Diseases of the hippocampus and their symptoms
Experts attribute the hippocampus to a rather vulnerable structure of the brain, its damage (including traumatic brain injury) and related diseases can cause various symptoms - neurological and mental.
Modern neuroimaging methods help to identify morphometric changes in the hippocampus (its volume), which are present in hypoxic damage and certain diseases of the brain, as well as in its reduction deformities.
Asymmetry of the hippocampus is considered an important clinical sign, since, presumably, the left and right hippocampus are affected differently during aging. According to some studies, the left hippocampus plays the main role in episodic verbal memory (verbal recollection of memories), and the right hippocampus plays a major role in the consolidation of spatial memory. According to the measurement, in people over 60 years of age, the difference in their volumes is 16-18%; it increases with age, and in comparison with women, men have more asymmetry. 
A slight decrease in the hippocampus, which occurs with age, is considered normal: atrophic processes in the medial temporal lobe and entorhinal cortex begin to occur closer to the seventh decade. But dramatically shrinking the brain's seahorse increases the risk of dementia, the early symptoms of which are brief episodes of memory loss and disorientation. Read more in the article - Symptoms of dementia
The reduction of the hippocampus is much more pronounced in Alzheimer's disease . However, it is not yet clear whether this is the result of this neurodegenerative disease or is a prerequisite for its development. 
According to studies, in patients with generalized depressive disorder and stress disorders of post-traumatic etiology, there is a bilateral and unilateral decrease in the volume of the hippocampus - by 10-20%. Long-term depression is also accompanied by a decrease or impairment of neurogenesis in the hippocampus. According to neurophysiologists, this is due to increased levels of cortisol. This hormone is produced and released by the adrenal cortex in response to physical or emotional stress, and its excess negatively affects the pyramidal neurons of the hippocampus, impairing long-term memory. It is because of the high level of cortisol that the hippocampus shrinks in patients with Itsenko-Cushing's disease . , 
A decrease in the number or alteration of nerve cells in the hippocampus can also be associated with inflammatory processes (neuroinflammation) in the temporal lobe of the brain (for example, with bacterial meningitis, with herpes simplex virus type I or II encephalitis) and prolonged activation of microglia, whose immune cells (macrophages ) release pro-inflammatory cytokines, proteinases and other potentially cytotoxic molecules.
The volume of this cerebral structure can be reduced in patients with brain gliomas , since tumor cells produce the neurotransmitter glutamate into the extracellular space, the excess of which leads to the death of hippocampal neurons.
In addition, a number of studies with MRI volumetric analysis of the hippocampus have recorded its decrease in traumatic brain injury, epilepsy, moderate cognitive impairment, Parkinson's and Huntington's diseases, schizophrenia , Down and Turner syndromes. 
Malnutrition of the nervous tissue - hypotrophy of the hippocampus - may have an ischemic etiology after suffering strokes; in drug addiction, in particular, opioid, hypotrophy is observed, due to disorders of dopamine metabolism by psychoactive substances.
Disorders caused by the lack of certain elements affect the trophism of the nervous tissue of the entire hippocampus, negatively affecting the functioning of the central nervous system. So, vitamin B1 or thiamine and the hippocampus are connected by the fact that in cases of chronic deficiency of this vitamin, the processes of formation of short-term memory are disrupted. It turned out that with a lack of thiamine (the risk of which is increased in alcoholics) in the dentate gyrus and hippocampal fields CA1 and CA3, the number of pyramidal neurons and the density of their afferent processes can decrease, which is why there are disruptions in the transmission of nerve impulses. , Long tiaminovaya failure can cause Korsakoff syndrome .
A progressive decrease in the volume of nervous tissue with the loss of neurons - atrophy of the hippocampus - occurs in almost the same diseases, including Alzheimer's and Itsenko-Cushing's diseases. Risk factors for its development are considered cardiovascular diseases, depression and stress conditions, status epilepticus, diabetes mellitus, arterial hypertension, obesity. Symptoms include memory loss (in Alzheimer's, prior to anterograde amnesia ) , difficulty with familiar processes, spatial definition, and verbal expression. 
If the structural organization of the cells of the fields of the Ammon's horn and the subiculum region is disturbed and a part of the pyramidal neurons (atrophy) is lost - with the expansion of the interstitium and the proliferation of glial cells (gliosis) - the sclerosis of the hippocampus is determined - mesial sclerosis of the hippocampus, mesial temporal or mesial temporal lobe sclerosis. Sclerosis occurs in patients with dementia (causing loss of episodic and long-term memory) and also leads to temporal lobe epilepsy . Sometimes it is defined as limbic temporal or hippocampal, that is, hippocampal epilepsy. Its development is associated with the loss of inhibitory (GABAergic) interneurons (which reduces the ability to filter afferent signals of the entorhinal cortex and leads to hyperexcitability), impaired neurogenesis and proliferation of axons of granular cells of the dentate zilin. More information in the article - Epilepsy and epileptic seizures - Symptoms
As evidenced by clinical practice, hippocampal tumors are rarely found in this cerebral structure, and in most cases it is a ganglioglioma or dysembryoplastic neuroepithelial tumor - a slowly growing benign glioneuronal neoplasm, consisting mainly of glial cells. Most often occurs in childhood and young age; the main symptoms are headache and intractable chronic convulsions.
Congenital anomalies of the hippocampus
With such malformations of the cerebral cortex, such as focal cortical dysplasia, hemimegalencephaly (unilateral enlargement of the cerebral cortex), schizencephaly (the presence of abnormal cortical clefts), polymicrogyria (decrease in convolutions), as well as accompanied by convulsions and visual-spatial disturbances of the periventricular nodule hippocampus.
Abnormal enlargement of the amygdala and hippocampus was identified by researchers in the presence of early childhood autism syndrome . Bilateral enlargement of the hippocampus is observed in children with lissencephaly of the brain , abnormal thickening of the gyrus (pachygiria), or with subcortical laminar heterotopy - a doubling of the cerebral cortex, a manifestation of which is epileptic seizures. More information in the materials:
The hypoplasia of the hippocampus, and often of the corpus callosum, associated with underdevelopment of the brain, is detected in newborns with severe encephalopathy with a mutation of the WWOX gene encoding the oxidoreductase enzyme. This congenital anomaly, leading to early death, is manifested by the infant's lack of spontaneous movement and response to visual stimuli, as well as seizures (which appear several weeks after birth).
Inversion of the hippocampus - a change in its anatomical position and shape - also represents an intrauterine malformation of the hippocampus itself (Cornu Ammonis), the formation of which from the folds of the gray matter of the archicortex is completed by the 25th week of pregnancy.
Incomplete inversion of the hippocampus, as well as malrotation of the hippocampus or inversion of the hippocampus with malrotation, is the formation of a spherical or pyramidal hippocampus, which is more often observed in the left temporal lobe - with a decrease in size. Morphological changes in nearby furrows may be observed. The anomaly is detected in patients with and without seizures, in the presence of other intracranial defects and in cases of their absence.
A congenital anomaly is also a hippocampal cyst - a small cavity filled with cerebrospinal fluid (an expanded perivascular space bounded by a thin wall) of a rounded shape. Residual cysts of the hippocampus, synonymous with remnant cysts of the sulcus (Sulcus hippocampalis), are formed with incomplete involution of the embryonic hippocampal fissure during intrauterine development. The characteristic localization of the cysts is laterally at the apex of the hippocampal groove, between Cornu Ammonis and Gyrus dentatus. They do not manifest themselves in any way and are most often discovered by chance during routine MRI studies of the brain. According to some reports, they are detected in almost 25% of adults.
Hippocampus and coronavirus
Since the beginning of the spread of covid-19, doctors have noted in many recovered patients forgetfulness, anxiety, depressive mood, they often hear complaints of "fog in the head" and increased irritability.
The coronavirus that causes covid-19 is known to enter cells through receptors in the olfactory bulb (Bulbus olfactorius), resulting in symptoms such as anosmia or loss of smell.
The olfactory bulb is associated with the hippocampus, and neurodegenerative disease researchers at the Alzheimer's Association argue that it is responsible for the cognitive impairment seen in covid-19 patients, in particular short-term memory problems.
Recently, it was announced that they intend to start a large-scale study of the effects of coronavirus on the brain and the causes of cognitive decline in the near future, in which scientists from almost four dozen countries will participate - under the technical guidance and coordination of the WHO.
Read also - Coronavirus lingers in the brain even after recovery
Diagnostics of the diseases of the hippocampus
The main methods for diagnosing diseases associated with certain damage to the structures of the hippocampus include the study of the neuropsychic sphere , magnetic resonance imaging and computed tomography of the brain .
Doctors prefer to visualize the hippocampus on MRI: with standard T1-weighted sagittal, coronal, diffusion-weighted axial images, T2-weighted axial images of the whole brain, and T2-weighted coronal images of the temporal lobes. To identify pathological changes in the fields of the hippocampus itself, dentate or parahippocampal gyri, 3T MRI is used; an MRI scan with a higher field may be required. 
Also carried out: Doppler ultrasonography of the vessels of the brain , EEG - encyphalography of the brain.
Details in publications:
Treatment of diseases of the hippocampus
Congenital anomalies of the hippocampus associated with underdevelopment and reduction deformities of the brain cannot be cured: children are doomed to disability due to cognitive impairments of varying severity and associated behavioral disorders.
How are some of the diseases listed above treated? Read in publications:
In cases where anticonvulsants, that is, antiepileptic drugs, do not cope with seizures in mesial temporal lobe epilepsy, they resort to at least surgical treatment.
Operations include: hippocampectomy - removal of the hippocampus; limited or extended ectomy of epileptogenic zones (resection or excision of the affected structures); temporal lobectomy with preservation of the hippocampus; selective resection of the hippocampus and tonsil (amygdala-hippocampectomy). 
According to foreign clinical statistics, in 50-53% of cases after surgery, epileptic seizures in patients stop, 25-30% of operated patients have seizures 3-4 times a year.
How to train the hippocampus?
Since the hippocampus (its dentate gyrus) is one of the few cerebral structures where neurogenesis or nerve regeneration occurs - the formation of new neurons, the process of memory impairment (provided that the underlying disease is treated) can be positively influenced by exercise.
It has been proven that aerobic sports exercise and any physical activity (and especially in old age) contribute to the survival of neurons and stimulate the formation of new nerve cells in the hippocampus. By the way, exercise reduces stress and improves depression. , ,
In addition, cognitive stimulation helps to train the hippocampus, that is, mental exercises: memorizing poetry, reading, doing crosswords, playing chess, etc.
How to enlarge the hippocampus, because in old age it becomes smaller? A proven remedy by researchers is exercise, which increases the perfusion of the hippocampus, and the formation of new cells of the nervous tissue is more active.
How to restore the hippocampus after stress? Engage in mindfulness meditation, which is a mind-training practice aimed at slowing down running thoughts, releasing negativity, and achieving peace of mind and body. A study from an East Asian university has shown that meditation can help lower blood cortisol levels.