What Is Incomplete Hippocampal Inversion?

The term “incomplete hippocampal inversion” often appears on a brain scan report, and it can sound concerning without proper context. This finding refers to a slight variation in the typical structure of the brain that occurs during development. It is not considered a disease or a sign of brain damage but rather an anatomical anomaly. For most individuals, this variation is discovered by chance during a medical imaging test performed for unrelated reasons.

Defining Incomplete Hippocampal Inversion

To understand what makes a hippocampal inversion “incomplete,” it is helpful to know about the hippocampus itself. This seahorse-shaped structure, located deep within the temporal lobes on both sides of the brain, has a role in forming new memories and in spatial navigation. During fetal development, the brain undergoes complex growth and folding processes, and the hippocampus performs a maneuver where it folds inward and rotates.

This developmental process typically occurs during the second trimester of pregnancy and is a normal part of forming the brain’s complex architecture. Incomplete hippocampal inversion, sometimes called hippocampal malrotation, occurs when this folding and rotation process halts before it is fully finished. This results in a hippocampus that has a slightly different shape and orientation than expected.

Instead of a well-defined, horizontally oriented oval shape, the hippocampus may appear more rounded, vertically positioned, and located more toward the midline of the brain. This structural variation can occur on one side of the brain or on both sides, though it is more common on the left side. The brain tissue itself is healthy, with normal cell size and signal intensity on medical scans, which is why it is classified as a developmental anomaly rather than a pathology.

Associated Neurological Conditions

While many people with incomplete hippocampal inversion (IHI) are neurologically healthy, research has identified correlations between this anatomical variation and certain neurological conditions. The most studied association is with temporal lobe epilepsy. Studies have found that IHI is more frequently observed in individuals with epilepsy compared to the general population, suggesting it might be a factor that contributes to seizure susceptibility.

The connection is not one of direct causation; IHI does not cause epilepsy. Instead, it is thought that the underlying developmental disturbance that led to the incomplete inversion might also affect other brain regions, creating a predisposition for seizures. The altered shape and position of the hippocampus could influence the neural networks within the temporal lobe, making them more prone to the abnormal electrical activity that defines an epileptic seizure.

Another notable association is with febrile seizures, which are seizures in young children triggered by a high fever. One study found that IHI was significantly more common in children who experienced prolonged febrile seizures compared to those with simple febrile seizures. This finding provides evidence that IHI may be a marker of a subtle disturbance in brain development that can lower the threshold for seizure activity under certain conditions.

The Diagnostic Process

Incomplete hippocampal inversion is a radiological finding, meaning it can only be identified through medical imaging of the brain. The primary tool for its diagnosis is Magnetic Resonance Imaging (MRI). An MRI uses powerful magnets and radio waves to create highly detailed, cross-sectional images of the brain’s soft tissues, allowing for a clear examination of its structure.

When reviewing a brain MRI, specifically the coronal views which show the brain in slices from front to back, a radiologist looks for a specific set of features to identify IHI. They assess the shape, orientation, and position of the hippocampus. A normally developed hippocampus appears as a horizontal, oval-shaped structure, whereas an incompletely inverted hippocampus often presents as more rounded or even pyramidal in shape and is oriented more vertically.

The radiologist may also observe other related anatomical markers, such as a deeper-than-usual groove known as the collateral sulcus located near the hippocampus. They will also confirm that the hippocampal tissue has a normal signal intensity, which indicates the cells are healthy, and that there is no evidence of volume loss, which would suggest a different condition like hippocampal sclerosis.

Clinical Significance and Management

The discovery of an incomplete hippocampal inversion on an MRI scan does not, in itself, necessitate medical treatment. The anatomical variation is not something that can be corrected or reversed, and management focuses entirely on any associated symptoms a person might have.

If an individual with IHI is asymptomatic, meaning they have no neurological symptoms such as seizures or memory problems, then no action or treatment is required. The finding is considered incidental—a chance discovery with no bearing on their current health. In these cases, physicians will reassure the individual that it is a common developmental anomaly and not a cause for concern.

Conversely, if a person with IHI also has a diagnosed neurological condition like epilepsy, the medical management will target that specific condition. For example, a neurologist would prescribe anti-seizure medications to control and prevent epileptic seizures. The treatment plan is determined by the symptoms, not by the presence of IHI on the brain scan. Therefore, a thorough discussion with a physician is important to interpret its relevance within an individual’s overall clinical picture.