Keywords
atlantoaxial instability - vertebral artery - CT angiography - 3D CT - Os odontoideum
Introduction
The craniovertebral junction (CVJ) is a complex area of multiple joints, bones, ligaments,
and vessels. Vertebral arteries (VAs) are the principal vessels closely placed to
the important bony and neural structures.[1] Because of the complex nature of CVJ, it is prone to various congenital and acquired
pathologies and often requires surgical correction. While dealing with these pathologies,
a complete understanding of anatomy is crucial. The VA is the critical structure concerned
during the surgery around CVJ. Congenital CVJ anomalies are often associated with
the anomalous course of VAs, including high-riding VA, fenestrated VA, persistent
first intersegment arteries, and persistent second intersegmental arteries.[2] Persistent second intersegmental artery associated with CVJ anomalies like atlantoaxial
dislocation (AAD) is uncommon. We report a case of management of an AAD with persistent
second intersegment artery and describe the role of VA mobilization during surgery.
Case Report
A 13-year-old boy presented with neck pain followed by progressive spastic quadriparesis
and imbalance while walking for 1 year. He was unable to do his day-to-day activities
without assistance. His general examination revealed a short neck, low hairline, right-sided
torticollis, cleft lip, and absent uvula. A neurological examination revealed modified
Ashworth Grade 3 hypertonia in all four limbs. His power in all four limbs was IV/V
(according to the Medical Research Council grading. His deep tendon reflexes in both
upper limbs and lower limbs were exaggerated. Babinski signs were positive, and posterior
column involvement was on both sides.
Magnetic resonance imaging (MRI) of CVJ showed significant anterior compression of
the cervicomedullary junction due to sharp bony indentation on sagittal T1-and T2-weighted
images and on axial T2-weighted images with widened atlantoaxial interval ([Fig. 1A, B]). Also, the hyperintense signal was noted in the cervicomedullary junction on sagittal
T2-weighted images due to chronic compression. Computed tomography (CT) scan of CVJ
on a sagittal view ([Fig. 1C]) and coronal view ([Fig. 1D]) revealed irregular cortical bone fragment at the posterior aspect of the anterior
arch of the atlas suggestive of Os odontoideum. The Wackenheim's line was seen transecting
the residual odontoid process representative of basilar invagination (BI). CT scan
of CVJ also revealed fixed AAD, abnormal configuration of left C1-C2 joints, and absence
of posterior arch of atlas and its spinous process ([Fig. 2A–E]). Three-dimensional (3D) CT scan CVJ also showed similar findings in the atlas and
atretic left C2 foramina transversarium ([Fig. 3A, B]). 3D CT angiography CVJ revealed an abnormal course of the left VA, which after
exiting the C3 neural foramina had subisthmic course passing beneath the C2 lamina
and not through its neural foramen ([Fig. 3C, D]). This was suggestive of the persistence second intersegmental artery on the left
side. In view of the congenital nature of AAD and associated bony and vascular anomalies,
we also prepared a 3D bony model using CT and CT angiography images. 3D bone model
showed bony abnormalities and an abnormal course of the left VA ([Fig. 3E]). It helped us to plan the surgical procedure, including mobilization of the left
VA and planning screw trajectories.
Fig. 1 Magnetic resonance imaging (MRI) of craniovertebral junction (CVJ) showing significant
anterior compression of the cervicomedullary junction due to sharp bony indentation
on sagittal T1 (A) and T2 (B)-weighted images. Also, hyperintense signals noted at the cervicomedullary junction
on sagittal T2-weighted images due to chronic compression. Computed tomography scan
of CVJ showing irregular, well cortical bone fragment at the posterior aspect of the
anterior arch of atlas suggestive of Os odontoideum on sagittal view (C) and coronal view (D).
Fig. 2 Computed tomography (CT) scan craniovertebral junction (CVJ) sagittal view in flexion
(A) and extension (B) images revealing fixed atlantoaxial dislocation. CT scan CVJ sagittal view in neutral
position showing abnormal configuration of left C1-C2 joint (C) and right C1-C2 joint (D) as well as axial images showing Os odontoideum, absent of posterior arch of atlas
and its spinous process (E).
Fig. 3 Three-dimensional computed tomography (CT) scan craniovertebral junction (CVJ) images
showing absent posterior arch of atlas on left side, absent C1 spinous process, and
atretic left C2 foramina transversarium (A, B). Three-dimensional CT angiography CVJ showing abnormal course of left vertebral
artery; after exiting C3 neural foramina it had subisthmic course passing beneath
the C2 lamina and not through its neural foramen (C, D). Three-dimensional bone model showing bony abnormalities and abnormal course of
left vertebral artery (E).
The patient underwent posterior C1-C2 fixation under general anesthesia. He was given
a prone position with his neck in slight extension. The skull traction was applied
using Gardner-wells tong with a 5-kg weight. Skin and facia were cut open by a midline
vertical incision from the inion to C3 spinous process. Subperiosteal dissection was
done, and the C1-C2 complex was identified. An anomalous course of the left VA was
identified and confirmed with the vascular Doppler. The artery was meticulously dissected
and mobilized medially ([Fig. 4A]) ([Video 1]). Subsequently, the C1-C2 joint was exposed and opened bilaterally, and end plates
were removed and filled with bone pieces. Also, by gentle manipulation, C1-C2 joints
were distracted, and distraction was maintained by impacting approximate-sized titanium
spacers filled with bone grafts. Bilateral C1 lateral mass and C2 pars screws were
inserted and connected with rods. A particular precaution was taken while inserting
the C2 pars screw on the left side as the medially mobilized VA was close to the entry
point ([Fig. 4B]). Both C1-C2 screws were inserted safely, preserving the bilateral VA. Occiput and
C2 lamina were drilled out and covered with bone graft harvested from the posterior
superior iliac crest, and the wound closed in layers.
Fig. 4 Intraoperative images; arrow showing medially mobilized left vertebral artery (A) and arrow relation of C2 pars screw entry point with mobilized left vertebral artery
(B).
The postoperative course was uneventful, and his neurological status slowly improved.
A postoperative MRI and CT scan of CVJ images revealed correction of BI and significant
decompression of the CV junction ([Fig. 5A–F]).
Fig. 5 Postoperative magnetic resonance imaging scan of craniovertebral junction (CVJ) T2
weighted sagittal view (A) and computed tomography scan of CVJ showing reduction in basilar invagination (sagittal
view, B and coronal view, C) and atlantoaxial dislocation sagittal views (left C1-C2 joint, D and right C1-C2 joint, E) with spacers and screws in situ on sagittal images and on axial images (D–F).
Discussion
Over the past two decades, CVJ surgeries have undergone a significant revolution.
This is applied even in irreducible AAD. Traditional surgeries like transoral odontoidectomy
with posterior fixation are less followed nowadays due to complications like the transoral
spread of wound infection.[3]
[4] Though various procedures are described in the literature, presently method of choice
for this pathology is the posterior fixation of the atlas and axis using lateral masses
and pars screws, respectively.[5] It is the shortest possible fixation and maintains mobility at the craniovertebral
joint. The surgery aims to decompress neural structures to achieve acceptable biomechanical
alignment and function by fusing C1-C2, thereby improving the region's local hemodynamics.[2] However, lateral mass screws and pars screw placement is technically demanding and
requires expertise.
VAs are the main arteries of the posterior circulations and supply blood to the brain
and spinal cord. Various anatomical variations exist in the development of these arteries,
including asymmetry, irregular and insufficient development; these include duplication
and fenestration of VA,[6]
[7] the variable origin of either one or both the arteries,[8] and high riding VA.[9] Prior understanding of these variations is very much essential to prevent intraoperative
catastrophic events.
The VA is the most common artery injured during atlantoaxial fixation. It becomes
even more susceptible to injury when the course is uncommon. VA injury is a rare complication
of CVJ surgery but is devastating when it occurs. The reported incidence of VA injury
during spinal surgery ranges from 0.1 to 0.5% in large series.[10] VA injury may occur in approximately 4.1% of patients during surgery at the CVJ.[11]
VA injury during surgery may lead to catastrophic bleeding, and the patient may develop
a postoperative neurological deficit. To prevent vascular insult, preoperative evaluation
of this region is imperative. CT scan CVJ with the 3D reconstruction of bone and CT
angiography are the most important investigative tools before planning safe CVJ surgery.
Also, the color-coded 3D model plays a vital role in better understanding bony and
vascular abnormalities in relation to each other.[12]
Embryologically, the VA develops from a long anastomotic channel that interconnects
the seven cervical intersegmental arteries.[13] The other intersegmental arteries involute during their development, and vertical
channels persist to form the normal VA. Depending on the abnormalities in involution,
various VA anomalies may occur. A persistent first intersegmental artery is one in
which the VA, after exiting the C2 foramen transversarium, passes below the C1 posterior
arch instead of its normal course through the C1 transverse foramen.[14] The VA anomaly described in our case is entirely different from the previously reported
ones. After exiting the corresponding C3 transverse foramen, the VA on the left side
passed medially and below the C2 isthmus in proximity to pars interarticularis. The
left-sided second intersegmental artery (between C2 and C3) might have persisted,
giving rise to this anomaly. The transverse foramina of C2 on the left side was also
atretic. Correctly identifying this anomaly in CT angiography has its management implications.
Here, the VA hugs the thin pars interarticularis of C2. A screw violating the C2 inferior
wall will likely injure the VA. Therefore, placing a transarticular or pars screw
may not be a good choice in these cases, as a bony breach inferiorly could be disastrous.
In such cases, skeletonizing the VA, V3 segment, and putting the screws under vision
is advisable. This way the chance of injuring the VA is minimal. A neuronavigation-guided
screw placement may further minimize the risk. The other option for such a VA anomaly
would be inserting translaminar screws, albeit with slightly inferior biomechanical
stability. In the presence of anomalous VA, occipital–cervical fusion is an alternative.
However, this hampers the neck movements adversely, which adds to the morbidity caused
to the patient and may have delayed complications.[15]
Conclusion
Congenital craniovertebral anomalies are often associated with VA anomalies. 3D CT
scans and CT angiographic images help in the early identification of these anomalies.
Preoperative knowledge about persistent second intersegment artery, early identification,
and intraoperative mobilization of the VA facilitate placement of C1-C2 intra-articular
screws with good clinical outcome.
Video 1
