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2	SUMMARY 1 INTRODUCTION ANATOMY and FUNCTION RADIOLOGICAL ANATOMY MR LESION PATTERNS CONGENITAL ANOMALIES Hippocampal Dysplasia in: -Agenesis of Corpus Callosum -Lissencephaly and Holoprosencephaly Focal Cortical Dysplasia Focal Malformations of Neuronal Migration Hippocampal Sulcus Remnant Cysts of Choroidal Fissure NEUROFIBROMATOSIS TYPE 1 HIPPOCAMPAL SCLEROSIS STATUS EPILEPTICUS
3	SUMMARY 2 TRANSIENT GLOBAL AMNESIA ISCHEMIA and INFARCTION VASCULAR MALFORMATIONS -Cavernous Angiomas -Arteriovenous Malformations MULTIPLE SCLEROSIS PRIMARY BRAIN TUMORS -Fibrillary Astrocytoma -Gangliogliomas -DNET -Anaplastic Astrocytoma and GBM LIMBIC ENCEPHALITIS NEURODEGENERATIVE DISORDERS CONCLUSIONS
4	INTRODUCTION
5	ANATOMY The hippocampus can be divided into three parts, according to its position with respect to the corpus callosum: the precommissural, the supracommissural, and the retrocommissural. The precommissural hippocampus and the supracom-missural hippocampus are vestigial structures and are relatively small; the former is situated in the caudal portion of the subcallosal area, immediately rostral to the septum verum, and the second, made up of indusium griseum, extends along the entire length of the corpus callosum ( Fig. 1).
6	ANATOMY The retrocommissural hippocampus, the most caudal portion of this structure, constitutes part of the internal surface of the temporal lobe. During development, it wraps around itself along the hippocampal fissure, protruding while conforming to the floor of the temporal horn. The hippocampal gyrus continues directly, in the posterior zone of the splenium of the corpus callosum, with the cingulate gyrus. The retrocommissural hippocampus is clearly divided into three structures lying longitudinally: the dentate gyrus, Ammon’s horn, and the subiculum. The most anterior portion of the retrocommissural hippocampus curves dorsally to form a rounded mass known as the uncus .
7	ANATOMY
8	ANATOMY The cortex of the hippocampus (Cornu Ammonis) can be subdivided, according to its thickness, size, and cellular density, into four zones: CA1, CA2, CA3, and CA4 .
9	ANATOMY The hippocampus, the parahippocampal gyrus, and the cingulate gyrus make up the limbic system. Two large bundles of association fibers, the fornix and the cingulum, also participate in both the external and internal arcs of the limbic system .
10	FUNCTION The components of the limbic system, together with the subcortical nuclei to which it is connected, are thought to be involved in the elaboration and expression of emotions and in the pathophysiology of affective disorders, certain dementias, and other neuropsychiatric disorders. Evidence also implicates the hippocampus and the limbic system as crucial to learning and memory, particularly in the formation of long-term memory, as it is directly or indirectly connected to all parts of the brain, especially to the enthorinal cortical area, septal nuclei, hypothalamus, and brainstem. The limbic system, especially the hippocampal struc-tures, fornix, and mammillary bodies, may be crucial in the pathogenesis of many cases of epilepsy.
11	RADIOLOGICAL ANATOMY
12	RADIOLOGICAL ANATOMY
13	CONGENITAL ANOMALIES Congenital anomalies of the hippocampus can be seen in association with agenesis of the corpus callosum , lissencephaly , and occasionally ho-loprosencephaly . The hippocampi are generally small in these disorders and have an abnormal vertical orientation consistent with incomplete inversion of the hippocampus during development; the ipsilateral temporal horn is generally enlarged and elongated vertically as well . Abnormal development of the hippocampus can be seen in up to 73% of cases of agenesis of the corpus callosum.
14	Hippocampus Dysplasia
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19	CONGENITAL ANOMALIES Abnormalities of other portions of the limbic system can also be identified on MR in association with septooptic dysplasia, including dysgenesis of the corpus callosum and hippocampal hypoplasia, as well as an abnormally low position of the fornix.
20	Hippocampal Dysplasia
21	Focal Cortical Dysplasia
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26	Hippocampal Sulcus Remnant
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33	Structural abnormalities of the hippocampus Lesion patterns
34	Structural abnormalities of the hippocampus Lesion patterns
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36	RM y ETM MR findings Hippocampal atrophy T2 high signal intensity Loss of internal structure Histopathology Neuronal loss (CA1, CA3, CA4) Glial proliferation
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40	"MR signal changes (increased..." MR signal changes (increased T2 signal intensity, increase volume of the affected grey matter) have been described in the hippocampus in patients during the periictal phase of status epilepticus . These lesions are usually hyperintense on diffusion-weighted images, and probably reflect transient cyto-toxic edema induced by the electrical activity.
41	"Follow-up MR exams demonstrate that..." Follow-up MR exams demonstrate that these signal abnormalities are usually reversible , a factor that helps to exclude a structural epileptogenic lesion.
42	"However" However, in some cases the reversibility of the high signal intensity is associated with some focal brain atrophy .
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48	"Ischemia" Ischemia, commonly associated with asphyxia in the perinatal and neonatal periods, is typically manifested by injury to the deep grey matter including the hippocampus ; this may be due to a relatively high level of N-methyl-D-aspartate receptors in the hippocampus, pos-sibly enhancing
49	"In adulthood" In adulthood, infarctions limited to the hippocampus and the ventro-posterolateral thalamus are produced in occlusions of the posterior cerebral artery near its origin, with distal flow from the posterior communicating artery . In these situations the infarction may be limited to the areas irrigated by the branches originating in the proximal part of the artery.
50	"More commonly" More commonly, hippocampal infarctions occur as a consequence of thrombotic or embolic occlusion of the proximal segment of the posterior cerebral artery or the basilar artery. In these situations the infarction is not limited to the hippocampus, but also affects other structures irrigated by the posterior circulation .
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54	PRIMARY BRAIN TUMORS Primary brain tumors can selectively affect the deep structures of the temporal lobe, including the hippo-campus and amygdala. These lesions constitute 10%-20% of cases of temporal lobe epilepsy, and mainly correspond to low-grade tumors such as fibrillary astrocytoma , ganglioglioma , dysembryoplastic neuroepithelial tumor (DNET) and glioblastoma multiforme . An accurate histological diagnosis is difficult with MR imaging, since these tumors share many characteristics: absence or mild edema, contrast enhancement and mass effect, well-circumscribed margins, and presence of calcifications. Some demographic ( Table 1) and radiological ( Table 2) data may be useful for differentiating the most common low-grade tumors affecting the hippocampus.
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74	CONCLUSIONS
75	CONCLUSIONS
76	CONCLUSIONS