The hippocampus is a small, seahorse-shaped complex anatomical structure located medially in the temporal lobe. It plays a crucial role in episodic and spatial memory. It has two gray matter folds: cornu ammonis/ hippocampus proper, and dentate gyrus. This bilaminar gray matter structure is formed of primitive allocortex with three, four, or five layers and starts to develop in early fetal life. Anatomically it has three parts: head (anterior), body (intermediate), and tail (posterior). According to the cellular composition, the hippocampus proper is divided into four parts: CA1, CA2, CA3, and CA4. Cornu ammonis inferomedially continues in the subiculum and Para-hippocampal gyrus. The posterior cerebral artery (PCA) supplies the hippocampus via three branches: the anterior hippocampal artery (supplies the head) and the middle and posterior hippocampal artery (supplies the body and tail).
The white matter fibers from the hippocampus form alveus which medially gather to form fimbria and continue with fornix. Several fissures (e.g., hippocampal fissures, choroidal fissures, etc.) surround the hippocampus and are referred to as peri-hippocampal fissures. The infolding of the hippocampal sulcus starts developing between dentate gyrus and cornu ammonis which gradually shifts to the area between dentate gyrus and subiculum. Magnetic resonance imaging (MRI) is the most preferred imaging protocol for the optimal evaluation of the hippocampal anatomy and pathology in patients.
The hippocampus can be affected by an array of congenital defects, vascular, inflammatory, degenerative, toxic metabolic pathologies, and tumors. Some of the hippocampal congenital anomalies are sulcal remnant cysts, choroidal fissure cysts, incomplete hippocampal inversion, etc. Sulcal remnant cysts and choroidal cysts are benign cerebral cysts located in the vestigial hippocampal sulcus and the choroid fissure respectively. Sulcal remnant cyst is caused due to lack of obliteration of the hippocampal sulcus and choroidal cyst generally forms due to the development error anywhere in the choroidal fissure that may be neuroepithelial or arachnoid type. Both the lesions are asymptomatic or very rarely symptomatic and appear isointense to CSF on MRI. Incomplete hippocampal inversion (IHI) results from a failure of hippocampal inversion during neurodevelopment. On imaging, they appear as abnormal globular or pyramidal shapes. They are unilateral and left-sided in most cases. The research on the connection between IHI and epilepsy is still in its early stages.
The hippocampus can be affected by inflammatory reactions and infections and may cause herpes simplex encephalitis (HSE), limbic encephalitis, etc. In the majority of cases, adults are affected by HSE by HSV type I virus whereas neonates are affected by type 2 virus. The olfactory route is the most probable way for HSV to enter the brain. This condition may include hallucinations, aphasia, seizures, personality changes, etc. as its symptoms at later stages. On MRI, cortical abnormalities, such as gyral enhancement and petechial hemorrhages, are observed. HSE has a poor prognosis, hence early antiviral treatment is essential. Limbic encephalitis (LE) is an autoimmune-mediated syndrome characterized by the subacute onset of confusion, temporal lobe seizures, anterograde amnesia, and behavioral changes. According to the location of the antigen (Ag), LE-associated neuronal antibodies (Ab) can be of two types: inside the neuron and in the cell membrane. The Ab against neuronal surface Ag associated with LE are voltage-gated potassium channels, AMPA, GABA, etc. They are often non-paraneoplastic. On MRI, LE reflects as uni- or bilateral swelling, medial temporal lobe atrophy, patchy enhancement, restricted diffusion, etc.
Hippocampal calcification, mesial temporal sclerosis, Alzheimer’s disease, and related dementias are some of the hippocampal degenerative diseases. The pathological status of hippocampal calcification reflects the latter stages of vascular fibrosis and is frequently found on computed tomography (CT). Mesial temporal Sclerosis is the most common cause of medically intractable partial complex epilepsy. It is pathologically characterized by neuronal loss and hippocampal gliosis. It also includes loss of internal hippocampal architecture and hippocampal head digitation, ipsilateral temporal horn dilatation, atrophy of the ipsilateral amygdala, ipsilateral mammillary body, ipsilateral fornix, contralateral cerebral hemisphere, and ipsilateral entorhinal area. It generally can be identified in MRI findings and treated with surgery. Alzheimer’s disease is a progressive neurodegenerative disorder that is pathologically characterized by the accumulation of senile plaques and neurofibrillary tangles. It is generally formed in the transentorhinal regions and spreads to the hippocampal cortex, mesial temporal lobes followed by temporal lobes, basal forebrain, and rest of the forebrain. On imaging, gradual bilateral hippocampal volume loss and mesiotemporal cortex volume loss can be observed.
As mentioned earlier, the entire hippocampus is supplied by branches of the PCA. Hence, PCA infarction can affect the entire hippocampus. Moreover, the anterior choroidal artery supplies the hippocampal head, and infarction in this artery can thus affect the hippocampal head. Transient global amnesia (TGA) is another vascular disease that can harm the hippocampus, in addition to arterial ischemic stroke. TGA is a syndrome clinically characterized by acute retrograde amnesia. It is generally seen in patients with old age (more than 60 years) and it lasts less than 24 hours.
Tumors in the mesial temporal lobe such as ganglioglioma and dysembryoplastic neuroepithelial tumors (DNET) etc. can affect the hippocampus. They are mostly found in children and young adults. These tumors are more frequent in the parahippocampal area and lateral occipitotemporal gyrus. They are located in the cortical gray matter or subcortical white matter. On MRI, it may represent a well-defined cystic mass with a mural nodule and variable calcification, and solid portion enhancement. On MRI, DNET reflects as a multiloculated or gyriform cyst.
The hippocampus is very vulnerable to excitotoxic brain injury and involved in various types of epileptic seizures primarily or secondarily. It is considered the most frequent area of acute seizure-induced brain abnormalities.
Reference:
Dekeyzer, S., De Kock, I., Nikoubashman, O., Vanden Bossche, S., Van Eetvelde, R., De Groote, J., Acou, M., Wiesmann, M., Deblaere, K., & Achten, E. (2017). "Unforgettable" - a pictorial essay on anatomy and pathology of the hippocampus. Insights into imaging, 8(2), 199–212. https://doi.org/10.1007/s13244-016-0541-2
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