is a cystic dilatation of the fourth ventricle or a cyst in communication
with the 4th ventricle, with a varying degree of vermian agenesis, often
associated with hydrocephalus.
The Dandy walker
malformation was initially described in 1914 by Dandy and Blackfan, who
described a case of an enlarged fourth ventricle which was presumed to
result from a fourth ventricle outlet obstruction.
In 1954, the term Dandy Walker malformation was introduced by
The exact pathogenesis of
Dandy walker syndrome has since been the subject of continuous debate.
The syndrome probably represents a nonspecific central nervous system
malformation, which may occur alone or with other malformations and which
may occur in single gene disorders, in chromosomal aberrations, or as a
consequence of an environmental insult. The cause of Dandy-Walker syndrome
is unknown. Despite the common association of other congenital anomalies
with this syndrome (including other central nervous system, cardiac,
gastrointestinal, urogenital, facial, and skeletal anomalies), this
condition has been only occasionally described in families, most often as
part of a syndrome inherited in an autosomal recessive manner.
This syndrome may also occur in the
context of chromosomal aberrations. Other associated nervous system
abnormalities may include agenesis of the corpus callosum, aqueductal
stenosis, cerebral heterotopias, holoprosencephaly, neural tube defects,
and rachischisis. Before 1984, with 300 reported cases of Dandy-Walker
syndrome in the literature, only 16 had been described in families,
suggesting that a genetic predisposition to this syndrome probably is
The syndrome probably
represents a nonspecific central nervous system malformation, which may
occur alone or with other malformations and which may occur in single gene
disorders, in chromosomal aberrations, or as a consequence of an
environmental insult. Four theories have been proposed for this
Dandy and Blackfan and
later Taggart and Walker f were of the opinion that congenital occlusion
of the foramina of Luschka and Magendie was the principal factor
responsible for the dilatation of the 4th ventricle and the secondary
maldevelopment of the cerebellar vermis. The reasons proposed for the
absence or presence of hydrocephalus included an abnormally small choroid
plexus producing less quantities of fourth ventricular fluid, the cyst
wall acting as a semipermeable barrier resulting in the escape of fluid
from the cyst into the subarachnoid spaces, and absorption of the cyst
fluid by vessels lying in the cyst wall. Gibson13 however
showed that the outlet foramina of the fourth ventricle could be patent.
Benda believed that the
malformation was principally a failure of fusion of the corpus cerebelli.
Others believed that a
neuroschistic cleft and bleb at the region of the corpus cerebelli,
without rupture of the ectoderm, resulted in adhesions with the inner
dural layer and inhibited the development of the vermis. This meant that
imperforation of the roof of the fourth ventricle was not the cause of the
anomaly but only a part of it.
delayed permeability of the fourth ventricle roof causes a foetal
hydrocephalus. This, together with an earlier developed posterior choroid
plexus causes excessive distending forces in the posterior fossa leading
to a dilated fourth ventrticle, a large posterior fossa, and a high
placed tentorium. Gardner also described a variant of the malformation,
'the Dandy-Walker cyst' In this anomaly ventricular fluid
accumulates between two layers of the ependyma to form a true cyst.
The cystic fourth
ventricle has a wall separate from the dura of the posterior fossa. The
outer layer of this wall is arachnoid and the inner layer ependyma. A
layer of tissue derived from the-cerebellum lies between the two layers.
This middle layer becomes more apparent laterally and superiorly. The
outlet foramina of the fourth ventricle are patent in 43-82 per cent of
the cases reported in the literature.
The choroid plexus of the
fourth ventricle is hypoplastic and is displaced caudally and lies near
the brainstemornearthedilated lateral recesses. The degree of involvement
of the inferior vermis is variable. Even when it seems that the vermis is
entirely replaced by the cyst wall, hypoplastic vermian tissue may be seen
on microscopy. The superior vermis is often displaced
superiorly, The cerebellar hemispheres are displaced laterally and
dorsally and show the effects of hypoplasia and secondary atrophy due to
chronic pressure. The brainstem is flattened antero-posteriorly.
Hydrocephalus is usually present with dilatation of the lateral and third
ventricles. The cause of the hydrocephalus may be aqueduct stenosdis,
fourth ventricular outlet obstruction to the CSF path ways.
accounts for about four per cent of all cases of hydrocephalus10.
The clinical features
depend on the effects produced by the cyst, the presence or absence of
hydrocephalus and associated anomalies.
62 per cent of patients
with the malformation present during the first year of life, 17 per cent
between the first ana fifth years and 11 per cent during the second decade
Infants present with
symptoms and signs of raised intracranial pressure, including vomiting, a
bulging fontanelle and sixth nerve paresis. The occipital region is
usually prominent, the shape of the skull tending towards dolichocephaly.
Transillumination of the posteriorfossa in infants is positive and will
also allow differentiation between the malformation and a posterior fossa
extra-axial arachnoid cyst.
Children over two years
of age present with headache and papiiloedema. Ataxia is present in
more than 50% of the patients
and delayed milestones and mental retardation is seen in upto 40 per cent
of these children.
motor deficits, cerebellar signs other than ataxiaand brainstem signs may
also occur. Often there may be only a macrocephaiy. Sutural diastasis is
commonly found. The lambdoid suture may be found split mere than the
others, occasionally to such an extent that the occipital bone is found
Dandy-Walker malformation develops at approximately 4 weeks gestation.
After the first trimester, ultrasound is useful in identifying the cyst
and other brain anomalies.
MRI is the
choice of imaging. Principal features on MRI include partial or complete
agenesis of vermian, dilatation of the 4th ventricle, enlarged posterior
fossa with elevation of the tentorium, and cerebellar hypoplasia. In mega
cisterna magna, the 4th ventricle is normal with no vermian hypoplasia.
The 4th ventricle is displaced in arachnoid cysts.
a detailed anatomic study of the structures in the posterior fossa and
also of all the associated anomalies present.
Barkovich et a
have, on the basis of MR studies of posterior fossa cysts and cyst-like
malformations, proposed a new classification in which the Dandy-Walker
malformation, the variant and the mega cisterna, magna, form part of a
continuum of a single developmental anomaly which they called the
''Dandy-Walker complex". The Dandy Walker variant is a less severe
malformation. the vermis is hypoplastic, but not absent, and the posterior
fossa is not enlarged.
syndrome is characterized clinically by ataxia, mental retardation,
episodic hyperpnoea, and abnormal eye movements, and is due to total
aplasia of the cerebellar vermis. On CT or MR1 where, in addition to
absence of the vermis. the fourth ventricle appears large and triangular
with the apex pointing backward at its mid portion. and large and 'bat's
wing' at a higher level.
of the vallecula suggests a Dandy-Walker malformation while a normal
vallecula and a compressed fourth ventricle suggest an arachnoid cyst. The
"key-hole sign" is seen with a cyst which is isolated from the ventricular
posterior fossa include,
malformation, and mega cisterna magna.
Post. fossa arch. cyst- MRI
Mega cisterna magna-MRI
An arachnoid cyst results in anterior displacement of the fourth
ventricle, but normal cerebellar development.
malformation is a cystic dilatation of the fourth ventricle or a cyst
in communication with 4th ventricle.
Mega cisterna magna is an anatomic variant with normal fourth
ventricle and small cerebellum.
using water soluble contrast material is perhaps the best for preoperative
assessment of the ventricular system and the CSF pathways. This not only
provides information on the patency of the aqueduct but will also clearly
rule out a posterior fossa arachnoid cyst.
cysts are asymptomatic and require no intervention. Treatment is
fourth ventriculostomy, as advocated by Dandy, was the treatment followed
till the advent of shunting devices. Shunting procedures are the treatment
of choice for the Dandy-Walker malformation. In the presence of
hydrocephalus, there is no question that a ventricular shunt has to be
inserted. What is under debate, however, is the need for shunting the
posterior fossa fluid collection. Ventriculoperitoneal shunt may allow
the cyst grow larger and herniated upward. Excising the cyst has been
tried virtually with no success. Combined shunting of the supra and infra
tentorial compartments is probably the optimal treatment.
many others feel that this double shunting is not necessary as a primary
procedure. Ventriculo peritoneal shunt may allow the cyst grow larger and
herniated upward. Excising the cyst has been tried virtually with no
success. Combined shunting of the supra and infra tentorial compartments
is probably the optimal treatment. The double shunt may be inserted, if
necessary, in two stages. In the absence of hydrocephalus, a cysto-peritoneal
shunt may be performed.
of the shunting procedures have been uniformly satisfactory. Adults and
older children have a better prognosis as they tend to have fewer
associated abnormalities than infants. The presence of associated
anomalies did not correlate with low IQ scores except in those pateints
with agenesis of the corpus callosum. The control of hydrocephalus is one
of the more important factors in determining the intellectual development
of these patients. Approximately, 50% of long term survivors have an IQ of
80 or more and approximately 30% have normal intelligence.