Os odontoideum: a rare cause of syncope

Abstract

Syncopal events are a concerning presentation and timely evaluation is warranted. Common aetiologies include cardiac and neurological pathology such as arrhythmias, vertebrobasilar arterial disease and vasovagal syncope. We describe the case of a 65-year-old man who presented to our emergency department with symptoms of vertigo and syncope. He was investigated extensively for both cardiac and neurological causes of his symptoms which returned negative results. An outpatient CT scan demonstrated the presence of Os odontoideum and dynamic instability of the atlantoaxial junction, with presumed dynamic obstruction of the vertebral arterial system. This was successfully managed with a posterior atlantoaxial lateral mass fusion with resolution of syncopal symptoms.

Background

Syncopal event accounts for a great portion of Emergency Department presentations, and cardiac and neurological testing is required to identify the cause; however, in many instances, a cause cannot be identified. Our case highlights a rare and underappreciated structural pathology which can be a cause of syncope.

Case presentation

Our patient was a 65-year-old retired man who presented with a 12-month history of recurrent episodes of syncope and presyncopal episodes. These episodes often developed seconds after our patient rotated his head away from the neutral position. These rotational movements would lead to brief episodes of either vertigo or fainting or a combination of the two. No other common triggers were identified on clinical history taking, which was conducted by several clinicians. Our patient denied mechanical neck pain, or any symptoms suggestive of a radiculopathy in the upper limbs. There were no symptoms to suggest systemic disease, and other than these episodes, he was well.

His prior medical history included a carotid sheath tumour resection, hypertension and type 2 diabetes mellitus. Our patient took candesartan, solifenacin and aspirin daily. A retired plumber, he drank alcohol only at social gatherings and was an ex-smoker of two pack years. Examination of our patient was unremarkable with no myelopathic signs demonstrable. He had normal power and sensation in all four limbs.

Investigations

• ECG—sinus rhythm.

• Blood pressure—no evidence of orthostatic hypotension.

• CT brain—normal.

• CT brain angiogram.

  • Left vertebral artery dominance.

  • Minor stigmata of atheroma at both bifurcations.

  • Minor calcification present at the cavernous carotids.

  • Appearances were within normal limits for a patient this age.

• Chest X-Ray—normal.

• Transthoracic echocardiogram.

  • Normal left ventricular size and systolic function.

  • Left atrium and pulmonary artery normal size

  • Right ventricular size and systolic function normal.

  • Aortic valve sclerosis.

  • No significant valvular disease.

• Holter monitor

  • No events recorded.

• Implanted loop recorder

  • No events recorded.

• CT cervical spine.

  • Non-union of odontoid to C2 vertebral body (figure 1).

Figure 1.

CT cervical spine—a CT cervical spine of our patient demonstrating non-union of the odontoid to the C2 vertebral body.

• Flexion extension cervical spine XR—C1/C2 instability—severe (figure 2).

Figure 2.

Preoperative X-ray of the cervical spine of our patient demonstrating dynamic instability of C1 and C2 with flexion and extension.

• MRI brain.

  • Normal MR angiogram.

  • Os odontoideum noted.

  • Left vertebral artery dominance with right hypoplastic vertebral artery (figure 3).

Figure 3.

An Magnetic Resonance Angiogram demonstrating our patient's hypoplastic right vertebral artery.

Differential diagnosis

A diagnosis of bow hunter’s syndrome was made. This was based on our history, examination and investigations which demonstrated atlantoaxial instability in the presence of Os odontoideum with presumed dynamic obstruction of the vertebrobasilar system. Although we did not conduct dynamic angiography or digital subtraction angiography to further confirm this diagnosis, the history of syncope and presyncope with head rotation combined with imaging findings including atlantoaxial instability, was enough for our team to make this diagnosis and proceed to treatment.

Treatment

Our patient underwent posterior atlantoaxial lateral mass fusion 3 weeks after presentation. A cervical spine XR demonstrated atlantoaxial stability of the cervical spine on flexion and extension (figure 4). He had an unremarkable recovery and was discharged on day 3 postoperatively.

Figure 4.

Postoperative X-ray of the cervical spine of our patient demonstrating fixation of C1 and C2 with confirmation of stability on flexion and extension views.

Outcome and follow-up

Our patient performed well postoperatively. At his 6-week follow-up, our patient had no further episodes of presyncope or syncope and returned to driving. At 1 year, our patient remained asymptomatic.

Discussion

This case study demonstrates the rare finding of Os odontoideum causing atlantoaxial instability and bow hunter’s syndrome. Os odontoideum describes a spinal abnormality where an ossicle with smooth cortical margins representing the odontoid process is not in continuity with the body of the C2 vertebrae. The ossicle itself can be in anatomic alignment with the dens of the C2 vertebrae, moving with the anterior arch of the atlas; or out of alignment functionally fused to the basion. These configurations are described as orthotopic and dystopic, respectively.¹ Regardless, a gap exists between the axis and the free ossicle above the level of the superior facets of the axis leading to incompetence of the cruciate ligament, and therefore atlantoaxial instability.²

First described in 1886 by Giocomini on cadaveric studies,³ the true prevalence of Os odontoideum is unknown and often the finding is incidental.⁴ The disorder is thought to be of both congenital and traumatic aetiology.⁵ Congenital theories centre around the failure of fusion between the dens and the body of the axis. Evidence for this include the presence of Os odontoideum in identical twins,⁶ an increased expression of genes in patients with Os odontoideum⁷; and the association of the disorder in patients with Down syndrome, Morquio syndrome and Wolcott-Rallison syndrome.^8–11

The traumatic hypothesis theories that an initial injury to the dens occurs and that there is subsequent avascular necrosis and osseous remodelling which lead to Os odontoideum. It is likely that two independent aetiologies exist for the development of Os odontoideum.¹ Regardless, the presence of atlantoaxial instability is of importance clinically as it may affect the neural and vascular tissues at this region.

The largest case series of Os odontoideum leading to atlantoaxial instability was described by Zhao et al. In their study of 279 patients, 84.9% of patients presented with pyramidal signs, and a large majority presented with weakness, numbness or neck pain. Interestingly, only 40.1% of patients had a history of trauma. In contrast to our patient, none of the patients in this study experienced syncopal events.¹

Bow hunter’s syndrome or rotational vertebrobasilar insufficiency describes transient occlusion or stenosis of the vertebral artery system associated with changes in head position. The symptoms of bow hunter’s syndrome range from vertigo like symptoms to ischaemic stroke.¹² A case like our own was described by Ford, where a 17- year-old man suffered symptoms of syncope, nystagmus, diplopia and ataxia on head rotation. Os odontoideum was discovered, and a C1–C2 fixation procedure resolved his symptoms.¹³ A further case has been described by Kikuchi where bilateral vertebrobasilar insufficiency due to atlantoaxial instability in the presence of Os odontoideum has led to ischaemic stroke.¹⁴

Posterior C1–C2 internal fixation and fusion is recommended for patients with Os odontoideum who suffer any neurological symptoms or signs or demonstrated C1–C2 instability. In cases where patients do not suffer symptoms or signs, clinical or radiographic surveillance may be an option in management.¹⁵ Where Os odontoideum is suspected as the cause of syncopal events, referral to a spinal surgeon is highly recommended to prevent possibly catastrophic outcomes.

Learning points.

• Atlantoaxial instability and bow hunter’s syndrome is a possible cause of syncopal events and must be considered where traditional investigations have been exhausted.

• Os odontoideum describes an ossicle with smooth cortical margins representing the odontoid process that is not in continuity with the body of C2. It is a cause of atlantoaxial instability and may be congenital or acquired.

• A cervical spine CT will demonstrate this abnormality and a flexion extension cervical spine XR will demonstrate the degree of instability at the atlantoaxial junction.

• Posterior fixation is a surgical option which can relieve the symptoms and help prevent serious sequelae.