Dandy-Walker malformation 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 Brenda.

Pathogenesis

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 uncommon.

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 developmental abnormality.

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.

Gardner¹² felt that 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.

Pathology

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.

Clinical Features

Dandy-Walker malformation 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 of life.

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.

Seizures, 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 "floating".

Investigations

The 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.

MR provides 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.

Joubert's 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.

The absence 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 system.

The differential diagnosis of a posterior fossa include, arachnoid cysts,  Dandy-Walker malformation, and mega cisterna magna.

CT-ventriculography 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.

Treatment

20%of the cysts are asymptomatic and require no intervention. Treatment is controversial.

Direct 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.

However, 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.

The results 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.