27.1 General Anatomy

The anterior nucleus of the thalamus (ANT) is a limbic system relay station which connects mesial temporal lobe structures to frontal cortex and cingulum and therefore was selected as a potential stimulation site for refractory epilepsy. ¹ , ² From the surgical perspective, ANT is a challenging target due to its unique anatomy compared to other deep brain stimulation (DBS) targets. The challenges consist of: (1) prominent veins on its anterior and superior surface; (2) partial bulging of the nucleus into the lateral ventricle; (3) poor visualization of ANT in routine magnetic resonance imaging (MRI) sequences; and (4) the anatomical variation in the location of the nucleus with respect to traditional stereotactic landmarks. ANT lies posterior to the junction where the superior choroidal vein and thalamostriatal vein join to form the internal cerebral vein, which instead runs through the foramen of Monro to the third ventricle. Therefore, ANT typically has the superior choroidal vein on its superior aspect, the thalamostriatal vein on its anterolateral aspect, and the internal cerebral vein on its medial aspect.

The ANT is surrounded by incomplete thin white matter layers (▶Fig. 27.1), namely the internal and external medullary lamina. ³ , ⁴ These white matter layers can be visualized using sophisticated MRI methods such as magnetization prepared rapid gradient echo (MP-RAGE) ⁵ or short tau inversion recovery (STIR) sequences ⁶ , ⁷ (▶Fig. 27.1), and may also be demonstrated using intraoperative microelectrode recording (MER). ⁸ The mammillothalamic tract is a major neuronal input pathway originating from the mamillary bodies (containing passing forniceal fibers from the hippocampus), which is clearly identified using MRI (▶Fig. 27.1 and ▶Fig. 27.2), and therefore may be used to guide surgical targeting. ⁵ , ⁶ , ⁷ The ANT comprises four distinct subnuclei, namely the anteromedial nucleus, the anterior principal/anteroventral nucleus, the anterodorsal nucleus, and the dorsal superficial subnuclei. ¹ , ² Occasionally, 3T MRI enables visualization of the anteromedial subnucleus (▶Fig. 27.1b) and dorsal superficial nucleus ⁶ apart from the anterior principal/anteroventral subnucleus. For surgical targeting, axial and coronal images typically show the ANT in several slices, while sagittal images are valuable for showing the course of the mammillothalamic tract (▶Fig. 27.2). Occasionally, ANT borders are delineated in the sagittal plane as well (▶Fig. 27.2). The ANT can be assessed in both 3T and 1.5T MRI, or using restricted imaging protocols for stereotactic MRI for those patients implanted with a vagus nerve stimulator or DBS. ⁷