Bow Hunter Syndrome (BHS) is a rare medical condition where specific movements of the head and neck restrict blood flow to the brain, causing temporary neurological symptoms. The condition affects non-hunters who must hold their heads in a rotated or extended position. While treatable, a fatal outcome is rare, though the risk of stroke makes early diagnosis important.
Understanding Positional Vertebral Artery Compression
BHS is medically known as Rotational Vertebral Artery Syndrome (RVAS) or Positional Vertebral Artery Compression. The condition involves a temporary mechanical narrowing or blockage of one of the two vertebral arteries in the neck. These arteries supply blood to the posterior circulation of the brain, including the brainstem and cerebellum, which control balance and coordination.
Compression of the artery is triggered by rotation or extension of the head, causing the vessel to be pinched or kinked. This mechanical obstruction is often caused by an underlying structural abnormality in the cervical spine, such as a bone spur (osteophyte), a herniated disc, or a fibrous band. The most common locations for this compression are at the C1-C2 level (atlantoaxial joint) or the C5-C7 level of the neck vertebrae.
The compression becomes symptomatic when blood flow is critically reduced, especially if the contralateral vertebral artery cannot compensate due to being naturally smaller (hypoplastic) or already narrowed. When the head returns to a neutral position, blood flow is usually restored, and symptoms quickly resolve. This positional and transient nature defines the syndrome.
The Direct Risk of Severe Neurological Injury
BHS carries a risk of posterior circulation stroke, also known as vertebrobasilar ischemia. This occurs when the temporary lack of blood flow is severe or prolonged enough to cause permanent damage to the brainstem or cerebellum.
Repetitive mechanical compression can injure the vessel wall, leading to the formation of a blood clot. If a clot forms, it can travel downstream and block a smaller artery in the brain, resulting in an ischemic stroke.
A review of patients with BHS-related injuries found that the long-term impairment or mortality rate was around 28% among those who suffered a stroke. The severity of the neurological injury depends on which part of the brain is affected and the extent of the blocked blood flow.
The risk is heightened in individuals who may already have other vascular risk factors, such as hypertension or diabetes, or who have limited collateral blood flow to the posterior brain. The condition is treated seriously because it is a surgically correctable cause of stroke, requiring immediate medical intervention to mitigate the potential for permanent neurological damage.
Identifying the Key Symptoms of BHS
The symptoms of BHS are transient and reliably triggered by a specific head movement, such as turning the head sharply to one side. The most frequently reported symptoms are positional vertigo (a spinning sensation) and general dizziness or lightheadedness.
Other common warning signs include blurred or double vision (diplopia), unsteadiness, or difficulty maintaining balance (ataxia). More severe episodes can cause slurred speech (dysarthria), nausea, or even fainting (syncope).
These neurological deficits resolve almost immediately upon returning the head to neutral. A sudden drop in blood pressure to the brainstem can cause a transient loss of consciousness. The predictability of the symptoms with head movement helps distinguish BHS from other causes of dizziness, such as inner ear disorders.
Diagnosis and Treatment Pathways
Diagnosing BHS requires demonstrating that the vertebral artery is mechanically compressed during head movement. Standard static imaging, such as an MRI or CT scan, is insufficient because the compression is only present when the head is turned; therefore, dynamic imaging studies are necessary to capture the occlusion in real-time.
The gold standard diagnostic method is Digital Subtraction Angiography (DSA) performed while the patient’s head is rotated to the symptomatic position. Less invasive alternatives include dynamic Magnetic Resonance Angiography (MRA) or Dynamic CT Angiography (CTA), which also image the blood vessels during head rotation. These tests confirm the diagnosis by showing a significant reduction or complete cessation of blood flow in the vertebral artery.
Treatment begins with conservative management, especially for patients with mild or infrequent symptoms. This includes patient education to avoid the specific head movements that trigger symptoms and, in some cases, the use of a soft cervical collar to limit neck rotation. Medications like antithrombotics may also be prescribed to reduce the risk of clot formation.
For severe cases where conservative measures fail, or where there is a high risk of stroke, surgical intervention is considered. The goal of surgery is either to decompress the artery by removing the bony spur or fibrous band causing the obstruction, or to stabilize the spine. Surgical options include vertebral artery decompression or, for cases involving instability at the C1-C2 joint, a cervical fusion to prevent the movement that causes compression.