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 (20–22). Selective neuronal injury by kainate-induced status epilepticus in rats was associated with focal reduction of NAA, determined by proton MRS imaging (MRSI), even before T2-weighted MRI changes were observed (23).
Najm et al. (24) 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 acid–treated 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 (24).
An early use of proton MRS in the context of human epilepsy was analysis of extracts of samples from temporal lobe tissue resected for treatment of drug-resistant temporal lobe epilepsy (TLE) (25). 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. (26) in two patients with Rasmussen's syndrome. Another study suggests that lactate accumulation results from repetitive seizures rather than from the disease process in Rasmussen's syndrome (27).
Subsequent proton MRS studies have shown focal reductions of NAA signal in patients with nonlesional TLE (25,26,28–42) and extratemporal partial epilepsies (27,43,44) with good correlation with EEG abnormalities 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 noninvasive localization of epileptogenic foci (25–39,43–46)(Table 1). The MRS findings may have prognostic value for seizure outcome as well (39,47,48).
Table 1. Proton MRS studies in temporal lobe epilepsy
|Achten et al. (45)||25a||24 (96%)||18 (72%)||24 (96%)||—|
|Breiter et al. (38)||7||7 (100%)||0||7 (100%)||0|
|Cendes et al. (31)||100||100 (100%)||54 (54%)||84 (84%)b||2 (2%)c|
|Connelly et al. (30)||25a||22 (88%)||10 (40%)||15 (60%)||3 (12%)|
|Cross et al. (35)||20a||15 (75%)||9 (45%)||11 (55%)||0|
|Duc et al. (76)||11a||11 (100%)||—||11 (100%)||0|
|Ende et al. (46)||16||16 (100%)||8 (50%)||16 (100%)||0|
|Hetherington et al. (37)||10||10 (100%)||4 (40%)||10 (100%)||0|
|Hugg et al. (29)||8||8 (100%)||—||8 (100%)||0|
|Kuzniecky et al. (39)||30||29 (97%)||7 (23%)||29 (97%)||1 (3%)|
|Ng et al. (36)||25||23 (92%)||3 (12%)||21 (84%)||2 (8%)|
|Vainio et al. (34)||7a||7 (100%)||—||7 (100%)||0|
|Total||284||272 (95.8%)||113 (43.8%)||243 (85.6%)||8 (3.1%)|
In a series of 100 consecutive patients with TLE (31), 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 (92.5%) of 93 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 MRI volumes of the hippocampus (MRI Vol) 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 histopathologic examination showed mild mesial temporal sclerosis.
The previously mentioned study (31) showed a direct correlation between the measure of NAA/Cr and MRI Vol. However, another study with 33 patients showed no correlation between the measure of hippocampal specific Cr/NAA and MRI Vol (39). Methodologic 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 of NAA abnormalities in patients who become seizure free after surgery (14,15).
It has been demonstrated that the degree of relative hypometabolism on fluorodeoxyglucose–positron emission tomography (FDG-PET) does not correlate with either MRI Vol (49) or histopathologic measure of cell loss (50). Potentially, if NAA and glucose metabolism are related, the component of decreased NAA that is believed to be related to disturbances in the metabolism might be better understood. Here too, findings from different studies are conflicting. One study with single-voxel proton MRS (1H-MRS) measure of mesial temporal NAA/(Cho+Cr) showed a significant correlation with normalized glucose metabolism in midtemporal (combined mesial and lateral temporal cortex) and temporal polar regions (51). Another study restricted to sampling only the hippocampus showed no correlation between measurement of Cr/NAA and normalized hippocampal glucose uptake (52). One further study (45) did not look at a direct correlation between NAA measures and glucose metabolism, but rather examined the sensitivity in relationship of lateralized abnormalities in MTLE. They found that mesial temporal single-voxel MRS measurement of NAA/(Cho+Cr) was abnormal in 42 temporal lobes, whereas FDG-PET showed only 25 with decreased metabolism, a finding that demonstrates a higher sensitivity for 1H-MRS in detection of metabolic disturbance than that with FDG-PET. Again, differences in methods, and in particular, region of analysis, make it difficult to compare the findings between the studies.
Cendes et al. (33) showed a significant increase in lactate/creatine+phosphocreatine (lactate/Cr) values, during and soon after complex partial seizures (CPSs), but not during or soon after absence seizures associated with generalized epilepsy (33). In patients with TLE, the NAA resonance relative to Cr (NAA/Cr) was low in one or both temporal lobes, indicating neuronal loss or damage. This was not observed in patients with primary generalized epilepsy. The regions with abnormal lactate/Cr and NAA/Cr values corresponded to the epileptogenic focus as defined by clinical-EEG investigation. There was no change in the NAA/Cr values in the temporal lobes between the interictal, ictal, or postictal states, for both individual patients and intergroup comparisons. The normal lactate levels observed during and after nonconvulsive generalized seizures are in keeping with the lack of or minimal postictal confusion in absence attacks. The lack of an increase in lactate levels after absence attacks suggests that these seizures may be less likely to result in cellular damage than are the CPSs of TLE (33). Similar results have been reported by Maton et al. (53) in individual patients between interictal and postictal studies.
Proton MRS studies (26,27,33,36,53) indicate that (a) partial seizures are associated with abnormally high lactate levels, but absence seizures are not, and (b) no short-term changes of NAA occur during or soon after CPSs (33,53) or absence seizures (33). These findings (33) 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 nonconvulsive 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 (54). It remains to be seen if this is relevant for the severity of epilepsy.
The NAA signal is often used as a parameter of neuronal integrity. The side of maximal NAA reduction often coincides with the side of EEG abnormality. The relationship between spiking frequency and underlying neuronal function and epileptogenic state is unclear. Serles et al. (55) showed trend toward higher interictal spike frequencies on surface EEG in regions of pronounced neuronal metabolic damage or dysfunction. This suggests that both variables parallel an underlying pathologic substrate, although the pathophysiologic processes may be distinct. Peeling and Sutherland (56) used high-resolution 1H-MRS to determine the concentrations of several metabolites (lactate, alanine, NAA, γ-aminobutyrate (GABA), glutamate, aspartate, creatine, choline, taurine, inositol, and succinate) in tissue from patients undergoing surgical treatment for intractable epilepsy. They correlated the metabolite profiles with the results of histopathologic analysis of the excised tissue and the spike activity. Surprisingly, they found no differences in metabolite levels from tissue with active spiking or nonspiking neocortical sites. Conversely, Maton et al. (57) found that bilateral Cr/NAA abnormalities were 3 times as frequent as the detection of bitemporal interictal spikes. Thus as with PET and MRI Vol, measures of interictal NAA disturbances do not appear completely linked with the EEG measure of neurophysiologic epileptiform disturbance.
Proton MRS studies (14,15,58) 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 of NAA 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 of TLE, 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 discussed earlier. Discovering the causes of reversible decreases in NAA concentration holds great promise for better understanding cellular dysfunction associated with epilepsy.
It has been a matter of dispute whether recurrent seizures can cause neuronal loss in human TLE, and whether TLE is a progressive disease. Studies using MRSI have produced seemingly conflicting results. Vermathen et al. (59) studied a group of patients with non–temporal neocortical epilepsy and showed that hippocampal NAA/Cr was not reduced, in contrast to that in patients with unilateral TLE. They argued that seizures did not cause secondary hippocampal damage. Garcia et al. (60) found a negative correlation between NAA and seizure frequency in patients with both frontal and temporal epilepsy, although no correlation with duration. Tash et al. (61) found that ipsilateral and contralateral NAA/Cr were negatively correlated with duration of TLE. Frequency of CPSs was not correlated with MRS or MRI Vol. Patients with frequent generalized tonic–clonic seizures had lower NAA/Cr bilaterally and smaller hippocampal volumes ipsilaterally than did patients with none or rare generalized tonic–clonic seizures (61).
Preliminary data suggest that MRS may be useful for the evaluation of other forms of partial epilepsies (40,43,44,54), including those associated with cortical developmental malformations (CDMs) (41,42). Li et al. (41) demonstrated that different types of CDM show different degrees of decrease of NAA. In cortical dysplasia, the relative NAA signal was very low. This disorder appears to result from 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, in which 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 consists of a large number of neurons that failed to initiate or complete the migration process. In heterotopia, because of the high number of neurons, one would expect a relative increase of the NAA signal. This assumption is based on histopathologic studies showing normal-appearing neurons and on early FDG-PET studies showing patterns of glucose uptake similar to those of normal cortex. However, proton MRS studies have shown NAA signal intensity to be variably normal or abnormal in patients with heterotopia. This suggests that at least some of these apparently normal neurons are dysfunctional.
In another study at high field (4.1 T), Kuzniecky et al. (42) 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 with seizure activity (42). Another study in a patient with a giant heterotopia (62) 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 MRI. It is still unclear whether the decreases in NAA are related only to the abnormal structure of the dysgenic cortex, or to the ongoing epileptogenicity, or to both. However, studies have failed to find a correlation between the degree and extent of EEG abnormalities and NAA values. The differences in relative NAA signals among the different types of CMD discussed earlier appear to reflect more the type of malformation than the amount of interictal EEG abnormality.