The pathogenesis of spontaneous carotid dissection is at present uncertain. In most reported cases, cystic medial necrosis has not been found on microscopic examination of the involved artery. It seems likely that some cases, even beyond those with known disorders of connective tissue and with fibromuscular dysplasia, are associated with genetic alterations that weaken the media of vessels. In some, there has been disorganization of the media and internal elastic lamina, but the specificity of these changes is in doubt, as Ojemann and colleagues (1972) noted similar changes in some of their control cases. In a small proportion of cases there are the changes of fibromuscular dysplasia, as noted earlier. Several groups have found structural collagen abnormalities in the skin biopsies of patients with dissection. A more thorough study of these vessels is needed.
Dissection of these arteries may originate in the neck and extend into the intracranial portion of the vessel or remain isolated to either of these segments as noted below. In both instances there is a tendency to form pseudoaneurysms, mostly with the intracranial type, and in the latter there is a risk of rupture through the adventitia leading to a subarachnoid hemorrhage. Rapid and extreme rotational movement of the neck is the most common identifiable cause of vertebral artery dissection, as in turning the head to back up a car or with chiropractic manipulation. Extending the neck to have one’s hair washed, swinging a golf club, and direct neck trauma have also been precipitants. Forceful coughing may also cause dissection, as it may in the carotid vessels. There is no clear female predominance (as there may be in carotid dissection) but the previously cited intrinsic weaknesses of the vascular wall from Ehlers-Danlos disease and fibromuscular dysplasia are risk factors.
The dissection most commonly originates in the C1-C2 segment of the vessel, where it is mobile but tethered as it leaves the transverse foramen of the axis and turns sharply to enter the cranium. The symptoms, mainly vertigo, are fragments of the lateral medullary syndrome, often with additional features referable to the pons or midbrain, particularly diplopia and dysarthria. The clinical manifestations in our experience have fluctuated over minutes and hours, quite unlike the usual vertebrobasilar TIA.
Less-common strokes include artery-to-artery embolism to the posterior cerebral territory or, a syndrome that has come to our attention several times in the past few years, a centrally placed infarction of the cervical spinal cord with bibrachial weakness, presumably from occlusion of the anterior spinal arteries. Another interesting but rare association with dissection has been the reversible cerebral vasoconstriction syndrome (RCVS). Mawet and colleagues reported on 20 cases they extracted from their experience but could not determine which process occurred first and could only speculate as to the relationship. Dissection of the vertebral artery was a more common association with RCVS than was carotid artery dissection.
The diagnosis of vertebral dissection should be suspected if persistent occipitonuchal pain and vertigo or related medullary symptoms arise following one of the known precipitants—such as chiropractic manipulation of the neck, head trauma, or Valsalva straining or coughing activities—but it may otherwise escape detection until the full-blown medullary or cerebellar stroke is established. The stroke may follow the inciting event by several days or weeks or even longer, obscuring the relationship. Axial MRI images, particularly the T1-weighted sequences, show a double lumen in the dissected vessel, as described for carotid artery dissection earlier, and skillful ultrasound investigation documents the same. Some patients will be found to have evidence of spontaneous or traumatic dissection of multiple extracranial vessels; this also occurs as a consequence of dissection of the aortic arch from chest trauma.