The vertebral arteries may be occluded by atherothrombosis in their intracranial portion or at their origin at the subclavian artery or the arch of the aorta. Because the vertebral arteries have a long extracranial course and pass through the transverse processes of the cervical vertebrae, entering at C6 to proceeding rostrally to the C1 vertebrae before entering the cranial cavity, they are subject to trauma, spondylotic compression, and a variety of other vertebral diseases. With the exception of arterial dissection, in our experience the other causes of vascular occlusion in this list happen only infrequently. We rarely see convincing examples of spondylotic occlusion but several such cases have been reported. Extreme extension of the neck, as experienced by people who are having their hair washed in salons, or during yoga positions, may give rise to transient symptoms in the territory of the vertebral artery.
Dissection of the vertebral artery declares itself by cervicooccipital pain ipsilateral to the dissection and deficits of brainstem function. One’s attention is drawn to the diagnosis of vertebral dissection where there have been vigorous and protracted bouts of coughing or trauma to the neck or head. Dissection of the extracranial vessels is discussed in more detail further on. Examples of posterior circulation stroke in children have been reported in association with odontoid hypoplasia and other atlantoaxial dislocations, causing the vertebral arteries to be stretched or kinked in their course through the transverse processes of C1-C2 (Phillips et al).
The results of acute vertebral artery occlusion are quite variable and there may be no symptoms if an artery is occluded extracranially and there is adequate flow from the opposite vertebral artery or other collateral supply. Occlusion of a vertebral artery low in the neck is usually compensated by anastomotic flow to the upper part of the artery via the thyrocervical, deep cervical, and occipital arteries or by reflux from the circle of Willis. In a configuration in which one vertebral artery is occluded just proximal to the origin of its PICA branch, and the opposite vertebral artery is open and sufficient in size, there may be no symptoms because the PICA is still filled by retrograde flow through its vertebral artery. If the occlusion of the artery is so situated as to block the posterior inferior cerebellar artery supplying the lateral medulla and inferior cerebellum (PICA), a characteristic syndrome results with vertigo being a prominent symptom (see “Lateral Medullary
Syndrome” described further on). If the subclavian artery is blocked proximal to the origin of the left vertebral artery, exercise of the arm on that side may draw blood from the right vertebral and basilar arteries, retrograde down the left vertebral and into the distal left subclavian artery—sometimes resulting in the symptoms of basilar insufficiency. This phenomenon, described in 1961 by Reivich and colleagues, was referred to by Fisher (1961) as the subclavian steal. Its most notable features are vertigo and other brainstem signs coupled with transient weakness on exercise of the left arm. There may also be headache and claudication or pain of the arm.