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Chapter 54 MRS for imaging neuronal dysfunction in epilepsy
Advances in Clinical Neurophysiology (Supplements to Clinical Neurophysiology Vol. 54) Editors: R.C. Reisin. M.R Nuwer, M. Hallett, C. Medina fg 2002 Elsevier Science B. V. AU rights reserved.
359
Chapter 54
MUS for imaging neuronal dysfunction in epilepsy Fernando Cendes", Li Min Li" and Douglas Arnold" 'Department ofNeurology, University of Campinas, Campinas, SP (Brazil) "Department ofNeurology and Neurosurgery; Montreal Neurological institute and Hospital, McGill University, Montreal, PQ H3A 2B4 (Canada)
Introduction
e
Proton magnetic resonance spectroscopy HMRS)
Magnetic resonance spectroscopy (MRS) permits chemically specific, non-invasive measurement of certain compounds in living tissue. MRS measurements in the brain of living animals became practical in 1980 (Ackerman et al. 1980), and the capability for human measurements was developed a few years later. By the early 1990s, applications of MRS to several neurologic disorders had been published (Petroff et al. 1995). The non-invasive nature of MRS means that repeated measurements can be made, so that kinetic and longitudinal studies are possible in a single subject. In addition, one can study human tissues that are inaccessible except by invasive techniques. In the human brain, phosphate energy stores, intracellular pH, lactate concentration, and the neuronal marker N-acetylaspartate (NAA) are examples of MRS-measurabl,e variables that are important for both clinical and scientific purposes and cannot be studied easily by any other technique (Petroff et al. 1995).
* Correspondence to: Dr. D.L. Arnold, MRS Laboratory, Brain Imaging Center, Montreal Neurological Institute and Hospital, 3801 University Street, Montreal, PQ H3A 2B4, Canada. E-mail:
Water-suppressed, localized MR spectra of normal human brain at 'long' echo times (TE 136272 ms) reveal four major resonances (Fig. 1): • One at 3.2 ppm, which arises from tetramethylamines (mainly from choline-containing phospholipids) (Cho); • One at 3.0 ppm, which arises primarily from creatine and phosphocreatine (Cr); • One at 2.0 ppm which arises from N-acetyl groups (mainly NAA); • One at 1.3 ppm, which arises from the methyl resonance of lactate, or, in certain pathological conditions, methyl groups oflipids (De Stefano et al. 1995; Petroff et al. 1995). Under normal physiologic conditions and with the small voxel size used for most in vivo MRS examinations ofthe adult human brain, the lactate peak is not visible above the baseline noise. Multiple lines of evidence support the use ofNAA as a neuronal marker. It is found exclusively in neurons and their processes in the mature brain (Moffett et al. 1991; Simmons et al. 1991). In human brain spectra, in vivo NAA is reduced in situations known to be associated with neuronal loss. When decreases in the relative NAA signal arise
360
NAA
NAA
Cho
~
Cho
Fig. 1. Spectra from the right and left mid temporal lobes in a patient with left TLE with normal MRI. Note the reduced NAA peak on the left temporal lobe (arrow). This patient underwent left temporal lobe resection and became seizure-free after surgery. The postoperative histopathology showed mild mesial temporal sclerosis.
from neuronal or axonal degeneration, irreversible changes are expected. However, there have been observations of reversible decreases in NAA in a number of conditions, emphasizing that neuronal dysfunction or transient relative volume changes can also lead to decreased NAA (De Stefano et al. 1995a; Hugg et al. 1996a; Cendes et a1. 1997a). The ability to quantify specifically neuronal loss or damage is one of the most interesting potential applications of MRS in cerebral disorders. Changes in the resonance intensity of Cho probably result mainly from increases in the steady-state levels of phosphocholine and glycerol-phosphocholine. These choline-containing membrane phospholipids are released during active myelin breakdown. Thus, the resonance intensity of Cho increases in acute demyelinating lesions in humans (De Stefano et a1. 1995), and in certain tumors, such as meningiomas (Preul et al. 1996). Total Cr concentration is relatively constant throughout the brain and tends to be relatively resistant to change. However, large changes can be seen with destructive pathology such as malignant tumors (Preul et al. 1996). It is reasonable to use creatine as an internal standard to normalize resonance intensities of NAA and Cho in order to correct for artifactual variations in signal intensity due to magnetic field and radiofrequency inhomoge-
neity. However, this must be done with caution. Use of an external concentration reference can be reliable if factors such as radiofrequency field inhomogeneity and coil tuning and coupling can be adequately controlled (Petroff et a1. 1995). Lactic acid is the end product of glycolysis and accumulates when oxidative metabolism cannot meet energy requirements. For instance, elevation of lactic acid in cerebral neoplasms correlates approximately with relative rates of glucose uptake. However, lactate is both intracellular and extracellular, and large amounts may be accumulated outside actively anaerobic tissue (e.g. in necrotic tissue or fluid- filled cysts) (De Stefano et al. 1995; Petroff et a1. 1995).
Proton MRS studies in partial epilepsies Animal studies Brain lactate is elevated by seizure activity, as demonstrated by conventional biochemical studies of excised tissue. In vivo animal studies using proton MRS allowed more dynamic studies and a better appreciation of how persistent lactate elevation is after even brief convulsive seizure (Petroff et al. 1995). Selective neuronal injury by kainate-
361
induced status epilepticus in rats was associated with focal reduction of NAA determined by proton MRSI, even before Tz-weighted MRI changes were observed (Ebisu et al. 1994). Najm et al. (1998) used proton MRS to identify specific in situ metabolic markers for seizures and seizure-induced neuronal damage in rat brains. They pretreated rats with placebo or cycloheximide 1 h before kainic acid injection and then scanned rat brains at the level of the hippocampus before, during, and 24 h after seizures. They found a significant increase in lactate ratios in kainic acidtreated rats during and 24 h after seizure onset, however, this increase was prevented by cycloheximide pretreatment. They suggest that in situ lactate increase is a marker of seizure-induced neuronal damage and that there is no significant increase of in situ lactate during seizures that do not lead to neuronal damage (Najm et al. 1998). Human studies
An early use of proton MRS in the context of human epilepsy was in analysis of extracts of samples from temporal lobe tissue resected for treatment ofdrug resistant temporal lobe epilepsy (TLE; Petroff et al. 1989). This type of in vitro study is a useful source of information about concentrations of several human brain metabolites that are observable by in vivo MRS studies. The first observations of elevated brain lactate (and also reduced NAA) in vivo by proton MRS associated with a seizure disorder was reported by Matthews et al. (1990) in two patients with Rasmussen's syndrome. Another study suggests that lactate accumulation result from repetitive seizures rather than from the disease process in Rasmussen's syndrome (Cendes et al. 1995). Subsequent proton MRS studies have shown focal reductions ofNAA signal in patients with nonlesional TLE (Petroff et al. 1989; Matthews et al. 1990; Hugg et al. 1993; Cross et al. 1996; Cendes et al. 1997a,b; Kuzniecky et al. 1997, 1998; Li et al. 1998, 2000) and extratemporal partial epilepsies (Cendes et al. 1995; Garcia et al. 1995; Stanley et al. 1998) with good correlation with EEG abnor-
malities and severity of cell loss. The results of published MRS studies suggest that in patients with partial epilepsy there is a metabolic abnormality throughout the brain, with patterns of asymmetry and focal accentuation that are useful for non-invasive localization of epileptogenic foci (Petroff et al. 1989: Matthews et al. 1990; Hugg et al. 1993; Cendes et al. 1995, 1997a,b; Garcia et al. 1995; Cross et al. 1996; Kuzniecky et al. 1998; Stanley et al. 1998). The MRS findings may have prognostic value for seizure outcome as well (Kuzniecky et al. 1998, 1999; Li et al. 2000). In a series of 100 consecutive patients with TLE (Cendes et al. 1997b), the NAA/Cr values were abnormally low in at least one temporal lobe in all but one patient and were low bilaterally in 54%. The asymmetry between right and left sides of NAA/Cr lateralized 86/93 (92.5%) of patients who had lateralization by ictal EEG. There were seven patients with no clear lateralization by EEG. The MRSI lateralization was ipsilateral to the EEG in all but two patients who had bilateral TLE and bilateral AM-HF atrophy greater on the same side as the MRSI. Twelve of 13 patients with normal MRIVol had a significant decrease of NAA/Cr within the mesial temporal lobe ipsilateral to the ictal EEG focus (Fig. 1). Seven of these underwent surgery and the histopathology showed mild mesial temporal sclerosis. The above mentioned study (Cendes et al. 1997b) showed a direct correlation between the measure of NAA/Cr and MRiVol. However, another study with 33 patients showed no correlation between the measure of hippocampal specific Cr/NAA and MRiVol (Kuzniecky et al. 1998). Methodological and statistical regional analysis differences are most likely responsible for the discrepancy between studies. Some degree of disassociation between severity of NAA abnormality and hippocampal volume or cell loss is not unexpected. This includes the finding of abnormal NAA measures in normal volume hippocampi (both ipsilateral and contralateral to site of seizure onset) and the reversibility or correction ofNAA abnormalities in patients who become seizure-free after surgery (Hugg et al. 1996; Cendes et al. 1997).
362
Proton MRS studies (Matthews et al. 1990; Cendes et al. 1995, 1997; Maton et al. 2001) indicate that (a) partial seizures are associated with abnormally high lactate levels, but absence seizures are not, and (b) no short-term changes ofNA occur during or soon after CPS (Cendes et al. 1997; Maton et al. 2001) or absence seizures (Cendes et al. 1997c). These findings (Cendes et al. 1997c) may be related to the lack of postictal confusion in patients with absence seizures, as well as with the more benign course of primary generalized epilepsy with non-convulsive attacks. The spatial relationship between the NAA decrease and the underlying mechanisms causing neuronal damage is unclear. It has been observed that the neuronal damage as measured by NAA can extend to areas at a distance from the lesion, and that the timing of the insult may contribute to the widespread neuronal damage (Li et al. 2000c). It remains to be seen if this is relevant for the severity of epilepsy. The NAA signal is used as a parameter of neuronal integrity. The side of maximum NAAreduction often coincides with the side of EEG abnormality, The relationship between spiking frequency and underlying neuronal function and epileptogenic state is unclear (Peeling and Sutherland 1993; Series et al. 1999). Proton MRS studies (Hugg et al. 1996; Cendes et al. 1997; Series et al. 2001) have shown recovery of relative NAA either ipsilaterally or contralaterally after successful temporal lobe removal. This suggests that structural or functional changes associated with seizure activity may lead to depression ofNAA in the ipsilateral or contralateral temporal lobe. This preliminary observation has potentially great significance for understanding the utility of imaging NAA in the presurgical lateralization ofTLE, as it suggests that reduction in NAA reflects not only the sequelae of the initial injury to temporal lobe structures, but also an effect of the seizure activity itself (or other factors associated with the ongoing epileptic state). Still, what remains most interesting is the component of NAA decrease that is not directly related to neuronal loss, atrophy, glucose hypometabolism, and neurophysiologic epileptiform disturbances as dis-
cussed above. Discovering the causes of reversible decreases in NAA concentration holds great excitement for better understanding cellular dysfunction associated with epilepsy. It has been a matter of dispute whether or not recurrent seizures can cause neuronal loss in human temporal lobe epilepsy, and whether TLE is a progressive disease. Studies using MRSI have produced seemingly conflicting results. Vermathen et al. (2000) studied a group of patients with nontemporal neocortical epilepsy and showed that hippocampal NAA/Cr was not reduced, in contrast to patients with unilateral TLE. They argued that seizures did not cause secondary hippocampal damage. Garcia et al. (1997) found a negative correlation between NAA and seizure frequency in patients with both frontal and temporal epilepsy, although no correlation with duration. Tasch et al. (1999) found that ipsilateral and contralateral NAA/Cr was negatively correlated with duration of temporal lobe epilepsy. Frequency of complex partial seizures was not correlated with MRS or MRIVol. Patients with frequent generalized tonic-clonic seizures had lower NAAICr bilaterally and smaller hippocampal volumes ipsilaterally than patients with none or rare generalized tonic-clonic seizures (Tasch et al. 1999). Preliminary data suggests that MRS may be useful for the evaluation of other forms of partial epilepsies (Garcia et al. 1995; Stanley et al. 1998; Li et al. 2000a,b), including those associated with cortical developmental malformations (COM; Kuzniecky et al. 1997; Li et al. 1998). Li et al. (1998) demonstrated that different types of COM show different degrees of decrease of NAA. In cortical dysplasia, the relative NAA signal was very low. This disorder appears to result form abnormal neuronal and glial cell differentiation and proliferation, and the lesion contains structurally abnormal neurons with abnormal synaptic activity and connectivity, thus explaining the reduced NAA values. In polymicrogyria, where the NAA values were normal or slightly abnormal, the malformation is due to an abnormal cortical organization caused by a postmigrational insult and the neurons are mature. Heterotopia consist of a large number of neurons that failed to initiate or complete the
363
migration process. In heterotopia, because of the high number of neurons, one would expect a relative increase ofthe NAA signal. This assumption is based on histopathologic studies showing normalappearing neurons and on early fluorodeoxyglucose PET studies showing patterns of glucose uptake similar to normal cortex. However, proton MRS have shown NAA signal intensity to be variably normal or abnormal in patients with heterotopia. This suggests that at least some ofthese apparently normal neurons are, in fact, dysfunctional. Kuzniecky et al. (1997) found that patients with focal cortical dysplasia had significant metabolic abnormalities in correspondence with the structural lesions, whereas patients with heterotopia and polymicrogyria demonstrated no subcortical MRSI abnormalities. They showed significant correlation between the metabolic abnormalities and the frequency of seizures but not with the degree of interictal EEG discharges. Quantitative neuronal and glial cell counts revealed no statistically significant correlation between cell loss and the abnormal metabolic ratios in those who underwent surgery. Their findings suggest that MRSI based metabolic abnormalities in patients with CDM are variable and are likely to be associated with complex cellular mechanisms involving the regulation of NAA, total Cr content, and Cho and perhaps to seizure activity (Kuzniecky et a1. 1997). Another study in a patient with a giant heterotopia (Preul et al. 1997) showed changes in NAA and creatine-phosphocreatine (Cr) levels, reflecting alterations in energy metabolism and neuronal dysfunction in the area of heterotopia and in regions of the ipsilateral hemisphere that appeared normal on MR!. It is still unclear if the decreases in NAA are related only to the abnormal structure of the dysgenic cortex, or to the ongoing epileptogenicity, or both. However, studies so far, have failed to find a correlation between the degree and extent ofEEG abnormalities and NAA values. The differences in relative NAA signals among the different types of CMD discussed above, appear to reflect more the type of malformation than the amount of intericta1 EEG abnormality.
References Ackerman, J.J .H., Grove, T.H., Wong, G.G., et al. Mapping of metabol ites in whole animals by 31P NMR using surface coils. Nature, 1980,283: 167-170. Cendes, F., Andermann, F, Silver, K. and Arnold, D.L. Imaging of axonal damage in vivo in Rasmussen's syndrome. Brain, 1995, 118: 753-758. Cendes, F, Andennann, F, Dubeau, F., Matthews, P.M. and Arnold, D.L. Normalization of neuronal metabolic dysfunction after surgery for temporal lobe epilepsy. Evidence from proton MR spectroscopic imaging. Neurology, 1997a, 49: 1525-1533. Cendes, F., Caramanos, Z., Andermann, F., Dubeau, F. and Arnold, D.L. Proton magnetic resonance spectroscopic imaging and magnetic resonance imaging volumetry in the lateralization of ternporallobe epilepsy: a series of 100 patients. Ann. Neurol., 1997b, 42: 737-746. Cendes, F., Stanley, 1.A., Dubeau, F, Andermann, F. and Arnold, D.L. Proton magnetic resonance spectroscopic imaging for discrimination of abscence and complex partial seizures. Ann. Neural, 1997c, 4\: 74-81. Cross, LH., Connelly, A., Jackson, G.D., Johnson, c.L., Neville, B.G.R. and Gadian, D.G. Proton magnetic resonance spectroscopy in chi Idren with temporal lobe epilepsy. Ann. Neurol., 1996, 39: 107-113. De Stefano, N., Matthews, P.M. and Arnold, D.L. Reversible decreases in N-acetylaspartate after acute brain injury. Magn. Reson. Med., 1995,34: 721-727. Ebisu, T., Rooney, W.O., Graham, S.H., Weiner, M.W. and Maudsley, A.A. N-Acety1aspartate as an in vivo marker of neuronal viability in kainate-induced status epilepticus: 'H magnetic resonance spectroscopic imaging. J. Cereb. Blood Flow Metab., 1994, 14: 373-382. Garcia, P.A., Laxer, K.D., van der Grond, J., Hugg, J.w., Matson, G.B. and Weiner, M.W. Proton magnetic resonance spectroscopic imaging in patients with frontal lobe epilepsy. Ann. Neurol., 1995, 37: 279-281. Garcia, P.A., Laxer, K.D., van der Grond, J., Hugg, J.w., Matson, G.B. and Weiner, M.W. Correlation of seizure frequency with N-acetylaspartate levels determined by 'H magnetic resonance spectroscopic imaging. Magn. Reson. Imaging, 1997, 15: 475--478. Hugg, J.w., Laxer, K.D., Matson. G.B., Maudsley, A.A. and Weiner, M.W. Neuron loss localizes human temporal lobe epilepsy by in vivo proton magnetic resonance spectroscopic imaging. Ann. Neurol., 1993,34: 788-794. Hugg, J.w., Kuzniecky, R.I., Gilliam, FG., Morawetz, R.B., Faught, R.E. and Hetherington, H.P. Normalization of contralateral metabolic function following temporal lobectomy demonstrated by H-I magnetic resonance spectroscopic imaging. Ann. Neurol., 1996,40: 236-239. Kuzniecky, R., Hetherington, H., Pan, J., Hugg, J., Palmer, C., Gilliam, F., Faught, E. and Morawetz, R. Proton spectroscopic imaging at 4.1 tesla in patients with malformations of cortical development and epilepsy. Neurology, 1997,48: 1018-1024. Kuzniecky, R., Hugg, 1.W., Hetherington, H., Butterworth, E., Bilir, E., Faught, E. and Gilliam, F Relative utility of 'H spectroscopic imaging and hippocampal volumetry in the lateralization of mesial temporal lobe epilepsy. Neurology, 1998,51: 66-71. Kuzniecky, R., Hugg, 1., Hetherington, H., Martin, R., Faught, E., Morawetz, R. and Gilliam, F Predictive value of 'H MRSI for outcome in temporal lobectomy. Neurology, 1999,53: 694-698.
364 Li, L.M., Cendes, E, Bastos, A.e., Andennann, E, Dubeau, F. and Arnold, D.L. Neuronal metabolic dysfunction in patients with cortical developmental malformations: a proton magnetic resonance spectroscopic imaging study. Neurology, 1998,50: 755-759. Li, L.M., Caramanos, Z., Cendes, E, Andermann, F., Antel, S.B., Dubeau, E and Arnold, D.L. Lateralization of temporal lobe epilepsy (TLE) and discrimination ofTLE from extra-TLE using pattern analysis of magnetic resonance spectroscopic and volumetric data. Epilepsia, 2000a, 41 : 832-842. Li, L.M., Cendes, E, Andermann, F., Dubeau, E, Antel, S., SerIes, w., Olivier, A. and Arnold, D.L. Prognostic value of proton MR spectroscopic imagng for surgical outcome in patients with intractable temporal lobe epilepsy and bilateral hippocampal atrophy. Ann. Neural., 2000b, 47: 195-200. Li, L.M., Cendes, F., Andermann, E, Dubeau, F. and Arnold, D.L. Spatial extent of neuronal metabolic dysfunction measured by proton MR spectroscopic imaging in patients with localizationrelated epilepsy. Epilepsia, 2000c, 41: 666-674. Maton, B., Londono, A., Sawrie, S., Knowlton, R., Martin, R. and Kuzniecky, R. Postictal stability of proton magnetic resonance spectroscopy imaging ((1 )H-MRSI) ratios in temporal lobe epilepsy. Neurology, 2001, 56: 251-253. Matthews, P.M., Andermann, F. and Arnold, D.L. A proton magnetic resonance spectroscopy study of focal epilepsy in humans. Neurology, 1990,40: 985-989. Moffett, 1.R.,Aryan, M.A., Namboodiri, M.A.A., Cangro, e.B. and Neale, 1.H. Immunohistochemical localization of N-acetylaspartate in rat brain. Neuroreport, 1991,2: 131-134. Najm, I.M., Wang, Y, Shedid, D., Luders, H.O., Ng, T.e. and Comair, YG. MRS metabolic markers of seizures and seizure-induced neuronal damage. Epilepsia, 1998, 39: 244--250. Peeling, 1. and Sutherland, G. 'H magnetic resonance spectroscopy of extracts of human epileptic neocortex and hippocampus. Neurology, 1993,43: 51;9-594. Petroff, O.A., Spencer. D.O., Alger, lR. and Prichard, 1.W. Highfield proton magnetic resonance spectroscopy of human cerebrum
obtained during surgery for epilepsy. Neurology, 1989,39: 11971202. Petroff, O.A.C., Pleban, L.A. and Spencer, D.O. Symbiosis between in vivo and in vitro NMR spectroscopy: the creatine, N-acetylaspartate, glutameate, and GABA content of the epileptic human brain. Magn. Reson. Imaging, 1995, 13: I197-1211. Preul, M.C., Caramanos, Z., Collins, D.L., Villemure, lG., Leblanc, R., Olivier, A., Pokrupa, R. and Arnold, D.L. Accurate, noninvasive diagnosis of human brain tumors by using proton magnetic resonance spectroscopy. Nature Med., 1996,2: 323-325. Preul, M.e., Leblanc, R., Cendes, E, Dubeau, F., Reutens, D., Spreafico, R., Battaglia, G., Avoli, M., Langevin, P., Arnold, D.L. and Villemure, lG. Function and organization in dysgenic cortex. Case report. J Neurosurg., 1997,87: 113-121. Series, W., Li, L.M., Caramanos, Z., Arnold, D.L. and Gotman, 1. Relation of interictal spike frequency to 'H-MRSI-measured NAA/Cr. Epilepsia, 1999,40: 1821-1827. Series, W., Li, L.M., Antel, S.B., Cendes, E, Gotman, J., Olivier, A., Andermann, F., Dubeau, E and Arnold, D.L. Time course of postoperative recovery of N-acetyl-aspartate in temporal lobe epilepsy. Epilepsia, 2001,42: 190-197. Simmons, M.L., Frondoza, C.G. and Coyle, J.T.lmmunocytochemical localization of N-acetyl-aspartate with monoclonal antibodies. Neuroscience, 1991,45: 37---45. Stanley, J.A., Cendes, E, Dubeau, E, Andennann, F. and Arnold, D.L. Proton magnetic resonance spectroscopic imaging in patients with extratemporal epilepsy. Epilepsia, 1998, 39: 267273. Tasch, E., Cendes, F., Li, L.M., Dubeau, F., Andennann, F. and Arnold, D.L. Neuroimaging evidence of progressive neuronal loss and dysfunction in temporal lobe epilepsy. Ann. Neural., 1999,45: 568-576. Vennathen, P., Laxer, K.D., Matson, G.B. and Weiner, M.W. Hippocampal structures: anteroposterior N-acetylaspartate differences in patients with epilepsy and control subjects as shown with proton MR spectroscopic imaging. Radiology, 2000, 214: 403---410.
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Chapter 54
MUS for imaging neuronal dysfunction in epilepsy Fernando Cendes", Li Min Li" and Douglas Arnold" 'Department ofNeurology, University of Campinas, Campinas, SP (Brazil) "Department ofNeurology and Neurosurgery; Montreal Neurological institute and Hospital, McGill University, Montreal, PQ H3A 2B4 (Canada)
Introduction
e
Proton magnetic resonance spectroscopy HMRS)
Magnetic resonance spectroscopy (MRS) permits chemically specific, non-invasive measurement of certain compounds in living tissue. MRS measurements in the brain of living animals became practical in 1980 (Ackerman et al. 1980), and the capability for human measurements was developed a few years later. By the early 1990s, applications of MRS to several neurologic disorders had been published (Petroff et al. 1995). The non-invasive nature of MRS means that repeated measurements can be made, so that kinetic and longitudinal studies are possible in a single subject. In addition, one can study human tissues that are inaccessible except by invasive techniques. In the human brain, phosphate energy stores, intracellular pH, lactate concentration, and the neuronal marker N-acetylaspartate (NAA) are examples of MRS-measurabl,e variables that are important for both clinical and scientific purposes and cannot be studied easily by any other technique (Petroff et al. 1995).
* Correspondence to: Dr. D.L. Arnold, MRS Laboratory, Brain Imaging Center, Montreal Neurological Institute and Hospital, 3801 University Street, Montreal, PQ H3A 2B4, Canada. E-mail:
Water-suppressed, localized MR spectra of normal human brain at 'long' echo times (TE 136272 ms) reveal four major resonances (Fig. 1): • One at 3.2 ppm, which arises from tetramethylamines (mainly from choline-containing phospholipids) (Cho); • One at 3.0 ppm, which arises primarily from creatine and phosphocreatine (Cr); • One at 2.0 ppm which arises from N-acetyl groups (mainly NAA); • One at 1.3 ppm, which arises from the methyl resonance of lactate, or, in certain pathological conditions, methyl groups oflipids (De Stefano et al. 1995; Petroff et al. 1995). Under normal physiologic conditions and with the small voxel size used for most in vivo MRS examinations ofthe adult human brain, the lactate peak is not visible above the baseline noise. Multiple lines of evidence support the use ofNAA as a neuronal marker. It is found exclusively in neurons and their processes in the mature brain (Moffett et al. 1991; Simmons et al. 1991). In human brain spectra, in vivo NAA is reduced in situations known to be associated with neuronal loss. When decreases in the relative NAA signal arise
360
NAA
NAA
Cho
~
Cho
Fig. 1. Spectra from the right and left mid temporal lobes in a patient with left TLE with normal MRI. Note the reduced NAA peak on the left temporal lobe (arrow). This patient underwent left temporal lobe resection and became seizure-free after surgery. The postoperative histopathology showed mild mesial temporal sclerosis.
from neuronal or axonal degeneration, irreversible changes are expected. However, there have been observations of reversible decreases in NAA in a number of conditions, emphasizing that neuronal dysfunction or transient relative volume changes can also lead to decreased NAA (De Stefano et al. 1995a; Hugg et al. 1996a; Cendes et a1. 1997a). The ability to quantify specifically neuronal loss or damage is one of the most interesting potential applications of MRS in cerebral disorders. Changes in the resonance intensity of Cho probably result mainly from increases in the steady-state levels of phosphocholine and glycerol-phosphocholine. These choline-containing membrane phospholipids are released during active myelin breakdown. Thus, the resonance intensity of Cho increases in acute demyelinating lesions in humans (De Stefano et a1. 1995), and in certain tumors, such as meningiomas (Preul et al. 1996). Total Cr concentration is relatively constant throughout the brain and tends to be relatively resistant to change. However, large changes can be seen with destructive pathology such as malignant tumors (Preul et al. 1996). It is reasonable to use creatine as an internal standard to normalize resonance intensities of NAA and Cho in order to correct for artifactual variations in signal intensity due to magnetic field and radiofrequency inhomoge-
neity. However, this must be done with caution. Use of an external concentration reference can be reliable if factors such as radiofrequency field inhomogeneity and coil tuning and coupling can be adequately controlled (Petroff et a1. 1995). Lactic acid is the end product of glycolysis and accumulates when oxidative metabolism cannot meet energy requirements. For instance, elevation of lactic acid in cerebral neoplasms correlates approximately with relative rates of glucose uptake. However, lactate is both intracellular and extracellular, and large amounts may be accumulated outside actively anaerobic tissue (e.g. in necrotic tissue or fluid- filled cysts) (De Stefano et al. 1995; Petroff et a1. 1995).
Proton MRS studies in partial epilepsies Animal studies Brain lactate is elevated by seizure activity, as demonstrated by conventional biochemical studies of excised tissue. In vivo animal studies using proton MRS allowed more dynamic studies and a better appreciation of how persistent lactate elevation is after even brief convulsive seizure (Petroff et al. 1995). Selective neuronal injury by kainate-
361
induced status epilepticus in rats was associated with focal reduction of NAA determined by proton MRSI, even before Tz-weighted MRI changes were observed (Ebisu et al. 1994). Najm et al. (1998) used proton MRS to identify specific in situ metabolic markers for seizures and seizure-induced neuronal damage in rat brains. They pretreated rats with placebo or cycloheximide 1 h before kainic acid injection and then scanned rat brains at the level of the hippocampus before, during, and 24 h after seizures. They found a significant increase in lactate ratios in kainic acidtreated rats during and 24 h after seizure onset, however, this increase was prevented by cycloheximide pretreatment. They suggest that in situ lactate increase is a marker of seizure-induced neuronal damage and that there is no significant increase of in situ lactate during seizures that do not lead to neuronal damage (Najm et al. 1998). Human studies
An early use of proton MRS in the context of human epilepsy was in analysis of extracts of samples from temporal lobe tissue resected for treatment ofdrug resistant temporal lobe epilepsy (TLE; Petroff et al. 1989). This type of in vitro study is a useful source of information about concentrations of several human brain metabolites that are observable by in vivo MRS studies. The first observations of elevated brain lactate (and also reduced NAA) in vivo by proton MRS associated with a seizure disorder was reported by Matthews et al. (1990) in two patients with Rasmussen's syndrome. Another study suggests that lactate accumulation result from repetitive seizures rather than from the disease process in Rasmussen's syndrome (Cendes et al. 1995). Subsequent proton MRS studies have shown focal reductions ofNAA signal in patients with nonlesional TLE (Petroff et al. 1989; Matthews et al. 1990; Hugg et al. 1993; Cross et al. 1996; Cendes et al. 1997a,b; Kuzniecky et al. 1997, 1998; Li et al. 1998, 2000) and extratemporal partial epilepsies (Cendes et al. 1995; Garcia et al. 1995; Stanley et al. 1998) with good correlation with EEG abnor-
malities and severity of cell loss. The results of published MRS studies suggest that in patients with partial epilepsy there is a metabolic abnormality throughout the brain, with patterns of asymmetry and focal accentuation that are useful for non-invasive localization of epileptogenic foci (Petroff et al. 1989: Matthews et al. 1990; Hugg et al. 1993; Cendes et al. 1995, 1997a,b; Garcia et al. 1995; Cross et al. 1996; Kuzniecky et al. 1998; Stanley et al. 1998). The MRS findings may have prognostic value for seizure outcome as well (Kuzniecky et al. 1998, 1999; Li et al. 2000). In a series of 100 consecutive patients with TLE (Cendes et al. 1997b), the NAA/Cr values were abnormally low in at least one temporal lobe in all but one patient and were low bilaterally in 54%. The asymmetry between right and left sides of NAA/Cr lateralized 86/93 (92.5%) of patients who had lateralization by ictal EEG. There were seven patients with no clear lateralization by EEG. The MRSI lateralization was ipsilateral to the EEG in all but two patients who had bilateral TLE and bilateral AM-HF atrophy greater on the same side as the MRSI. Twelve of 13 patients with normal MRIVol had a significant decrease of NAA/Cr within the mesial temporal lobe ipsilateral to the ictal EEG focus (Fig. 1). Seven of these underwent surgery and the histopathology showed mild mesial temporal sclerosis. The above mentioned study (Cendes et al. 1997b) showed a direct correlation between the measure of NAA/Cr and MRiVol. However, another study with 33 patients showed no correlation between the measure of hippocampal specific Cr/NAA and MRiVol (Kuzniecky et al. 1998). Methodological and statistical regional analysis differences are most likely responsible for the discrepancy between studies. Some degree of disassociation between severity of NAA abnormality and hippocampal volume or cell loss is not unexpected. This includes the finding of abnormal NAA measures in normal volume hippocampi (both ipsilateral and contralateral to site of seizure onset) and the reversibility or correction ofNAA abnormalities in patients who become seizure-free after surgery (Hugg et al. 1996; Cendes et al. 1997).
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Proton MRS studies (Matthews et al. 1990; Cendes et al. 1995, 1997; Maton et al. 2001) indicate that (a) partial seizures are associated with abnormally high lactate levels, but absence seizures are not, and (b) no short-term changes ofNA occur during or soon after CPS (Cendes et al. 1997; Maton et al. 2001) or absence seizures (Cendes et al. 1997c). These findings (Cendes et al. 1997c) may be related to the lack of postictal confusion in patients with absence seizures, as well as with the more benign course of primary generalized epilepsy with non-convulsive attacks. The spatial relationship between the NAA decrease and the underlying mechanisms causing neuronal damage is unclear. It has been observed that the neuronal damage as measured by NAA can extend to areas at a distance from the lesion, and that the timing of the insult may contribute to the widespread neuronal damage (Li et al. 2000c). It remains to be seen if this is relevant for the severity of epilepsy. The NAA signal is used as a parameter of neuronal integrity. The side of maximum NAAreduction often coincides with the side of EEG abnormality, The relationship between spiking frequency and underlying neuronal function and epileptogenic state is unclear (Peeling and Sutherland 1993; Series et al. 1999). Proton MRS studies (Hugg et al. 1996; Cendes et al. 1997; Series et al. 2001) have shown recovery of relative NAA either ipsilaterally or contralaterally after successful temporal lobe removal. This suggests that structural or functional changes associated with seizure activity may lead to depression ofNAA in the ipsilateral or contralateral temporal lobe. This preliminary observation has potentially great significance for understanding the utility of imaging NAA in the presurgical lateralization ofTLE, as it suggests that reduction in NAA reflects not only the sequelae of the initial injury to temporal lobe structures, but also an effect of the seizure activity itself (or other factors associated with the ongoing epileptic state). Still, what remains most interesting is the component of NAA decrease that is not directly related to neuronal loss, atrophy, glucose hypometabolism, and neurophysiologic epileptiform disturbances as dis-
cussed above. Discovering the causes of reversible decreases in NAA concentration holds great excitement for better understanding cellular dysfunction associated with epilepsy. It has been a matter of dispute whether or not recurrent seizures can cause neuronal loss in human temporal lobe epilepsy, and whether TLE is a progressive disease. Studies using MRSI have produced seemingly conflicting results. Vermathen et al. (2000) studied a group of patients with nontemporal neocortical epilepsy and showed that hippocampal NAA/Cr was not reduced, in contrast to patients with unilateral TLE. They argued that seizures did not cause secondary hippocampal damage. Garcia et al. (1997) found a negative correlation between NAA and seizure frequency in patients with both frontal and temporal epilepsy, although no correlation with duration. Tasch et al. (1999) found that ipsilateral and contralateral NAA/Cr was negatively correlated with duration of temporal lobe epilepsy. Frequency of complex partial seizures was not correlated with MRS or MRIVol. Patients with frequent generalized tonic-clonic seizures had lower NAAICr bilaterally and smaller hippocampal volumes ipsilaterally than patients with none or rare generalized tonic-clonic seizures (Tasch et al. 1999). Preliminary data suggests that MRS may be useful for the evaluation of other forms of partial epilepsies (Garcia et al. 1995; Stanley et al. 1998; Li et al. 2000a,b), including those associated with cortical developmental malformations (COM; Kuzniecky et al. 1997; Li et al. 1998). Li et al. (1998) demonstrated that different types of COM show different degrees of decrease of NAA. In cortical dysplasia, the relative NAA signal was very low. This disorder appears to result form abnormal neuronal and glial cell differentiation and proliferation, and the lesion contains structurally abnormal neurons with abnormal synaptic activity and connectivity, thus explaining the reduced NAA values. In polymicrogyria, where the NAA values were normal or slightly abnormal, the malformation is due to an abnormal cortical organization caused by a postmigrational insult and the neurons are mature. Heterotopia consist of a large number of neurons that failed to initiate or complete the
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migration process. In heterotopia, because of the high number of neurons, one would expect a relative increase ofthe NAA signal. This assumption is based on histopathologic studies showing normalappearing neurons and on early fluorodeoxyglucose PET studies showing patterns of glucose uptake similar to normal cortex. However, proton MRS have shown NAA signal intensity to be variably normal or abnormal in patients with heterotopia. This suggests that at least some ofthese apparently normal neurons are, in fact, dysfunctional. Kuzniecky et al. (1997) found that patients with focal cortical dysplasia had significant metabolic abnormalities in correspondence with the structural lesions, whereas patients with heterotopia and polymicrogyria demonstrated no subcortical MRSI abnormalities. They showed significant correlation between the metabolic abnormalities and the frequency of seizures but not with the degree of interictal EEG discharges. Quantitative neuronal and glial cell counts revealed no statistically significant correlation between cell loss and the abnormal metabolic ratios in those who underwent surgery. Their findings suggest that MRSI based metabolic abnormalities in patients with CDM are variable and are likely to be associated with complex cellular mechanisms involving the regulation of NAA, total Cr content, and Cho and perhaps to seizure activity (Kuzniecky et a1. 1997). Another study in a patient with a giant heterotopia (Preul et al. 1997) showed changes in NAA and creatine-phosphocreatine (Cr) levels, reflecting alterations in energy metabolism and neuronal dysfunction in the area of heterotopia and in regions of the ipsilateral hemisphere that appeared normal on MR!. It is still unclear if the decreases in NAA are related only to the abnormal structure of the dysgenic cortex, or to the ongoing epileptogenicity, or both. However, studies so far, have failed to find a correlation between the degree and extent ofEEG abnormalities and NAA values. The differences in relative NAA signals among the different types of CMD discussed above, appear to reflect more the type of malformation than the amount of intericta1 EEG abnormality.
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