The hippocampus is a paired structure deep within the medial temporal lobe, known for its role in processing and forming new memories and regulating emotional responses. During the prenatal period, this structure develops through a complex process of folding and rotation. Incomplete Hippocampal Inversion (IHI), also called hippocampal malrotation, is a common anatomical variation resulting from an arrest in this normal developmental rotation. IHI is a physical variation, not a disease, typically detected through advanced brain imaging. Understanding IHI requires clarifying its origins, its appearance on a scan, and its association with certain neurological conditions.
The Developing Hippocampus
The hippocampus begins development early in gestation as an unfolded layer of tissue. Normal development involves inward folding and rotation, known as inversion, occurring between the first and second trimesters. This folding happens as the dentate gyrus and the cornu ammonis, the main components, roll into the temporal lobe. The process is typically complete around the 21st to 25th week of gestation, resulting in the characteristic adult shape.
The inversion pushes the structure deeper into the temporal lobe, creating a final, flattened, and elongated structure. This rotation also influences the formation of surrounding sulci, or grooves, in the brain tissue. A fully inverted hippocampus is positioned horizontally. Consequently, the collateral sulcus, the groove located just beneath it, remains relatively shallow.
Identifying Incomplete Inversion
Incomplete Hippocampal Inversion occurs when the normal rotation process stops prematurely, leaving the structure resembling its earlier fetal state. This anatomical variation is recognized using specific visual markers on high-resolution Magnetic Resonance Imaging (MRI). While a fully inverted hippocampus appears flat and horizontal on a coronal view, an IHI-affected hippocampus appears distinctly rounded or bulbous.
IHI is also identified by the vertical orientation of the hippocampus, contrasting with the normal horizontal positioning. Furthermore, the surrounding collateral sulcus appears significantly deeper than usual, reflecting the incomplete folding. IHI is relatively common, present in approximately 17 to 19 percent of otherwise healthy individuals. This difference is frequently unilateral, affecting only one side of the brain, and is observed more often in the left hemisphere.
Clinical Relevance and Associated Disorders
The primary question regarding IHI is whether this anatomical variation holds any clinical significance or if it is merely an incidental finding. While IHI does not cause noticeable symptoms for the majority of people, its presence correlates with several neurological and psychiatric conditions. This suggests IHI may sometimes be a marker of a broader developmental disruption.
The strongest and most consistent association is with temporal lobe epilepsy (TLE), a disorder characterized by recurrent seizures originating in the medial temporal lobe. Studies demonstrate that IHI is more prevalent in patients with TLE than in the general population, particularly when IHI is found on the same side as the seizure focus. This suggests that the developmental failure resulting in IHI may also create an environment that lowers the seizure threshold in that area of the brain.
IHI has also been investigated as a potential neurodevelopmental marker in psychiatric disorders, most notably schizophrenia. Research indicates that IHI is more common and often more pronounced in individuals diagnosed with schizophrenia, supporting the hypothesis of early brain development issues. While these links are correlational and not causative, IHI may contribute to subtle differences in hippocampal function, a region strongly implicated in the symptoms of both TLE and schizophrenia.
Diagnosis and Monitoring
IHI is almost always detected incidentally when a person undergoes a brain scan, such as Magnetic Resonance Imaging (MRI), for unrelated reasons like evaluating headaches or trauma. The diagnosis is made by a radiologist who uses the high-resolution images. They specifically look for the constellation of features, including the characteristic rounded shape, the vertical orientation, and the deep collateral sulcus.
The medical management following an IHI diagnosis depends entirely on the patient’s symptoms. If IHI is found in an asymptomatic individual, no specific treatment or intervention is typically required. If a patient is experiencing symptoms like seizures, the presence of IHI may guide further investigation into the temporal lobe as a potential site of seizure onset. In these symptomatic cases, IHI can help neurologists and neurosurgeons plan specialized epilepsy protocols or evaluate the potential prognosis of treatment.