[Preoperative Simulation for Complication Control in Spinal Surgery].

The craniovertebral junction not only contains anatomically important structures such as the medulla oblongata, upper cervical spinal cord, and vertebral artery, but also controls the dynamic movements of flexion, extension, and rotation of the head and neck. Consequently, instability and spinal deformities can easily occur in the craniovertebral region, and appropriate treatment should be selected based on the specificity of the lesion. Basilar invagination often involves bone and vascular anomalies and fusion surgery is often required. Therefore, careful pre-operative simulations are necessary. The creation and use of three-dimensional bone models, including image navigation, are useful for surgical simulation.

Atlanto-occipital ligament calcification: a novel imaging finding in pediatric rotational vertebral artery occlusion.

We describe a 2-year-old girl with bow hunter syndrome complicated by vertebral artery dissection and multiple ischemic infarcts. Pediatric bow hunter syndrome is a rare and likely under-recognized disorder. Interestingly, our patient had atlanto-occipital ligament calcification on CT scan, an imaging finding that has not been reported in association with bow hunter syndrome and one that might help increase recognition of this dynamic disorder of the posterior circulation.

Basilar invagination in a child with atlanto-occipital subluxation and suspected prenatal Dandy-Walker malformation.

BACKGROUND AND PURPOSE: Although advances in imaging have allowed earlier and more accurate diagnosis of various fetal anomalies, Dandy-Walker malformation (DWM) remains one of the more challenging central nervous system anomalies to diagnose accurately before birth. Basilar invagination (BI), which is a dislocation of the dens in an upward direction, is occasionally accompanied by Klippel-Feil syndrome (KFS). We report a pediatric case of BI caused by atlanto-occipital subluxation (AOS) in KFS, suspected of having DWM prenatally but head magnetic resonance images (MRI) showed no evidence of that at 7 months of age. CASE: At 28 weeks of gestation, fetal MRI study revealed a small cerebellar vermis, leading us to suspect a DWM. The patient was born at 40 weeks of gestation. Head CT showed inferior vermian hypoplasia without findings of hydrocephalus. Cervicothoracic CT showed cervical lamina assimilations, thoracic hemivertebrae, and cervicothoracic scoliosis. He was diagnosed with Dandy-Walker variant and KFS. At 7 months of age, head MRI showed near normal cerebellum and vermis and there was no evidence of the DWM. He did not have intellectual or developmental delay and imaging studies were performed periodically. At 9 years of age, an already existing cough headache deteriorated. Three-dimensional reconstructed images from CT scan showed C1 hypoplasia, fusion of C1 and C2, BI, and AOS. Sagittal T2-weighted MRI showed protrusion of cerebellar tonsils inferiorly to the level of the posterior arch of C2. Serum calcium, phosphate, and parathyroid hormone levels were normal. The diagnosis was tonsillar herniation related to BI following AOS in KFS. Posterior occipitocervical fixation was performed under traction. CONCLUSIONS: We found out two important clinical issues: DWM findings after birth can be disappearing and BI can present sequential deterioration because of AOS in KFS. Our observation indicated the possible prognosis of pediatric BI with long follow-up and can help us decide on its surgical treatment timing when associated with AOS.

Instrumented fusion in a 12-month-old with atlanto-occipital dislocation: case report and literature review of infant occipitocervical fusion.

BACKGROUND: The treatment of atlantoaxial dislocation in very young children is challenging and lacks a consensus management strategy. DISCUSSION: We review the literature on infantile occipitocervical (OC) fusion is appraised and technical considerations are organized for ease of reference. Surgical decisions such as graft type and instrumentation details are summarized, along with the use of bone morphogenic protein and post-operative orthoses. ILLUSTRATIVE CASE: We present the case of a 12-month-old who underwent instrumented occipitocervical (OC) fusion in the setting of traumatic atlanto-occipital dislocation (AOD). CONCLUSION: Occipitocervical (OC) arthrodesis is obtainable in very young infants and children. Surgical approaches are variable and use a combination of autologous grafting and creative screw and/or wire constructs. The heterogeneity of pathologic etiology leading to OC fusion makes it difficult to make definitive recommendations for surgical management.

Traumatic atlanto-occipital dislocation: do children and adolescents have better or worse outcomes than adults? A narrative review.

BACKGROUND: Traumatic atlanto-occipital dislocation is an uncommon, severely unstable pathology, which can lead to detrimental or even fatal neurological impairment. Specifically, children have consistently been reported to be more susceptible to this type of injury because of their disproportionately larger head, ligament laxity, and injury mechanisms. However, to date, rates of missed injury and outcomes including neurologic recovery of pediatric and adult populations following this insult have not been comparatively evaluated. METHODS: Standard search engines were used to investigate outcomes of traumatic atlanto-occipital dislocation in children and adolescents compared to adults. CONCLUSIONS: Based on case reports and small series from the literature, it seems that children and adolescents tend to have a better likelihood of survival with the possibility of long-term neurological complications. Comparatively, adults who suffer traumatic atlanto-occipital dislocation either succumb to their injuries or survive with very little if any neurological complications.

[Occipitocervical junction: Aanatomy, craniometry and pathology]. / Okzipitozervikaler Übergang: Anatomie, Kraniometrie und Pathologien.

The occipitocervical junction comprises of the occiput condyles, the atlas, and the axis. The radiological evaluation of this region is supported by craniometric measurement methods which are based on predefined anatomical landmarks. The main pathologies of the occipitocervical junction are traumatic injuries, congenital anomalies or normal variants, infections, arthropathies, and tumors. In this article, the anatomy of the occipitocervical junction as well as the most important craniometric measurement methods are explained. Moreover various pathologies and similar appearing normal variants are presented.

Imaging of Atlanto-Occipital and Atlantoaxial Traumatic Injuries: What the Radiologist Needs to Know.

Approximately one-third of all cervical spine injuries involve the craniocervical junction (CCJ). Composed of the occiput and the first two cervical vertebrae, this important anatomic landmark, in conjunction with an intricate ligamentous complex, is essential to maintaining the stability of the cervical spine. The atlantoaxial joint is the most mobile portion of the spine, predominantly relying on the ligamentous framework for stability at that level. As acute onsite management of trauma patients continues to improve, CCJ injuries, which often lead to death onsite where the injury occurred, are increasingly being encountered in the emergency department. Understanding the anatomy of the CCJ is crucial in properly evaluating the cervical spine, allowing the radiologist to assess its stability in the trauma setting. The imaging findings of important CCJ injuries, such as atlanto-occipital dissociation, occipital condyle fractures, atlas fractures with transverse ligament rupture, atlantoaxial distraction, and traumatic rotatory subluxation, are important to recognize in the acute setting, often dictating patient management. Thin-section multidetector computed tomography with sagittal and coronal reformats is the study of choice in evaluating the extent of injury, allowing the radiologist to thoroughly evaluate the stability of the cervical spine. Furthermore, magnetic resonance (MR) imaging is increasingly being used to evaluate the spinal soft tissues and ligaments, and to identify associated spinal cord injury, if present. MR imaging is also indicated in patients whose neurologic status cannot be evaluated within 48 hours of injury. .

Craniovertebral tuberculosis in children: experience of 23 cases and proposal for a new classification.

PURPOSE: The aim of this study was to review cases of paediatric patients with craniovertebral junction (CVJ) tuberculosis with a view to try and stratify the cases into different groups which would help plan treatment and hence develop a protocol for treatment of a fairly uncommon condition still widely seen in the developing world. METHODS: Twenty-three cases of paediatric craniovertebral tuberculosis had their clinical features and radiology reviewed. The treatment plan in each case was analyzed according to their presentation. The results of treatment after 1 year were assessed in each case. RESULTS: The clinical presentations varied from neck pain with hypoglossal nerve palsy to frank spastic quadriparesis, and frank instability at the atlanto-axial junction was seen in five out of 23 patients. Cold abscesses were seen in 18/23 cases, and severe torticollis was the presentation in 6/23. We graded the children with this condition into three groups: 1) those with instability and gross neurodeficit who required early operative intervention (1 needed trans-oral decompression with posterior fixation and 4 required posterior fixation) 2) those who had severe torticollis and large cold abscesses who were treated with trans-oral aspiration of cold abscess followed by neck immobilization (6/23) and 3) those who did not have significant neck muscle spasm or torticollis and who were treated with immobilization alone (12/23). Only one child in group 3 required delayed intervention for instability which developed after completion of the course of anti-tubercular medication. CONCLUSIONS: We concluded that children with craniovertebral tuberculosis should be treated according to their clinical presentation coupled with evidence of radiological instability. Those with gross deficit and instability need early stabilization, those with minimal deficit and no instability but severe pain or torticollis need aspiration of the cold abscess with external immobilization, whereas those without deficit, instability or severe pain may be managed by external orthoses alone. Of course, medical treatment for tuberculosis is necessary in each case.

Motion-related vascular abnormalities at the craniocervical junction: illustrative case series and literature review.

The craniocervical junction (CCJ) functions within a complicated regional anatomy necessary to protect and support vital neurovascular structures. In select instances, vascular pathology can be attributed to this complicated interplay of motion and structure found within this narrow space. The authors report 3 cases of complex vascular pathology related to motion at the CCJ and detail the management of these cases. Two cases involved posterior circulation vascular compression syndromes, and one case involved a vascular anomaly and its relation to aneurysm formation and rupture. The patient in Case 1 was a 66-year-old man with a history of syncopal episodes resulting from the bilateral vertebral artery becoming occluded when he rotated his head. Successful microsurgical decompression at the skull base resulted in patent bilateral vertebral artery V3 segments upon head movement in all directions. The patient in Case 2 was a 53-year-old woman who underwent elective resection of a right temporal meningioma and who experienced postoperative drowsiness, dysphagia, and mild right-arm ataxia. Subsequent MRI demonstrated bilateral posterior inferior cerebel-lar artery (PICA) strokes. Cerebral angiography showed a single PICA, of extradural origin, supplying both cerebellar hemispheres. The PICA exhibited dynamic extradural compression when the patient rotated her head; the bilateral PICA strokes were due to head rotation during surgical positioning. In Case 3, a 37-year-old woman found unconscious in her home had diffuse subarachnoid hemorrhage and evidence of a right PICA aneurysm. A right far-lateral craniectomy was performed for aneurysm clipping, and she was found to have a dissecting aneurysm with an associated PICA originating extradurally. There was a shearing phenomenon of the extradural PICA along the dura of the foramen magnum, and this microtraumatic stress imposed on the vessel resulted in a dissecting aneurysm. This series of complex and unusual cases highlights the authors' understanding of vascular pathology of the CCJ and its management.

Magnetic resonance imaging of traumatic injury to the craniovertebral junction: a case-based review.

OBJECT: The craniovertebral junction (CVJ) is unique in the spinal column regarding the degree of multiplanar mobility allowed by its bony articulations. A network of ligamentous attachments provides stability to this junction. Although ligamentous injury can be inferred on CT scans through the utilization of craniometric measurements, the disruption of these ligaments can only be visualized directly with MRI. Here, the authors review the current literature on MRI evaluation of the CVJ following trauma and present several illustrative cases to highlight the utility and limitations of craniometric measures in the context of ligamentous injury at the CVJ. METHODS: A retrospective case review was conducted to identify patients with cervical spine trauma who underwent cervical MRI and subsequently required occipitocervical or atlantoaxial fusion. Craniometric measurements were performed on the CT images in these cases. An extensive PubMed/MEDLINE literature search was conducted to identify publications regarding the use of MRI in the evaluation of patients with CVJ trauma. RESULTS: The authors identified 8 cases in which cervical MRI was performed prior to operative stabilization of the CVJ. Craniometric measures did not reliably rule out ligamentous injury, and there was significant heterogeneity in the reliability of different craniometric measurements. A review of the literature revealed several case series and descriptive studies addressing MRI in CVJ trauma. Three papers reported the inadequacy of the historical Traynelis system for identifying atlantooccipital dislocation and presented 3 alternative classification schemes with emphasis on MRI findings. CONCLUSIONS: Recognition of ligamentous instability at the CVJ is critical in directing clinical decision making regarding surgical stabilization. Craniometric measures appear unreliable, and CT alone is unable to provide direct visualization of ligamentous injury. Therefore, while the decision to obtain MR images in CVJ trauma is largely based on clinical judgment with craniometric measures used as an adjunct, a high degree of suspicion is warranted in the care of these patients as a missed ligamentous injury can have devastating consequences.

Atlantoaxial dislocation in the setting of NMLFS.

BACKGROUND: Nablus mask-like facial syndrome (NMFLS) is an extremely rare genetic syndrome characterized by facial dysmorphia as well as developmental delay. In the present report we describe a potential association between non-traumatic atlanto-occipital dislocation and NMFLS in an 11-year old female lacking typical facial features of NMFLS. CASE DESCRIPTION: An 11-year-old female with autism presented with symptoms of persistent headache and vomiting as well as neck stiffness. Further investigation and CT imaging revealed congenital malformation of the skull base and craniocervical junction with complete posterior subluxation of the left occipital condyle. MRI findings later corroborated the findings on CT. CONCLUSIONS: The patient was successfully treated with occipitocervical fusion. The findings in this case suggest the possibility that atlanto-occipital instability and generalized occipitocervical may be associated with NMFLS.

Exploring the Pathogenesis of Atlanto-Occipital Instability in Chiari Malformation With Type II Basilar Invagination: A Systematic Morphological Study.

BACKGROUND: Our previous study suggested that atlanto-occipital instability (AOI) is common in patients with type II basilar invagination (II-BI). OBJECTIVE: To further understand the pathogenesis of AOI in Chiari malformations (CM) and CM + II-BI through systematic measurements of the bone structure surrounding the craniocervical junction. METHODS: Computed tomography data from 185 adults (80 controls, 63 CM, and 42 CM + II-BI) were collected, and geometric models were established for parameter measurement. Canonical correlation analysis was used to evaluate the morphological and positional relationships of the atlanto-occipital joint (AOJ). RESULTS: Among the 3 groups, the length and height of the condyle and superior portion of the lateral masses of the atlas (C1-LM) were smallest in CM + II-BI cases; the AOJ had the shallowest depth and the lowest curvature in the same group. AOJs were divided into 3 morphological types: type I, the typical ball-and-socket joint, mainly in the control group (100%); type II, the shallower joint, mainly in the CM group (92.9%); and type III, the abnormal flat-tilt joint, mainly in the CM + II-BI group (89.3%). Kinematic computed tomography revealed AOI in all III-AOJs (100%) and some II-AOJs (1.5%) but not in type I-AOJs (0%). Morphological parameters of the superior portion of C1-LM positively correlated with those of C0 and the clivus and significantly correlated with AOI. CONCLUSION: Dysplasia of the condyle and superior portion of C1-LM exists in both CM and II-BI cases yet is more obvious in type II-BI. Unstable movement caused by AOJ deformation is another pathogenic factor in patients with CM + II-BI.

Imaging of torticollis in children.

The normal anatomy of the cervical spine and various imaging techniques for the evaluation of torticollis are reviewed, and possible causes of torticollis in infants and children are discussed, with an emphasis on relevant imaging findings. Torticollis is a congenital or acquired deformity characterized by rotational deformity of the cervical spine with secondary tilting of the head. Although torticollis is a sign of an underlying disease process, its presence does not imply a specific diagnosis, and the cause should be sought if torticollis persists or is associated with other symptoms. Congenital torticollis, seen in neonates and infants, usually results from craniocervical vertebral anomalies or muscular causes, although ocular abnormalities such as congenital paralytic squint (strabismus) and congenital nystagmus should also be considered. Acquired torticollis, seen in older children and adolescents, is often secondary to trauma, infection, or tumors. Imaging should be used as a general screening tool only after a complete medical history and clinical findings have been obtained. In newborns or infants with congenital torticollis, ultrasonography (US) is the modality of choice. In cases of acquired torticollis resulting from trauma, conventional radiography (lateral and anteroposterior views) should be the first-line imaging modality. In nontraumatic acquired torticollis, computed tomography (CT) of the neck or cervical spine is the initial imaging study. If CT findings are negative, magnetic resonance (MR) imaging of the brain and cervical spine should be performed. The use of multiple imaging modalities (conventional radiography, US, CT, and MR imaging) is common in the radiologic work-up of torticollis, and radiologists must understand the role of each imaging modality in patients of various ages.

Syringomyelia associated with arachnoid septum at the craniovertebral junction, contradicting the currently prevailing theory of syringomyelia formation.

Despite a number of various hypotheses in the literature, the pathophysiology of syringomyelia is still not well understood. In this article, we report two cases of cervical syringomyelia not associated with Chiari I malformation. Both cases had a septum-like structure in the subarachnoid space on the dorsal side of the cord at the craniovertebral junction. Cardiac-gated phase-contrast cine-mode magnetic resonance imaging (MRI) demonstrated decreased cerebrospinal fluid (CSF) flow on the dorsal side of the spinal cord. Surgical excision of this septum, restoring the CSF flow, resulted in a prompt reduction of the syrinx size in both cases. Findings in these cases contradict the currently prevailing hypothesis of syrinx formation that postulate that the piston-like movement of the cerebellar tonsils enhance the pulsatile CSF flow in the spinal subarachnoid space, driving the CSF into the syrinx through the perivascular space of Virchow and Robin. The authors propose that a mechanism based on the decreased pulsatile CSF flow in the spinal subarachnoid space will be more suitable as a hypothesis in studying the pathophyisiology of syringomyelia. These cases also provide an important lesson in managing the patients with syringomyelia not associated with Chiari I malformation.

Endoscope-assisted microsurgical transoral approach to the anterior craniovertebral junction compressive pathologies.

At the present time, an update to the classical microsurgical transoral decompression is strongly provided by the most recent literature dealing with the introduction of the endoscopy in spine surgery. In this paper, we present our experience on the endoscope-assisted microsurgical transoral approach to anterior craniovertebral junction (CVJ) compressive pathology. We analysed seven patients (3 paediatrics and 4 adults ranging from 6 to 78 years) operated on for CVJ decompressive procedures using an open access, microsurgical technique, neuronavigation and endoscopy. All techniques mentioned were simultaneously employed. Among the endoscopic routes described in the literature, we have preferred the transoral using 30° endoscopes. In all the cases endoscopy allowed a radical decompression compared to the microsurgical technique alone, as confirmed intraoperatively with contrast medium fluoroscopy. In conclusion, endoscopy represents a useful complement to the standard microsurgical approach to the anterior CVJ; it provides information for a better decompression with no need for soft palate splitting, hard palate resection, or extended maxillotomy. Moreover, intraoperative fluoroscopy helps to recognize residual compression. Virtually, in normal anatomic conditions, no surgical limitations exist for endoscopically assisted transoral approach, compared with the pure endonasal and transcervical endoscopic approaches. In our opinion, the endoscope deserves a role as "support" to the standard transoral microsurgical approach since 30° angulated endoscopy significantly increases the surgical area exposed at the level of the anterior CVJ.

Proposed Grading System for Posterior Atlantooccipital Membrane Complex Injuries on Cervical Magnetic Resonance Imaging.

BACKGROUND: The posterior atlantooccipital membrane complex consists of the posterior atlantooccipital membrane and posterior atlantoaxial membrane. Posttraumatic, posterior atlantooccipital membrane complex injuries may have varied appearances on cervical magnetic resonance imaging. The purpose of this study was to identify the different types of posterior atlantooccipital membrane complex injuries that occur in trauma patients. METHODS: Patients who suffered a posterior atlantooccipital membrane complex injury were identified retrospectively using key word searches of cervical magnetic resonance imaging reports between 2013 and 2020 using Nuance mPower software. All relevant imaging studies were reviewed by 2 neuroradiologists. A description of the location and type of posterior atlantooccipital membrane complex injury was recorded, along with additional osteoligamentous trauma of the craniocervical junction and relevant clinical history. RESULTS: Forty-one patients were identified with acute posterior atlantooccipital membrane complex injury. Four distinct patterns of posterior atlantooccipital membrane complex injury were observed. CONCLUSIONS: A grading system for posterior atlantooccipital membrane complex injuries is proposed on the basis of these data: grade 1-edema confined to the posterior atlantooccipital and atlantoaxial membrane; grade 2-edema confined to the posterior atlantooccipital and atlantoaxial membrane and ligamentum nuchae; grade 3-stripping injury of the posterior atlantooccipital membrane with C1-C2 dorsal epidural hematoma; and grade 4-frank disruption of the posterior atlantooccipital membrane at C1 with edema in the remaining posterior atlantooccipital membrane complex.

Are MRI high-signal changes of alar and transverse ligaments in acute whiplash injury related to outcome?

BACKGROUND: Upper neck ligament high-signal changes on magnetic resonance imaging (MRI) have been found in patients with whiplash-associated disorders (WAD) but also in non-injured controls. The clinical relevance of such changes is controversial. Their prognostic role has never been evaluated. The purpose of this study was to examine if alar and transverse ligament high-signal changes on MRI immediately following the car accident are related to outcome after 12 months for patients with acute WAD grades 1-2. METHODS: Within 13 days after a car accident, 114 consecutive acute WAD1-2 patients without prior neck injury or prior neck problems underwent upper neck high-resolution proton-weighted MRI. High-signal changes of the alar and transverse ligaments were graded 0-3. A questionnaire including the impact of event scale for measuring posttraumatic stress response and questions on patients' expectations of recovery provided clinical data at injury. At 12 months follow-up, 111 (97.4%) patients completed the Neck Disability Index (NDI) and an 11-point numeric rating scale (NRS-11) on last week neck pain intensity. Factors potentially related to these outcomes were assessed using multiple logistic regression analyses. RESULTS: Among the 111 responders (median age 29.8 years; 63 women), 38 (34.2%) had grades 2-3 alar ligament changes and 25 (22.5%) had grades 2-3 transverse ligament changes at injury. At 12 months follow-up, 49 (44.1%) reported disability (NDI > 8) and 23 (20.7%) neck pain (NRS-11 > 4). Grades 2-3 ligament changes in the acute phase were not related to disability or neck pain at 12 months. More severe posttraumatic stress response increased the odds for disability (odds ratio 1.46 per 10 points on the impact of event scale, p = 0.007) and so did low expectations of recovery (odds ratio 4.66, p = 0.005). CONCLUSIONS: High-signal changes of the alar and transverse ligaments close after injury did not affect outcome for acute WAD1-2 patients without previous neck problems. High-resolution upper neck MRI has limited value for the initial examination and follow-up of such patients.

Geometry of the articular facets of the lateral atlanto-axial joints in the case of occipitalization.

This study investigates if atlanto-occipital fusion affects the size and geometrical configuration of the articular facets of the atlanto-axial joint. Morphometric analysis was performed on the male adult skull, the occipital bone of which is assimilated with the first cervical vertebrae (the atlas). The perimeter, Feret's diameter, surface area, and circularity of the inferior articular fa-cets were measured. However, we did not observe significant bilateral differences in size of the inferior articular facets of the assimilated atlas compared to normal first cervical vertebrae. Geometrical conformation of the articular facets of the atlas and axis was assessed using a coordinate measuring machine (PMM - 12106, Leitz). The results obtained from this machine indicated that the inferior articular facets of the assimilated atlas presented asymmetrical orientation compared to the normal anatomy of the atlas. Hence, in the case of occipitalization, the gap between the articulating facets of the atlas and the axis was measured to be greater than in the normal atlanto-axial joint. Computer assisted tomography was applied to visualise the anatomical relationship between the inferior articular facets of the assimilated atlas and the corresponding facets located on the axis. In this case, radiographic examination revealed that the bilaterally articulating facets (inferior and superior) showed disproportion in their adjustment within the lateral atlanto-axial joints. Thus, we concluded that the fusion of the atlas with the occipital bone altered the geometry of the inferior articular facets of the atlas and influenced the orientation of the superior articular facets of the axis.

[Isolated hypoglossal nerve palsy secondary to an atlantoccipital joint synovial cyst. Case report and literature review]. / Parálisis aislada del nervio hipogloso secundaria a quiste articular atlanto-occipital. Presentación de un caso y revisión de la literatura.

Hypoglossal nerve palsy is commonly associated with the involvement of other cranial nerves. His injury is rarely isolated. We present a patient in which paralysis is due to the presence of a "juxtafacet cyst" of the atlanto-occipital joint. We review the anatomy of the hypoglossal nerve, different therapeutic options, the differential diagnosis and papers published to date.

Atlanto-occipital Dissociation in the Setting of Relatively Normal Radiologic Findings.

BACKGROUND: Craniocervical junction (CCJ) dislocations are often fatal. Atlanto-occipital dissociation can be challenging to diagnose, especially in patients who present with absent or subtle radiologic signs. CASE DESCRIPTION: A neurologically intact 37-year-old patient presented to the hospital following a high-speed motor vehicle accident. Initial computed tomography scans showed normal CCJ anatomy, but magnetic resonance imaging (MRI) of the CCJ was performed to further evaluate perimesencephalic subarachnoid hemorrhage. MRI revealed partial disruption of the anterior atlantoaxial membrane and tectorial membrane as well as complete disruption of the posterior atlanto-occipital membrane, ligamentum flavum, and apical ligament, signifying atlanto-occipital dissociation. Halo spinal immobilization was performed in preparation for stabilization with posterior occipitocervical fusion; however, the CCJ distracted widely during surgery owing to the accident-related dislocation, signifying an unstable fracture. Posterolateral fusion was performed, and the distraction injury was corrected via posterior surgical instrumentation. CONCLUSIONS: Normal occiput-C1 craniometric parameters in the setting of unexplained perimesencephalic subarachnoid hemorrhage does not eliminate the possibility of missed or delayed diagnosis of traumatic atlanto-occipital dissociation injuries. Cervical MRI without contrast should be considered in patients with vertebral artery dissection or perimesencephalic subarachnoid hemorrhage after a blunt injury with neck pain. When MRI shows evidence of disruption of ≥2 atlanto-occipital ligaments, surgical stabilization should be considered, as these are clinically very unstable injuries.