Microsurgical anatomic features of the lamina terminalis

OBJECTIVE: The lamina terminalis (LT) is a structure of considerable interest for microneurosurgery, and precise knowledge regarding its normal anatomic features and the variations thereof is required. The purpose of this study, which was based on microanatomic dissection of human cadaveric specimens, was to review the microsurgical anatomic features of the LT and its neurovascular relationships. The surgical implications of the morphometric data are discussed. METHODS: The region of the LT was examined in 10 human cadaveric heads, obtained from 8 fresh adult cadavers and 2 formalin-fixed adult cadavers, and in 10 formalin-fixed, isolated, adult brains. An operating microscope was used for all dissections and measurements. RESULTS: Assuming the LT to be a triangular structure, we performed measurements of the distance between the midportion of the upper edge of the chiasm and the lower edge of the anterior commissure (height), which averaged 8.25 mm. The distance between the medial edges of the optic tracts (base) averaged 12.81 mm. The area averaged 52.84 mm(2). A minimal amount of retraction was needed to fully expose the LT, and generally there was no need to mobilize the anterior cerebral artery-anterior communicating artery complex. Perforating branches to the hypothalamus and optic apparatus are laterally located and do not interfere with LT incision and/or fenestration. CONCLUSION: The LT constitutes a clearly identifiable microsurgical target. The allowable extent of LT opening is sufficient to provide wide access into and free cerebrospinal fluid flow from the third ventricle. Fenestration of the LT is a safe procedure, provided that the relevant anatomic landmarks are identified and respected.