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290
Chapter 8 Anomalies of the Cerebellum
TH
Vermis
A
Figure 8–9. MRI of fetal Dandy-Walker malformation. The information is similar to that provided with ultrasound in Figure 8–8. There is a better
demonstration of the high-riding tentorium and torcular herophili (TH). (Reproduced, with permission, from the Visual Encyclopedia of Ultrasound in
Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
normally shaped cerebellum 4 ( Figure 8–10 ). Arachnoid
cysts growing in the posterior fossa have a mass effect
B
closely a Dandy-Walker malformation or vermian hypoplasia ( Figure 8–12 ). 35
C
and cause asymmetrical distortion of the cerebellum or
pressure on the vermis, resulting in flattening of its dorsal
border and in severe cases compression of the aqueduct
and ventriculomegaly
34
( Figure 8–11 ). The greatest difficulty encountered is posterior fossa hemorrhage, which
results at times in destruction of the cerebellar vermis
and enlargement of the cisterna magna, mimicking very
Cerebellum
Cerebellum
Implications for Targeted Examination
The Dandy-Walker complex has genetic implications, and
sonography is usually offered to couples at risk. Although
the most severe forms of this condition can be recognized
as early as 14 weeks,
Cisterna magna
22
caution is necessary because of
Vermis
A
Figure 8–10. Sonography of cerebellar hypoplasia. ( A ) Second trimester fetus with multiple anomalies. The cerebellum has a normal shape, but the
transverse diameter is about –3 standard deviations (SDs) from the mean. ( B, C ) Third trimester fetus with severe cerebellar hypoplasia. The transverse cerebellar diameter is below –4 SDs from the mean, and the cisterna magna is enlarged ex vacuo. (Reproduced, with permission, from the Visual
Encyclopedia of Ultrasound in Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
B
C
Chapter 8 Anomalies of the Cerebellum
3v
Brain stem
A
Figure 8–11. Sonography of a posterior fossa arachnoid cyst. ( A, B ) The cerebellum is displaced laterally by a fluid collection ( arrow ). ( C ) A cyst
is interposed between the tentorium and the surface of the cerebellum ( arrow ) (3v, third ventricle). (Reproduced, with permission, from the Visual
Encyclopedia of Ultrasound in Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
Cerebellum
B
Cerebellum
C
Cerebellum
291
the ambiguous images caused by the incomplete development of the cerebellum at this gestational age.
20 , 21
W e
recommend limiting the diagnosis of Dandy-Walker complex to those cases in which either a very large cisterna
magna or other abnormal findings are encountered. In the
midtrimester, it is stressed that the transcerebellar view
only demonstrates the superior part of the vermis and
therefore is not sufficient to rule out the Dandy-Walker
complex. A lower scan is required to demonstrate the
“closure” of the inferior vermis.
a median view should be obtained as well.
4 , 12 , 23 , 36
In cases at risk,
4 , 23 , 36
Couples
should be informed that false-negative and false-positive
Frontal horns
Tentorium
A
B
cases have been reported and that an antenatal diagnosis
is not possible in all cases, particularly with minor anatomical alterations.
Implications for Sonographic Screening
In the midtrimester, a cisterna magna measuring > 10 mm
and/or an “open” fourth ventricle
5 , 19 , 38
alert to the possibility of Dandy-Walker complex. The transcerebellar view,
which is recommended as part of the standard sonographic
examination of the fetal brain,
29
will not detect all cases of
Dandy-Walker complex.
C
D
30 , 37
Figure 8–12. In this third trimester fetus, the association of severe ventriculomegaly, open fourth ventricle, small rotated vermis, and communication
of the frontal horns had prompted the diagnosis of Dandy-Walker malformation associated with obstructive hydrocephaly and possibly a telencephalic
malformation. After birth autopsy demonstrated the sequelae of a posterior fossa hemorrhage with secondary hydrocephaly and disruption of the septum
pellucidum. In retrospect, the normal insertion of the tentorium was in contrast with the diagnosis of Dandy-Walker malformation. (Reproduced, with
permission, from the Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology,
2010, www.isuog.org. )
292
Chapter 8 Anomalies of the Cerebellum
Prognosis
Neurosurgical series that report the combination of
Dandy-Walker malformation and hydrocephaly describe
abnormal neurologic development in 40% to 70% of
survivors.
genetic syndromes, with a wide spectrum of outcomes.
The clinical significance of Dandy-Walker malformation
without ventriculomegaly is debated.
however, that the neurologic outcome is mainly related to
the appearance of the cerebellar vermis.
is of normal size and morphology, a normal development
has been reported in 85% of cases. Conversely, when the
vermis is abnormally lobulated, and/or there are associated
cerebral anomalies, the prognosis is poor.
multiple anomalies and genetic syndromes. When isolated,
it may be asymptomatic, but precise risk figures are not
available.
which Blake’s pouch cyst has been found in association
with obstructive hydrocephaly,
natally have in general a good outcome and tend to
undergo intrauterine remission.
associated anomalies, including chromosomal aberrations, are not infrequent. Indeed, in our own, albeit limited experience, the main difference between the three
varieties of Dandy-Walker complex depends mostly on
the rate of associated anomalies. Isolated cases usually
have a good outcome.
8 – 10
Dandy-Walker malformation is also part of many
39
It would seem,
20 , 30 , 39
When this
24 , 25
Vermian hypoplasia/agenesis is also frequently part of
31 , 39
Despite small series in the pediatric literature in
11 , 40
cases diagnosed pre-
5 , 12–14 , 39
In our experience,
Etiology, Pathogenesis, and Pathology
This condition is most frequently recognized by diagnostic
imaging ( Figure 8–13 ), and pathologic data are scant. It has
been suggested that the enlargement of the cisterna magna
may be secondary to a distention of the Blake’s pouch that
does not displace the cerebellar vermis.
14
Associated Anomalies
Megacisterna magna is usually an isolated finding. When
the diagnosis is made in fetal life, an association with
trisomy 18 has been reported. 37 This, however, may represent the consequence of cerebellar hypoplasia, which
is seen frequently in these cases, rather than a primary
enlargement.
33
Diagnosis
Megacisterna magna was originally described in postnatal patients using purely subjective criteria. In the
obstetric literature, the term has been used to indicate
cases with a cisterna magna depth in excess of 2 standard
deviations (SDs) above the mean, or 10 mm ( Figures 8–14
and 8–15 ).
However, there is most likely a discrepancy between the
classic postnatal radiologic definition of the condition
(that identifies a rare finding) and the obstetric one (that
implies a prevalence of ∼2.5%). Indeed, most fetuses with
a prenatal diagnosis of megacisterna magna are found to
be normal after birth. The value of the obstetric definition
30 , 37
The definition is now well established. 29
Obstetric Management
The diagnostic workup must include a detailed search for
associated anomalies, including karyotyping. In counseling couples, it is stressed that any of these anomalies
can have a good outcome when isolated and that Blake’s
pouch cyst is most frequently a normal variant. Serial
scans are suggested because of the potential for cerebral
maldevelopment, including ventriculomegaly. With the
exception of those cases associated with hydrocephaly
and macrocrania that may require cesarean delivery,
19
no modification of the standard obstetric management
is indicated.
MEGACISTERNA MAGNA
Excludes
Dandy-Walker complex, cerebellarhypoplasia
Definition
A large cisterna magna, in the absence of cerebellar
anomalies
Incidence
Unknown. If the obstetric definition of cisterna magna
is used (a depth ≥ 10 mm in fetuses at 20–40 weeks’
gestation),
29 , 30 , 37
a prevalence of ∼2% is expected.
Figure 8–13. In the late second trimester, the depth of the cisterna
magna of this infant was found to be in excess of 4 SDs above the mean.
It progressively diminished in size, and at birth it was found to be only
at the upper limit of normal by the neuroradiologist who performed
this scan. The infant had a normal neurologic and intellectual postnatal
follow-up. (Reproduced, with permission, from the Visual Encyclopedia
of Ultrasound in Obstetrics and Gynecology, International Society of
Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
Cisterna magna
1
Chapter 8 Anomalies of the Cerebellum
Vermis
293
A
Figure 8–14.
Encyclopedia of Ultrasound in Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
Sonography of megacisterna magna at 30 weeks’ gestation (same case as in Figure 8–12). (Reproduced, with permission, from the Visual
appears to be the identification of a generic risk factor for
associated anomalies.
B
cerebellum) ( Figure 8–10 ), and posterior fossa arachnoid
cysts (mass effect with asymmetric distortion of the cerebellum) ( Figure 8–11 ).
Differential Diagnosis
This condition should be differentiated from the DandyWalker complex (which is characterized by an open fourth
ventricle) (see Figure 8–1 ), cerebellar hypoplasia (small
Prognosis
In the absence of associated anomalies, the prognosis
is good.
39
The largest antenatal series thus far available
AB
Figure 8–15. MRI of a neonatal megacisterna magna. (Reproduced, with permission, from the Visual Encyclopedia of Ultrasound in Obstetrics and
Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
294
Chapter 8 Anomalies of the Cerebellum
does suggest that these infants may be at a slightly
increased risk of mild developmental delay.
41
However,
severe neurologic complications are rarely encountered.
Indeed, megacisterna magna is frequently an incidental finding in radiologic examinations performed after
birth.
Obstetric Management
Other than the need for a detailed US scan, no modification of standard obstetric management is indicated. It
remains uncertain whether fetal karyotyping is indicated.
It would seem that the risk is only increased for trisomy
18, which is unlikely to result in a completely normal sonogram ( Figure 8–16 ).
CEREBELLAR HYPOPLASIA/ATROPHY
Excludes
Dandy-Walker malformation, megacisterna magna, unilateral cerebellar hemisphere hypoplasia
Definition
A small cerebellum
Incidence
Rare
Etiology and Pathogenesis
Variable. Cerebellar hypoplasia or atrophy may be the consequence of many different processes, including acquired
42
lesions.
In our experience, it is usually found either in
the context of multiple anomalies or as a consequence
of pontocerebellar hypoplasia. This is a group of neurodegenerative disorders featured by the concomitance
of cerebellar and brainstem hypoplasia with autosomal
recessive transmission. Different types exist, and they
usually develop either postnatally or in late gestation
43 , 44
( Table 8–3 ).
Pathology
Hypoplasia refers to a morphologically normal but small
cerebellum; atrophy refers to inadequate growth of the
cerebellum with a progressive increase in the size of
the fissures in comparison to the size of the foliae.
Differentiation of these two rare entities prenatally is probably impossible.
Diagnosis
Hypoplasia of the cerebellum, with a secondary increase
in the size of the cisterna magna, may be obvious, particularly in late gestation ( Figures 8–10 and 8–17 ). Associated
cerebral anomalies, including microcephaly and cortical
malformations, may be present. The association with a
thin brainstem that does not display the typical protrusion
of the pons is indicative of pontocerebellar hypoplasia.
Normative data of the fetal pons have been published.
our own experience, however, the diagnosis is more easily
made using MRI ( Figure 8–18 ).
Polyhydramnios, fetal paralysis, and seizures may
be present. Particularly in early gestation, the diagnosis
is extremely difficult or impossible. In at least one of
our cases, the cerebellum had a normal appearance and
dimensions at 20 weeks, and the condition could only be
demonstrated in late gestation. Even in familial cases with
26
In
4
4
AB
Figure 8–16. Megacisterna magna in a fetus with trisomy 18. The head appears round, the cavum septi pellucidi is enlarged, the vermis has an unusual
appearance, and multiple extracranial anomalies were present. (Reproduced, with permission, from Atlas of Obstetric Ultrasound, 2009. The Global
Library of Women’s Medicine, www.glowm.com. )
Chapter 8 Anomalies of the Cerebellum
295
Table 8–3. PONTOCEREBELLAR HYPOPLASIAS:
DIFFERENTIAL DIAGNOSIS OF THE
THREE MOST COMMON TYPES
Type 1 Type 2 Type 4
Presentation at birth
Polyhydramnios
Jitteriness, clonus –
Muscle stiffness – –
Respiratory
insufficiency
Follow-up findings
Microcephaly
Gross delay of all
milestones
Spinal anterior
horn involvement
Chorea or
spasticity
Main findings at autopsy
Pontocerebellar
hypoplasia
Regressive
neuronal changes
Demyelination – – –
Spinal anterior
horn cells affected
Genes affected Unknown tRNA
Modified from Barth P. Pontocerebellar hypoplasia. In: Gilman S, editor-inchief. MedLink Neurology. San Diego: MedLink Corporation. Available at
http://www.medlink.com . Republished by permission.
+/–
+/–
++ +
++ +
+
–
++ +
++ +
+
Rare
+++
Rare
––
+
––
endonuclease
subunit
TSEN54;
less
frequently
TSEN2,
TSEN3
+/–
++
+
–
TSEN54
measurement. The available experience is limited to a
handful of cases diagnosed in utero, and providing risk
estimates is impossible. It is likely, however, that most
cases will not be positively identified in early gestation.
Implications for Sonographic Screening
Most cases of cerebellar hypoplasia tend to develop
throughout gestation, and we anticipate that it will be
impossible to recognize them in utero, particularly in
early gestation. The identification of a small or borderline
cerebellum in a fetus represents a diagnostic dilemma.
The available experience does not allow for establishing
an optimal quantitative cutoff. A reasonable approach is to
use –2 SD from the mean (2.5th centile). A careful search
for associated anomalies, including the visualization of the
brainstem, is indicated in these cases.
Implication for Sonographic Diagnosis
Cerebellar hypoplasia should be suspected when a small
cerebellum is seen, usually with an ex vacuo enlargement of
the cisterna magna. 33 In borderline cases, polyhydramnios,
other cerebral anomalies, and abnormal fetal movements,
including contractures and seizure activity, increase the
index of suspicion. It is important to stress that cerebellar
hypoplasia is frequently evolutive, and sonographic diagnosis may fail particularly in early gestation. Follow-up
examinations in patients with a familial history or with
apparently small cerebellums during second trimester
routine examinations seem indicated. Genetic testing is
available for some of the conditions associated with cerebellar hypoplasia, and it should be considered in pregnancies at risk.
44
Prognosis
In general, prognosis is poor. Most fetuses we have diagnosed in utero died in the early postnatal period.
exception was the case of a fetus with cerebellar hypoplasia
from a mother who had cerebellar hypoplasia herself and
moderate mental retardation. The infant survived and is
doing well.
39,43,44
One
Obstetric Management
When the diagnosis is made within the temporal limits of
voluntary pregnancy termination, this can be offered to
the couples. We recommend storing fetal DNA for subsequent analysis, as cerebellar hypoplasia is frequently part
of genetic conditions with a high recurrence rate.
an increased risk of recurrence, the diagnosis may be very
difficult or even impossible during pregnancy .
Cerebellar hypoplasia is a rare condition, and it is
unclear which is the optimal quantitative threshold to
make this diagnosis. Most growth curves of the cerebellum report the 90% prediction interval,
45 , 46
and certainly
the 5th centile is not a reasonable cutoff to use, as it
would include a disproportionate number of normal
fetuses. The most difficult clinical problem is certainly
represented by second trimester fetuses with borderline
RHOMBENCEPHALOSYNAPSIS
Excludes
Dandy-Walker complex, vermian agenesis
Definition
Fusion of the cerebellar hemispheres with vermian
agenesis
296
Figure 8–17. (A, B) Magnetic resonance of cerebellar hypoplasia, same case of the Figure 8–10 (B, C) . The cisterna magna is enlarged ex vacuo, as well
as the entire subarachnoid space. (Reproduced, with permission, from the Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, International
Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
Chapter 8 Anomalies of the Cerebellum
Cerebellum
AB
Etiopathogenesis
The formation and development of the midline portion of the cerebellum is thought to be controlled by the
“isthmic organizer,” a band of neuroepithelium that is
located between the mesencephalon and the metencephalon. A defect in the isthmic organizer is considered to be
the primary cause of rhombencephalosynapsis, and FGF8
AB
and Lmx1a have been suggested as candidate genes. The
condition, however, seems to occur sporadically, and the
recurrence risk is probably very small.
47 – 50
Pathology
Rhombencephalosynapsis is featured by variable degrees
of agenesis of the vermis, dorsal fusion of the cerebellar
Figure 8–18. In this fetus with cerebellar hypoplasia, MRI demonstrated a very thin brainstem with no bulging of the pons ( arrow ). The forehead is
sloping, suggesting microcephaly. This is an example of pontocerebellar hypoplasia. (Courtesy Rabih Chaoui, Berlin, Germany.)
Figure 8–19. Rhombencephalosynapsis. Autoptic specimen demon-
strates the absence of the vermis and the dorsal fusion of the cerebellar
hemispheres. (Reproduced, with permission, from the Visu al Encyclopedia
of Ultrasound in Obstetrics and Gynecology, International Society of
Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
hemispheres and of the dentate nuclei, and superior cerebellar peduncles ( Figure 8–19 ).
47 , 49
Chapter 8 Anomalies of the Cerebellum
297
syndrome, or cerebello-trigeminal-dermal dysplasia
(OMIM 601853) and VACTERL-H association (spinal and
forearm abnormalities).
49
Diagnosis
Postnatally, the diagnosis with MRI is prompted by the
presence of a single-lobed cerebellum with a small keyhole shaped fourth ventricle. The most important views
are the axial and coronal, which demonstrate a single
cerebellar mass without the typical cleavage lines that are
normally seen between the vermis and hemispheres.
The cerebellum is usually smaller than normal. The sagittal views are not diagnostic for the diagnosis, but they
are helpful in that they demonstrate the absence of the
normal anatomical landmarks of the vermis: the fastigium
points and vermian fissures. Only a few cases have been
recognized antenatally; these probably represent the most
severe end of the spectrum of this condition and usually
are seen in association with other cerebral and extracerebral anomalies.
4 , 48 , 49
The main sonographic finding is a
small triangular cerebellum ( Figure 8–20 ). However, there
are no absolute sonographic criteria to differentiate the
vermis from the surrounding hemispheres, and the diagnosis of rhombencephalosynapsis in the fetus is usually
difficult and at times may be impossible.
4
Demonstration
of an abnormal folial pattern crossing from one hemisphere to the other in a coronal view is an important clue
to the diagnosis, but this is usually possible only in late
gestation. 4
3 , 42
Associated Anomalies
Other central nervous system (CNS) anomalies are frequently found, and these cases are predominant in prenatal series. 49 Association with extraneural anomalies is
also well established, including Gomez-Lopez-Hernandez
AB
Figure 8–20. Prenatal findings of rhombencephalosynapsis. ( A ) Sonography. ( B ) MRI. The cerebellum is small, with an abnormal triangular shape,
and the cleavage lines between the hemispheres and vermis are not demonstrated. (Reproduced, with permission, from the Visual Encyclopedia of
Ultrasound in Obstetrics and Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
Differential Diagnosis
The differential diagnosis includes cerebellar hypoplasia
(identification of rhombencephalosynapsis within a small
cerebellum may be impossible) and vermian agenesis with
molar tooth abnormality (again, a very difficult differential
298
Chapter 8 Anomalies of the Cerebellum
diagnosis; however,vermian agenesis is usually associated
with an open fourth ventricle).
Prognosis
When associated anomalies are identified, such as severe
ventriculomegaly, the prognosis is poor. Infants with isolated rhombencephalosynapsis usually have impairment
of cognitive functions. The extent of the deficit, however,
is variable and in some cases may be mild. Out of five isolated cases, two were indeed found to have normal IQs.
50
eye movements, and head stereotypies are the most common neurologic disturbances. Attention problems are the
most common behavioral disturbances.
50 , 53
Implications for Sonographic Screening
It is likely that in the absence of other intracranial anomalies or cerebellar hypoplasia, rhombencephalosynapsis can
not be identified in a nontargeted examination of the fetal
brain .
Implications for Sonographic Diagnosis
The diagnosis of rhombecephalosynapsis is a difficult one
even for expert sonologists. The most important views for
the diagnosis are the axial and coronal ones, demonstrating an abnormal triangular configuration of the cerebellum. The condition should be suspected whenever the
cerebellar dimensions are small.
Obstetric Management
Exclusion of associated anomalies is the most important
part of management. Some of the cerebral anomalies that
have been found in association with rhombencephalosynapsis are difficult or impossible to demonstrate antenatally,
particularly in early gestation. In continuing pregnancies,
no modification of standard obstetric management is
required.
JOUBERT SYNDROME AND RELATED
CEREBELLAR DISORDERS
Includes
Vermian agenesis with Joubert syndrome; vermian agenesis with “molar tooth” anomaly; coloboma, oligophrenia/
developmental delay, ataxia, cerebellar vermis hypoplasia, and hepatic fibrosis (COACH); cerebello-oculo-renal
syndrome (CORS); oral-facial-digital syndrome type VI
(OFD-VI); Senior-Løken syndrome.
Excludes
Vermian hypoplasia
51,52
neuoradiologic features are associated with extraneural
anomalies and constitute different syndromes.
Etiopathogenesis and Associated Anomalies
Joubert and related cerebellar disorders are transmitted
as an autosomal recessive trait. The genetics is complex, with at least eight genes involved.
51–53
The different
genotypes are responsible for the different clinical manifestations that have been categorized in a number of syndromes: Joubert syndrome,
52
COACH, CORS, OFD-VI,
and Senior-Løken syndrome. The gene products that are
affected in these conditions are known to take part in the
development of the primary cilium and/or basal body and
centrosome apparatus. Essentially, Joubert and related
disorders are now considered part of the general group of
ciliopathies.
52,53
Pathology
There is a variable degree of deficit of the cerebellar vermis,
from hypoplasia to complete agenesis; consequently, the
two hemispheres come in close contact in the midline. 54
The frequency of breathing disorders suggests the coexistence of lesions of the brainstem. Dysmorphic features
are usually present, including a large head, prominent
forehead, and typical facies (high, rounded eyebrows, epicanthal folds, and upturned nose).
51
Diagnosis
Postnatally, the predominant finding in neuroimaging of
the head is the typical appearance of the mesencephalon in
the axial plane (commonly referred to as the molar tooth
sign) that is the consequence of a deep posterior interpeduncular fossa with thick and elongated superior cerebellar peduncles, associated with hypoplasia or agenesis of the
cerebellar vermis ( Figure 8–21 ).
with sonography is difficult.
handful of cases, we have never been able to demonstrate
with certainty the molar tooth sign, probably because
of the limited contrast resolution of US, which does not
allow a clear view of the cisterns surrounding the mesencephalon. In pregnancies at risk, an open fourth ventricle
in the axial plane has been described as the most important finding.
28
The authors of the original report have not
found sagittal planes to be of value. Indeed, the absence
of the vermis results in apposition of the two cerebellar
hemispheres in the midline that mimics the presence of
a normal vermis. In low-risk pregnancies, this condition
can be easily missed. An open fourth ventricle may be
identified at least in some cases, but in our experience it
is extremely difficult to assess the underdevelopment of
the vermis ( Figure 8–22 ). Furthermore, in the absence of a
proband, the clinical manifestation of these disorders cannot be clearly predicted. Molecular genetics can be helpful
in some cases.
56
28 , 54 , 55
4 , 28
In our own experience in a
Prenatal diagnosis
Definition
Hypoplasia of the cerebellar vermis associated with the
characteristic neuroradiologic “molar tooth” sign; these
Differential Diagnosis
The main intrauterine finding may be an open fourth ventricle; therefore, the entity that is most similar to Joubert
Chapter 8 Anomalies of the Cerebellum
ABC D
Figure 8–21. Joubert syndrome. ( A, B ) Postnatal MRI in the coronal planes demonstrates the absence of the vermis. ( C ) In the midsagittal plane, the
anatomical landmarks of the vermis (fastigium point of the fourth ventricle, fissures) could not be demonstrated. ( D ) In the axial plane, the pathognomonic “molar tooth” sign is demonstrated ( arrow ). (Reproduced, with permission, from the Visual Encyclopedia of Ultrasound in Obstetrics and
Gynecology, International Society of Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
299
syndrome and related disorders is the Dandy-Walker
complex. Careful examination in the axial and sagittal
planes usually allows identification of a superiorly displaced vermis in the latter condition. MRI may be helpful
because of its unique ability to document the molar tooth
sign, which is pathognomonic of Joubert syndrome.
55
Implications for Targeted Examinations
In pregnancies at increased risk because of a previous
affected child, neurosonography should be performed to
assess the presence and integrity of the vermis. An open
fourth ventricle after 20 weeks’ gestation and failure to
4v
visualize the main landmarks of the cerebellar vermis
are strongly suggestive of a recurrence. On the basis of
the available evidence, MRI should be performed in that
it allows a better demonstration of the abnormalities of
the mesencephalon (molar tooth sign). In the absence
of a familial history, the diagnosis of Joubert and related
syndromes is a major challenge even for an expert using
either US or MRI.
Implications for Sonographic Screening
Joubert and related syndromes are associated with very
subtle prenatal findings, and we expect that in low-risk
4v
AB
Figure 8–22. Sonography of Joubert syndrome. In this third trimester fetus, we were unable to clearly identify the cerebellar vermis between the two
cerebellar hemispheres ( arrow ) ( A ), and the fourth ventricle appeared open ( B ). ( C ) In the midsagittal plane, the anatomical landmarks of the vermis
(fastigium point of the fourth ventricle, fissures) could not be demonstrated ( arrows ). The diagnosis of Joubert syndrome was considered and was con-
firmed after birth. (Reproduced, with permission, from the Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, International Society of
Ultrasound in Obstetrics and Gynecology, 2010, www.isuog.org. )
C