Anatomy And Pathophysiology

The role of the ventrolateral thalamus in movement disorders and the optimal location for therapeutic lesions have long been the subject of debate. Part of the difficulty lies in the fact that numerous anatomical classification schemes are in use. The most widely used schemes are the Anglo-American terminology and Hassler terminology. In the Anglo-American terminology, the ventrolateral nuclei are divided from anterior to posterior into the ven-tralis anterior (VA), ventralis lateralis (VL), and ventralis posterior (VP), whereas in the Hassler terminology the same nuclei are divided into lateral polaris (LPO), ventralis oralis anterior (Voa), ventralis oralis posterior (Vop), ventralis intermedius (Vim) and ventralis caudalis (VC) (Fig. 1). The Hassler nomenclature will be used in this discussion as it appears to correlate with the relevant anatomy and physiology.

The main pallidal output pathways terminate in the ventrolateral thalamus, particularly the Voa nucleus, which in turn projects back to the pre-motor cortex. In contrast, the Vop and Vim nuclei receive multiple inputs,

Hassler Thalamic Abbreviations

Figure 1 Axial section through the thalamus at 1.5 mm above the AC-PC plane. Abbreviations are as follows: Gpe—globus pallidus externa; Gpi—globus pallidus interna; IC—internal capsule; VC—ventralis cau-dalis; Vim—ventralis intermedius; Voa—ventralis oralis anterior; Vop— ventralis oralis posterior. L, H, and F refer to the somatotopic representation of the leg, hand, and face in the VC and Vim nuclei.

Figure 1 Axial section through the thalamus at 1.5 mm above the AC-PC plane. Abbreviations are as follows: Gpe—globus pallidus externa; Gpi—globus pallidus interna; IC—internal capsule; VC—ventralis cau-dalis; Vim—ventralis intermedius; Voa—ventralis oralis anterior; Vop— ventralis oralis posterior. L, H, and F refer to the somatotopic representation of the leg, hand, and face in the VC and Vim nuclei.

including afferents from the contralateral cerebellum through the brachium conjunctivum. The Vop and Vim nuclei then project to the motor and pre-motor cortices. The VC nucleus is the primary thalamic relay for the medial lemniscal and spinothalamic tracts, and projects to the somatosensory cortex. Microelectrode recordings in the area of Vim reveal neurons that respond to kinesthetic stimulation, that is, movement of the joints and squeezing of muscle bellies, whereas neurons in VC respond to tactile stimulation [8,38]. Furthermore, neurophysiological recordings have also revealed cells that fire synchronously with the observed tremor (tremor cells), further implicating the Vim in the pathology of tremor [38]. Whether the presence of these cells indicates that the genesis of tremor lies within Vim, or that Vim is simply part of a larger loop mediating tremor is not clear. In either case, there is a considerable amount of clinical evidence that Vim lesions are effective in alleviating tremor. Lesions placed more anteriorly, in the Vop nucleus, or large lesions including both Vim and Vop lead to improvement in tremor and rigidity, although bradykinesia is often unaffected or sometimes worsened by such lesions [18,34,39,43]. These findings support the contention that thalamotomy is best reserved for patients with Parkinson's disease in whom tremor is the predominant complaint or in patients with intractable essential tremor.

In patients with ET, the goal of thalamotomy is to permanently abolish tremor by placing a small lesion in the Vim nucleus of the thalamus. In thalamic stimulation, the goal is to place the probe 2 to 3 mm anterior to the junction of Vim and VC so that the stimulating current inactivates a portion of Vim while avoiding stimulation of the VC nucleus or the internal capsule.

Was this article helpful?

0 -1

Post a comment