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Asian Spine J > Volume 10(1); 2016 > Article
Srivastava, Nemade, Aggarwal, and Bhoale: Congenital Absence of Posterior Elements of C2 Vertebra with Atlanto-Axial Dislocation and Basilar Invagination: A Case Report and Review of Literature

Abstract

Developmental anomalies of the axis are commonly encountered, especially anomalies involving the odontoid process. Anomalies of the posterior elements are uncommon. We describe a unique case of agenesis of posterior elements of C2 with basilar invagination and atlanto-axial dislocation. An obese 8-year-old boy presented with symptoms of cervical myelopathy. Radiological workup revealed a craniovertebral junction anomaly with occipitalised atlas, absent posterior elements of axis, and hypertrophied C3 spinous process. Atlanto-axial instability and basilar invagination was present. Magnetic resonance angiography revealed hypoplastic left vertebral artery. Traction with cervical tongs failed to improve the alignment and symptoms. Anterior trans-oral release, followed by posterior decompression and custom-made instrumentation, was done. The patient recovered completely and was asymptomatic at the end of two years. X-ray and computed tomography scan demonstrated reduction of basilar invagination and maintenance of alignment. This is the first case to be reported of agenesis of posterior elements of axis associated with basilar invagination. One should look for this condition in patients with hypertrophied spinous process of C3. Utilization of hypoplastic pedicle of axis serves as an additional fixation point to increase the stability of the construct.

Introduction

Developmental anomalies of the axis are commonly encountered, especially anomalies involving the odontoid process [1,2]. These range from anomalies of fusion to complete aplasia [2]. Anomalies of the posterior elements are uncommon. These include invagination of lamina of C2 and agenesis of the posterior elements of C2 [3,4,5,6,7,8,9,10,11]. Patients usually present with progressive myelopathy or, rarely, as mechanical neck pain. We describe a case involving absence of posterior elements of C2 having occipitalisation of C1 with mobile atlanto-axial dislocation (AAD) and basilar invagination. To our knowledge, this is the first case to be reported of agenesis of posterior elements of axis, presenting as cervical myelopathy due to basilar invagination.

Case Report

An 8-year boy presented to the outpatient department with neck pain, progressive imbalance and difficulty in walking, since 6 months duration. On examination, he was found to be obese, with a body mass index of 31.42 kg/m2. He had a low hairline and stiffness of neck movements. Power and sensations in all extremities were normal; however, he had spasticity and exaggerated deep tendon reflexes in all four extremities. The plantar response was extensor. Static and dynamic radiographs of cervical spine revealed mobile AAD, absent spinous process of axis and hypertrophied C3 spinous process (Fig. 1). Due to occipitalisation of atlas and basilar invagination, the atlanto-dens interval (ADI) was difficult to calculate in dynamic radiographs (ADI could be marked on digitally magnified images). However, the inter-spinous process distance between C1 and C3 was more in flexion, and decreased to normal alignment in extension (Fig. 1B, C). Both these parameters demonstrated a mobile ADD. The C5–C6 spinous processes were fused with each other.
The computed tomography (CT) scan of cervical spine with 3D reconstruction further illustrated the pathoanatomy of the condition (Figs. 2, 3). Occipitalisation of atlas, absent laminae and spinous process of the axis was seen; the pedicles were hypoplastic. On CT scan, the narrowest pedicle diameter was 2.9 mm. The transverse process and the vertebral foramen were well formed. The tip of the odontoid process was proximal to the Mcrae's line, indicating basilar invagination. Magnetic resonance imaging (MRI) scan revealed the compression of medulla and spinal cord till the C3 level (Fig. 4). MR angiography revealed a hypoplastic left vertebral artery. The patient was put on skeletal traction for a week; however, it failed to improve the alignment or symptoms. The patient then underwent an anterior transoral release, followed by posterior instrumented occiput–C4 fusion. The rudimentary C2 pedicles were utilised as additional fixation points, after manufacturing custom-made 2.7 mm screws. Thorough cord decompression and posterior iliac crest bone grafting was done between occiput and C3. Prior to grafting, the cancellous bone between the two tables of the occiput was exposed by burring the outer table. An "H" shaped graft was put between C3 and occiput (Fig. 5). Additional cancellous bone graft was put laterally between C2 and occiput to promote fusion. The patient was mobilised on day 14-postoperative, with the help of a sterno-occipto-mandibular immobiliser brace. At the end of two years, the patient was asymptomatic; stability and alignment were maintained as confirmed by dynamic radiographs and CT scan (Figs. 6, 7).

Discussion

Developmental anomalies are common in odontoid process. However, anomalies of posterior elements of axis are uncommon. These include invagination of lamina of axis causing cord compression and absence of posterior elements of axis [3,4,5,6,7,8,9,10,11]. There are 7 reported cases of absent posterior elements of axis (Table 1). Additionally, Vangilder et al. [9] reported a case of "spina bifida of axis" in 1987; however details of the case are unavailable.
The axis develops from the second spinal sclerotome in three essential stages [12,13,14]. The stages of development of neural arch of axis and the accompanying defect due to failure of the stage have been summarized in Table 2. The defect in the posterior elements of the axis may be caused by the failure of the extension of the chondrification centers in the posterior arch, or by the failure of the ossification process [3,15]. In our patient, the absence of the cartilaginous remnant of posterior elements of axis indicated the failure of dorsal migration of cells from the second spinal sclerotome, or failure of chondrification leading to failure of formation of the neural arch.
Most of the cases reported till date had varying degrees of anterolisthesis of C2 over C3 [7,10,11]. However, the causal relationship of absent posterior elements to the anterolisthesis has not yet been postulated. We believe that hypoplasia or aplasia of the posterior elements could have compromised the structural integrity of C2–C3 facet joints, leading to instability and subsequent anterolisthesis. The present case, however, is unique because of the presence of basilar invagination. Occipitalisation of atlas and multiple fused vertebrae could have concentrated the flexion extension force on C1–C2 leading to AAD. The hypertrophied spinous process of C3 represents a compensatory mechanism to provide attachment to interspinous ligament and paraspinal muscles, otherwise attached to spinous process of axis.
Being a congenital anomaly, we included MR angiography in the preoperative workup. Angiography revealed hypoplastic left vertebral artery. This had important clinical implications, as we had to be extra cautious while inserting the right-sided screws.
Our patient had a basilar invagination, which was irreducible on cervical traction. Thus, we opted for anterior transoral release of contracted structures, as described by Wang et al. [16]. After confirming reduction of basilar invagination on fluoroscopy, we did posterior O-C4 fixation and fusion in the same setting. Hypoplastic pedicles of C2 were utilized as extra fixation points. Hypoplastic pedicles were exposed and cortical bone overlying the pedicle was burred. Entry was made using a pedicle finder and a ball tip probe was used to palpate the tract of pedicle screw. Check X-ray was taken to confirm the trajectory and placement. Since the diameter was small, 2.7 mm polyaxial screws were passed in the pedicles of C2. The diameter was decided preoperatively, after careful measurement on CT scan. This increased the strength of the construct and aided in maintenance of reduction.
Almost all cases of agenesis of posterior elements of C2 have presented with myelopathy, and no asymptomatic cases have been reported in literature [6,7,8,9,10,11]. Thus, it is prudent to believe that a patient with missing posterior elements of C2 may/will develop symptoms some time in their lifetime. One must closely follow-up such patients and consider performing instrumented fusion if patient develops even early clinical evidence of myelopathy, because once myelopathy becomes severe, the chances of complete neurological recovery are poor.
We present a unique case of absent posterior elements of axis with occipitalised C1, mobile AAD and basilar invagination, presenting as cervical myelopathy. Absent posterior elements of axis is a rare condition. One should look for this condition in patients with hypertrophied spinous process of C3. Close follow up is necessary, and fusion may be considered in patients who develop myelopathy. Careful preoperative evaluation should be done. Angiography must be considered to identify anomalies in vertebral artery. Utilization of hypoplastic pedicle of axis serves as additional fixation point to increase the stability of construct. Further experience is mandatory to formulate an appropriate treatment protocol for this complex anomaly.

Conflicts of interest

Conflict of Interest: No potential conflict of interest relevant to this article was reported.

References

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Fig. 1

(A) Lateral X-ray of cervical spine demonstrating absence of posterior elements of C2 vertebra with hypertrophied spinous process of C3. (B, C) Flexion and extension view of cervical spine demonstrating mobile atlanto-axial dislocation. Black line denotes the atlanto-dens interval, which increases in flexion and reduces in extension. White dashed arrows denote the interspinous process distance between C1 and C3, which is more in flexion and decreases in extension. Both these parameters indicate a mobile atlanto-axial dislocation.

asj-10-170-g001.jpg
Fig. 2

Computed tomography scan showing tip of odontoid process above the Mcrae's line, suggesting basilar invagination. There is occipitalisation of atlas.

asj-10-170-g002.jpg
Fig. 3

(A) Reference sagittal section showing the level at which axial cut (B) has been taken. (B) Axial cut showing hypoplastic pedicles of axis with absent lamina and spinous process. (C) Three-dimensional reconstruction of C2 vertebra showing absent spinous process and lamina with hypoplastic pedicles (blue arrows).

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Fig. 4

Magnetic resonance imaging showing compression of medulla and spinal cord till C3 level.

asj-10-170-g004.jpg
Fig. 5

Intra-operative image showing occiput-C4 instrumentation. A cortico-cancellous bone graft has been placed between occiput and spinous process of C3 (dotted yellow arrow). Additional cancellous bone graft has been placed laterally between C2 and occiput (white arrows).

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Fig. 6

Follow-up X-ray demonstrating maintenance of alignment.

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Fig. 7

Follow-up computed tomography scan showing maintenance of reduction of basilar invagination, shown by tip of odontoid process being below the Mcrae's line.

asj-10-170-g007.jpg
Table 1

Table showing cases reported of absence of posterior elements of C2

asj-10-170-i001.jpg

Cases have been listed in chronological order of date of publication.

NA, not available; AAD, atlanto-axial dislocation; O–C4, occiput to C4.

Table 2

Table showing stages in development of neural arch of C2 vertebra and accompanying defect in case of failure of the particular stage

asj-10-170-i002.jpg


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