temporal lobe epilepsy. Human brain biopsy samples were separated into crude and refined synaptosomes, neuronal cell bodies, and glia using density gradient centrifugation. Neuronal fractions were enriched in glutamate and N-acetylaspartate. Glial cell fractions were enriched in lactate, glutamine, and inositol. The creatine content was the same in biopsied epileptic cortex (8.8-8.9 mmol/kg) and normal in vivo occipital lobe (8.9 mmol/kg). Glutamate content was higher in epileptic cortex at biopsy (10.1-10.5 mmol/kg) than normal in vivo occipital lobe (8.8 mmol/kg). GABA content was higher in biopsies of epileptic cortex (2.3-2.2 mmol/kg) than in normal in vivo occipital lobe (1.2 mmol/kg). N-acetylaspartate content was lower in biopsied epileptic temporal cortex (5.8-6.8 mmol/kg) than normal in vivo occipital lobe (8.9 mmol/kg). Paired in vivo and ex vivo measurements are critical for a firm understanding of the changes seen in the 1H-spectra from patients with epilepsy.