Keywords
occipital bone - atlas - atlanto-occipital - synostosis
Introduction
Knowledge of cranio-vertebral abnormalities and their morphology is important for
clinicoradiological studies. This leads to clinical assessments with more accurate
diagnoses, as in those cases where synostosis is present, i.e the atlanto-occipital
fusion that affects the mentioned joint and that is formed by the junction between
the lateral mass superior articular facet of the atlas and the corresponding occipital
condyle. The atlanto-occipital membrane joins these two bones.[1] The atlanto-occipital joint is a synovial joint with a loose and thin articular
capsule. Atlanto-occipital synostosis is defined as a congenital fusion of the atlas
to an occipital base.[2]
[3] This anomaly was first described by Rokitansky in 1884, and since then, several
terminologies such as synostosis, occipitalization of Atlas, fusion of atlanto-occipital
or ankylosis of atlanto-occipital fusion have been used.[2]
Cervico-capital rotation process involves important joints with respect to the neurological
field. Among these joints, the atlanto-occipital joint accounts for 40% of the total
rotation process,[4] and anatomical changes may affect the joint i.e when synostosis of both bones occurs.
This anomaly has a 0.14% to 0.75% of reported incidence in world population.[2] Different varieties of this anomaly have been described. These include complete
or incomplete types, complete synostosis being the most common.[5] Synostosis may be associated with other skeletal malformations, such as spina bifida
of atlas, occipital vertebra, basilar invagination, Klippel-Feil syndrome, Arnold-Chiari
malformation and cervical stenosis.[6]
Knowledge of atlanto-occipital synostosis is important from the clinical point of
view due to the vertebral artery position and the first spinal nerve (C1). Vertebral
artery compression may cause a cerebral blood flow (CBF) decrease and C1 compression
can cause neurological symptoms as well.[7] Different clinical manifestations of this synostosis have been described, namely
cephalea, cervical pain, abnormal neck position anomalies, decreased range of movement,
dizziness, dysphagia and dysarthria.[8]
Materials and Methods
This was an observational and cross-sectional study with a descriptive coverage. Quantitative
and qualitative information was collected. 105 adult skulls without reported injury
of the occipital region were randomly selected. These skulls belonged to the Departments
of Anatomy of Caldas, Manizales and Autonoma universities. The presence of synostosis
between the occipital bone and C1 was assessed. There was no distinction between ethnicity
or sex. Among exclusion criteria were those skulls with presence of trauma or gross
malformations in the cervico-occipital region. Synostosis was assessed between the
occipital bone and the C1. A Vernier caliper and a camera were used for morphometric
descriptions.
Results
Out of 105 skulls studied, 1.9% showed synostosis between the lower portion of the
occipital bone and the C1. One of skulls had complete synostosis and another one hemisinostosis,
as follows.
Skull 1 exhibited a hemisinostosis between the occipital bone and the first cervical
vertebra (C1), characterized by complete fusion on the right side and incomplete fusion
on the left side ([Figs. 1] and [2]). This region did not exhibit posterior condyloid foramina ([Fig. 2]). This suggests the presence of synostosis in corresponding ducts of the emissary
veins within the sigmoid sinus. On the right side, a developed channel between the
lateral mass and the lateral occipital bone was observed, which makes way for the
course of the right vertebral artery. Its diameter was 4.5 mm at its greatest end
and 2.5 mm at its lowest end. On the left side, a lateral mass fusion of the vertebrae
C1 with occipital bone was observed. The transverse foramen and the extra space makes
way for the course of the left vertebral artery. Its diameter was 4.5 mm at its greatest
end and 2 mm at its lowest end. The anterior tubercle of C1 vertebra showed no synostosis
with the occipital bone. However, it was deviated from the median line of 4 mm, with
reference to the pharyngeal tubercle ([Fig. 1]).
Fig. 1 (Standard basal skull with C1 and occipital synostosis. Synostosis on the right side
of the lateral masses and hemisinostosis on the left side is highlighted.
Fig. 2 Incomplete synostosis between C1 and occipital bone.
Skull 2 exhibited a bilateral synostosis between the occipital bone and the first
cervical vertebra (C1) ([Figs. 3] and [4]). This region presented no posterior condyloid foramina, which suggests the presence
of synostosis in corresponding ducts of emissary veins within sigmoid sinus, similarly
to case 1. On the left side, a developed channel between lateral mass and occipital
bone is identified which makes way for the course of the left vertebral artery. Its
diameter was 6 mm at its greatest end and 5mm at its lowest end. On the right side,
there was a complete fusion of the lateral mass that prevented the development of
the C1 foramen transversarium. This suggests that the artery had an intracranial course
from the C2 foramen transversarium. The anterior tubercle of C1 suffered no synostosis
with the occipital, although it was deviated from the midline, 1.5 mm, with reference
to the pharyngeal tubercle.
Fig. 3 Complete synostosis: in a standard basal skull, a deviation of the above process
C1 to the right side is noted. Intrajugular process fused with occipital bone is also
noted. This divides jugular foramen into two.
Fig. 4 Complete synostosis.
Discussion
The frequency of atlanto-occipital synostosis in this study was 1.9%. This is significantly
higher compared with the information reported in the literature, which references
values in range from 0.14 to 0.75%.[9]
[10]
[11]
In the skull with hemisinostosis, the reduced diameters of the small ends of the ducts
formed by the passage of the vertebral artery in its exocraneal course lead to a suspected
blood flow decrease due to stenosis of both arteries. This is due to the contrasting
values of the diameter of the vertebral artery reported in the literature from 3.5
to 4 mm.[12] This would affect the movement of the anterior, posterior, inferior posterior, cerebellar
and basilar arteries. On the other hand, the skull with atlanto-occipital bilateral
synostosis shows a transverse foramen diameter within normal parameters in its left
side, in contrast to the right side transverse agenesis foramen which suggests that
the vertebral artery undergoes no narrowing in its course toward the inside of the
skull after piercing the atlantoaxial membrane.
Atlas deviations of the anterior tubercle suggest an asymmetrical cruciate ligament,
since the fovea for the odontoid process moves unilaterally. This could have generated
instability on the atlantoaxial joint. The case of bilateral synostosis in one of
the skulls is related to a deep invagination of the basilar region, consistent with
reports in the literature.[6]
Conclusion
In a direct morphological study in skulls of a Colombian sample, it was determined
that the frequency of the atlanto-occipital synostosis is higher by 253% compared
with that reported in the literature. This type of synostosis alters the occipital
and spinal region morphologically and can generate variations in the course of the
vertebral artery compatible with some clinical manifestations.
