Temporal Lobe Sclerosis Associated With Hippocampal Sclerosis in Temporal Lobe Epilepsy: Neuropathological Features (original) (raw)
Abstract
Widespread changes involving neocortical and mesial temporal lobe structures can be present in patients with temporal lobe epilepsy and hippocampal sclerosis. The incidence, pathology, and clinical significance of neocortical temporal lobe sclerosis (TLS) are not well characterized. We identified TLS in 30 of 272 surgically treated cases of hippocampal sclerosis. Temporal lobe sclerosis was defined by variable reduction of neurons from cortical layers II/III and laminar gliosis; it was typically accompanied by additional architectural abnormalities of layer II, that is, abnormal neuronal orientation and aggregation. Quantitative analysis including tessellation methods for the distribution of layer II neurons supported these observations. In 40% of cases, there was a gradient of TLS with more severe involvement toward the temporal pole, possibly signifying involvement of hippocampal projection pathways. There was a history of a febrile seizure as an initial precipitating injury in 73% of patients with TLS compared with 36% without TLS; no other clinical differences between TLS and non-TLS cases were identified. Temporal lobe sclerosis was not evident preoperatively by neuroimaging. No obvious effect of TLS on seizure outcome was noted after temporal lobe resection; 73% became seizure-free at 2-year follow-up. In conclusion, approximately 11% of surgically treated hippocampal sclerosis is accompanied by TLS. Temporal lobe sclerosis is likely an acquired process with accompanying reorganizational dysplasia and an extension of mesial temporal sclerosis rather than a separate pathological entity.
Introduction
In the earliest descriptions of hippocampal sclerosis (HS) in temporal lobe resection specimens, it was recognized that “diffuse and disseminated lesions” may be present in association with this pathology involving the uncus, amygdala, and lateral temporal neocortex (1). The term “mesial temporal sclerosis” as an alternative to HS was introduced in recognition of the frequent involvement of mesial limbic structures adjacent to the hippocampus (2). The cause, severity, frequency, and distribution of neocortical neuronal loss and gliosis (temporal lobe sclerosis [TLS]) has been little documented in the neuropathology literature. It remains relatively undefined and is likely to be underreported.
Attention has recently refocused on the presence of additional pathology in the temporal neocortex in patients with HS in relation to its pathogenesis, the process of epileptogenesis, and possible explanations for poor outcomes after surgery. Aside from the identification of a second distinct lesional (or dual) pathology in addition to HS, modern neuroimaging studies identify temporal lobe abnormalities (3-6) and even extratemporal neocortical changes in patients with HS that may involve hippocampal projection pathways (7-9). The precise pathological correlate of these more widespread neocortical changes remains unclear, as does its significance in relation to generation of seizures. There is also a recognized association between maldevelopmental abnormalities of the temporal lobe, including focal cortical dysplasias (FCDs), and HS. Furthermore, previous descriptions of TLS-like changes have also alluded to features of cortical dysplasia (10, 11). Therefore, there are fundamental questions regarding the relationship of TLS to HS as well as to cortical dysplasia.
Our aim was to review and clarify the neuropathological features and criteria for TLS in a large series of temporal lobe resections for HS and to study the clinical and neuroimaging features to address some of these issues.
Materials and Methods
Case Material
We reviewed our series of temporal lobe resections carried out for the treatment of refractory temporal lobe epilepsy (TLE) due to HS at the National Hospital for Neurology and Neurosurgery (London, UK) in the period 1994 to 2006 to identify cases with TLS. The local ethics committee of the National Hospital for Neurology and Neurosurgery and Institute of Neurology granted approval for neuropathological studies in epilepsy, and individual patient consent was obtained as necessary. The original slides were available from 272 cases with HS as the main pathological diagnosis; cases with FCD, tumors, traumatic or other lesions were not included. In each case, the entire temporal lobe specimen from pole to caudal resection margin was assessed. This invariably included the middle and inferior temporal gyri, fusiform gyrus, and in many cases also part of the superior gyrus. Standard stains on each tissue block included hematoxylin and eosin, Luxol fast blue/cresyl violet, and glial fibrillary acidic protein (GFAP) immunohistochemistry.
Additional Immunohistochemistry
In selected cases, the coronal tissue block taken at approximately 1 to 1.5 cm caudal to the temporal pole was identified as described in detail (12). Additional sections were cut at 20 and 7 μm for additional immunohistochemistry. Briefly, sections were dewaxed, rehydrated, and washed; endogenous peroxidase was quenched with 3% hydrogen peroxide in water. Sections were then either microwaved with antigen retrieval buffer, enzyme treated, or left with no treatment. Sections were stained either overnight or for 1 hour at room temperature with primary antibodies. DAKO Envision horseradish peroxidase kit (Dako, Glostrup, Denmark) was used for visualization, with 3,3′diaminobenzidine as substrate. Sections were washed with PBS buffer with 0.5% Tween-20 in each step. Primary antibodies included NeuN (Chemicon International, Inc, Temecula, CA; 1:1000 microwaved), parvalbumin (Swant, Bellinzona, Switzerland; 1:4000 microwaved), calretinin (Swant; 1:4000 microwaved), neurofilament 2F11 (Dako; 1:500), and glutamic acid decarboxylase (Sigma, St Louis, MO; 1:5000 microwaved). Care was taken to ensure that slides were included on a single immunohistochemistry cycle to standardize cell-staining intensity.
Qualitative Analysis of Cases
The cortical myeloarchitecture and cytoarchitecture was assessed on both the original and new sections. The severity of neuronal loss was graded as mild, moderate, or severe, according to the region with most severe changes. The anterior-posterior extent of observed cortical changes was assessed by recording any gradation in severity of neocortical changes at different coronal levels as described (12). Similarly, the numbers of gyri (superior, middle, inferior temporal, and fusiform gyri) involved were also noted.
Quantitation of Cortical Neuronal Staining in the Middle Temporal Gyrus
On the 20-μm NeuN-stained sections 1 cm from the pole, we delineated a comparable region of interest (ROI) in all cases using Histometrix image analysis software (Kinetic Imaging, Liverpool, UK). The ROI included the full thickness of cortex on crest of the middle temporal lobe gyrus extending from the pial border to the white matter interface (Fig. 1A). The middle temporal lobe gyrus had been identified by the neurosurgeon (A.W.M.) in theater and permanently inked in the laboratory when the specimen was received. The percentage area of immunostaining, or field fraction, within this ROI was estimated (Figs. 1B, C). The image analysis red-green-blue detection threshold and light source intensity were kept constant in all measurements; the field fraction of staining in each ×40 field was measured and the mean value over the entire ROI calculated. Identical measurements were carried out on an equal number of TLS and non-TLS cases. The non-TLS cases were selected from the same series of patients with temporal lobectomies carried out for HS in which there was no evidence of neocortical pathology identified. When a TLS case was identified, the nearest sequential case without TLS was selected as a control. A mean field fraction control value was calculated from non-TLS cases, and the percentage reduction in each TLS case compared with this control value was calculated. This gave an estimation of “bare or unstained areas” in the cortex, as a result of neuronal loss and excessive clustering or cell grouping.
FIGURE 1.
Quantitative measurements in evaluation of temporal lobe sclerosis (TLS) and controls. (A) Sequential 5-mm coronal slices of a fixed temporal lobe specimen with the pole to the left. “A” indicates the anterior block, “P” the posterior, “S” the superior and “I” the inferior resection margin. The region of interest used for quantitative analysis is highlighted in red on the midtemporal gyrus cortex at 1.5 cm from the pole. (B) NeuN-immunostained section of cortical layers III to V from a case with hippocampal sclerosis but without TLS as viewed on the image analyzer. (C) The same field as shown inpanel B using field fraction analysis to detect the percentage of NeuN immunostaining highlighted in green. (D) NeuN-stained coronal section taken 1.5 cm from the pole and showing TLS with severe neuronal loss from the superficial cortex, visible at this magnification affecting all gyri, including the superior and inferior temporal gyri (arrows).
Quantitation of Neuronal Distribution in Cortical Layer II
Neuronal distribution and the degree of neuronal clustering in cortical layer II in the 20-μm NeuN-stained sections were determined using Image Pro software (Media Cybernetics, London, UK, version 6.2); 20 fields were analyzed with a ×63 objective along the upper part of layer II using a Nikon (Eclipse 80i) microscope. Care was taken to exclude layer I, which is hypocellular. For each field, a compressed image was formatted from images stepped at 5-μm intervals through the z axis of the section, to ensure all NeuN-positive neurons and nuclei were in maximal focus. The x, y coordinates of the central position of each nucleus in each field were tagged, and a running filter was applied to connect each coordinate with others in the immediate area. Based on the principles of Euclidean geometry, polygons were drawn to connect the midpoints between any neuron (N) and its neighbors (Voronoi tessellation). In essence, any point within the polygon is nearer to N than any other neuron in the section. Polygons touching the borders of the field were excluded. Similar mathematical methods have been previously used to study neuronal distribution in the cortex in epilepsy (13). The area of each polygon was measured, and the coefficient of variation (CV) of polygon areas over all the fields was calculated. It has been shown that for an even distribution of cells, the CV is less than 0.33, and for a clustered distribution, the CV is greater than 0.64 (14). Identical measurements were carried out on 12 TLS and an equivalent number of non-TLS cases. The method was repeated in 6 TLS cases with near identical results.
Clinical and Neuroimaging Correlations
The clinical records of each TLS and non-TLS patient were reviewed and compared with the pathological findings. The blinded review of the original magnetic resonance imaging (MRI) scans (obtained on a 3-T GE scanner in 12 patients and a 1.5-T GE scanner in the remaining patients) was performed by specialists in neuroimaging for the presence of temporal lobe atrophy, altered T2 signal, and blurring of the gray-white matter interface, as described in previous studies (3-6). Mann-Whitney test for nonparametric data was used for statistical analysis (SPSS version 16) and comparison of clinical and pathological data between TLS and non-TLS cases.
Results
We identified 30 cases from 272 temporal lobe resections (11%) with the features of TLS based on previous descriptions (1, 10). Diagnostic criteria for TLS were then formulated based on the observations as detailed below and summarized in Table 1. In the remaining reviewed cases, the temporal lobe neocortex showed an ordered, preserved hexalaminar architecture with no definite neuronal loss and only varying degrees of cortical gliosis. Diffuse gliosis of the temporal lobe white matter and variable degrees of perivascular white matter atrophy were common findings in lobectomies for HS; these features were not included in the criteria for TLS (Table 1).
TABLE 1.
Neuropathological Criteria for Temporal Lobe Sclerosis
TABLE 1.
Neuropathological Criteria for Temporal Lobe Sclerosis
Neuropathological Features of TLS
In the 30 cases with TLS, NeuN staining revealed marked neuronal depletion from the deeper part of cortical layer II extending to the upper part of layer III in 12 cases (Figs. 1D, 2B, C, G). More moderate degrees of superficial neuronal loss were present in the remaining cases (Figs. 2D, H). In cases with milder neuronal loss, this was more readily appreciated on NeuN than the hematoxylin and eosin stain (Figs. 2H, I). The extent of involvement within the temporal lobe in the coronal axis varied from patchy changes involving 1 gyrus (10 cases) to extensive involvement throughout the temporal lobe resection (6 cases). In the rostrocaudal axis in 12 cases, there seemed to be a gradient of the sclerosis with more severe involvement of the anterior temporal pole compared with the caudal lobe (Table 1). In half of the cases, the extent of neuronal loss varied between the gyrus and sulcus (Fig. 2C), either with greater neuronal loss in the sulcus or more rarely, greater loss on the gyral crests.
FIGURE 2.
Distribution and severity of neuronal loss in temporal lobe sclerosis (TLS). (A) Gyrus from the crown to the depth in a case with hippocampal sclerosis (HS) but without TLS showing preservation of normal hexalaminar architecture. (B) A gyrus from a case with severe TLS showing a laminar band of neuronal loss in layers II and III (arrowhead) and apparent hypercellularity and crowding of neurons in the outer part of layer II at the interface with layer I, which forms a dense cell band (arrow). (C) TLS case showing gyral to sulcal variation of neuron loss in layer II. In addition to the overlying cell crowding of layer II (arrow) in this region, there is more apparent loss in the depth of the gyrus (arrowhead); the loss extends half way up the gyrus. (D) Mild TLS case showing crowding and clustering of cells in layer II through the gyral depth (arrow); neuronal loss is not perceivable at this magnification. (E) Control normal section from an adult male patient with no history of temporal lobe epilepsy for comparison to epilepsy cases. The temporal lobe was removed for the management of acutely increased intracranial pressure after head trauma. The laminar architecture in layers I to III is normal with no evidence of neuronal loss or dysplasia. (F) Cortical layers I to IV from a patient with HS and temporal lobe epilepsy but without evidence of TLS. (G) Severe TLS case with marked reduction in neurons from layers II and III compared with E and F. (H) Case with less marked TLS compared with panel G. Patchy and incomplete depletion of neurons from layers II and III is appreciated in a NeuN-stained section. (I) Same region as panel G on corresponding hematoxylin and eosin-stained section demonstrates than neuronal loss is much less easily detectable in comparison to NeuN staining. All cases are stained for NeuN except I. Bars: (A-D) 1,000 μm; (E-I) 100 μm.
The remaining neurons in the upper part of layer II appeared aggregated in clusters or groups, imparting an apparent hypercellularity (Figs. 2B-D, G, 3A, E); increased intensity of immunostaining of residual layer II neurons was noted in NeuN-stained sections. There was also a qualitative impression of enlarged cell size, more fusiform morphology, and horizontal alignment of neurons within the superficial cortex (Fig. 3A). These cytologic features were present to variable degrees in all cases, but they fell short of the dysmorphic and hypertrophic neuronal changes that characterize FCD type II (15). Moreover, abnormal patterns of neurofilament labeling were not present in any subject. In most cases, abnormal horizontal myelinated fibers were present in the vicinity of the cortical clusters in layer II, as previously described (10).
FIGURE 3.
Layer II dysplasia in temporal lobe sclerosis (TLS) cases. (A) A horizontal alignment, clustering, and malorientation of residual neurones at the interface of layers I and II are appreciated in a NeuN-immunostained section. (B) Glial fibrillary acidic protein (GFAP)-immunostained section highlights a “tramline” pattern of gliosis at low power involving mainly layers II and III. (C) GFAP immunostaining in layers I to III of a non-TLS case showing gliosis mainly in layer I and Chaslin's subpial band with delicate radial fibers extending toward the deeper cortical layers. (D) A TLS case shows a dense band of cellular and fibrillary gliosis in layers II and III in addition to superficial subpial gliosis. (E) TLS case with a region of marked neuron loss from deeper layer II extending into layer III with some clustering of remaining neurons in outer layer II at the interface with layer I as seen on NeuN immunostaining. (F) The corresponding section to E stained with calretinin immunohistochemistry shows a normal distribution and preservation of immunopositive interneurons. (G) A calretinin-stained section of postmortem tissue from the inferior temporal lobe gyrus of a normal adult control shows comparable distribution, morphology, and number of calretinin-immunostained cells in the superficial cortical layers as in the TLS case in F. (H) Focal cortical dysplasia IIB case for comparison to TLS/layer II dysplasia cases demonstrating hypercellularity of layer II but with enlarged hypertrophic and dysmorphic neurons through the cortex, including layer II. Bars: (A) 20 μm; (B) 1,000 μm; (in all other figures) 100 um.
The laminar neuronal depletion was accompanied by a band of laminar gliosis in layers II and III as visualized with GFAP immunostaining corresponding to the region of laminar neuronal loss (Figs. 3B, D). This gliosis pattern was not observed in the less affected areas in the TLS cases or in any controls in which GFAP expression was restricted to Chaslin's subpial band (cortical layer I), often with extension of well-defined radially aligned processes into the deeper cortical layers (Fig. 3C).
Calretinin-positive neurons appeared preserved with normal morphologies, laminar distribution, and no evidence of clustering in layer II even in areas with abnormal NeuN immunostaining (Figs. 3E-G). In a few TLS cases, occasional calretinin-positive bipolar neurons showed a horizontal rather than vertical alignment in the cortex. Parvalbumin immunolabeling showed normal cell morphology and laminar organization in TLS (not shown), and immunolabeling for glutamic acid decarboxylase showed normal cortical patterns in all TLS cases with no reduction of staining in regions of NeuN-determined neuronal loss.
Patterns of HS in the TLS cases are indicated in Table 2. There were more TLS than non-TLS cases that showed severe hippocampal neuronal loss (total HS, neuronal loss from CA2, and dentate gyrus in addition to CA1 and CA4; 11/30 vs 2/30). Classical HS (neuronal loss mainly restricted to CA4 and CA1) and atypical HS patterns were not different between the groups.
TABLE 2.
Summary of Hippocampal Pathology and Clinical Data
TABLE 2.
Summary of Hippocampal Pathology and Clinical Data
Quantitative Analysis of Cortical NeuN Staining
Field fraction analysis was carried out in 16 randomly selected TLS and 16 control cases in similar anatomical regions. This confirmed a variable percentage reduction in cortical NeuN staining in the middle temporal gyrus, ranging from 0 to 44% (mean, 19% reduction) in TLS cases compared with controls. This supported the qualitative impression that neuronal loss extended to this gyrus in some of the TLS cases.
Quantitative Analysis of Neuronal Distribution in Layer II
The Voronoi tessellation method was carried out in 12 randomly selected TLS and non-TLS cases as an objective measurement of abnormal neuronal clustering in layer II (Table 3; Fig. 4). The mean CV of polygon areas was 0.51 in TLS cases and 0.45 in non-TLS cases (p < 0.001). These data indicate a more clustered distribution of neurons in TLS cases.
TABLE 3.
Results of Measurements of Voronoi Polygon Tessellations for Distribution of Neurons in Layer II
TABLE 3.
Results of Measurements of Voronoi Polygon Tessellations for Distribution of Neurons in Layer II
FIGURE 4.
Voronoi tessellation measurements. NeuN fields from cortical layer II in temporal lobes (A, C, E) with corresponding Voronoi polygon diagrams for the 3 fields (B, D, F). (A, B) is a non-temporal lobe sclerosis (TLS) case with a relatively even distribution of polygon sizes. C and D as well as E and F are TLS cases that demonstrate clustering on NeuN staining and correspondingly greater variability in polygon sizes. Bar = 10 μm.
Clinical Correlations
The clinical data for the 60 cases (30 TLS; 30 non-TLS) are presented in Table 2. Although the median age at the time of initial precipitating injury (IPI) and onset of frequent seizures was younger in the group with TLS, this difference was not significant. The durations of frequent epilepsy were similar in the 2 groups. Seventy-three percent of TLS patients had a history of febrile seizures as an IPI, which was more frequent than in the non-TLS group (p = 0.005; Table 2). There was no difference in the frequency of the frequent complex partial seizures or the occurrence of generalized tonic-clonic seizures, or of episodes of status epilepticus between the 2 groups. The seizure outcomes after anterior temporal lobe resection were also similar in the 2 groups, with 73% of TLS and non-TLS patients being International League Against Epilepsy class 1 (seizure-free) at 2-year follow-up (16).
Correlations With Preoperative Neuroimaging
Blinded review of preoperative MRI revealed 1 patient in the TLS group with diffuse ipsilateral atrophy involving the temporal lobe. There was no atrophy, blurring of gray-white matter interface, or other temporal neocortical changes in any of the other cases in either TLS or non-TLS groups.
Discussion
Atrophy of the temporal neocortex in patients with TLE was recognized in the earliest surgical series from the 1950s (1, 17). Based on these early reports and our current findings, we have designated the term “temporal lobe sclerosis (TLS)” and outlined the diagnostic criteria for this pathology in a large series of patients with HS and TLE. Temporal lobe sclerosis was identified in 11% of surgically treated patients in our series, indicating that this pathology is relatively frequent. Mesial TLS was introduced as a term to encompass sclerosis extending beyond the hippocampus (HS) to involve adjacent medial structures, including the amygdala. We consider that TLS involving the lateral temporal lobe represents part of the spectrum or extension of this process of “mesial temporal lobe sclerosis,” rather than a distinct separate or “dual” pathology.
The hallmark pathological features of TLS in all of our cases are bare areas of neuronal loss in the deeper parts of layer II and upper parts of layer III, as described and illustrated in early studies (17). This is accompanied by laminar gliosis in this region, suggesting that this likely represents an acquired, nondevelopmental process. The pathology, particularly in more subtle cases, was best appreciated with GFAP and NeuN immunostains. Temporal lobe sclerosis is reminiscent of the laminar atrophy observed at postmortem examination in patients after status epilepticus in whom neuronal loss and gliosis involving “middle cortical layers” (18) or, occasionally, layers V and VI (19) is observed. In experimental models of TLE, such as the pilocarpine model, neuronal loss is also apparent in layers II/III and IV (20). Neocortical atrophy was a relatively frequent finding in one of the first temporal lobe surgical series of Cavanagh and Meyer (1), present in 17 of 19 cases with HS. In their series, 13 had a history of status epilepticus, which was thought to be important in the causation of both the HS and the TLS. Furthermore, in that series, IPIs were documented, some of which occurred early (1). In the present series, although episodes of status epilepticus were less frequent, IPIs were frequently recorded; the majority (73%) were febrile seizures in early life; 27% of these were prolonged (status epilepticus). Overall, a history of febrile seizure in this series was significantly more frequent in cases with TLS than in cases without TLS. One possibility, therefore, is that the changes of TLS are a manifestation of an enhanced vulnerability of superficial cortical neurons to an early cerebral event in the maturing neocortex of a subset of patients.
Some of the architectural features we observed in cortical layer II of TLS cases suggest a disturbance in normal cortical development or maturation through comparison with described cortical malformations in human and animal models (15, 21-23). Indeed, we and others have previously interpreted these features as a form of cortical dysplasia or microdysgenesis (10, 11); they do not, however, easily fit into current classification schemes for cortical dysplasia (15). Layer II hypercellularity may be observed in more severe cortical malformations, including hemimegalencephaly (24, 25), polymicrogyria (26), FCD II (27) (Fig. 2H), and experimental models of cortical dysplasia (28, 29). Layer II is the last cortical lamina to be formed during development. Maturation of both the hippocampus (30) and the neocortex continues after birth, likely continuing into adolescence (31). It is conceivable that an early IPI or seizure event could interfere with such a process of late cortical development and maturation, resulting in layer II architectural disorganization in addition to neuronal injury, TLS, and HS. In our experience and based on this series, this type of layer II architectural disorganization is always observed in the setting of TLS and HS and not in isolation. We also noted that numbers, overall orientation, morphology, and distribution of inhibitory interneurons, including calretinin-positive cells that predominate in the upper cortical layers, seem to be preserved in this condition, as noted in previous studies (10, 11). This suggests that within the superficial cortical layers, specific subsets of late, radially migrating neurons are preferentially affected. As such, we consider the layer II architectural abnormalities as “acquired” or reorganizational dysplasia or cortical dysmaturation that is entwined with the process of TLS/HS and distinct from other FCD types. We therefore advocate the routine use of NeuN for the assessment of such cytoarchitectural abnormalities in the temporal lobe, for the identification of TLS and its distinction from other cortical dysplasia types.
We noted variability in the distribution of specific features among TLS cases. One common pattern showed an anterior-posterior gradient, the temporal pole being more severely affected. In previous MRI studies, temporopolar or anterior temporal lobe abnormalities in HS, including volume loss, increased T2 signal, and loss of demarcation between the gray and white matter in this region, have been identified (3-6). The pathological correlates of these changes remain to be clearly characterized, but an abnormality of white matter myelination has been suggested (3,5,6). In vivo qualitative MRI identified temporal lobar abnormalities in only 1 patient in the present series, suggesting that the cortical atrophy associated with TLS may be subtle and require quantitative MRI for detection. For example, a previous subregional quantitative MRI study in HS demonstrated the greatest volume reduction to involve the ipsilateral temporal pole compared with other temporal gyri (32). We have, however, recently been unable to detect any temporal lobe changes in individual patients in a subgroup of the TLS patients included in the current study using voxel-based-morphometry and statistical parametric mapping (33).
Electroencephalographic studies using depth electrodes have also supported a pivotal role for the temporopolar cortex in TLE with preceding or concurrent discharges with those from the hippocampus arising from this site (34). Preferential involvement of temporopolar structures in TLS/HS may relate to known connections between the temporal pole and the hippocampus (35); these pathways have also been implicated in extratemporal cortical abnormalities in HS as seen on MRI (8). The temporopolar cortex has a distinct cytoarchitecture corresponding to Brodmann area 38 and has connections with the perirhinal cortex, entorhinal cortex, and parahippocampal gyrus (36) and amygdala, which are also regions known to show variable sclerosis in patients with HS/mesial temporal sclerosis (37, 38). In the present series, there were limited samples of amygdala and parahippocampal gyral tissue available for any assessment of associated pathology of these regions. The hippocampus, however, more often showed severe degrees of sclerosis in TLS patients that could indicate greater susceptibility to neuronal injury in this subset. We also noted sulcogyral variations in the extent of neuronal loss in some cases, as also noted in early reports (17). This pattern was initially interpreted as compatible with “anoxia” and comparisons drawn with the ulegyric distribution of cortical infarction seen in neonates. This may indicate common pathogenic pathways for ulegyria and TLS as a result of an early insult. An alternative hypothesis is that it reflects chemoarchitectonic patterns, for example, the sulcal-gyral variation in _N_-methyl-D-aspartic acid receptors on layer II pyramidal neurons that we have previously shown (39), or regional variations in vulnerability to neuronal injury.
In addition to anatomical distribution within the temporal lobe, the severity of TLS may also be variable; therefore, it is conceivable that subtle cases of TLS with mild focal neuronal loss are overlooked. Quantitative methods (as used in the present study) may prove to be more sensitive and objective methods for the routine detection of the abnormal cytoarchitecture in subtle TLS, which could then also be more precisely localized and correlated with preoperative quantitative neuroimaging (40). Here, we selected the middle temporal gyrus 1 cm caudal to the pole as a comparable anatomical area for measurement between cases and controls, as in previous studies (40). Because this region was not always affected by TLS, some cases failed to show a reduction with field fraction measurements. Therefore, targeting these quantitative measures to the cortical region of maximal perceived neuronal loss by qualitative inspection (with comparison to control values for that region) may be more appropriate for assessment of individual cases. At the other end of the spectrum, some cases showed severe and extensive neuronal loss throughout the temporal lobe. It is conceivable that this process of laminar sclerosis may extend to extratemporal regions, and this would be relevant to continuation of seizures after anterior temporal lobe resection. In the present series, however, there was no suggestion of poorer postsurgical outcome in patients with TLS, with 73% being seizure-free at 2 years.
There are several limitations to this pathological study. Although we reviewed a large series of cases, the number of TLS cases identified is relatively small. In addition, it is possible that episodes of status epilepticus and IPI were inaccurately or underreported. As with all quantitative and immunohistochemical studies, there is an assumption that fixation, processing, and staining methods are comparable between cases; wherever possible, we have aimed to standardize these steps.
In summary, we report the neuropathological features of TLS in 11% of patients with TLE and HS. We consider this to represent part of the spectrum of mesial temporal sclerosis rather than a second or dual pathology. The associated layer II dysplastic features are likely to be reorganizational changes as part of the process of TLS rather than a primary or separate cortical malformation. In this series, childhood febrile seizures were a frequent event that could be of etiological significance. The presence of TLS does not seem to influence outcome after surgery, but these initial clinical, radiological, and pathological correlations could be addressed in future larger series of patients.
Acknowledgments
We are most grateful to John Stevens, Neuroradiologist at the Chalfont Centre for Epilepsy, for reviewing the radiology, to James Francis at Media Cybernetics for devising the Image Pro macro to measure neuronal clustering, and to Robert Courtney and Jane de Tisi. We are grateful to Waney Squier, Neuropathologist at the John Radcliffe Hospital, for her help.
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Author notes
We acknowledge the financial support of the Wellcome Trust (066185) and Medical Research Council grant G79059.
This work was undertaken at University College London Hospital/University College London, which received a proportion of funding from the Department of Health's National Institute for Health Research Biomedical Research Centres funding scheme.
L.O.C. is supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brazil. The authors declare no conflicts of interest.
Copyright © 2009 by the American Association of Neuropathologists, Inc.