Posterior Circulation Ischemic Stroke From Atlantoaxial Instability and Lateral Dislocation due to Os Odontoideum: Case Report and Review of Literature

ABSTRACT

Background

Strokes in young individuals often stem from unusual causes. Posterior circulation ischemic stroke caused by vertebral artery insufficiency due to atlantoaxial instability or dislocation is rare. We present a case of posterior circulation ischemic stroke due to an unstable os odontoideum and review the current literature. The clinical features and imaging manifestations are described to promote awareness of etiology, early diagnosis, and assessment.

Case Presentation

A 24‐year‐old male presented with recurrent right‐sided limb numbness and weakness and cerebellar ataxia due to posterior circulation ischemic stroke. The work‐up revealed thrombosis reformation in the tortuous left vertebral artery. It is noteworthy that the patient developed compression and chronic damage of the vertebral artery secondary to atlantoaxial instability and lateral dislocation due to an os odontoideum. He underwent antiplatelet and anticoagulant therapy, cervical traction, and posterior atlantoaxial screw fixation and fusion with iliac crest autograft. The postoperative course was uneventful. At 6‐month follow‐up, the patient had a solid fusion mass and rigid stability of the atlantoaxial joint without neurologic deficits or ischemic sequelae.

Conclusions

For unexplained posterior circulation ischemic stroke, it is important to consider unstable os odontoideum as a potential etiology, especially in pediatric and young adult male patients. Atlantoaxial instability and dislocation with os odontoideum, especially when occurring laterally, may cause insufficiency of the vertebral artery and subsequent posterior circulation ischemic strokes. The significance of lateral atlantoaxial dislocation in the genesis of vertebral artery injury and the necessity for specific positional imaging are emphasized.

Atlantoaxial instability and dislocation due to unstable os odontoideum can be an underlying cause of unexplained posterior circulation ischemic stroke in young adults. Recognizing this condition is crucial for diagnosis and treatment to prevent vertebral artery insufficiency and subsequent strokes.

1. Introduction

Posterior circulation ischemic stroke (PCIS) is ischemia associated with the vertebrobasilar arterial system, commonly affecting the brainstem, cerebellum, and occipital lobes [1]. Unlike in the elderly, strokes in younger populations are often caused by cardiogenic emboli, vascular dissection, and other rare causes such as vasculitis or thrombophilia [2]. PCIS from vertebral artery (VA) insufficiency due to atlantoaxial instability (AAI) or atlantoaxial dislocation (AAD) is rare. This report describes a case of recurrent PCIS from AAI and lateral dislocation due to an unstable os odontoideum (OO) to promote awareness, early diagnosis, and assessment of PCIS with an etiological origin in the upper cervical spine.

2. Case Report

A 24‐year‐old male presented with recurrent right‐sided limb numbness and weakness, dizziness, and cerebellar ataxia over the past 4 years. He reported no history of trauma nor a family history of early‐onset stroke. The initial onset was relieved after thrombolytic therapy, with dizziness reoccurring 1 week later. Digital subtraction angiography (DSA) demonstrated a winding, “string‐of‐beads” change in the V3 segment of the left VA (Figure 1a). Magnetic resonance imaging (MRI) demonstrated acute infarctions in bilateral cerebellum and a mural thrombus formation in the V3 segment of the left VA, possibly compressed by the lateral mass of the atlas (Figure 1b). Rivaroxaban, aspirin, and atorvastatin were prescribed by a local hospital. Two and a half years later, the aforementioned medications were discontinued because a follow‐up showed complete thrombosis absorption (Figure 1c). Unfortunately, symptoms reappeared 1 year later, with thrombosis reformation (Figure 1d), prompting referral to our hospital.

FIGURE 1.

Angiography images over the past 4 years. (A) Anteroposterior view of the left VA angiograms during the second episode, demonstrating a winding “string‐of‐beads” change (arrow) in the V3 segment. (B) Black blood sequence, showing a mural thrombus formation (arrow). (C) The follow‐up magnetic resonance angiography after conservative treatment, showing complete absorption of the thrombosis in the left VA. (D) Anteroposterior view of the left VA angiogram during the third episode, demonstrating thrombosis reformation (arrow) in the V3 segment.

Physical examination revealed normal limb muscle strength during interictal periods. Neurologic examination revealed bilaterally hyperactive deep tendon reflexes, normal sensation, intact cranial nerves, and negative pathological signs. MRI showed old infarcts in the bilateral posterior inferior cerebellum and myelopathy at C1 level (Figure 2a,b). Compression and narrowing of the left VA were indicated in the black blood sequence (Figure 2c). Cervical computed tomography (CT) revealed OO and a leftward dislocation of the atlas relative to the axis (Figure 2d,e). Flexion‐extension lateral cervical x‐rays revealed AAI and increased anteroposterior mobility (Figure 2g,h). The head‐turning test was negative in the neck vascular ultrasound. The echocardiogram indicated a patent foramen ovale (PFO).

FIGURE 2.

Preoperative radiographic images. (A) A transverse view of T2‐weighted MRI, showing old cerebellar infarcts in bilateral PICA territory and left SCA territory. (B) Sagittal view of T2‐weighted MRI showing OO and myelopathy (arrow) at C1 level. (C) Black blood sequence showing lateral AAD (red arrow) and compressed left VA (yellow arrow). (D) CT sagittal multiplanar reconstruction with OO. CT coronal multiplanar reconstruction in pre‐traction (E) and post‐traction (F) states compares the alignment of the atlas (red arrow) and axis (yellow arrow). (G,H) Flexion‐extension lateral X‐rays of cervical spine revealing AAI with OO.

After admission, the patient was transitioned from oral anticoagulants to low molecular weight heparin and underwent 1 week of cervical traction to correct lateral AAD (Figure 2f). With comprehensive preoperative preparation, the patient underwent posterior atlantoaxial screw fixation and fusion under general anesthesia. The patient was positioned prone and secured with Gardner‐Wells tongs for traction. An O‐arm scanner verified the optimal reduction of C1–C2. The posterior arch of C1 was resected for decompression. Internal fixation of C1–C2 was performed with pedicle screws and titanium rods. Supplementally, iliac crest bone grafts were placed for fusion between posterior structures and secured with sutures. The patient had an uneventful course without complications and was discharged on the seventh postoperative day. Afterwards, the patient was immobilized with a head–neck‐chest plaster cast for 3 months and continued aspirin therapy considering PFO. Six months postoperatively, follow‐up images (Figure 3) indicated that, although anatomical reduction was not completely achieved, compression of the left VA had been released. Good stability, decompression of the spinal cord, and a solid union of the grafted autologous bone were obtained. The patient is well without neurologic deficits, ischemic sequelae, or new cerebral infarction lesions.

FIGURE 3.

Postoperative follow‐up images at 6 months. (A,B) Anteroposterior and lateral x‐rays showing posterior atlantoaxial fixation and fusion. (C) MRI showing satisfactory posterior decompression (arrow) of the spinal cord. (D) CT sagittal multiplanar reconstruction showing a solid union of the grafted autologous iliac bone (arrow). (E) CT angiography coronal multiplanar reconstruction showing the release of compression of the left VA (arrow).

3. Discussion

OO is a rare anomaly of the craniocervical junction, defined as an independent ossicle separated from the foreshortened odontoid peg at the body of the second cervical vertebrae (C2) [3]. There is still debate about its etiology. The congenital hypothesis posits a failure of the odontoid process or its apex to fuse with the main section of the axis body [4]. In contrast, the traumatic hypothesis links OO to unrecognized odontoid fractures and bone remodeling [5].

The odontoid process plays a critical role in stabilizing the atlantoaxial joint. In OO, the failure of the transverse ligament of the atlas due to the loss of bony support can subsequently lead to AAI or AAD. Clinical presentation of OO is variable depending on the resulting upper cervical stability. Some patients are asymptomatic or experience milder symptoms, such as neck pain, while some can present with neurological dysfunction, such as myelopathy or stroke‐like symptoms [6, 7]. It is noted that the latter is exceedingly rare and may present as an initial symptom.

Among the cases reported so far (Table S1), the population of strokes associated with unstable OO is almost exclusively male, with the majority being pediatric or young adult patients. These cases fall into two categories: one where infarction results from occlusion of the VA itself or reduced blood flow in collateral branches, and another where chronic intimal damage of the VA leads to thrombus formation and arterio‐arterial embolism in the supplied region. A commonality is that the lesion is mostly identified at the V3 segment. Most patients exhibited symptoms of headache, vomiting, cerebellar ataxia, and motor disturbance such as hemiplegia and dysarthria. Few patients experienced consciousness disturbance or cranial nerve dysfunctions, including nystagmus and painless visual loss.

In this report, the infarcts were located in the posterior inferior regions of the cerebellum, which the VAs predominantly supply. Moreover, thrombosis formation was observed in the V3 segment of the left VA at the onset of symptoms, which provides strong evidence of AAI and AAD as a cause of PCIS. The embolus could also stem from a right‐to‐left shunt caused by the PFO. However, we found no evidence of deep vein thrombosis in the lower limbs. The exact mechanism of PCIS caused by OO remains unclear. It is generally thought that repeated kinking and stretching of a hyper‐curved VA between the transverse foramina in an unstable atlantoaxial joint leads to intimal damage, promoting thrombosis formation and vertebrobasilar occlusion [8, 9]. Although increased anteroposterior mobility is the most common type of AAI or AAD, the distinguishing feature of this case is the concurrent lateral dislocation of the atlantoaxial joint. Previous reports have not mentioned lateral AAD, which can be a critical factor in VA injury. The lateral displacement of the atlas causes definitive compression on the VA (Figure 4), leading to contact‐induced and friction‐mediated microtrauma between the lateral mass and the VA during flexion–extension and rotational movement of the cervical spine. This situation presents a more direct mechanism of injury and may account for the patient's higher recurrence rate and frequency.

FIGURE 4.

(A,B) 3D‐printed cervical model, illustrating the leftward dislocation of the atlas relative to the axis and the positions of VAs in relation to the atlas. The left VA is compressed by the inferior edge of the lateral mass (B, arrow).

Although myelopathy occurs more commonly, instability of the upper cervical spine can also lead to compression or occlusion of the vertebrobasilar arterial system, with PCIS potentially presenting as the initial symptom. However, due to the lack of distinctive symptoms, the dynamic compression of VA caused by AAI or AAD is easy to be overlooked in patients with PCIS. To enhance the identification rate, Vupputuri et al. [10] proposed a management algorithm and advised a flexion‐extension x‐ray for all patients of PCIS. Moreover, thrombosis or dissection occurring in the V3 segment of the VA warrants particular attention, given the higher mobility of bone structures surrounding this location. Sawlani et al. [11] described one VA exhibiting a shortened and straighter course. It is defined as the “Stretched loop sign”, predisposed to thrombosis and occlusion. For patients with confirmed or suspected instability of the upper cervical spine, it is imperative to conduct CT angiography under specific positional conditions according to the potential mechanism of injury. This facilitates the acquisition of evidence for VA compression or occlusion. Furthermore, this case highlights the importance for spine surgeons to assess not only high‐riding VA but also the history of the cerebral infarction, the presence of AAI, and lateral dislocation.

The treatment for unstable OO includes conservative management and surgical intervention. Conservative treatment encompasses cervical immobilization with a neck brace and antiplatelet or anticoagulation therapy. The presence of neurologic symptoms is typically a clear indication for surgical intervention regarding instrumented stabilization and C1–C2 fusion. Prophylactic fusion may not be suitable for asymptomatic patients, as there was no significant difference in outcome compared with conservative treatment [12, 13]. Furthermore, Dai et al. [14] contended that asymptomatic patients, once exhibiting clear signs of instability, should also receive fusion surgery to prevent possible neurological complications. Previous studies [6, 15] have discussed the efficacy of posterior atlantoaxial fusion surgery and internal fixation. Although complete reduction of the AAD is preferable, strong stabilization of the atlantoaxial joint is the goal of surgery. The majority of patients experienced favorable outcomes. Occipitocervical fusion or fixation is not recommended unless atlantoaxial fusion fails or bone quality is poor [6, 15], as it may significantly restrict neck mobility.

4. Conclusion

A rare case of PCIS secondary to unstable OO is described. Upper cervical instability may cause not only spinal cord injury but insufficiency of the VA as well. Instability or dislocation of the atlantoaxial joint, particularly when occurring laterally, is a key factor in the genesis of VA injury. Posterior atlantoaxial screw fixation and fusion yielded good clinical results with the 6‐month follow‐up. More attention should be paid to the OO and resultant vertebrobasilar artery insufficiency in patients of PCIS, especially in pediatric and young adult male patients.

Author Contributions

All authors had full access to the data in the study and take responsibility for the integrity of the data. Conceptualization: Hongli Wang and Xiang Han. Methodology: Hongli Wang, Weijun Tang, and Xiaosheng Ma. Investigation: Xianghe Wang, Yao Li, and Minghao Shao. Formal Analysis: Jixian Lin and Xiang Han. Resources: Jingjuan Liang and Xiaosheng Ma. Writing – Original Draft: Xianghe Wang. Writing – Review and Editing: Yao Li, Hongli Wang, and Xiang Han. Visualization: Xianghe Wang and Weijun Tang. Supervision: Xiaosheng Ma and Jingjuan Liang.

Ethics Statement

This study was exempt from ethics approval. The exemption was granted by the Huashan Hospital Institutional Review Board (HIRB), Fudan University.

Consent

Written consent was obtained for surgical procedure. Consent for publication is waived as all patient's information is de‐identified.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Table S1. PCIS associated with os odontoideum.

Data Availability Statement

Data and material mentioned in the submitted work are available by contacting the corresponding author.