Key-words:
Cervical - dystopic - myelopathy - Os odontoideum
Discussion
OO is a rare disorder of the odontoid process of C2 vertebra as described by Giacomini.[[1]] It is defined radiographically as an oval or round-shaped ossicle that has no continuity
with the body of C2. It has smooth circumferential cortical margins representing the
remnants of odontoid process. Fielding et al[[2]] have classified it into two types based on anatomic location: Orthotopic and dystopic.
It is usually found on in asymptomatic individuals with an estimated prevalence of
0.7%,[[3]] but fulminant myelopathy is also described secondary to minor trauma. The TAL functions
by restraining atlantoaxial motion. In OO, there is failure of this ligament secondary
to the loss of bony support by the odontoid process. This becomes important as the
person then suffers from mobile or insufficient dens, and it may cause translation
of the atlas on the axis and may compress the cervical cord or vertebral arteries.[[4]] The exact etiology still remains obscure because those malformations mostly are
incidentally detected in asymptomatic patients or are diagnosed only when patients
become symptomatic. There are several reports of patients with OO becoming quadriparetic
after minor trauma.[[5]] Although OO is a rare condition, it is exact frequency remains unknown since no
large-scale screening studies have been performed. Nevertheless, in a study made by
Perdikakis and Skoulikaris[[3]] they described the MR appearance of the odontoid process and calculated the prevalence
of its morphological variants. They retrospectively reviewed 133 patients, age range
between 19 and 81 years, which were examined within a period of 7 years and found
OO in only one case (0.7%) whereas Sankar et al.[[6]] in their study detected it in 3.1% of children with abnormal cervical radiographs.
This lesion usually presents clinically in the pediatric population; moreover, most
authors today believe it might represent an unrecognized fracture or damage to the
epiphyseal plate during the first few years of life.[[7]] Some believe that it may represent a congenital anomaly instead of occult trauma.
During the embryological development, part of the odontoid process is derived from
the fourth occipital sclerotome giving rise to the apex of the odontoid, which is
called the ossiculum terminale or the apical odontoid epiphysis. The first and second
cervical sclerotomes are the ones which contribute to the odontoid and axis bodies.
The odontoid usually has an epiphyseal growth plate, which separates the first and
second cervical sclerotomes, and it is frequently known as the neurocentral synchondrosis.
This structure lies below the level of the superior articular facets of the axis and
is usually visible in children younger than 3 or 4 years but disappears by 8 years
of age. The odontoid process carries a unique blood supply because of its dependence
from the terminal apical arcade for the majority of the vasculature. This anastomoses
with the caudal supply from the deep penetrating branches arising off the posterior
ascending arteries which in turn comes from the vertebral artery. This also provides
an explanation for the increased risk of ischemia due to the precarious blood supply
of the odontoid process. Since no vessels pass through the transient epiphyseal plate
between the odontoid process and the axis, this arterial apparatus is extremely crucial
early in life. The relatively fixed position of the dens as the atlas rotates provides
insufficient vascularization by the anterior and posterior branches of the vertebral
arteries. Consequently, there is a great dependence of odontoid process on a terminal
descending supply superiorly (the apical arcade). This apparent vascular insufficiency
of the odontoid blood supply predisposes it particularly vulnerable to ischemia and
necrosis, especially in traumatic events. Moreover, the blood vessels traverse closely
alongside the odontoid process; hence, the blood supply of the odontoid process may
be unstable because it can be easily obstructed. It is hypothesized that such an obstruction
might lead to ischemia which consequently might contribute to poor fracture healing
and callus formation. It is associated with many congenital malformations such as
Morquio's disease,[[8]] the Klippel-Feil syndrome,[[9]] multiple epiphyseal dysplasia,[[8]] achondroplasia,[[10]] the Larsen syndrome,[[11]] the Wolcott–Rallison syndrome,[[12]] and chondrodystrophia calcificans. The other hypothesis for development of OO is
originated mainly from the work of Fielding and Griffin which is the posttraumatic
or acquired hypothesis. They have concluded that OO forms after an unrecognized fracture
to the odontoid with the subsequent contraction of the apical and alar ligaments,
the distraction of the fractured fragment, and the abrupt termination of blood supply,
leading to the formation of an ossicle. This is also supported by the fact that OO
is most commonly located at the base of the dens and not at the synchondrosis where
a fusion failure is expected, as reported in other studies. There at times can be
multiple ossicles instead of a single-ossicle. Several other studies have supported
the posttraumatic etiology.[[13]],[[14]]
Most patients have their first onset of symptoms in childhood with neck pain or neurologic
symptoms due to cord compression from the posterior translation of the Os into the
cord in extension or the odontoid into the cord in flexion. There is, however, another
group of patients presenting in adulthood with symptoms of cervical myelopathy secondary
to mechanical cord compression often secondary to minor trauma which cannot be recollected.
Increased motion at the C-1 to C-2 level can lead to symptoms of central cord syndrome,
Brown-Sequard syndrome, hypoventilation syndrome (Ondine's curse), and cardiorespiratory
failure in severe cases. In extremely rare scenarios it can also cause cerebellar
infarction secondary to mechanical effects of compression leading to embolization.[[15]] The presence of atlanto-axial instability in adults is usually a result of traumatic
ligamentous rupture and consequent instability and as a result of rheumatoid arthritis.
The presence of basilar invagination and syringomyelia together with atlanto axial
instability typically leads to a diagnosis of Chiari malformations which should also
be kept in mind while investigating such patients. The diagnosis of OO is primarily
radiographic. It can be clearly visualized using plain radiographs with the open mouth,
anteroposterior, and lateral views. In the lateral radiographs dynamic views performed
in flexion and extension should be done for further evaluation of atlanto-axial instability
and reducibility. Although quite useful in diagnosis, the exact sensitivity and specificity
of standard plain radiographs in diagnosing OO have not been studied.[[15]] For a detailed anatomical analysis CT scans and MRI scans are important for a better
illustration of osseous abnormalities and spinal cord compression and pathology, respectively.
Addition of CT angiography to these studies is important to look for vertebral artery
position and anomalies which can be commonly found with syndromic patients. It is
important both for diagnosis and further surgical planning which is frequently a posterior
approach.[[16]] Furthermore, Hughes et al. recommended the use of kinematic MRI in diagnosing OO,
given the advantage of directly visualizing the motions of joint components and the
surrounding soft tissues.[[17]] However, an initial examination of patients with myelopathy using a conventional
MRI scan can occasionally lead to a misdiagnosis of an intramedullary spinal cord
tumor in cases of chronic cervical spine instability secondary to OO.
The management of symptomatic OO is usually a well-defined surgical indication in
terms of instrumented stabilization and C1 C2 fusion. Due to paucity of literature
regarding the natural history of untreated OO, the course of treatment for asymptomatic
individuals with incidentally detected OO is still debatable. There have been reports
of conservative management and close observation as reported by Dai et al. who have
reported that at 1-year follow-up it remained stable. However, some authors[[18]] believe that all asymptomatic patients even those with a “stable” OO, should undergo
C1-C2 fusion to avert any neurological complications. This is also better appreciated
when we consider reports in the literature on sudden death,[[19]] significant neurological complications[[8]] following minor injuries in previously undiagnosed OO, and patients who suffer
late neurological deterioration.
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