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  • Updated 06.01.2021
  • Released 02.21.2007
  • Expires For CME 06.01.2024

Hippocampal and parahippocampal seizures

Introduction

Overview

Among the focal epilepsies, mesial temporal lobe epilepsy associated with hippocampal sclerosis (MTLE-HS) and characterized by seizures originating in the hippocampal formation is the most common. Data accumulated from surgical series strongly suggest that MTLE-HS represents a distinctive epileptic syndrome. Early diagnosis of this epilepsy syndrome is important because disabling seizures and their consequences can be prevented by surgical treatment, either by resection or ablation. Resection has a well-established seizure-free rate of 70% to 90% of patients and low complication rates at epilepsy surgery centers. Ablation is a newer technique that does not have as well-established seizure-free or complication rates. The correct identification of hippocampal and parahippocampal seizures is often noninvasive, but intracranial EEG recording is sometimes required. However, the diagnosis of MTLE-HS or associated with other mesial temporal lesions can be achieved without invasive methods based on seizure history, clinical presentation, progressive nature, intractability, and special features in electrophysiological, neuropsychological, structural, and functional imaging. Differentiation from other temporal lobe epilepsy types and evaluation of the memory is important if resection of the hippocampal formation is considered. Medial is alternatively used for mesial in this situation.

Key points

• Hippocampal and parahippocampal seizures are common manifestations of mesial temporal lobe epilepsy.

• Hippocampal and parahippocampal seizures have characteristic features, including auras with paramnestic features and impaired awareness without convulsive motor features.

• Hippocampal and parahippocampal seizures are associated with localized structural and functional abnormalities.

Historical note and terminology

In 1880, John Hughlings Jackson described the symptomatology of "dreamy states," and in 1889, he described other variants of “uncinate fits" as "a particular variety of epilepsy," which subsequently were called "psychomotor seizures" by Gibbs and colleagues and "temporal lobe seizures" by Jasper and Kershman (100; 145). Although Anderson and then Jackson and Beevor had noted the association of temporal lobe tumors with olfactory hallucinations and dreamy states (04; 139), it was the postmortem finding of a small cystic lesion restricted to the uncinate gyrus in a patient who had suffered from seizures with dreamy states, elaborated automatisms, and amnesia (140) that led Jackson and Stewart to the concept of "uncinate fits" with "origin of the discharge... in a region of which this gyrus [ie, the gyrus uncinatus] is part..." (141).

Jackson and Stewart described masterly the most characteristic symptomatology of the mesial temporal lobe seizures (141). Insights into temporal lobe function with description of the nowadays classical aura-types, automatisms (mainly of the oroalimentary type and including deambulation), and amnesia were fostered by results of stimulation and ablation in monkey experiments reported by Ferrier (Ferrier 1876). However, the verification of the tight relation in humans of certain temporal lobe structures, specifically the anterior mesial ones, with the observed and described variety of symptoms and signs did not happen until the era of epilepsy surgery, with reproduction of these symptoms by electrical brain stimulation as well as electroencephalographic, pathological, anatomical, and physiological studies (243; 232; 245; 90; 186; 240; 241; 242; 30; Feindel and 241; 241; 248; 13; 65; 302; 01; 102; 144; 174; 88; 62; 104; 110; 14; 257; 190; 228; 269; 346; 191; 323).

Based on a stereoelectroencephalographic study on the electroclinical features of psychomotor seizures, Wieser proposed 5 localization-related subtypes: the temporobasal limbic type, the temporal pole type, the frontobasal-cingulate type, the opercular (insular) type, and the posterior neocortical temporal type (323).

Bouchet and Cazauvieilh described the pathology of mesial temporal lobe epilepsy as early as 1825 and presented autopsy data with abnormalities in the Ammon's horn in 7 out of 18 patients who suffered from epilepsy and psychiatric symptoms ("mental alienation seizures“) (Bouchet and Cazauvieilh 1825; 37). At that time, the abnormalities were believed to be an effect, rather than a cause, of epilepsy. Jackson recognized limbic-type seizures and associated them with lesions in mesial temporal structures but not with Ammon’s horn sclerosis (Jackson 1931-1932). Sommer and Bratz both suggested that Ammon’s horn sclerosis might be an epileptogenic lesion (282; 38), and subsequent evidence has accumulated suggesting that distinctive structural damage, ie, hippocampal sclerosis, is typical for temporal lobe epilepsy. This pathology was accurately described and illustrated by Bratz, with destruction of the pyramidal cells in Sommer’s sector (currently termed, sector CA1), preservation of the cells in the neighboring subiculum, and cell loss in the hilus of the dentate gyrus and adjacent sector CA3 but preservation of neurons in sector CA2 and of the dentate granule cells (38). Granule cell dispersion is a common finding in hippocampal sclerosis in patients with intractable focal epilepsy. It is considered to be an acquired, post-developmental rather than a pre-existing abnormality, involving dispersion of either mature or newborn neurons, but the precise factors regulating it and its relationship to seizures are unknown. Thom and colleagues presented 2 cases of granule cell dispersion with associated CD34-immunopositive balloon cells, a cell phenotype associated with focal cortical dysplasia type IIB, considered to be a developmental cortical lesion promoting epilepsy (308).

The ILAE Commission of Classification and Terminology 2005-2009 revised concepts, terminology, and approaches for classifying seizures and forms of epilepsy (23) without making changes in epilepsy syndromes already recognized and updated in the 2006 classification (80). The Commission’s work was aided by the Montreal workshop (45). In the new classification, the concept of generalized and focal with regard to seizures was redefined. Generalized seizures were defined as seizures occurring in and rapidly engaging bilaterally distributed networks, whereas focal seizures emerge within either discretely localized or more widely distributed networks that are limited to 1 hemisphere. Function MRI connectivity studies have provided evidence for this more distributed abnormality in mesial temporal lobe epilepsy with hippocampal seizures (212; 117). The new scheme includes lists of generally agreed-upon epileptic seizure types and epilepsy syndromes. The description of focal seizures in the new classification adopts the main principals of the diagnostic scheme of the Task Force approved in Buenos Aires in 2001 and is based on the ictal phenomena, which has been drawn from the glossary of ictal semiology (35) and is revised from the original 1981 classification. The Cleveland group proposed a somewhat similar seizure classification based exclusively on ictal semiology and conceptually concentrated on the so-called symptomatogenic zone (185). The symptomatogenic zone is 1 of 5 zones that must be determined to define the "epileptic focus." The epileptogenic zone, on the other hand, is more closely related to the epileptic syndrome. None of the classification revisions so far have negated the 1981 classification of epileptic seizures.

The focal onset (partial) seizures as before were classified according to the degree of impairment of the consciousness during seizures:

Without impairment of consciousness or awareness

With observable motor or autonomic components. This roughly corresponds to the concept of “simple partial seizure.” “Focal motor” and “autonomic” are terms that may adequately convey this concept depending on the seizure manifestations.

Involving subjective sensory or psychic phenomena only. This corresponds to the concept of an aura, a term endorsed in the 2001 Glossary.

With impairment of consciousness or awareness. This roughly corresponds to the concept of complex partial seizure. ‘‘Dyscognitive’’ is a term that has been proposed for this concept (35).

Evolving to a bilateral, convulsive seizure (involving tonic, clonic, or tonic and clonic components). This expression replaces the term “secondarily generalized seizure.”

In the new classification, the distinction of complex and simple partial seizures is eliminated. Nevertheless, the Commission recognized that such distinction based on dyscognitive features can be further used for specific purposes in individual patients. Dyscognitive seizures with or without automatisms were not exactly synonymous with the term "complex partial seizures," which were defined on the basis of impaired consciousness only and do not necessarily involve limbic areas. Dyscognitive seizures with or without automatisms, as well as the term "psychomotor," conforms more to the original intent of the term "complex partial seizures" in the 1970 ILAE Classification of Epileptic Seizures. It was implied that mesial temporal limbic areas and their immediate connections are involved in the clinical manifestations, although seizures may have been initiated elsewhere.

The ILAE Commission on Classification and Terminology uses the terms “structural,” “genetic,” “infectious,” “metabolic,” “immune,” and “unknown” as the categories of etiological factors of the disease (265). For other than well described distinct electroclinical syndromes with characteristic clinical, electrographic, developmental, and genetic features, the Commission proposed the method of defining other clinical entities based on constellations of the clinical, electrographic, and specific cerebral lesions. The category of these types of clinical entities includes mesial temporal lobe epilepsy, hypothalamic hamartoma with gelastic seizures, epilepsy with hemiconvulsion and hemiplegia, and Rasmussen syndrome (23).

Hippocampal and parahippocampal seizures as seen in association with mesial temporal lobe epilepsy are typically characterized by auras evolving into focal seizures with impaired awareness with initial behavioral arrest (motionless stare), followed by oroalimentary and manual automatisms. However, MTLE-HS probably consists of more than one syndrome, and it is not certain whether features of patients with hippocampal sclerosis clearly differentiate them from those with other mesial temporal lesions. Nevertheless, observation by Heuser and colleagues showed distinct phenotypic tendency in MTLE-HS patients: higher incidence of simple partial seizures and seizures with ictal psychiatric and autonomic manifestations as well as higher incidence of childhood febrile seizures and lower age of onset than temporal lobe epilepsies without hippocampal sclerosis (126). In the current classification, this syndrome is listed under the category “epilepsy syndromes by age of onset and related conditions, with less specific age relationship” (80).

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