Bilateral Bow Hunter Syndrome Associated with Loss of Cervical Physiological Curvature

Introduction

In 1978, Sorensen reported a case of dorsolateral medulla syndrome caused by head-turning archery and named it bow hunter syndrome [1]. Bow hunter syndrome is a rare syndrome characterized by insufficient blood circulation due to mechanical compression or occlusion of the vertebrobasilar artery during rotation or hyperextension of the head and neck. It is now referred to officially as rotational occlusion of the vertebral artery syndrome. The most common causes of bow hunter syndrome include osteophytes, fibrous band formation, cervical disc herniation, tumors, and other anatomical abnormalities of the cervical spine [2–4]. While bow hunter syndrome is commonly observed in mechanical occlusion of 1 vertebral artery, bilateral occlusion is rare [5,6]. Clinical symptoms of bow hunter syndrome include vertigo, nausea, vomiting, nystagmus, and syncope. In severe cases, it can lead to hemiblindness, ataxia, dysphagia, consciousness disorder, and other posterior circulation infarction [7,8]. The patient described in the present case report underwent digital subtraction cerebral angiography (DSA), which confirmed mechanical compression of the head and neck of both vertebral arteries during rotation, resulting in obvious hemodynamic disorder and posterior circulation ischemia. However, magnetic resonance imaging (MRI) and computed tomography (CT) examinations did not reveal common causes such as osteophytes, suggesting a possible association between the change in cervical vertebrae curvature and the occurrence of bow hunter syndrome.

Case Report

A 25-year-old woman was admitted to the hospital primarily due to experiencing dizziness for a period of 10 hours. The dizziness worsened when the right side of the neck was turned and the body position changed. Additionally, the patient experienced visual rotation, nausea, and vomiting.

Past history: The patient reported no history of cervical trauma or injury and had no vascular risk factors. However, she did mention prolonged use of computers and mobile phones with poor posture for more than 8 hours daily.

Physical examination: The patient’s vital signs were within normal range. Her neurological examination revealed no abnormalities, except for the presence of suspicious horizontal nystagmus in both eyes during head rotation to the right. There were no other indications of brainstem involvement.

Auxiliary examination: Random blood glucose level upon admission was 6.1 mmol/L, and routine blood tests showed no abnormalities. Head CT and electrocardiogram results were also normal. A cervical spine MRI revealed the disappearance of the physiological curvature of the cervical spine (Figure 1). Color Doppler ultrasound indicated a maximum flow velocity of 44.6 cm/s in the left vertebral artery in the standard body position, which decreased to 18.2 cm/s when the head was turned to the right (Figure 2). Cerebral angiography showed that the main branches of the bilateral vertebral arteries, distal basilar arteries, bilateral posterior cerebral arteries, and other main branches were normal. However, there was a decrease in blood flow and delayed development in the distal end of the bilateral vertebral arteries in cephalometric angiography. When the head was turned to the right, the blood flow in the left vertebral artery was slow, leading to immediate dizziness symptoms in the patient. Computed tomography angiography (CTA) revealed varying degrees of compression in the circum-axial junction area of the bilateral vertebral arteries when the head was turned (Figures 3–5).

Bow hunter syndrome was diagnosed based on the clinical symptoms and the above examinations. The patient was given a cervical collar to limit excessive twisting of the neck and instructed to avoid large-angle deflection of the neck after discharge. During the 3-month follow-up, no symptoms such as dizziness reappeared.

Discussion

According to the literature, bow hunter syndrome primarily affects individuals between the ages of 50 and 70 [9]. However, it can also occur in adolescents and young adults, with the left vertebral artery being commonly affected [7]. Bow hunter syndrome is categorized into atlantoaxial type, subaxial type, and mixed type based on the site of vertebral artery compression [4]. In this case, the patient has atlantoaxial type bow hunter syndrome, with compression occurring at the C1-C2 level, affecting both bilateral vertebral arteries. When the patient turned her neck, the bone margin of the axis compressed the vertebral artery, leading to stenosis and reduced blood flow. Consequently, there was insufficient blood supply to the posterior circulation, resulting in dizziness. The right vertebral artery was slightly thinner than the left, with the left vertebral artery being the dominant supplier of blood to the posterior circulation. Therefore, compression of the left vertebral artery led to noticeable symptoms, including dizziness. Both angiography and color Doppler ultrasound confirmed significant impairment of posterior circulation blood flow during neck rotation in this patient.

Jost and Dailey conducted a review and analysis of 126 patients with bow hunter syndrome. Their findings indicated that syncope, syncope-like symptoms, and dizziness were the most common symptoms associated with bow hunter syndrome [5]. However, it is important to note that these symptoms are not exclusive to bow hunter syndrome, as they can also be present in conditions such as vestibular disease, carotid stenosis, and vertebral artery-type cervical spondylosis. Therefore, relying solely on clinical symptoms for diagnosis can be challenging. Currently, DSA is considered the criterion standard for diagnosing bow hunter syndrome. This invasive examination allows for the visualization of vascular conditions, specific compression sites, and collateral circulation in both neutral and head-turned positions [10]. However, it is important to consider that DSA may not be suitable for all patients due to contraindications. In this particular case, the patient underwent vertebral artery ultrasound examination in both the cephalic and right positions. The examination revealed significant hemodynamic changes in the left vertebral artery during right neck rotation. These hemodynamic changes during head and neck rotation are believed to be the underlying cause of bow hunter syndrome. This finding was subsequently confirmed by DSA, which showed a slow decrease in the flow rate of the left vertebral artery during right neck rotation, thus establishing a correlation between the ultrasound examination and the DSA results. Orlandi et al reported clear hemodynamic changes in dynamic vascular ultrasound results for patients with bow hunter syndrome [11]. Following the identification of hemodynamic changes, the next step is to assess vascular morphology using CTA imaging. CTA can provide visualization of the anatomical structure between cervical blood vessels and cervical verte-brae, and comparing CTA images in different head and neck positions can further reveal vascular changes. In this particular case, CTA confirmed compression of the vertebral artery after cervical rotation. Therefore, the combination of vascular ultra-sonography and CTA can serve as a non-invasive and effective diagnostic method. Compared with DSA, this approach eliminates the need for radiation exposure and is simple to perform. Notably, dynamic vascular ultrasound can be incorporated as a routine screening tool for suspected cerebrovascular disease, enabling early detection of blood flow changes associated with head position alterations.

Bow hunter syndrome is a condition that occurs when the vertebral artery is compressed by abnormal bony structures during head and neck rotation or hyperextension. The compression can be categorized into primary and secondary causes. Primary causes include osteophytes, idiopathic bone hyper-plasia, congenital dysplasia, intervertebral disc herniation, ligament hyperplasia, and ossification, as well as instability of the uncinate vertebral joint [4]. Secondary causes refer to bow hunter syndrome resulting from complications of iatrogenic operations (such as vertebral surgery) or head and neck trauma [12]. In this particular case, the patient had not undergone any surgery nor had she experienced trauma, ruling out secondary compression. The patient, a young woman, had well-developed cervical vertebrae as observed in the CTA scan, with no signs of osteophytes, atlanto-occipital fusion, or vertebral joint instability. The MRI also revealed no significant disc herniation, ligament hyperplasia, or ossification. This case is unique as the patient presented with only loss of cervical physiological curvature. The reason behind this may be the compression of the vertebral artery’s extensible space due to the loss of cervical physiological curvature. When her neck was turned, the limited extensible space of the vertebral artery and the restriction caused by the vertebral bone margin resulted in passive compression and compression stenosis of the artery. Consequently, blood flow became difficult, posterior traffic was not opened, and there was no compensatory blood flow supplement in the anterior circulation, leading to symptoms of posterior circulation ischemia such as dizziness. Di Stefano et al suggested that the abnormal entry of the vertebral artery into the transverse foramen may be more prone to compression during long-path rotation, indicating a possible link between the vertebral artery pathway and the occurrence of bow hunter syndrome [13]. In the present case, it is likely that the loss of cervical physiological curvature altered the vertebral artery pathway, making it more susceptible to compression and the occurrence of bow hunter syndrome during rotation.

There are no unified treatment guidelines for bow hunter syndrome. The currently reported treatment methods include conservative, surgical, and interventional treatments [4,14,15]. In general, the primary treatment goal for patients with bow hunter syndrome is to reduce or prevent disease recurrence. Common treatments include neck protection, such as wearing a neck retractor to limit excessive neck twisting and to avoid large-angle deflection of the neck. In some severe cases, surgical intervention may be required to correct the vertebral artery compression. Some patients achieve good results with interventional treatments [16,17]. In our case, the patient’s symptoms were relieved after restricting excessive twisting and stretching of the neck, in line with the current treatment principles.

The prevalence of bow hunter syndrome remains unclear, with most previously reported cases occurring in individuals between 50-70 years of age. There is a limited number of case reports involving young patients, possibly due to a lack of understanding of the disease, leading to missed or incorrect diagnoses. The occurrence of bow hunter syndrome in this young patient suggests that it may not be an extremely rare condition. Utilizing dynamic vascular Doppler ultrasound or transcranial Doppler (TCD) to screen patients experiencing dizziness, syncope, and/or other related nervous system symptoms may aid in timely detection and diagnosis of bow hunter syndrome, contributing to a better understanding of its prevalence. This case report aims to provide the medical community with additional knowledge and information about bow hunter syndrome, while also stimulating further research interest to improve the diagnosis, treatment, and overall medical care for patients.

Conclusions

Bow hunter syndrome is a rare clinical posterior circulation compression syndrome with complex etiology. The present case suggests that loss of cervical physiological curvature may be related to the occurrence of bow hunter syndrome. The dynamic monitoring of blood flow by color Doppler ultrasound and TCD in different head positions provides clear clues to confirm or refute suspected bow hunter syndrome. With the help of CTA, the diagnosis of bow hunter syndrome may be obtained by noninvasive examination.