Basal ganglia and thalamus

Surface anatomy for frame-based stereotaxy

Stereotaxy makes use of a three-dimensional coordinate system relative to a fixed frame of reference on the body to target deep lesions with high precision. Stereotactic brain surgery is commonly performed using a frame-based method (e.g. Leksell, Cosman–Roberts–Wells), although frameless alternatives are available. The stereotactic frame sits on a base ring, which must be applied to the patient's skull immediately preoperatively ( Fig. 27.1 ) using four skull-pins screwed under local anaesthesia with sedation in the sitting position. The base ring should be placed parallel to a line extending from the lateral canthus to the tragus or the zygomatic arch to approximate the plane of the anterior commissure–posterior commissure (AC–PC) line. This aligns the stereotactic CT to the preoperative MRI, which is normally aligned from nasion to occiput. Once the equipment is attached, high-resolution head CT is commonly performed and fused with the preoperative MRI in specific computer software to generate appropriate stereotactic coordinates, such that the trajectory avoids sulci that contain vessels, visible cortical vessels, eloquent cortex, and a transventricular route. Such software can also incorporate standardized stereotactic brain atlases to aid planning (e.g. Schaltenbrand et al ).

Tips and Anatomical Hazards

• For stereotactic targeting of deep structures, acquire a preoperative high-quality MRI (under general anaesthesia, if required), to be fused with intraoperative head CT (taken once the base ring has been attached to the skull) in the target planning software.

• Fit the stereotactic base ring parallel to the orbitomeatal line and symmetrically, to aid image fusion.

• Ensure multiple checks of coordinates and frame setting are performed by multiple surgeons.

• Check coordinate accuracy first, using a phantom target if available.

• Avoid or minimize loss of cerebrospinal fluid on dural puncture, to reduce brain shift.

• Do not choose an entry route where cortical vessels are visible on susceptibility-weighted imaging MRI and avoid sulcal entry.

• Do not choose a transventricular route because this often results in more medial targeting and increased bleeding risk with ependymal violation.

Gross anatomy of the basal ganglia and thalamus

The basal ganglia are thought to be critical for adaptive behaviours (including goal-directed action selection, habit formation and motor control). The thalamus is the ‘gateway to the cortex’, and thalamo­cortical neurones are the only route by which basal ganglia output, cerebellar output and ascending sensory information (except olfaction) can reach the cortex. Historically, the basal ganglia included the corpus striatum (striatum, globus pallidus and internal capsule), claustrum and amygdala, but modern scientific and clinical usage is restricted to the following telencephalic and diencephalic structures: striatum (split into the caudate and putamen by the internal capsule), globus pallidus (pars interna and pars externa), subthalamic nucleus and substantia nigra (pars compacta and pars reticulata). With growing knowledge about the functional connectivity of each of these deep nuclei and the ability to identify them individually on modern MRI scans ( Fig. 27.2 ), older terminology derived from gross pathological appearances, such as ‘lentiform nucleus’ (putamen and globus pallidus) and ‘corpus striatum’ (‘striped body’: caudate, putamen and globus pallidus), is becoming less common.

The striatum is considered to be the principal ‘input’ structure of the basal ganglia since it receives the majority of afferents from other parts of the brain – predominantly from the cortex but also midbrain dopaminergic neurones and intralaminar/ventral thalamus ( Table 27.1 ). It is divided into dorsal (caudate nucleus and putamen) and ventral (nucleus accumbens and olfactory tubercle) parts, which regulate sensorimotor control/learning and reward-related behaviours, respectively. The caudate nucleus is a C-shaped structure that is closely associated with the lateral wall of the lateral ventricle via its large head (anterior pole), tapering body and tail terminating at the amygdala in the temporal lobe ( Fig. 27.3A ). Anteriorly and inferolaterally, it is separated from the putamen (which has the same structure and function) and from the globus pallidus by the anterior limb of the internal capsule ( Fig. 27.3B ). Medially, the caudate is separated from the thalamus by the sulcus terminalis, where the striae terminalis (fibres from the amygdala to the thalamus/hypothalamus), striae medullaris (hypothalamus/anterior thalamus to habenula) and thalamostriate vein reside ( Fig. 27.3C ). Medially, the putamen is separated from the globus pallidus by an external medullary lamina and laterally it is separated from the claustrum by the external capsule. Within the globus pallidus, the internal medullary lamina divides pars externa (GPe) from pars interna (GPi). The GPi lies in the medial aspect of the ‘lenti­form nucleus’, immediately lateral to the genu of the internal capsule and above the optic tract; it sends efferent inhibitory projections to the thalamus via the ansa lenticularis and lenticular fasciculus ( Fig. 27.3D ). The subthalamic nucleus (STN) is separated from the thalamus by the zona incerta and fields of Forel superiorly; the substantia nigra (‘black substance’) lies anteroinferolaterally, the red nucleus posteromedially and the internal capsule laterally. The substantia nigra has a pars compacta (SNc), formed by dopaminergic neurones (containing neuromelanin) that project to the caudate–putamen, and a less dense pars reticulata (SNr), containing GABAergic (GABA, gamma-aminobutyric acid) projection neurones that project to the thalamus, pedunculopontine nucleus and superior colliculus.

The thalamus is a 4 cm egg-shaped mass with a long axis at 30° to the midline. Anteriorly, it is separated from the head of the caudate nucleus by the genu of the internal capsule. Medially, it forms the lateral wall of the third ventricle. It is almost always connected to the contralateral thalamus by the interthalamic adhesion (which can impede a third ventriculostomy). Superiorly, from medial to lateral, it is covered by tela choroidea, forms the floor of the lateral ventricle, and is separated from the body of the caudate nucleus by the stria medullaris, stria terminalis and thalamostriate vein (see Fig. 27.3C ). Laterally, it is separated from the lentiform nucleus (globus pallidus and putamen) by the genu and posterior limb of the internal capsule. Inferiorly, it is separated from the subthalamic nucleus by the fields of Forel and zona incerta (together these structures form the ‘subthalamus’ or ‘ventral thalamus’). Posteriorly, the thalamus forms the anterior wall of the atrium of the lateral ventricle. All known connections between thalamus and cerebral cortex are reciprocal, two-way radiations (thalamocortical and corticothalamic), running in the internal capsule and corona radiata. The medial and anterior thalamic nuclei produce the anterior thalamic peduncle (radiation), which runs towards the anterior and inferior frontal cortical areas. Axons of the superior thalamic peduncle run vertically to connect the thalamus with posterior frontal and parietal cortical areas. The inferior thalamic peduncle courses towards the temporal lobe cortex and includes the auditory radiation. The posterior thalamic peduncle and the optic radiation run backwards to the occipital and inferior parietal cortex. The important connections between basal ganglia, thalamus and cortex are summarized in Table 27.2 .