Extrahippocampal gray matter loss and hippocampal deafferentation in patients with temporal lobe epilepsy

Hippocampal sclerosis (HS) is the most common histological abnormality observed in patients with medial temporal lobe epilepsy (MTLE) (Margerison & Corsellis, 1966). It usually represents marked cell loss in the hippocampal regions CA1 and hilus, with less intense loss in the end folium (CA3/4) and relative sparing of CA2 (Blumcke et al., 2002). Magnetic resonance imaging (MRI) can identify in vivo signs that are reliably associated with the neuronal loss encountered in HS (Jack et al., 1990; Cendes et al., 1993). In particular, hippocampal atrophy or loss of hippocampal internal structure on T1-weighted images and increased hippocampal T2 signal are consistently associated with HS (Cendes et al., 1993). More refined MRI analyses have advanced the understanding of the pathological mechanisms underlying MTLE. Postprocessing computerized analyses of brain MRI demonstrate that patients with MTLE exhibit subtle brain abnormalities outside the hippocampus that are not fully appreciated by qualitative visual inspection of diagnostic images. Careful manual and automated morphometrical studies showed that patients with MTLE show significant extrahippocampal atrophy that involves the temporal lobe and extratemporal brain structures (Keller et al., 2002; Bernasconi et al., 2003; Bonilha et al., 2003; Bernasconi et al., 2004; Bonilha et al., 2004; McDonald et al., 2008). Notably, the distribution of brain atrophy in MTLE preferentially affects a network of regions that are functionally and anatomically connected to the hippocampus (Spencer, 2002; Bonilha et al., 2005; Keller & Roberts, 2008; Riederer et al., 2008). Morphometrical MRI studies of patients with epilepsy have also demonstrated that extrahippocampal gray matter loss likely follows a progressive course (Bonilha et al., 2006). Cognitive deficits commonly exhibited by patients with MTLE, in particular memory impairment, are directly related to the degree of medial temporal and frontal lobe atrophy (Alessio et al., 2006; Bonilha et al., 2007; Focke et al., 2008). Even though hippocampal and extrahippocampal gray matter atrophy are directly related to seizure control and cognitive performance, the mechanisms underlying brain damage in patients with MTLE remain largely unknown. One theory postulates that seizure toxicity plays a major role in the distribution of damage (Spencer, 2002; Sutula et al., 2003; Riederer et al., 2008). However, this is still controversial and a recent study suggested that seizure control and gray matter atrophy may be unrelated (Liu et al., 2003), and it is unknown what the main reasons leading to damage are. Another complementary theory suggests that loss of hippocampal connections can lead to remote deafferentation and thereby neuronal damage, particularly since structures directly connected to the hippocampus, such as the thalamus, typically exhibit a large degree of atrophy (Bonilha et al., 2005). This theory suggests that the impoverishment of hippocampal connections can lead to reduction in the complexity of remotely located neocortical circuitry. Interestingly, remote neuronal damage related to seizure spread (but not to deafferentation) may depend on the integrity of hippocampal connections, as this is the route for seizure travel. Therefore, it is possible that the two mechanisms, i.e., direct toxicity from seizure spread and deafferentation may contribute to extrahippocampal damage. The extent to which each mechanism contributes to the overall neuronal loss in patients with MTLE is largely unknown. It is also unclear whether both processes interact and result in the overall pattern of damage. This study aimed to investigate the relationship between the loss of hippocampal fibers and extrahippocampal gray matter loss in patients with MTLE. In order to comprehensively evaluate to pattern of loss of connectivity from the hippocampus, this study employed a combination of automated voxel-based morphometrical analyses, diffusion tensor imaging (DTI) and hippocampal probabilistic tractography. DTI assesses the diffusion properties of water molecules and provides indirect information regarding white matter fibers. Traditional investigation of DTI data comprises the voxel by voxel assessment of mean diffusivity (MD) and fractional anisotropy (FA), which can therefore provide information regarding the location and extent of disconnection between brain structures in association with disease. DTI studies involving the assessment of FA and MD have been successfully used in patients with MTLE. For instance, Focke et al. (2008) recently confirmed that patients with MTLE exhibit reduction in FA and increase in MD, changes traditionally considered compatible with neuronal loss, in the ipsilateral temporal lobe of patients with MTLE. These results suggest loss of connections from the atrophied hippocampi. DTI data can also be employed in the probabilistic reconstruction of white matter tracts, which can be quantified and also analyzed on a voxel-by-voxel manner. This technique, termed probabilistic tractography, provides information about specific tracts as opposed voxel-wise MD and FA analyses, which provide information on the integrity of fibers within a region, not taking into account the direction of information travel. Probabilistic tractography has not been extensively investigated in MTLE. This manuscript employed a cross-sectional study aiming to investigate the relationship between hippocampal connection loss and gray matter atrophy. We aimed to confirm that: (1) patients with MTLE would exhibit significant gray matter atrophy involving extrahippocampal structures, particularly affecting limbic regions, and (2) patients with MTLE would also exhibit loss in white matter integrity and reduction of fibers in the hippocampal and limbic pathways. Finally, we hypothesized that patients with MTLE would show a relationship between gray matter atrophy and loss of hippocampal connections, indicating a deafferentation mechanism underlying part of extrahippocampal atrophy.