Year : 2007 | Volume
: 2 | Issue : 1 | Page : 10--15
Aqueductal stenosis and mental retardation associated with O S odontoideum, "hypermobile" atlantoaxial dislocation and congenital stenosis of atlas leading to spastic quadriparesis
Rupant K Das, Sanjay Behari, Namit Singhal, Awadesh K Jaiswal, Ashok K Mahapatra
Department of Neurosurgery, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India
Department of Neurosurgery, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Rae Bareli Road, Lucknow - 226 014, UP
Background: The authors report a unique concomitant occurrence of mental retardation, hydrocephalus due to aqueductal stenosis, a freely floating atlas (having anterior, posterior and transverse atlantoaxial dislocation [AAD]) consequent to an orthotopic os odontoideum and stenosis of atlantal ring. There were no features of coexisting chondroskeletal dysplasias or other systemic diseases. To the best of the authors«SQ» knowledge, this is the first reported case of its type in the literature. Case Report: This 17-year-old girl with delayed milestones had enlarging head with vomiting at 1 year of age. Her CT scan revealed hydrocephalus with aqueductal stenosis, and she underwent a ventriculoperitoneal shunt. She subsequently had progressive quadriparesis. A minor fall led to transient unconsciousness and aggravation of her symptoms. Radiology of the craniovertebral junction revealed os odontoideum with anterior, posterior and transverse C1-2 subluxation and atlantal ring stenosis with marked cervical compression. A C1 laminectomy with occipitocervical contoured rod fusion with onlay autologous bone graft under guidance of intraoperative image intensifier was performed. Conclusion: The concomitant presence of hydrocephalus due to aqueductal stenosis, mental retardation and congenital stenosis of the atlantal ring points towards a congenital origin for the os odontoideum. The free floating atlantal ring on the axis led to anteroposterior and transverse AAD, necessitating intubation and occipitocervical stabilization in absolutely neutral position of the neck since both flexion as well as extension movements would have been deleterious. Congenital stenosis of atlas is an extremely rare entity; it contributed to cervical canal compromise even in neutral position of the cervical spine when the AAD had been adequately reduced, requiring an additional C1 laminectomy. The simultaneous presence of all these anomalies merited unique management considerations.
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Das RK, Behari S, Singhal N, Jaiswal AK, Mahapatra AK. Aqueductal stenosis and mental retardation associated with O S odontoideum, "hypermobile" atlantoaxial dislocation and congenital stenosis of atlas leading to spastic quadriparesis.J Pediatr Neurosci 2007;2:10-15
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Das RK, Behari S, Singhal N, Jaiswal AK, Mahapatra AK. Aqueductal stenosis and mental retardation associated with O S odontoideum, "hypermobile" atlantoaxial dislocation and congenital stenosis of atlas leading to spastic quadriparesis. J Pediatr Neurosci [serial online] 2007 [cited 2022 Dec 9 ];2:10-15
Available from: https://www.pediatricneurosciences.com/text.asp?2007/2/1/10/31999
Os odontoideum is characterized by a small corticated ossicle separated from the base of the odontoid. , It was first described by Giacomini in 1886.  In our patient, an orthotopic os odontoideum (an os in close proximity to the anterior arch of the atlas) was associated with a hypermobile AAD. In flexion of the neck, the ring of atlas along with the attached os moved forwards relative to the remaining attached part of the odontoid and the body of axis; and in extension of the neck, the ring of the atlas with the attached os slid backwards into the cervical canal. In flexion of the neck, therefore, the part of the odontoid attached to the body of the axis; and in extension of the neck, the os odontoideum held in close proximity to the anterior arch of atlas were causing cervicomedullary compression. ,, There was also a concomitant transverse C1-2 joint dislocation. In addition, there was congenital hypoplasia of the ring of the atlas, leading to cervical canal stenosis even in neutral position of the neck (when the os was aligned with the attached part of the odontoid and there was no AAD). ,,,,,, The anomalies had led to cervical spastic quadriparesis. The patient had earlier presented with raised intracranial pressure caused by hydrocephalus due to aqueductal stenosis for which she had undergone a ventriculoperitoneal shunt. She also had a mild mental retardation, with the intelligent quotient being in the range of 60-70.  To the best of the authors' knowledge, this syndrome complex of mental retardation, hydrocephalus due to aqueductal stenosis, hypermobile anteroposterior and transverse AAD due to os odontoideum and stenosis of atlantal arch without any associated features of chondroskeletal dysplasias has not been described before.
This 17-year-old girl had a history of enlarging head with recurrent vomiting and delayed milestones at 1 year of age. Her computed tomographic scan revealed hydrocephalus with periventricular lucency due to aqueductal stenosis, and she underwent a "Chhabra" right ventriculoperitoneal shunt. Following the shunt, the symptoms of raised intracranial pressure improved but she continued to have delayed social milestones and showed persistent mental retardation. She continued to follow simple commands but had difficulty in walking and running fast and suffered frequent falls. At 14 years of age, she again had repeated episodic bifrontal headache with vomiting. The computed tomographic scan showed recurrence of hydrocephalus with ventriculoperitoneal shunt obstruction. After shunt revision, she noticed increased spasticity in all four limbs with occasional tingling in the trunk and limbs. She was also unable to walk unsupported with increasing unsteadiness. Two years prior to admission, after a minor fall, she was unconscious for 1.5 hours and then developed increase in spastic quadriparesis, with inability to hold objects in hands and to bear her weight on her lower limbs. She also started having difficulty in lifting arms above her shoulder and developed flexor spasms. There was no history of severe neck pain or neck movement restriction. There was no sphincter disturbance or respiratory discomfort. There was neither family history of mental retardation nor of gestational or neonatal infections, and the patient's delivery had been full term and normal.
Her neurological examination revealed mental retardation, with an intelligent quotient between 60 and 70; and spastic quadriparesis grade III (left-sided weakness greater than right) with severe grip weakness and flexor spasms. There was no sensory impairment. There was mild lateral head tilt towards the right side. There was no lower cranial nerve impairment, and her respiration was normal. There was no associated joint abnormality or kyphoscoliosis. The right ventriculoperitoneal shunt was functioning well.
Her plain and contrast computed tomographic (CT) scan of the head revealed the right ventriculoperitoneal shunt tip passing through the Keene's point into the left trigone, but functioning well with adequately decompressed ventricles. The axial scan at the level of the midbrain shows nonvisualization of the aqueduct; and at the level of the upper pons, it shows a narrowed upper IV ventricle [Figure 1]. Her computerized lateral radiograph of the cervical spine in neutral position revealed an orthotopic os odontoideum, with the posterior arch of the atlas being well anterior to the spinolaminar line without the presence of AAD [Figure 2]A. Her lateral cervical radiograph of the cervical spine in flexed position of the neck showed an AAD with decrease of the dynamic cervical canal diameter between the posteriorly displaced attached part of the odontoid and the posterior arch of the atlas. The latter had displaced anterior to the spinolaminar line [Figure 2]B. On extension, there was retrolisthesis of the anterior arch of the atlas-os odontoid complex relative to the body of axis, causing cervical canal compromise; and the posterior arch of atlas was displaced posterior to the spinolaminar line [Figure 2]C.
Her magnetic resonance imaging revealed significant cervical compression with T2 hyperintense changes in the cord opposite the tip of the attached part of the odontoid. There was no Chiari I malformation or syringomyelia [Figure 3]. The neutral midsagittal and axial computed tomographic reconstructed images revealed an effective canal diameter of 12 mm at the level of the atlas [Figure 4]A and B. The os odontoideum was confirmed. The C1-2 joints were symmetrical, with a relative horizontal orientation of the facet joint surfaces. There was also a lateral displacement towards the right side of the lateral masses of the atlas relative to the body of C2 [Figure 5].
The patient was placed on a cervical collar. She underwent surgery in prone position with the head resting on a horseshoe head rest. The neutral position of the neck with proper alignment of the os with the attached portion of the odontoid was confirmed by an intraoperative x-ray image intensifier. Then, she underwent an occipitocervical contoured rod fusion using a 3.5 mm stainless steel contoured rod secured to the occipital bones and the C2 and C3 lamine using Aesculap 5.0 stainless steel wires. , Autologous split rib graft was placed along the decorticated occipitocervical bony surfaces [Figure 6]A and B. Postoperatively, she was kept on overnight elective ventilation. Following extubation, her neck movements were restricted using a hard cervical collar for 3 months.
She had significant reduction in spasticity, and her flexor spasms disappeared following surgery. Her postoperative lateral radiograph of the craniovertebral junction showed a stable construct with reduction of the AAD [Figure 7].
In our patient, the os odontoideum appeared as a rounded, corticated and smooth-margined ossicle that was clearly separated from the base of the odontoid process. ,, Both acquired and congenital mechanisms have been suggested in the etiology of os odontoideum. Embryologically, the axis vertebra originates from the proatlas and the C1 and C2 sclerotomes. Failure of the odontoid process (the C1 sclerotome) or the apex of the odontoid process (the proatlas) to fuse with the main portion of the odontoid process results in os odontoideum. , Os odontoideum has also been attributed to a disunited fracture following trauma during early childhood. ,, The failure of non-union may be due to the proximal fragment being pulled away from rest of the odontoid complex by the apical and alar ligaments, and due to disruption of the blood supply to the odontoid process.  In our patient, although a history of trauma was present, the concomitant presence of hydrocephalus due to aqueductal stenosis, mental retardation and congenital stenosis of the atlantal ring point towards a congenital origin for the os odontoideum. In consonance with this clinicoradiological picture, the patient had features of myelopathy since childhood that got aggravated following the fall at the age of 14 years. Other congenital occipitocervical anomalies like Klipper Feil anomaly have been associated with os odontoideum, as has been reported by Sherk et al ., suggesting that at least in some cases, os odontoideum has a congenital origin. 
The stability of the atlantoaxial joints essentially depends upon the integrity of the odontoid process and the attached ligaments. In patients with os odontoideum but with an intact transverse ligament, the atlas will be displaced along with the odontoid process, leading to atlantoaxial instability. Usually the anterior arch of atlas-os odontoideum complex moves forwards relative to the attached part of the odontoid-C2 body during flexion of the neck and reduces to its anatomical position in extension. This results in reducible AAD. , Occasionally, the attached part of the odontoid-C2 body complex telescopes into the cervical canal directly behind the anterior arch of atlas-os odontoideum complex.  In this case, there is persistent cervical compression both in flexion and extension of the neck as the relative position of the axis does not change relative to the anterior arch of the atlas during dynamic movements of the neck, resulting in fixed AAD. In the former case, a posterior fusion suffices; while in the latter, patients may require a transoral decompression prior to the posterior fusion.  Our case was unique and the atlantal ring was almost freely floating on the axis. The anterior arch of atlas-os odontoideum complex moved forwards relative to the axis during flexion of the neck so that the remaining part of the odontoid attached to the body of the axis caused cervical canal compromise; and the entire complex moved backwards relative to the axis, compromising the cervical canal during neck extension. Thus, there were both forward and backward atlantoaxial subluxations during dynamic movements of the neck. There was also a transverse C1-2 dislocation, with the atlas dislocating laterally relative to the axis at the C1-2 facet joints. The simultaneous presence of stenosis of the atlantal ring (canal diameter at the C1 level being 12 mm, while the normal range in Asians is between 16 and 18 mm) further aggravated the canal compromise at the C1-2 level. ,,,,,, The extension movements of the neck further increased the thecal compression due to increase in this retrolisthesis.
There could be several reasons for this abnormally free movement of the atlas on the axis. Os odontoideum leads to the free movement of remaining hypoplastic dens (attached to the body of axis), which was no longer constrained by the transverse ligament. The apical and alar ligaments usually pull the os odontoideum away from the attached part of the dens, permitting unrestricted forward and backward movements of both the segments.  On coronal CT and MR images, the C1-2 joints normally have an oblique orientation. In our patient, the C1-2 joints had relatively flatter facet joints, which perhaps permitted the free sliding of joint surfaces on each other.  The dorsal caudal portion of the proatlas forms the superior portion of the C1 posterior arch, and the first spinal sclerotome forms its inferior portion. The dorsal caudal segment of the neural arch of proatlas also leads to the development of the lateral atlantal masses. Neural arch of the second spinal sclerotome forms the lamina of axis as well as its lateral masses. The existence of atlantal hypoplasia and os odontoideum may, therefore, be associated with maldeveloped and therefore incompetent facet joints (due to the common origin of the C1 and C2 neural arches and facet joints from their respective sclerotomes), as was seen in this case.  The coexistence of laxity of joint ligaments perhaps also contributed to the exaggerated anteroposterior movements of these segments along with the free transverse dislocation of the facet joints in our patient.
The associated hydrocephalus was an uncommon finding in this patient. Various skeletal dysplasias, such as Morquio syndrome, Dyggve-Melchior-Clausen spondyloepimetaphyseal dysplasia, spondyloepiphyseal dysplasia congenita (type Spranger-Wiedemann) and achondroplasia, may be associated with odontoid hypoplasia and subluxation of the first and second cervical vertebrae as well as ventriculomegaly. , Shohat et al . reported 12 patients with metatropic dysplasia with odontoid hypoplasia, subluxation of the first and second cervical vertebrae and ventriculomegaly. The ventriculomegaly in these cases is usually caused by overcrowding of the tentorial incisura due to a relatively small posterior fossa and obstruction to the outlets of the fourth ventricle, caused by foramen magnum stenosis. In 6 of these patients, the lateral flexion-extension radiographs of the cervical spine demonstrated subluxation; 3 had subluxation even in neutral position; and sudden odontoid dislocation developed in another patient after a simple fall.  Metatropic dysplasia is characterized at birth by normal length associated with long trunk and short limbs with prominent joints. Short trunk dwarfism subsequently develops in these patients due to progressive kyphoscoliosis. Radiologic studies show marked flattening of the vertebrae; short, broad, "barbell-like" metaphyses and irregular epiphyses.  In our patient, the presence of hydrocephalus, mental retardation, os odontoideum, C1-2 hypermobile subluxation including a transverse facet subluxation and C1 arch stenosis point towards a syndromic variety of atlantoaxial subluxation, but none of the distinguishing clinical features of the usual skeletal dysplasias including abnormal limb joints coexisted. Moreover, aqueductal stenosis was responsible for the hydrocephalus rather than foramen magnum stenosis, leading to IV ventricular outlet obstruction as is usually seen in skeletal dysplasias.  No etiology could be assigned to the coexistence of mental retardation in our patient since the ventriculomegaly had been adequately managed since childhood and the patient neither had a family history of any congenital disorder nor had any evidence of developmental, metabolic or hormonal disorder of the nervous system. , The maternal gestational and obstetric history had been unremarkable.
The surgical management in our patient involved several unique considerations. At the age of 1 year, she underwent a right ventriculoperitoneal shunt for features of progressive hydrocephalus. The shunt had subsequently been revised as its blockage had led to the reappearance of symptoms of raised intracranial pressure. The development of features of raised intracranial pressure in our patient indicates that she had actual cerebrospinal pathway blockage and not mild ex-vacuo ventriculomegaly due to cortical atrophy. At that time, the hypermobile AAD was missed as the overt manifestations of myelopathy in the form of frequent falls and unsteady gait were attributed to growth retardation associated with mental retardation. The myelopathy got aggravated following her minor fall 2 years ago. At that time, the characteristic history of transient loss of consciousness with progressive quadriparesis led to the investigation of the craniovertebral junction and to the diagnosis of atlantoaxial subluxation. In our patient, positioning and intubation for surgery had to be performed while maintaining her neck in absolutely neutral position. The alignment of the atlantoaxial joints was checked using an intraoperative X-ray image intensifier.  Some authors have suggested placement of the patient in a halo brace prior to surgery and continuing with the halo brace during surgery to ensure continued maintenance of proper alignment of the C1-2 joints. 
In our patient, although the posterior arch of the atlas was well developed, a C1-2 sublaminar fusion (Brooks' technique) was not performed. , This was because of the coexisting canal stenosis at the level of atlas that would have been further compromised by sublaminar wiring. Moreover, closely approximating the C1 posterior arch with the lamina of the axis would have led to the risk of dislocating the C1 anterior arch-os odontoideum complex backwards into the spinal canal.  Thus, a C1 laminectomy was performed with an occipito-C2-C3 stainless steel contoured rod and onlay autologous rib bone graft stabilization. , During surgery, the occipitoatlantal segment was freely mobile on the axial and subaxial segment of the spine. Care also had to be taken, therefore, to ensure that the metal rod was adequately contoured and fixed in situ under fluoroscopic guidance so that the axial-subaxial segment was not inadvertently pushed anteriorly towards the thecal sac to cause posterior compromise of the cord; the anterior arch of the atlas-os odontoid complex did not undergo retrolisthesis into the spinal canal to cause anterior cod compression; and the occiput-atlas complex was adequately aligned to the axial-subaxial segments with no rotational or transverse dislocation to prevent torticollis or transverse neck tilt. ,, In view of her symmetrical C1-2 facet joints, a Mageryl's transarticular C1-2 fixation , or the Goel's C1-2 lateral C1-2 mass fixation  would perhaps have sufficed in stabilizing the C1-2 joints while preserving the occipitoatlantal and C2-3 motion segments; an occipitocervical construct was, however, preferred in view of the hypermobility (including transverse mobility) of the C1-2 segments and because an additional C1 laminectomy was also required in our patient in view of the coexisting atlas hypoplasia. In these circumstances, the occipitocervical construct that included one joint superior and inferior to the hypermobile one was considered to be biomechanically superior and more stable than the one that only included the apposing mobile segments. 
To conclude, a unique case of a patient with ventriculomegaly due to aqueductal stenosis, mental retardation, os odontoideum, atlas hypoplasia and hypermobile (anterior, posterior and transverse) atlantoaxial subluxation without other features of coexisting chondroskeletal dysplasias is described. The clinicoradiological features and unique management considerations are discussed. To the best of our knowledge, this is the first reported case of its type in the literature.
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