Microsurgical Anatomy of the Thalamus

The thalamus is located in the center of the lateral ventricle at the upper end of the brainstem. It is positioned deep to the posterior half of the insula and the lower part of the pre- and postcentral gyri, and deep to the adjacent part of the superior temporal gyrus ( Fig. 2.1a-e ). ⁹ The anterior thalamic tubercle, the prominence overlying the anterior thalamic nucleus, forms the posterior edge of the foramen of Monro. The thalamus reaches the level of the posterior commissure posteriorly and the hypothalamus sulcus inferiorly. Its upper margin forms the floor of the lateral ventricle. The stria terminalis and thalamostriate veins run along the striothalamic sulcus, at the junction of the thalamus and caudate nucleus, and the choroid plexus attaches along the choroidal fissure, which is the cleft between the thalamus and fornix. Each lateral ventricle wraps around the superior, inferior, and posterior surfaces of the thalamus. ¹⁰ The prominent posterior part, the pulvinar, presents in the wall of three different supratentorial compartments: the posterolateral part of the pulvinar forms the lateral half of the anterior wall of the atrium; the posteromedial part of the pulvinar is covered by the crus of the fornix and the part medial to the fornix forms part of the anterior wall of the quadrigeminal cistern; and the inferolateral part of the pulvinar in the region of the geniculate bodies forms part of the roof of the ambient cistern. The medial part of the thalamus forms the upper part of the lateral wall of the third ventricle. ¹⁰

Blood is supplied to the thalamus through anterolateral, lateral, posterolateral, medial, and dorsal arteries ( Fig. 2.1f, g ). ¹¹ The anterolateral arteries of the thalamus arise from the premammillary branch of the posterior communicating artery; the lateral arteries arise from the anterior choroidal artery; the posterolateral arteries arise from the thalamogeniculate artery; and the medial arteries arise from the thalamoperforating artery. The thalamogeniculate and the thalamoperforating arteries are two of the larger perforating branches of the posterior cerebral artery. ⁹ The thalamoperforating arteries enter the brain through the posterior perforated substance to supply structures in the floor and lateral walls of the third ventricle, including the anterior two-thirds of the thalamus in the area below the floor of the body of the lateral ventricle. They also supply the cerebral peduncle, hypothalamus, midbrain, and internal capsule. The thalamogeniculate arteries arise in the ambient cistern, enter the brain in the region of the geniculate bodies, and send branches into the posterolateral part of the thalamus, including the geniculate bodies and the adjacent part of the internal capsule.

The thalamic veins are divided into superficial and deep veins ( Fig. 2.1h, i ). Some of the thalamic veins course on the ventricular surface and some course in the basal cisterns. ^{12 , 13} Anterior and superior superficial thalamic veins cross the surface of the thalamus and drain into the internal cerebral vein. ¹⁴ The superficial thalamic veins course along the ventricular surface of the thalamus in a subependymal location and drain into the adjacent veins in the ventricle, velum interpositum, or basal cisterns. ¹⁴

The deep thalamic veins are divided into anterior, superior, inferior, and posterior thalamic veins. ¹⁴ The anterior thalamic vein drains the anterosuperior part of the thalamus and terminates in the adjacent part of the internal cerebral, anterior septal, thalamostriate, or anterior caudate vein, or other smaller veins in the region. The superior thalamic vein is the largest of the thalamic veins. It arises in the central superior part of the thalamus, runs medially to emerge from the medial surface of the thalamus near the striae medullaris thalami, runs posteriorly below the internal cerebral vein in the velum interpositum, and ends in the internal cerebral or great cerebral vein. The inferior thalamic veins arise in the anteroinferior part of the thalamus and traverse the posterior perforated substance to drain into the posterior communicating or peduncular vein. The posterior thalamic veins drain the posterior inferolateral portion of the thalamus and empty into the posterior part of the basal vein or into the veins coursing on the posterolateral surface of the midbrain.

Microsurgical Anatomy of the Pineal Region

The pineal gland is an extra-axial structure that is surrounded superiorly by the splenium of the corpus callosum and the vein of Galen and anteriorly by the habenular commissure and posterior commissure ( Fig. 2.2 ). The pineal gland extends posteriorly into the quadrigeminal cistern from its stalk. The internal cerebral vein, basal vein of Rosenthal, anterior calcarine vein, and superior vermian vein converge on the vein of Galen before it drains into the straight sinus above the pineal gland. The blood supply to the pineal gland is from branches of the medial and lateral choroidal arteries through anastomoses to the pericallosal, posterior cerebral, superior cerebellar, and quadrigeminal arteries. ¹⁵ The inferior margin of the posterior part of the falx cerebri slopes toward the splenium to meet the tentorium in the midline.

The straight sinus originates below the splenium of the corpus callosum and travels on the superior surface of the tentorium to where both sides of the tentorium come together at an angle to meet the falx cerebri. ¹⁶

Surgical Approaches

Depending on the location of the lesion in the thalamus, the approach may be the anterior interhemispheric transcallosal approach (including the transventricular, transforaminal, and transchoroidal or transforniceal variations), or the posterior interhemispheric transcallosal, parieto-occipital transcortical transventricular, or infratentorial supracerebellar approach ( Fig. 2.3 ). ^{3 , 17}

A number of different types of tumors can occupy the pineal region, including tumors originating in the pineal body (pinealoblastomas/pineocytomas, teratomas, and germinomas), in the splenium of the corpus callosum (intrinsic glial tumors), in the velum interpositum (meningiomas), or in the fornix. ¹⁶ The surgical approaches used to access lesions in the pineal region are dependent on the complex anatomical relationship of the surgical target to surrounding structures, the location of the arteries feeding the lesion, anatomical variations, and the extent of resection goals. A wide variety of approaches to this region have been described, which can be tailored to the morphology of the target lesion. These approaches include the infratentorial supracerebellar approach, the posterior-interhemispheric transtentorial approach, the occipital interhemispheric approach, the parieto-occipital interhemispheric transcallosal approach, the posterior transcortical approach via the angular gyrus and lateral ventricle, the posterior subtemporal approach, and the combined supra- and infratentorial transsinus approaches ( Fig. 2.3 ). ^{18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28}

Vascularization of the Brainstem

The brainstem is supplied by the posterior circulation arising from the vertebral arteries. The two vertebral arteries come together in the midline to form the basilar artery, most c ommonly at the level of the pontomedullary sulcus. Three neurovascular complexes can be defined: an upper complex related to the superior cerebellar artery (SCA); a middle complex related to the anterior inferior cerebellar artery (AICA); and a lower complex related to the posteroinferior cerebellar artery (PICA) ( Fig. 2.4 and Fig. 2.5 ). ^{29 , 30} The upper complex includes the SCA, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum, and the oculomotor, trochlear, and trigeminal cranial nerves ([CNs] III-V). ²⁹ The SCA arises anterior to the midbrain, passes inferior to the oculomotor and trochlear nerves and superior to the trigeminal nerve to reach the cerebellomesencephalic fissure, where it runs on the superior cerebellar peduncle and terminates by supplying the tentorial surface of the cerebellum. ²⁹ The middle complex includes the AICA, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum, and the abducens, facial, and vestibulocochlear nerves (CNs VI-VIII). ²⁹ The AICA arises at the level of the pons, courses alongside the abducens, facial, and vestibulocochlear nerves to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum. ²⁹ The lower complex includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum, and the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves. The PICA arises at the level of the medulla, encircles the medulla, and courses past the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves to reach the surface of the inferior cerebellar peduncle, where it dips into the cerebellomedullary fissure and terminates by supplying the suboccipital surface of the cerebellum. ²⁹

Long Tracts of the Brainstem

The brainstem is divided into ventral and dorsal parts by the medial lemniscus (ML) ( Fig. 2.6a ). ^{1 , 31}

Related posts:

3 Development of the Human Brainstem and Its Vasculature 4 Pathology of the Brainstem 9 Approaches to the Dorsal Brainstem, Thalamus, and Pineal Region 15 Tumors of the Thalamus 13 Adult Brainstem Gliomas 21 Radiotherapy for Pineal, Thalamic, and Brainstem Tumors

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