| Cardiovascular regulation in epilepsy with emphasis on the interictal state | ||
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TLE is a syndrome presented as quite a unique type of epilepsy. Whereas partial epilepsy comprises slightly more than 50% of all seizure types, the relative incidence and prevalence of different partial seizure types have not been adequately determined (Keränen et al. 1988, Hauser 1997, Williamson et al. 1997). However, TLE is considered the most common epileptic syndrome and it is estimated that approximately 80% of patients with partial seizures have temporal lobe epilepsy (Dreifuss 1987, Williamson et al. 1987). Kun and his co-workers interviewed all male citizens in Singapore at 18 years of age and surprisingly discovered TLE as comprising only 16.9% of all epilepsies being the most common syndrome in this population based study (Kun et al. 1999). TLE can be subclassified into mesial temporal lobe epilepsy (MTLE) and lateral temporal neocortical epilepsy. This subclassification is useful, because MTLE comprises the majority of the cases of epilepsy refractory to pharmacotherapy (Babb & Brown 1987). However, it may be remediable to surgery because hippocampal sclerosis can often be seen as an underlying pathology in MTLE (Thadani et al. 1995, Benbadis et al. 1996). In fact, surgical treament may abolish seizures in 80-90% of patients with MTLE (Wieser & Williamson 1993).
In MTLE, the first seizures often begin at early childhood to cessate after the childhood years. At that time the seizures are usually well controlled with AEDs. However, the seizures start again after a few years in the adolescence or early adulthood and sometimes they become more severe and progress eventually to refractory epilepsy. Of these patients, many have had febrile seizures in the early childhood (French et al. 1993, Wieser & Williamson 1993, Mathern et al. 1995). Typical simple partial seizures of the temporal lobe include dejá vu, gustatory, taste or olfactory sensations, panic attacks or sensations of fear or rage. The complex partial seizures consist of a short deterioration of consciousness and oro-alimentary and gestural automatisms. When the seizures become secondary generalized, tonic-clonic convulsions occur. (Engel et al. 1997)
Epilepsy arising from the lateral temporal neocortex cannot always be differentiated from MTLE and overlapping may occur (Burgerman et al. 1995, Walczak 1995). Although exact statistics are not available, a reasonable estimate would be that less than 10% of patients with TLE have seizure origin in the lateral temporal cortex. Moreover, distinguishing seizure characteristics do not exist (Williamson et al. 1997). Sometimes TLE can be difficult to differentiate from psychiatric diseases, since a temporal epileptic seizure may manifest itself even as psychosis. Moreover, TLE, as well as other epilepsies, may be associated with psychiatric co-morbidity (Engel & Taylor 1997).
In 1880, Sommer described an obvious cell loss seen with a microscope in certain area of the hippocampus in patients with seizures originating from the temporal lobe area (Sommer 1880). It has been established that hippocampal damage is the most common pathology underlying TLE (Babb & Brown 1987).
Neuron loss is usually located in the fields H1 and H3 of the hippocampi, and when the neuron loss is restricted to those areas, it is regarded as the classic hippocampal cell loss (Sutula et al. 1989, Babb & Brown 1987, de Lanerolle et al. 1992). However, more wide spread neuron loss is often seen in resected temporal lobes of patients with TLE (Margerison & Corsellis 1966, Bruton 1988). The structures suffering from neuron loss in addition to hippocampi include the amygdala, the uncus of the hippocampus and the parahippocampal gyrus. This form of neuron damage is called mesial temporal lobe sclerosis. (Engel 1992, French et al. 1993, Wieser et al. 1993, Williamson et al. 1993, Thadani et al. 1995)
In hippocampal and mesial temporal lobe sclerosis (MTS) the damaged neurons are replaced by glial cells to form gliosis that shrinks the volume of the hippocampus and other structures leading to atrophia and sclerosis seen as volume loss and signal changes on MRI. The pathologic changes also include mossy fiber sprouting, dentate gyral dispersion or duplication, and a wide range of neurochemical alterations. Epileptogenic human hippocampus is not a nonfunctioning diseased region of the brain. A number of changes in electrophysiologic, biochemical, neurotransmitter and gene regulation occuring in the surviving cells probably contribute to seizure generation in TLE (Mathern et al. 1997).
Whether the hippocampal sclerosis is the “cause” or “consequence” of seizures has been a matter of controversy for over 100 years. Several studies have shown that prolonged febrile and partial seizures, as well as status epilepticus may cause hippocampal damage (Cavanagh & Meyer 1956, Falconer et al. 1964, Margerison & Corsellis 1966, Bruton 1988). The evidence exists that the neuronal reorganization continues with recurrent seizures, and clinical observations on the development of medical intractability of MTLE also suggest an ongoing process (French et al. 1993, Engel et al. 1997). On the other hand, recent studies have shown that, at least in some patients there is an association between an initial precipitating injury (e.g. any significant brain insult) prior to habitual seizure onset and hippocampal sclerosis (Trenerry et al. 1993, Mathern et al. 2002). However, patients with episodes of generalized tonic-clonic status epilepticus and prolonged partial seizure activity may develop progressive hippocampal neuronal loss in a widespread distribution that is dissimilar to classic Ammon´s horn sclerosis (Pedley & Engel 1997). Today it is concluded that hippocampal sclerosis is presumably both the cause and effect of seizures (Bruton 1988, Gloor 1991, Armstrong 1993, Kälviäinen & Salmenperä 2002, Mathern et al. 2002).
Focal cortical dysplasias (e.g. microdysgenesis, heterotopic gray matter and ectopic single neurons), may be encountered in patients with focal epilepsies. These anatomical changes may be detectable in MRI, but for example, microdysgenesis can only be detected histologically. Focal cortical dysplasias may accompany other changes, such as hippocampal sclerosis (dual pathology). (Levesque et al. 1991)
The development of the MRI has without doubt been the most important new diagnostic tool for the evaluation of the individual with epilepsy, and has revolutionized our understanding of the basic mechanisms of epilepsy. MRI-based hippocampal volumetry has been shown to quantitatively indicate the presence of hippocampal volume loss. (Jack et al. 1990, Jack et al. 1992)
Majority of patients with partial epilepsy have temporal lobe seizures (Dreifuss 1987, Williamson et al. 1987), and approximately 90% of patients with nonlesional temporal lobe epilepsy have localization of the ictal onset zone in the amygdala or hippocampus (Spencer et al. 1992, Spencer et al. 1993). There is now a consensus that MRI is a reliable indicator of MTS in patients with TLE (Jack et al. 1990, Jackson et al. 1990, Berkovic et al. 1991, Cascino et al. 1991, Lencz et al. 1992, Cascino et al. 1993, Spencer et al. 1993, Jack et al. 1995). The neuroimaging alterations associated with MTS include hippocampal formation atrophy, an increased mesial temporal signal intensity and loss of hippocampal internal structure (Jackson et al. 1990, Berkovic et al. 1991, Jackson et al. 1994,).
The optimal MRI technique for visualizing hippocampal anatomy and other mesial temporal structures includes a heavily T1-weighted sequence through an oblique-coronal plane (Jack et al. 1992, Jack et al. 1995). In most patients, hippocampal atrophy coexists with a medial temporal signal intensity alteration. The signal alteration can best be appreciated on the T2-weighted image and using the FLAIR sequence. The T1-weighed image, in addition to showing hippocampal atrophy, may also reveal a “black hole” in the hippocampus, representing a region of increased signal (Jackson et al. 1990, Jackson et al. 1994).