platybasia - magnetic resonance imaging - cephalometry
platibasia - imagem por ressonância magnética - circunferência craniana
The cranio-vertebral transition (CVT) is composed by the base of the occipital bone
and the first two cervical vertebrae, atlas and axis (C1 and C2, respectively). This
region is susceptible to conformational abnormalities, such as platybasia – flattening
of skull base – and basilar invagination – protrusion of the tooth of the C2 to the
posterior fossa. They are mutually associated, and both may also relate to cerebellar
tonsil herniation. Clinical repercussions are varied and derive from compression of
nervous structures or obstruction of cerebrospinal fluid circulation, with consequent
hydrocephalus and syringomyelia[1].
Imaging exams are indispensable in the assessment of CVT, and the gold-standard is
magnetic resonance imaging (MRI), especially because it can demonstrate both musculoskeletal
and nervous elements involved. Craniometric imaging parameters were first established
last century, notably from German radiograph-based studies, but some are still used
for evaluating CVT by computerized tomography (CT) or MR[1].
Chamberlain’s line extends from the posterior portion of the hard palate up to the
posterior margin of the foramen magnum (ostium). The distance between the apex of
the odontoid and the Chamberlain’s line (DOCL) is used to measure the projection of
the axis into the posterior fossa to diagnose basilar invagination.
The Welcker basal angle (WBA) and the clivus-canal angle (CCA) are also largely used
for evaluation of platybasia[1].The WBA measures angulation of the skull base and is obtained by two lines: one
traced from the nasion up to the tubercle of sella turcica and another line from the
latter to the anterior margin of the foramen magnum (basion). Values above a normal
limit of 140º have been considered diagnostic of platybasia[1],[2]. The CCA is measured between the line that follows parallel to the clivus and another
that borders the posterior surface of the odontoid process[2]. Values lower than 150º are considered abnormal[1]. A narrower angle increases the risk nervous compression.
For several decades, researchers have reported a high prevalence of platybasia and
basilar invagination in the northeastern Brazil population when compared with other
regions within and beyond Brazilian borders[3],[4],[5]. Studies conducted in northeast Brazil by Silva et al.[3],[4], and Alves et al.[6], have also found an association with brachycephaly[3],[4],[6],[7] – a condition in which the lateral diameter of the skull is disproportionately large.
In a corpse-based study, also conducted in northeastern Brazil, there was a surprisingly
high rate of brachycephaly (77.6%), higher than that reported in other populations
associated with brachycephaly[8]. This condition is also popularly referred to as “cabeça-chata” (flat head), a term often used in a pejorative manner to define the northeastern
population[9].
Brachycephaly and its association with platybasia may not be pathological, but rather
a phenotypic tendency of a population, apparently related to ethnic-anthropological
factors in northeast Brazil[10]. Thus, diagnosing platybasia and basilar invagination with craniometric parameters
derived from overseas samples may not be appropriate. However, there have not previously
been imaging studies assessing CVT parameters in the northeastern population.
The authors aimed to assess craniometry in a sample from the northeastern Brazil population
via MR imaging. Specific objectives were: 1) outline a profile of the sample for the
DOCL, WBA and CCA measures; 2) verify if there were correlations between these three
parameters.
METHODS
Data was obtained from head MRIexaminations, conducted at an imaging service at outpatient
level in “Sertão” (drylands) of the state of Paraíba (NE). The MR imaging was carried
out by spontaneous demand on that service between January 2011 and December 2012.
All patients aged 18 years or above were included. Exclusion criteria were: manifested
clinical suspicion of CVT disease by the referring physician, surgery to the base
of the skull or important previous cranio-vertebral trauma, technical difficulties
in the identification of the parameter structures, resulting from positioning errors,
misangulation and imaging artifacts, such as those generated by metallic material
or patient movement. If a patient had more than one examination during the period
of the study, only the first one was included.
All examinations were carried out in an open device, of 0.35 T (Magnetom C!, Siemens
Medical Solutions, Erlangen, Germany, 2011). The parameters were outlined in the mid
sagittal plane of T1 3D sequence (magnetization-prepared rapid gradient-echo or MP-RAGE)
acquired at the sagital plane, with thickness varying from 0.9 to 1.1 mm. Images were
analyzed in digital DICOM format, by means of an open version of the visualization
software Osirix® in its free version 3.9.2 (Mac-Apple® platform).
The DOCL, CCA and WBA were measured according to the techniques described by Smoker[1] and Amaral et al.[2] ([Figures 1], [2] and [3]).
Figure 1 Distance between the odontoid apex and Chamberlain’s line (DOCL).
Figure 2 Clivus-canal angle (CCA).
Figure 3 Welcker basal angle (WBA).
The data was analyzed by means of the statistical software R3.2.0. The Shapiro-Wilk
test was used to assess normality. The comparison between genders was made using the
unpaired Wilcoxon’s test. Spearman’s correlation was used to verify the relation between
the variables, with the p-value corrected by Holm’s method. The value of p < 0.05
was considered as statistically significant.
The study was substantiated on secondary data (tests), guaranteeing privacy and confidentiality
of the patients, ensuring the non-use of the information to the detriment of the people,
according to Resolution 466/12 of the National Council of Health. The project was
approved by the Ethics in Research Committee of the institution (CAAE: 30255914.1.0000.5188),
with substantiated exemption of the consent term. The use of the images was authorized
by the direction of the service. There was no interference in the protocol of the
examinations or any clinical or surgical intervention in function solely for this
research.
RESULTS
A total of 181 head MRIs were studied. Eighty participants (44.2%) were men and 101
(55.8%) were women. The median age was 46 years (IQR = 32–63). The craniometric variables
are described in [Table].
Table
Craniometric parameters.
|
Craniometry
|
Percentiles
|
Mean (SD)
|
|
|
2.50
|
25
|
50
|
75
|
97.50
|
|
WBA (°)
|
117.1
|
124.3
|
128.5
|
133.4
|
142.65
|
128.96 (6.51)
|
|
DOCL (mm)
|
-7.55
|
1.23
|
2.27
|
4.47
|
15.5
|
2.25 (5.06)
|
|
CCA (°)
|
121.7
|
143.2
|
150.5
|
157.3
|
172.5
|
150.14 (11.37)
|
WBA: Welcker basal angle; DOCL: distance between odontoid and Chamberlain’s line;
CCA: clivus-canal angle; SD: standard deviation.
There was no significant difference in craniometric parameters for gender and age.
The WBA showed a parametric distribution, with an average of 128.96º (confidence interval
of 95%, 128.01–129.91).
There was no significant correlation between the CCA and DOCL (p = -0.215; p = 0.26), but there was a positive correlation between the WBA and the DOCL
(p = 0.366; p < 0.0001), as well as an inverse correlation between the WBA and the CCA
(p = -0.415; p < 0.0001) ([Figures 4], [5] and [6]).
Figure 4 Relationship between clivus-canal angle (CCA) and distance between odontoid apex
and Chamberlain’s line (DOCL).
Figure 5 Relationship between Welcker basal angle (WBA) and distance between odontoid apex
and Chamberlain’s line (DOCL).
Figure 6 Relationship between Welcker basal angle (WBA) and clivus-canal angle (CCA).
DISCUSSION
The use of lines, distances and angles that relate to anatomical landmarks of a CVT
helps in the diagnosis of alterations such as platybasia and basilar invagination[2],[11]. Despite most of these measures having been designed when radiographs were the only
imaging method available[1],[12],[13],[14], they are currently adopted for CT and MR imaging analysis.
There has been considerable variation in craniometric parameters across studies. The
WBA has varied broadly, with one reported average of 115.41° (±8.45°) for Netto et
al.[7]. Other authors have reported relatively large amplitudes: 103.5° to 131.5° for Boogard[15]; 115° a 140° for List[16]; 115° to 150° for Walsh et al.[17]; 120° to 147° for McGreger[14]; 121.5° to 148.5° for Brailsford[13]; 115º to 140º Royo-Salvador[18]; and 97º to 133.2º for Batista et al.[19]
There is still no consensus over the upper limit for DOCL values, which range from
2.0 to 6.6 mm[1],[20],[21]. This difference is not insignificant when compared to the small dimensions of CVT
and hence could lead to considerable changes in specificity in the diagnosis of basilar
invagination, as well as in the estimated prevalence[11],[22].
Taking into consideration the DOCL upper limit of 2 mm[20], more than half of the participants in this study would be diagnosed with basilar
invagination, which would be unlikely for individuals who come to imaging services
by spontaneous demand.
According to Smoker, a CCA value lower than 150º is considered abnormal[1]. Botelho and Ferreira[11] analyzed the CCA via MRI, obtaining an average of 148º ± 9.8º (with a range from
129º to 179º). Batista et al.[19], analyzing with the use of CT, obtained an average of 153.6º ± 7.6º (varying from
132.3º to 173.9º).
Our study showed a CCA with an average of 150.14º ± 11.37º – very close to the values
cited by Botelho and Ferreira[11] and Batista et al.[19], which is apparently higher than that suggested by Smoker[1]. Despite Smoker[1] having demonstrated significant correlation between the CCA and DOCL, none was found
in this study. Thus, it can be speculated that the values of the CCA obtained by MRI
or CT of asymptomatic patients seem to be higher than the ones published using radiographs,
from which normality standards were defined[1],[19]; or, alternatively, it is possible that each population naturally presents with
different values.
Northeastern Brazilian population tendency toward brachycephaly as well as other CVT
peculiarities have been addressed in previous clinical[4],[5],[23], and anthropological studies[9]. A strong relationship between brachycephalia and alterations of the CVT in patients
submitted to surgeries in this region was also found by other researchers [4],[5].
Nonetheless, some of the variance between samples may be also attributed to different
imaging methods used in each study, as well as to a lack of validation and standardization
of the old craniometric techniques as they are adapted to new imaging resources, such
as CT and MRI.
In this study, an inverse correlation between the WBA and CCA suggests that flattening
of the skull base may accentuate thebulbo-pontine transition in its passage in the
CVT. A similar correlation was referred to by Joaquim et al.[22], who demonstrated that the presence of platybasia and a decreased clivus seem to
favor a protrusion of the cervical spine to the inside of the skull base (basilar
invagination)[24]. Previous studies have already hypothesized that the presence of platybasia may
be associated with compression of posterior fossa structures by mechanical opposition
to the apex of the odontoid process[1],[4],[5].
CVT imaging analysis is still poorly-described in the northeastern population, especially
those from the countryside. Despite all participants included in this studywere chosen
from a single reference imaging center in the countryside of the state of Paraíba,
their place of birth was not verified.Thus, it is not possible to know how much of
the sample was indeed born in that region and how accurately this sample represents
Paraíba or Brazilian Northeast populations as a whole. Therefore, this study could
provide further evidence to help define northeastern craniometric standards and its
peculiarities.
In conclusion, the inverse relation of the WBA and the CCA, demonstrated here, suggests
that platybasia, despite not being directly involved in nervous compressions, may
be a factor associated with this, as it can contribute to the narrowing of the passage
of the neuroaxis in the CVT. The correlation found between the WBA and DOCL demonstrates
the relationship between platibasya and basilar invagination, already referred to
in literature, and reinforces the clinical importance of platybasia.
Descriptive results of all three parameters also suggest that their normal ranges
may be different from those previously shown in international studies, which demands
further research involving larger sampling of normal individuals, not only to confirm
a difference, but also to propose new appropriate limits for people from Northeast
region.
