Obstetrical Randy William
K. Knutzon, E. Brant,
MD MD
#{149} John
P. McGahan,
MD
Fetal Cisterna Magna Anatomic Finding’ A linear hyperechoic structure in the cisterna magna, previously described as the straight sinus, was investigated during routine obstetric ultrasound (US) examination. One hundred three fetal US examinations were performed to determine the prevalence and appearance of this finding. Also, US water-bath examination of 18 fixed postmortem specimens and fetuses was performed, and results were correlated with those of gross and microscopic examination of the fetal brain. The structure was identified in 95 (92%) prenatal examinations between 15 and 38 menstrual weeks. It was located posteriorly between the cerebellar hemispheres in the cisterna magna. In those 95 cases in which it was identified, it appeared as a single echogenic line in 31 cases (33%), as two echogenic lines in 47 (49%), or as three or more lines in 17 (18%). When two echogenic lines were identified, they appeared “cystlike” in 17 (18%) of the 95 examinations. Correlation of the gross and histologic findings demonstrated these lines to represent normal subarachnoid septa rather than the straight sinus. Index terms: tus, abnormalities, vous system, 856.1298 Radiology
Brain, US studies, 856.87 856.874
1991;
153.1298
Fe-
#{149}
Fetus, central #{149} Fetus, US studies,
ANY
fossa
is important
for
detecting
received May 3; accepted Address reprint requests to J.P.M. C RSNA, 1991
cen-
tral nervous system anomalies such as Arnold-Chiari II malformation and indirect evidence of open neural tube defects such as meningomyebocele (2-5). The fetal posterior fossa has been investigated fairly extensively (6-13). During
the
fetal
prenatal posterior
cystic-appearing
US examination fossa, a linear structure
has
of or been
noted
posterior to the cerebellar vermis and along the midline in the cisterna magna. This structure has been described as the straight sinus (9,14) and torcular Herophili (12). Its bocation and variable appearance made it seem unlikely to represent a dural venous sinus. We undertook a study to determine the appearance and prevalence of this finding during routine obstetric US examination and to identify the anatomic basis of this US finding. AND
(R.K.K.,
May
6.
examinations
in fetuses
PhD
A Normal
between
All US examinations with an electronically
were performed focused phasedarray sonographic system (Acuson, Mountam View, Calif) with 3-5-MHz transducem. Color Doppler flow imaging and pulsed Doppler examination of the postenor fossa of the first iO fetuses was performed to check for flow within the linear structures. Results of all obstetric examinations in these fetuses were normal, and no abnormality pertaining to the central nervous
system
included obstetric 15
and 38 menstrual weeks. The examinations included comprehensive US evaluation of the fetus by one of three examiners (R.K.K., J.P.M., or W.E.B.) and US examination of the posterior fossa and the axial plane at an inclination of approximately ioo from the canthomeatal line. US scans of the posterior fossa were reviewed for the presence or absence of a linear hyperechoic structure in the cisterna magna. If present, the appearance of this structure was classified as a single line, two lines, or three or more lines. If two lines were present, the structure was further classifled as to whether it appeared “cystlike.”
was
identified
after
birth.
The second portion of this study included postmortem scanning of 18 formalin-fi.xed fetuses by means of a 7.5-MHz linear-array
transducer
in a water
bath.
These fetuses ranged in gestational age from 16 to 20 weeks. Subsequently, all these fetuses were examined with either gross dissection or en bloc paraffin embedment of the isolated posterior cranial fossa,
followed
by hematoxylin-eosin
staining of serial sections. Sectioning was performed in a horizontal plane. Gross and histologic findings were correlated with the postmortem US findings.
RESULTS Posterior fossa septa were identifled at US in 95 (92%) of 103 normal fetuses between the 15th and 38th menstrual
weeks.
These
septa
varied
in appearance from a single echogenic line in 31 of 95 (33%) of the fetuses, to two echogenic lines in 47 of 95 (49%),
METHODS
The first portion of this study performance of 103 prospective
of Radiology
MD,
believe that the routine prenatal obstetric ultrasound (US) examination of the fetal head should include a transventricular view, a transthalamic view to determine biparietal diameter, and a transcerebellar view to examine the cerebellum and cisterna magna (i,2). The transcerebellar view through the posterior
MATERIALS
J.P.M., WEB.) and Pathology (M.S.S.), University of California, Davis, Medical Center, 2516 Stockton Blvd, TICON H, Sacramento, CA 95817. From the 1990 RSNA scientific assembly. Received March 12, 1991; revision requested April 18; revision
Salamat,
ner-
#{149}
180:799-801
From the Departments
Shahriar
Septa:
M
Us
t
#{149} M.
Ultrasound
to three
or more
echogenic
lines in the cisterna magna in 17 of 95 (18%). When present, the single echogenic line was often located in the midline and extended posteriorly from the cerebellar vermis straight to the occiput (Fig la). Occasionally, septa deviated in either direction or had an origin that was not midline (Fig 2a). When two echogenic septa were present, they usually had parallel courses and originated centrally on either side of the cerebellar vermis at its junction with the cerebellum. The configuration of these two septa cystlike in i7 of 95 (18%) of the
tuses
examined
(Fig
ib).
With
was fe-
three
or 799
more more
septa, the configuration random (Fig id). The
cisterna
not
identi-
fled in eight of 103 (8%) of the We believe that nonvisualization
fetuses. of
these
magna
septa
septa
was echogenic
was
due
were
to technical
prob-
lems, such as maternal body habitus, position of the fetal head, or shadowing from densely ossified calvana in older fetuses. There was no correlation between number of lines and menstrual age. Flow was not detected in the 10 fetuses examined with color Doppler flow or pulsed Doppler US. The results of postmortem gross dissection or histologic examination were correlated with those of US in 18 fetal brains. Gross dissection revealed subarachnoid trabeculae and transparent semivertical septa traversing the cisterna magna in a sagittal orientation. These trabeculae were multiple in most cases, even when the US examination
disclosed
only
one
a.
b.
or two
septa. Gross and histologic correlation with results of a premortem US study are shown in Figure 2. During histologic preparation, much normal subarachnoid space of the cisterna magna is lost.
The
tissue
in the
remaining
space is arachnoid. The midline internab occipital protuberance can be seen on the histologic section, and a corresponding echogenic round protuberance can be seen on the US scan (Fig 2c, 2d).
DISCUSSION Prenatal diagnosis of central nervous system anomalies is of considerable importance and has been the topic of numerous scientific articles. Central nervous system anomalies are among the most common yet devastating congenital malformations. Attempts to detect these anomalies have included acquisition of standardized views of the fetal cranium. Among the screening surveys of fetal anatomy recommended for all obstetric US examinations is a view through the posterior fossa that includes the cerebellar hemispheres and cisterna magna (1,3,4). The main objective of evaluation of the cisterna magna is to better identify open neural tube defects such as myebomeningocele (4). Examination of the posterior fossa may also help detect other abnormalities such as Dandy-Walker syndrome or occipital encephaloceles. Many articles have provided precise measurements of the anatomic structures in the posterior fossa (7,13) and detailed anatomy of the posterior fossa,
but
literature 800
some
confusion
concerning
#{149} Radiology
exists
cystlike
in the
or linear
d. magna septa. (a) Single echogenic vermis (C) through the cisterna
C.
Figure
row)
1. Variable US appearances of the cisterna extends posteriorly from the midline cerebellar
(b )
Cystlike which may hemispheres,
(d) Multiple
structures
posterior
to the
cerebellum.
These structures have been recognized and thought to represent the straight sinus (9,14) or the torcular Herophili (12). Unfortunately, to our knowledge,
line (ar-
magna. configuration (arrow) in the cisterna magna in a fetus at 25 menstrual weeks, be mistaken for a vascular structure such as the straight sinus. C = cerebellar A = anterior. (c) Three parallel echogenic lines (arrows). C = cerebellum. echogenic lines in a random configuration (arrows). C = cerebellum.
no
sonographic-patho-
logic correlation has been undertaken to discern if these lines are normal or abnormal anatomic findings and what their significance is. In our initial experience, we found a cystlike structure posterior to the cerebellum (Fig lb). This was thought to represent a vascular structure, as previously noted. Within the structure, however, no arterial or venous pulsations could be identified with pulsed Doppler US, and no flow could be detected with color Doppler flow imaging. We initially considered
lack of sensitivity detecting cerebral
of color venous
imaging pulsations
as an
for
It became
explanation
evident,
however,
pulsations venous
flow
could structures
or pulsed
Our as
Doppler
study
also
a variable two
commonly, curred, multiple As
study,
this
demonstrated
was fluid
study,
US, and
not
that
always
be expected
would
brospinal
venous in small Doppler
of the posterior no color flow.
structure
had
normal
be identified with color
cystlike structure fossa demonstrated this
this.
that
in
tubular,
with
venous
a cere-
structure,
appearance. lines
were
but
other
but
In our observed
most
variations
oc-
such as one line in 31 cases septa in 17 cases. we
continued
it became
this
apparent
or
prospective by
the
September
1991
tures.
Their
lack
of venous
flow
may
cause them to be misdiagnosed as straight sinus thrombosis. In summary, normal single to multiple echogenic lines are routinely identifled within the posterior fossa and are normal anatomic structures corresponding to leptomeninges within the cisterna magna. #{149} Acknowledgment: Anderson
Special
for manuscript
thanks
to Karen
preparation.
References 1. Nyberg b. 2.
3.
4.
5.
6.
7.
d.
C.
Figure 2. (a) Routine US examination of a fetus at 15 menstrual weeks demonstrates a single eccentric line in the cisterna magna (arrow) posterior to the cerebellum (C). A = anterior. (I:) Gross dissected specimen demonstrates appearance similar to the eccentric line in Figure
ib, which represents rior. (c) Postmortem
the subarachnoid septa (arrow) posterior to the cerebellum (C). A = anteUS examination shows two echogenic cisterna magna septa (arrows). The echogenic round structure (B) posteriorly between these lines corresponds to the internal occipital protuberance. C = cerebellum. (d) Histologic section corresponding to c shows the bony internal occipital protuberance (B) and the periosteal (P) and meningeal (M) dura with intervening (between ulae with arachnoid
dural sinus (curved arrow). The normal subarachnoid space of the cisterna black arrows) is lost during processing but contains arachnoid and arachnoid the cerebellum (C) anteriorly. The septa (white arrow) represent concentrations trabeculae and correspond to the echogenic line seen at US examination.
magna trabecof
8.
9.
10.
11.
nearly 100% occurrence that they are a normal
of these septa finding. Water-
bath examination of the fetal head and brain was performed with meticulous histopathologic correlation and revealed these septa to be normal bridging arachnoid. These subarachnoid septa are extremely fragile and were difficult to dissect and photograph. After repeated efforts, however,
we
were
able
to demonstrate
these structures with regularity 2). The brain is normally covered very thin layer of adherent pia,
Volume
180
#{149} Number
3
(Fig by a which
forms
the
internal
margin
of the
sub-
arachnoid space. This space is bound externally by arachnoid and is traversed by numerous arachnoid trabeculae. These subarachnoid septa that we have identified with US most likely represent focal concentrations of arachnoid trabeculations in the subarachnoid space. The importance of anatomic identification of these structures is multifold. First, these septa are normal and should not be confused with pathologic entities. Second, it is important to note that these structures are not normal intracranial vascular struc-
12.
13.
14.
DA. Recommendations for obstetric sonography in the evaluation of the fetal cranium. Radiology 1989; 172:309311. Campbell J, Gilbert WM, Nicolaides KH, Campbell S. Ultrasound screening for spina bifida: cranial and cerebellar signs. Lancet 1986; 2:72-74.
Filly RA, Cardoza
JD, Goldstein
RB, Bark-
ovich AJ. Detection of fetal central nervous system anomalies: a practical level of effort for a routine sonogram. Radiology 1989; 172:403-408. Goldstein RB, Podrasky AE, Filly RA, Callen PW. Effacement of the fetal cisterna magna in association with myelomeningocele. Radiology 1989; 172:409-413. Nicolaides KH, Campbell 5, Gabbe 5G. Guidetti R. Ultrasound screening for spina bifida: cranial and cerebellar signs. Lancet 1986; 2:72-74.
Comstock
CH, Boal DB.
Enlarged
fetal
cisterna magna: appearance and significance. Obstet Gynecol 1985; 66 (Suppl 3): 255-285. Goldstein I, Reece EA, Pilu G, Bovicelli L, Hobbins JC. Cerebellar measurements with ultrasonography in the evaluation of fetal growth and development. Am J Obstet Gynecol 1987; 156:1065-1069. Goodwin L, Quisling RG. The neonatal cisterna magna: ultrasonic evaluation. Radiology 1983; 149:691-695. Mahony BS, Callen PW, Filly RA, Hoddick WK. The fetal cisterna magma. Radiology 1984; 153:773-776. McLeary RD, Kuhns LR, Barr M Jr. Ultrasonography of the fetal cerebellum. Radiology 1984; 151:439-442. Pilu G, De Palma L, Romero R, Bovicelli L, Hobbins JC. The fetal subarachnoid cisterns: an ultrasound study with report of a case of congenital communicating hydrocephalus. J Ultrasound Med 1986; 5:365372. Pilu G, Romero R, De Palma L, Jeanty P. Burdine C, Hobbins JC. Ultrasound investigation of the posterior fossa in the fetus. Am J Perinatol 1987; 4:155-159. Smith PA, Johansson D, Tzannatos C, Campbell S. Prenatal measurement of the fetal cerebellum and cisterna cerebellomedullaris by ultrasound. Prenat Diagn 1986; 6:133-141. Nyberg DA, Pretorius DH. Cerebral malformations.
Pretorius of fetal
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