The pathological effects of decades of seizures on the brain have been little explored. In this post-mortem series of patients with mainly long histories of drug-resistant epilepsy, we have confirmed age-accelerated tau-protein accumulation. The extent of tau-pathology correlated with clinical cognitive decline although few cases with higher Braak Stages (V/VI), associated with high likelihood of dementia (Braak and Braak, 1997
; Duyckaerts et al., 1997
; Iqbal and Grundke-Iqbal, 2008
) and based on National Institute of Ageing-Reagen Institute (NIA-RI) criteria (1997), were identified in our series. The lack of Braak Stage VI cases in our post-mortem series could reflect the younger age range and higher standard mortality rates in chronic epilepsy (Gaitatzis et al., 2004b
), with premature death occurring before any progression to higher Braak stages.
Alzheimer's disease is a multifactorial and heterogenous condition (Iqbal et al
., 2010) and known risk factors for sporadic Alzheimer's disease include age, genetic factors including ApoE ε4 genotype, and head trauma. The influence of seizures on Alzheimer's disease pathology is unknown. Alzheimer's disease pathology in young adults has been reported in epilepsy surgical tissues (Mackenzie and Miller, 1994
; Gouras et al., 1997
; Geddes et al., 1999
; Sen et al., 2007a
) and dysregulation of cdk5 and its activators, known to regulate tau-phosphorylation (Iqbal and Grundke-Iqbal, 2008
), has been shown in hippocampal sclerosis in epilepsy (Sen et al., 2006
). This raises the possibility that seizures, or their cellular effects, might influence degenerative neuronal pathways or promote a susceptibility to Alzheimer's disease pathology. In this post-mortem series, however, we failed to show a correlation between the maximum frequency of seizures, seizure number, type, age of onset or duration of epilepsy and the extent of neurofibrillary pathology. Although syndrome classification was not possible in all cases, partial and symptomatic epilepsies were more often associated with higher Braak stages than idiopathic epilepsies or those with a known genetic cause. This supports the notion that an underlying structural cortical abnormality, as the cause for epilepsy, is more relevant to promoting Alzheimer's disease pathology, but requires further investigation.
Our study supports a relationship between head injury occurring in patients with epilepsy and progressive neurofibrillary tangle pathology. Our findings invite comparison to the chronic traumatic encephalopathy occurring in sports people following repetitive closed head injuries (Corsellis and Brierley, 1959
; Corsellis, 1989
; McKee et al., 2009
). Chronic traumatic encephalopathy is characterized by accumulation of tau protein particularly in mesial temporal structures, as recently reviewed (McKee et al., 2009
). In chronic traumatic encephalopathy, a preferential localization of neurofibrillary tangles in a perivascular location, sulcal depths or superficial cortex is noted. Astrocytic tangles are prominent in superficial layers, and subpial and periventricular zones, with amyloid-β deposition being a less constant feature (McKee et al., 2009
). In the current epilepsy series, a history of head injury was recorded in nearly half of the patients, although detailed information of the number of trauma episodes per patient was not available. In only three cases was head trauma considered to be a cause of seizures, injury pre-dating the onset of epilepsy. Pathological evidence of traumatic brain injury, mainly in the form of old frontotemporal contusions, was present in 30%. We demonstrated an association between traumatic brain injury and Braak stage. Furthermore, 66% of cases were amyloid-β negative and the presence of amyloid-β did not correlate with traumatic brain injury. In addition, astrocytic tau deposits were noted in a third of cases in our series in a periventricular or subpial location similar to reports in cases with chronic traumatic encephalopathy (McKee et al., 2009
) and their presence was significantly associated with traumatic brain injury. Although morphologically similar to the thorn-shaped astrocytes associated with ageing (>60 years) (Schultz et al., 2004
), in our epilepsy series tau-positive astrocytes were noted as early as 15 years of age. Although the precise relationship between single head injury, cognitive decline and Alzheimer's disease is ambiguous (Breteler et al., 1995
; Jellinger, 2004
), the findings in the present series all underscore the observation that head injury, or more probably repetitive head injury, acquired as a result of drug-resistant epilepsy, is associated with the development of neurofibrillary tangle pathology. In clinical practice, our findings support recommendations that promote prevention of risk of head injury due to seizures.
Cerebrovascular disease was noted in 40% of this series. Epidemiological studies have shown that cerebrovascular disease is more common in chronic epilepsy than control groups (Gaitatzis et al., 2004a
; Hermann et al., 2008b
). Antiepileptic drugs as well as lifestyle factors in epilepsy patients may have adverse effects on cerebral vasculature (Hermann et al., 2008b
). We demonstrated an association between cerebrovascular disease and Braak stage, which became less significant when the patient's age of death was considered. In contrast, traumatic brain injury, which was not significantly associated with patient's age, remained highly associated with Braak stage following multivariate statistical analysis.
The hippocampus is affected early in both Alzheimer's disease and chronic traumatic encephalopathy. Alzheimer’s disease is associated with an increased incidence of unprovoked seizures (Palop and Mucke, 2009
), which are reported to develop late in the course of the disease (Mendez et al., 1994
). In this series, we were careful not to include patients in whom a neurodegenerative illness, in particular Alzheimer's disease, was the primary diagnosis with secondary symptomatic seizures. Sclerosis of the hippocampus is one of the most common and well-characterized pathologies identified in both post-mortem (Corsellis, 1957
; Margerison and Corsellis, 1966
; Meencke et al., 1996
) and surgical series of patients with epilepsy, particularly temporal lobe epilepsy (Bruton, 1988
; Blumcke, 2009
). In surgical series, hippocampal sclerosis is typically observed in young adulthood, in the context of refractory seizures with sclerosis visible on MRI and confirmed in resected specimens (Wieser, 2004
). The neuronal loss is centred on CA1 with more variable loss in other subfields and is accompanied by mossy fibre pathway reorganization (Houser et al., 1990
). Hippocampal sclerosis pathology may also arise in the elderly due to heterogeneous causes including anoxic-ischaemic injury and varied neurodegenerative conditions and is associated with slowly progressive amnesia and dementia without seizures (Probst et al., 2007
; Zarow et al., 2008
). The prevalence of hippocampal sclerosis in non-epilepsy elderly post-mortem series is ~16% (Dickson et al., 1994
) and is bilateral in 50% of these (Zarow et al., 2008
). The pattern of neuronal loss typically involves both CA1 and the subiculum.
Our post-mortem series represents patients with varied epilepsy syndromes and without systematic or serial MRI examination in the majority so that we cannot confirm the time course for the development of hippocampal sclerosis. The mean age of onset of epilepsy was 10.2 years overall (and 7 years in cases with hippocampal sclerosis). The pattern of hippocampal neuronal loss was characterized by sparing of subicular neurons with associated mossy fibre sprouting in the dentate gyrus, typical of hippocampal sclerosis in epilepsy (Thom et al., 2009
). In addition, the paucity of AT8 labelling in the region of sclerosis including threads, ghost tangles or astrocytes supports the view that the sclerosis predated the tau accumulation. All these features are evidence that favours an epilepsy-associated, rather than Alzheimer's disease-associated, pathogenesis of hippocampal sclerosis in our series. We identified hippocampal sclerosis in 40% overall, bilateral in 48%, which is comparable to a previous series of 650 post-mortem epilepsy cases that documented hippocampal sclerosis in 30.5%, with bilaterality in 56% (Meencke et al., 1996
). Hippocampal sclerosis patterns reflected those reported in surgical temporal lobe epilepsy series (Bruton, 1988
; Blumcke et al., 2007
; Thom et al., 2010a
), albeit with greater representation of atypical patterns as previously noted (Thom et al., 2009
). There was no association between the presence or pattern of hippocampal sclerosis and the Braak stage.
Tau accumulation in both Alzheimer's disease and normal ageing progresses through the hippocampus in a stereotypical sequential and hierarchical fashion (Braak et al., 2006
; Duyckaerts et al., 2009
; Frost et al., 2009
), which may reflect anterograde propagation along neuroanatomical pathways (Duyckaerts et al., 2009
; Lace et al., 2009
). Six distinct stages of hippocampal involvement have been proposed that correspond with known connections between the hippocampus proper and the entorhinal cortex (Lace et al., 2009
). Tau accumulation commences in CA1/subicular border, then to the outer followed by inner molecular layer of the dentate gyrus, next CA1 and CA2 and lastly CA4/3. In the present series, six patients with unilateral hippocampal sclerosis, reorganization of the mossy fibre pathway as confirmed with dynorphin immunohistochemistry (Houser et al., 1990
) and higher Braak scores (III–V) showed asymmetry of AT8 staining in hippocampal subfields. Reduced AT8 staining on the sclerotic side was apparent in CA1 with the accumulation in other subfields paralleling that on the preserved side, albeit reduced. Known hippocampal projection regions, including the frontal and temporal polar cortex, appeared symmetrically affected between hemispheres in these cases. In addition, in the non-sclerotic hippocampus, direct comparison between AT8 and dynorphin sections highlighted that neurons in the trajectory of a normal mossy fibre pathway [a component of the indirect hippocampal pathway with major input from the entorhinal cortex (Duvernoy and Cattin, 2005
)], were relatively delayed in tau accumulation compared with adjacent subfields. These findings could argue against the ‘pathway propagation’ theory of tau accumulation but favour intrinsic cellular ‘time-switches’ or other mechanisms for this selective neuronal vulnerability of neurodegeneration.
There is an intriguing convergence between cellular regulatory pathways and factors that determine normal cortical development which, when deregulated, can lead to neuronal degeneration (Bothwell and Giniger, 2000
; Wang and Liu, 2008
; Mattsson et al
., 2010). Cdk5 is one such developmental regulatory protein (Lim and Qi, 2003
), also pivotal in tau hyper-phosphorylation (Iqbal and Grundke-Iqbal, 2008
). Cdk5 has been previously shown to be abnormally expressed in epilepsy-associated developmental pathologies such as focal cortical dysplasia (Sen et al., 2008
). Whether brains harbouring malformations are more vulnerable to superimposed neurodegenerative processes has been little explored. We have previously noted premature neurofibrillary tangle accumulation in the dysmorphic neurons of focal cortical dysplasia in patients with epilepsy (Sen et al., 2007a
). Overall, in the current series the presence of malformation of cortical development was not associated with higher Braak staging. A further study of susceptibility to neurodegenerative processes within the regions of cortical malformation is required.
Our study also highlights that in around half of the cases in the series, cognitive decline was not associated with tau accumulation, suggesting that other factors may play a role in cognitive decline associated with epilepsy. Developmental delay is a feature of some childhood-onset epilepsies although the influence of this on subsequent dementia is uncertain (Helmstaedter and Elger, 2009
), and likely to be both heterogeneous and complex. In our series, a history of learning disability was not associated with a higher Braak stage. Furthermore, widespread cortical volume changes may be detected on MRI (Keller and Roberts, 2008
), which may also relate to cognitive decline (Hermann et al., 2008b
). In a recent small series of patients with epilepsy and hippocampal sclerosis without neurodegenerative disease, gliosis preferentially involved frontal, temporal and orbitofrontal cortices, which we argued also reflected subtle traumatic brain injury (Blanc et al., 2011
). Classifications of epilepsies also recognize the existence of the concept of ‘epileptic encephalopathy’ where cognitive impairment, which may be progressive, is not explained by the underlying pathology at the light microscopic level but may be a manifestation of the seizures (Berg et al., 2010
). This is highlighted in the present series in three patients with Dravet syndrome with cognitive decline in which we have reported an absence of neuropathological or degenerative changes (Catarino et al
This study is limited in that we have, by necessity, studied cases with more severe epilepsy, which will not represent the broader epilepsy population. In addition, there are no genetic data available, in particular ApoE ε4 genotype; there is evidence that effects of head trauma are more severe in patients with this genotype (Friedman et al., 1999
; McKee et al., 2009
). Complete clinical data were not available in a proportion of cases, in particular for seizure frequency and total number of seizures. Clinical investigation patterns have varied over the 20-year-period of this brain collection. In a small number of cases, limited brain sampling at post-mortem did not allow a categorical exclusion of old trauma or vascular disease and such cases were not included in the analysis. For control data, we have used large published post-mortem series from a normal ageing, non-epilepsy population (Braak and Braak, 1997
). This utilized silver methods rather than more sensitive AT8 immunostaining for Alzheimer's disease staging as currently recommended (Braak et al., 2006
). However, in a selection of cases, Gallyas silver staining showed a good correlation with AT8. In addition, as our Braak staging in young adults was very similar to the Braak (2006)
study and we had few Braak V and no Braak VI stages overall, we consider that Braak staging has not been overestimated in the current study.
In conclusion, this study supports the occurrence of accelerated brain ageing in chronic epilepsy although progression to high Braak stages was infrequent, possibly because of higher rates of premature mortality. Traumatic brain injury, rather than seizures themselves, is identified as an associated factor for AT8 accumulation in this series, suggesting that it is important to protect the head in drug-resistant epilepsy causing falls. It is likely that Alzheimer's disease pathology is not the sole explanation for cognitive decline associated with epilepsy, the cause of which requires further investigation.