Bilateral reorganization of the dentate gyrus in hippocampal sclerosis: a postmortem study

Abstract

Background: Hippocampal sclerosis (HS) is the most common surgical pathology associated with mesial temporal lobe epilepsy (MTLE). HS is typically characterized by mossy fiber sprouting (MFS) and reorganization of neuropeptide Y (NPY) fiber networks in the dentate gyrus. One potential cause of postoperative seizure recurrence following temporal lobe surgery may be the presence of seizure-associated bilateral hippocampal damage. We aimed to investigate patterns of hippocampal abnormalities in a postmortem series as identified by NPY and dynorphin immunohistochemistry.

Methods: Analysis of dentate gyrus fiber reorganization, using dynorphin (to demonstrate MFS) and NPY immunohistochemistry, was carried out in a postmortem epilepsy series of 25 cases (age range 21-96 years). In 9 patients, previously refractory seizures had become well controlled for up to 34 years prior to death.

Results: Bilateral MFS or abnormal NPY patterns were seen in 15 patients including those with bilateral symmetric, asymmetric, and unilateral HS by conventional histologic criteria. MFS and NPY reorganization was present in all classical HS cases, more variably in atypical HS, present in both MTLE and non-MTLE syndromes and with seizure histories of up to 92 years, despite seizure remission in some patients.

Conclusion: Synaptic reorganization in the dentate gyrus may be a bilateral, persistent process in epilepsy. It is unlikely to be sufficient to generate seizures and more likely to represent a seizure-induced phenomenon.

Figure 1 Grading of immunohistochemistry staining in the dentate gyrus for neuropeptide Y and dynorphin (A) grade 1 is equivalent to a normal staining pattern (case 17). (C) Grade 2 NPY confirms an increase in sprouted fibers in the dentate gyrus, in particular the granule cell layer (GCL), inner molecular layer (IML), and outer molecular layer (OML). However, there is still a clear demarcation between the horizontal fiber plexus density in the OML compared to the IML (case 6). (E) Grade 3 NPY shows an increase in fiber plexus, mainly running radially through the granule cell layer and into the IML and OML, obscuring any boundaries between these zones (case 18). (G) Grade 4 is similar to grade 3 but with a more extensive increase in hilar NPY sprouted fibers in addition (case 16). (B) Macroscopic coronal section: a case of unilateral hippocampal sclerosis (left side) at the level of the lateral geniculate nucleus, the approximate level that both hippocampi were sampled in all cases. Dynorphin grading for mossy fiber sprouting in dentate gyrus: (D) grade 1 dynorphin is equivalent to the normal staining pattern with labeling in the hilar region on mossy fiber terminals around neurons but no labeling in the IML (case 25). (F) Grade 2 dynorphin shows focal, patchy staining in the IML in addition to subgranular zone (case 8). (H) Grade 3 shows widespread, diffuse punctuate staining in the IML (case 8). (I) NPY: dynorphin double labeling: confocal image of the dentate gyrus in hippocampal sclerosis with mossy fiber sprouting and NPY fiber reorganization confirming a lack of colocalization between NPY (green) and dynorphin (red) in the molecular layer. Dapi (blue) = granule cell nuclei. Bars in A to D = 10 μm, in E to G = 100 μm, and in H = 20 μm.

Figure 2 Bilateral mossy fiber sprouting and neuropeptide Y changes in dentate gyrus in different HS grades Bilateral mossy fiber sprouting in case 16: (A) right-sided classical hippocampal sclerosis (CHS) with grade 3 dynorphin and (B) left-sided CA1-predominant (CA1p) hippocampal sclerosis with grade 3 dynorphin. Bilateral NPY fiber sprouting in case 1: (C) right-sided CHS with grade 4 NPY pattern and (D) left-sided CHS with grade 4 NPY pattern. Fiber sprouting in the absence of hippocampal sclerosis in case 25: (E) grade 2/3 NPY fiber sprouting with visible hilar neurons and (F) grade 2 dynorphin pattern, both on the right side. Evidence of fiber sprouting with prolonged seizure-free periods in case 12: right-sided end folium sclerosis (EFS) (G) and left-sided CHS (H), both with mossy fiber sprouting on dynorphin staining. Prolonged seizure-free periods prior to death also occurred in cases 16 and 1, as illustrated in A to D (bars = 100 μm).