The anatomy of the arterial blood supply of the brainstem is depicted in ▶Fig. 4.57, and the territories of individual arteries supplying the medulla, pons, and midbrain are shown in ▶Fig. 4.58. A thorough discussion of the arterial blood supply and venous drainage of the brainstem is found ▶Arteries of the Posterior Fossa, Chapter 11. Knowledge of the pattern of blood supply is essential for an understanding of the vascular syndromes to be described in this section.
Inadequate perfusion of discrete regions of the brainstem can be either transient (e.g., the transient ischemia of subclavian steal syndrome, see below) or permanent (causing tissue necrosis, i.e., brainstem infarction). Infarction is usually due to arterial occlusion. It produces different patterns of clinical deficits, depending on the particular vessel that has been occluded (vascular syndromes). Because the nuclei and fiber pathways of the brainstem are numerous, compactly arranged, and highly diverse in function, a correspondingly wide variety of vascular syndromes can be observed. To understand each vascular syndrome, one must first understand the complex topographical anatomy of the brainstem in the region that it affects. This is why the brainstem vascular syndromes are presented here in the chapter on the brainstem, rather than in ▶Chapter 11 together with the vascular disorders of the rest of the brain.
This syndrome occurs as the result of occlusion of the right or left subclavian artery proximal to the origin of the vertebral artery. Despite the occlusion, the cardiovascular system maintains adequate perfusion of the ipsilateral arm by “tapping” the ipsilateral vertebral artery in retrograde fashion: blood flows up the contralateral vertebral artery to its junction with the ipsilateral vertebral artery (where the two arteries join to form the basilar artery), and then back down the ipsilateral vertebral artery into the axillary artery and onward into the brachial artery. In rare cases, a situation may arise in which exercise of the arm diverts so much blood from the vertebrobasilar system that clinically evident brainstem ischemia ensues. The diagnosis of subclavian steal syndrome requires both the characteristic clinical manifestations and a clinically correlated angiographic finding of retrograde flow in the vertebral artery. Subclavian artery occlusion needs to be treated only if it causes ischemia in the hand or frank subclavian steal syndrome, with manifestations of ischemia in the vertebrobasilar territory, such as loss of consciousness or vertigo.
Infarction in the vertebrobasilar distribution, as in the carotid distribution, is usually due to embolism. The responsible emboli may arise from the heart, from atheromatous plaques in the vertebral arteries, or from an arterial dissection with secondary thrombosis.
A number of different brainstem vascular syndromes can be identified on clinical and radiological grounds. High-resolution MRI with T2-weighted and diffusion-weighted sequences allows the direct visualization of brainstem infarcts in their acute phase. Although there is a degree of variation among individuals, the vascular architecture of the brainstem is sufficiently uniform that the syndromes described here are well-defined clinical entities.
Brainstem infarction in a number of different locations often becomes manifest clinically as alternating hemiplegia (crossed weakness), which is defined as a combination of cranial nerve deficits on the side of the lesion with weakness of the opposite hemibody. In ▶Fig. 4.59, three different alternating hemiplegia syndromes are shown, each the result of ischemia in a particular region of the brainstem, with corresponding clinical deficits.
We now list the individual vascular syndromes that can be considered, in simplified terms, to be “variations” of the alternating hemiplegia syndrome, albeit with extraordinarily diverse manifestations. To make the presentation as clear as possible, the discussion of each syndrome is accompanied by a drawing of the affected structures in the brainstem and by a schematic diagram of the resulting clinical deficits.
Dorsolateral medullary syndrome (Wallenberg′s syndrome). See ▶Fig. 4.60 and ▶Fig. 4.61. Cause: occlusion or embolism in the territory of the PICA or vertebral artery. Clinical features: sudden onset with vertigo, nystagmus (inferior vestibular nucleus and inferior cerebellar peduncle), nausea and vomiting (area postrema), dysarthria and dysphonia (nucleus ambiguus), and singultus (respiratory center of the reticular formation). For further details, see ▶Fig. 4.60.