External cortical landmarks for localization of the hippocampus: Application for temporal lobectomy and amygdalohippocampectomy

Abstract

Background: Accessing the hippocampus for amygdalohippocampectomy and minimally invasive procedures, such as depth electrode placement, require an accurate knowledge regarding the location of the hippocampus.

Methods: The authors removed 10 human cadaveric brains from the cranium and observed the relationships between the lateral temporal neocortex and the underlying hippocampus. They then measured the distance between the hippocampus and superficial landmarks. The authors also validated their study using magnetic resonance imaging (MRI) scans of 10 patients suffering from medial temporal lobe sclerosis where the distance from the hippocampal head to the anterior temporal tip was measured.

Results: In general, the length of the hippocampus was along the inferior temporal sulcus and inferior aspect of the middle temporal gyrus. Although the hippocampus tended to be more superiorly located in female specimens and on the left side, this did not reach statistical significance. The length of the hippocampus tended to be shorter in females, but this too failed to reach statistical significance. The mean distance from the anterior temporal tip to the hippocampal head was identical in the cadavers and MRIs of patients with medial temporal lobe sclerosis.

Conclusions: Additional landmarks for localizing the underlying hippocampus may be helpful in temporal lobe surgery. Based on this study, there are relatively constant anatomical landmarks between the hippocampus and overlying temporal cortex. Such landmarks may be used in localizing the hippocampus during amygdalohippocampectomy and depth electrode implantation in verifying the accuracy of image-guided methods and as adjuvant methodologies when these latter technologies are not used or are unavailable.

Keywords: Anatomy; epilepsy surgery; hippocampectomy; landmarks; neurosurgery; temporal lobe.

Figure 1

The relationship of right hippocampus and temporal horn to the superficial structures. Copyright, The Neurosurgical Atlas, used with permission

Figure 2

Left hemisphere with pins noting the anterior, middle, and posterior parts of the hippocampus

Figure 3

Right hemisphere with pins noting the anterior, middle, and posterior parts of the hippocampus

Figure 4

Axial section through the right temporal lobe for verification of outer cortical landmarks to internally positioned hippocampus

Figure 5

Left hemisphere with pins marking the head and tail of the hippocampus. Here, the head (anterior pin) is seen at the level of the inferior temporal sulcus and the tail (posterior pin) is seen at the level of the lower part of the middle temporal gyrus

Figure 6

Right hemisphere with pins marking the head and tail of the hippocampus. In this specimen, the head (anterior pin) of the hippocampus is deep to the inferior temporal sulcus. However, the tail (posterior pin) is located in the superior temporal sulcus

Figure 7

Sagittal, axial, and coronal sequences were used to identify the anatomical landmarks. Using the IntelliLink tool, the hippocampal head was marked on the axial plane and that position was identified on the sagittal plane. A measurement was taken from the anterior temporal tip to the hippocampal head and recorded

Figure 8

Disconnection of the occipitotemporal fasciculus along the lateral aspect of the hippocampus will allow lateral mobilization the temporal cortex (instead of its resection), to provide enough exposure to allow for a posterior hippocampectomy. A corticotomy within the middle temporal gyrus, along the mid portion of the temporal lobe, will expose the ventricle. Copyright, The Neurosurgical Atlas, used with permission