Pearls & Oy-sters: Bow hunter syndrome: A rare cause of posterior circulation stroke

Discussion

Sorensen¹ coined the term “bow hunter syndrome” in 1978 to describe a case of rotational VA compression causing ischemic brainstem stroke. The dominant VA is frequently compressed, and collateral flow is limited in the presence of predisposing factors such as congenital atresia, hypoplasia, fenestration, or stenosis of the contralateral VA, though this was not evident in our patient.^2,–,4 Prior studies have also shown the left VA to be more commonly involved than the right. This is because left VA dominance is seen in about half the population and its involvement is more likely to be symptomatic.^5,6 Osteophytes or bone spurs were found to be the commonest underlying etiology of VA compression in one systematic review of 142 reported adult cases of BHS.⁵ Other reported atraumatic etiologies include congenital abnormalities such as fibrous bands tethering the VA or atlanto-axial bony anomalies, atlanto-axial instability, cervical disc herniation, cervical spondylosis, and rheumatoid arthritis.^7,–,13 Isolated case reports also exist of BHS after surgical correction of subclavian aneurysms or VA sacrifice.^5,14 The ischemic mechanism may not always be hemodynamic; rarely, it may be thromboembolic, as demonstrated in our patient. It has been postulated that repeated neck rotation and VA compression against bony or fibrous structures may injure the vessel wall, leading to thrombosis and infarction either due to direct injury to the intimal lining or through local complications such as dissection, perforation, and pseudoaneurysm formation and subsequent distal embolization.¹⁵ The VA can be compressed at all vertebral levels, but it is most common in the high cervical spine, C1-2 being the most common site.⁵ The VAs as they ascend superiorly pass through the foramina transversarium of 6 cervical vertebrae and atlas, they then exit travelling posteriorly and medially over the superior articular processes to penetrate the atlanto-occipital ligament and enter the skull through the foramen magnum. The atlanto-occipital joint is a major site for neck rotation and the VA is relatively immobilized at its exit from the C1 transverse process and its entry into the atlanto-occipital ligament; this anatomical relationship makes the VA prone to occlusion or injury at this site. Congenital cervical abnormalities are also more prevalent at this level and increase the likelihood of rotational injury.¹⁵ Of note, vascular injury from bony elements is not unique to the vertebrobasilar circulation and may occur in the carotid circulation. Previous studies have suggested an association with cervical carotid dissections and increased styloid process length.¹⁶ Typically, patients present with presyncope, syncope, vertigo, diplopia, and horizontal nystagmus directed towards the side of the compressed vessel. The ischemic symptoms are frequently transient; however, irreversible deficits including medullary and cerebellar infarctions have also been described.¹⁷ The direction of head rotation does not reliably predict the side of vessel compression.¹⁸ The condition may be demonstrated on vascular ultrasound, CTA, or MRA. DSA is the preferred method of diagnosis.¹⁸ In one series, half of those determined to have BHS did not show evidence of dynamic occlusion on ultrasound, though this was the most commonly used preoperative imaging modality.¹⁸ Angiography provides precise localization of the flow limiting lesion, allows assessment of collateral flow, and confirms the presence of dynamic occlusion or compression.⁵ Occasionally there can be more than one compressive site.¹⁹ In those cases where dynamic occlusion cannot be demonstrated, potential thromboembolic mechanisms (nonocclusive) such as pseudoaneurysm formation, thrombosis, or dissection should be sought.¹⁵ CTA may be equal or superior to MRA with fat suppression imaging of the neck to detect a VA dissection.^20,21 A detailed history and physical examination is essential to exclude other alternative diagnoses that may lead to posterior circulation ischemia. Common mimics include VA dissection; vasculitis; reversible cerebral vasoconstriction syndrome; mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes; and subclavian steal phenomenon or cardiac or paradoxical embolism.²²

BHS is a rare disorder and its management has not been standardized, but it is most often surgical.⁵ An initial conservative approach may include neck immobilization using a cervical collar or neck brace. In addition, the patient should be instructed to avoid rotating the neck in the symptomatic direction.¹⁷ Both antiplatelets and anticoagulation have been used for secondary stroke prevention in cases of BHS.¹⁷ A previous randomized clinical trial of 250 patients comparing antiplatelet agents with anticoagulation in cases of extracranial carotid and vertebral dissection concluded that there was no difference in efficacy between the 2 groups in terms of secondary stroke prevention and death.²³ Other authorities have suggested benefit from anticoagulation during the acute period of cervical or vertebral dissections as most strokes happen within a 7-day period after brain ischemia.^23,24 However, it is not clear if these results are applicable to all cases of ischemic stroke due to BHS as evidence of vascular injury (i.e., dissections or pseudoaneurysms) may not be evident. Surgical correction can be potentially curative. Frequently used procedures include surgical decompression of the VA or corrective fusion of C1-2.¹⁸ More recently, success with endovascular stenting of VAs or coil embolization of a symptomatic nondominant VA have also been reported (with patent dominant VA).^4,25,26 Management must be individualized, based on the underlying mechanism of vascular compression and injury.