Sonographic Measurements of the Fetal Fastigium Between 20 and

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Article
Sonographic Measurements of the
Fetal Fastigium Between 20 and 40
Weeks’ Gestation
Ronnie Tepper, MD, Devora Kidron, MD, Reli Hershkovitz, MD
Objective. The purpose of this study was to establish a new reference angle chart for fastigial biometric measurements throughout gestation in normal singleton pregnancies. Methods. A prospective cohort study was designed. A total of 505 pregnant women between 20 and 40 weeks’
gestation were included in the study. Excluded were those with multiple pregnancies, congenital
anomalies, abnormal karyotypes, and polyhydramnios or oligohydramnios. Transvaginal or transabdominal sonography was performed in the midsagittal plane of the brain. Results. The angle of the
fastigium was found to be 30° to 60° throughout pregnancy. These data were found to be constant
during the progression of pregnancy without any significant changes. Conclusions. Fastigial measurements throughout pregnancy are presented. Its angle is constant throughout pregnancy.
Evaluation of the fastigium may assist in cases of subtle posterior fossa anomalies. Key words:
fastigium; nomograms; posterior fossa; sonography.
Abbreviations
GA, gestational age
Received July 20, 2009, from the Departments of
Obstetrics and Gynecology (R.T.) and Pathology
(D.K.), Meir Medical Center, Sackler Faculty of
Medicine, Tel Aviv University, Kfar-Saba, Israel (R.T.);
and Department of Obstetrics and Gynecology,
Soroka University Medical Center, Ben Gurion
University of the Negev, Be’er Sheva, Israel (R.H.).
Revision requested July 23, 2009. Revised manuscript
accepted for publication August 4, 2009.
Address correspondence to Reli Hershkovitz, MD,
Ultrasound Unit, Department of Obstetrics and
Gynecology, Soroka University Medical Center, PO
Box 151, 84101 Be’er Sheva, Israel.
E-mail: ralikah@bgumail.bgu.ac.il
S
onographic imaging of the fetal posterior fossa has
become an important part of brain surveillance.
The fully matured cerebellum and especially the
cerebellar vermis are apparent after 18 weeks’ gestation and can be shown sonographically.1–4 The normal
appearance of the vermis and its biometric characteristics have been described in a few previous studies.1–3
Studying sonograms of the normal structure may assist in
the diagnosis of posterior fossa anomalies.5–7
The vermis can be diagnosed mainly in the sagittal
plane of the brain.2 It is a very complex structure composed of several lobes and fissures.8 The vermis grows
faster than the cerebellar hemispheres between 19 and 37
weeks gestation,8 and many posterior fossa anomalies are
associated with the vermis rather than the cerebellar
hemispheres.7–9
One of the important structures of the posterior fossa is
the fastigium. It is a thin roof plate of the fourth ventricle,
originally contiguous with the posterior border of the
cerebellum, and projects inward toward the cavity of the
ventricle.10 It has been suggested that the prognosis of
© 2009 by the American Institute of Ultrasound in Medicine • J Ultrasound Med 2009; 28:1657–1661 • 0278-4297/09/$3.50
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Sonographic Measurements of the Fetal Fastigium
the Dandy-Walker malformation may be associated with changes in the fastigium.5 When the
vermis is shown to be normal in appearance with
the fastigium in place and with a cyst in the posterior fossa, the prognosis is better in comparison
with patients who have an abnormal vermis with
the fastigium displaced from its normal position.
Moreover, it is well known that even subtle
changes in the vermis may be associated with
abnormal development later in life.11–13 Because
the fastigium may be involved in these changes,
the purpose of this study was to evaluate and create nomograms for the fastigium.
Materials and Methods
Study Population
A prospective cohort study was designed, and
measurements of the fastigium were obtained in
consecutive patients during routine sonography.
Each patient participated in the study only once.
Low-risk pregnant women between 20 and 40
weeks’ gestation were included in the study.
Inclusion criteria were an accurate gestational
age (GA) based on early sonographic measurements during the first trimester, singleton pregnancies, a normal amount of amniotic fluid
based on the amniotic fluid index, and the
absence of structural or chromosomal anomalies. We also included 3 patients with an abnormal vermis. The local Institutional Review Board
approved the study, and patients gave their consent orally.
Fastigial Anatomy
The fourth ventricle has a characteristic diamond shape in cross sections of the human
brain. It is located within the pons or in the
upper part of the medulla. Cerebrospinal fluid
entering the fourth ventricle through the cerebral aqueduct can exit to the subarachnoid
space of the spinal cord through 2 lateral
foramina of Luschka and a single midline foramen of Magendie. The fourth ventricle has a
“roof” dorsally and a “floor” ventrally. The roof
of the fourth ventricle is formed by the cerebellum, the floor by the rhomboid fossa, and
the side “walls” by the cerebellar peduncles.
The angle in the roof of the fourth ventricle is
the fastigium.
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Sonographic Measurements
Sonographic examination was performed with a
transvaginal 9–5 MHz or a transabdominal 4–2
MHz convex probe and Voluson 730 ultrasound
equipment (GE Healthcare, Milwaukee, WI).
Depiction of the fastigium was performed in the
midsagittal plane. The measurement was done
when the corpus callosum was shown throughout
its length with the cavum septum pellucidum. At
that same midsagittal plane, the vermis can be seen
with the fastigium (Figures 1–4). All of the images
were direct 2-dimensional images. Measurement of
the fastigial angle was performed online with the
ultrasound machine by using the angle mode of the
calculations on the keyboard. To measure the
fastigium, the midsagittal image described above
was used. The fastigium was measured online during the sonographic examination. The first caliper
was placed exactly at the middle upper border of
the fastigium; the second caliper was placed along
the upper border of the fastigium; and the third
caliper was placed along the lower border of the
fastigium. The numeric result of the angle then
appeared on the sonographic screen. Depiction
was successful in all patients. Measurements were
performed by 2 authors (R.T. and R.H.).
Statistical Analysis
Statistical analysis was performed with an SPSS
package (SPSS Inc, Chicago, IL) according to the
methods of Silverwood and Cole.14 Nomograms
were constructed by using a linear regression
model. Our assumption was that at each GA, the
measurement of interest had a gaussian distribu-
Figure 1. Two-dimensional midsagittal appearance of the normal fastigium at 24 weeks: transabdominal approach.
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Figure 2. Two-dimensional midsagittal appearance of the normal fastigium at 24 weeks: transvaginal approach.
Figure 4. Two-dimensional midsagittal appearance of the normal fastigium at 31 weeks: transvaginal approach.
tion with a mean and an SD. Normality was
assessed by a z score (z = observed y value – mean
GA/SD GA). The percentile curve at a given GA
was calculated by percentile GA = mean GA + K ×
SD GA. The mean and SD model was used to represent how the mean and SD changed with the
GA. K was the desired normal equivalent deviate.
A normal equivalent deviate of 0 was the 50th
percentile, and the 5th and 95th percentiles
would require K = ±1.645. The mean was modeled
by fitting a polynomial curve to the raw data.
sample using the z score (P = .9978). Figure 6 is a
scatterplot of our data showing no variability of
the angle throughout pregnancy. Table 1 and
Figure 7 show the mean angle of the fastigium for
each GA and the standardized residual. Table 1
presents the values of the fastigial angle throughout pregnancy. Figure 8 shows 2-dimensional
images of fastigial angles that are above the normal values as described in Table 1 and Figure 7.
Results
Figure 5. Normality of sample: normal plot of calculated z
scores in our database.
A total of 505 patients were included in the study;
412 were successful with the transabdominal
approach and the others with the transvaginal
approach. Figure 5 shows the normality of our
Figure 3. Two-dimensional midsagittal appearance of the normal fastigium at 28 weeks: transabdominal approach.
Figure 6. Scatterplot of fastigial angles by GA.
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Sonographic Measurements of the Fetal Fastigium
Table 1. Fastigial Angles During Gestation
GA, wk
n
Mean, °
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
22
25
25
42
25
23
24
20
25
20
28
23
25
22
21
21
25
20
24
22
22
44.2
43.4
46.4
43.7
41.3
42.1
46.2
44.2
42.4
41.0
40.9
44.9
43.9
40.8
41.4
44.9
44.8
41.7
44.2
42.6
42.8
SD, °
6.4
5.9
7.7
8.0
8.7
9.3
7.6
7.9
9.7
5.9
6.7
7.2
8.1
8.5
10.1
9.9
8.8
8.5
9.5
6.8
7.9
Minimum, °
Maximum, °
Mean +
SD, °
Mean –
SD, °
Mean +
2 SD, °
Mean –
2 SD, °
36.0
37.0
29.7
32.0
27.9
32.0
36.0
34.0
30.0
31.0
31.3
35.0
30.0
28.0
26.0
27.0
29.0
29.0
27.0
28.0
27.0
58.0
59.0
59.7
60.0
56.1
60.3
58.0
60.0
60.0
51.0
59.7
59.0
54.0
56.0
60.0
59.0
60.0
59.0
59.0
55.0
55.0
50.6
49.3
54.1
51.7
50.1
51.4
53.9
52.1
52.1
46.9
47.6
52.1
51.9
49.3
51.5
54.7
53.6
50.2
53.7
49.4
50.8
37.8
37.6
38.7
35.7
32.6
32.8
38.6
36.2
32.7
35.1
34.2
37.7
35.8
32.4
31.3
35.0
36.0
33.2
34.7
35.9
34.9
57.0
55.2
61.8
59.7
58.8
60.6
61.5
60.0
61.8
52.8
54.3
59.2
60.0
57.8
61.6
64.6
62.4
58.8
63.2
56.2
58.7
31.3
31.7
31.0
27.7
23.9
23.6
31.0
28.3
23.0
29.2
27.5
30.6
27.8
23.9
21.2
25.1
27.2
24.6
25.2
29.1
26.9
Discussion
Our study shows that the fastigial angle is constant throughout pregnancy. To the best of our
knowledge, this finding has not been reported
previously. This angle can be measured sonographically in the midsagittal plane. The sagittal
plane allows excellent visualization of the posterior fossa and enables differentiation between
the vermis itself and the fourth ventricle and cisterna magna. This sonographic examination is
easy to perform and enables the sonographer to
determine whether the vermian position and
ventricular systems, as shown by the fourth ventricle, are normal.
Figure 7. Fastigial angle measurements throughout pregnancy.
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The various abnormalities of the posterior
fossa may be gross in appearance, such as the
Dandy-Walker malformation and Joubert syndrome.6–9 The prognosis of the Dandy-Walker
malformation is not always easy to counsel. It is
known that patients with the Dandy-Walker
malformation who have 2 fissures, 3 lobes, and
a normal fastigium have less severe developmental delays than those with a malformed
vermis or only 1 fissure in the vermis.5 The
prognosis of Joubert syndrome is also not
always straightforward.6,15,16
Nevertheless, subtle anomalies of the posterior fossa are either difficult to diagnose or
difficult to counsel once diagnosed. The prognosis of patients with an isolated DandyWalker variant is not well known. Romano et
al16 described a group of 13 patients with superior vermian dysplasia. The patients had relatively heterogeneous clinical symptoms.
Patients with a dilated fourth ventricle had different symptoms from those with a normal
fourth ventricle.
In cases with subtle changes, sonographic
diagnosis can be difficult. Therefore, depiction of specific structures of the posterior
fossa, which are easy to see, may be of help.
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Nomograms for the fastigium that to our knowledge have not been presented previously are
presented here. This structure, which is simple
to depict, may be used as a sign in cases of subtle changes in the posterior fossa. Additional
studies are required to measure other structures
in the posterior fossa objectively to clarify other
posterior fossa abnormalities.
Figure 8. Examples of abnormal fastigial angles (A–C).
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A
B
C
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