Dandy walker syndrome or malformation: What we as a practicing Obstetricians must know??

 

 

: What changes happen in post fossa?? Ans :-Dandy walker syndrome or malformation is a malformation of the posterior fossa consisting of marked cystic dilatation of the fourth ventricle with anterior and superior displacement.

What is Dandy Walker variant or   Dandy walker complex   ? The term  Dandy walker complex is used to indicate the spectrum of abnormal  findings that share  in common  an enlargement   of the posterior fossa  and the impression  of a V shaped cleft  in the cerebellar vermis in  the axial  plane . When other anomalies are excluded however   careful evaluation of the posterior   fossa anatomy may help determine   the specific  abnormality and   prognosis.

History :---The term Dandy walker syndrome or malformation was originally described in 1914 by Dandy  and Blackfan in which  a rare malformation of the posterior fossa consisting of marked cystic dilatation  of the fourth ventricle with anterior and superior displacement  of  vermis with complete or partial   agenesis   was seen. Since the original description a number of other abnormalities of the posterior fossa with similarities to the Dandy Walker  malformation have  been described including the Dandy Walker  variant   with  mega-cisterna magna and Blake’s  pouch cyst .   Some   investigators have suggested that all of the these entities be grouped and referred to as the Dandy walker complex.

 

Prevalence ??  Dandy  Walker  malformation is exceedingly rare with an estimated incidence of about 1,30,000 births  and is found in 4%  to 12%  of all cases of infantile hydrocephalus. However minor variations of this condition are frequently encountered attested by an increasing number of reports both in the prenatal and pediatric literature.

 

Enlargement  of the cistern magna referred to a  megacisterna magna  was diagnosis   introduced by Some   investigators as to describe a series of adult patients with markedly enlarged   posterior fossa cisterns due to cerebellar  atrophy . Since  the original description   the term has been loosely  applied to describe an enlarged  retrocerebellar   space  and a normal and intact vermis and normal  cerebellar  hemispheres. In prenatal sonograms it the cistern magna depth is greater   than 10 mm and the  cerebellar hemispheres.

 

In prenatal  sonograms it the cistern magna  depth  is greater   than 10 mm and the cerebellar vermis appears  intact and in its   normal position  the condition is usually  referred to as  megacisterna  magma although  this is a risk  factor for associated malformations including  aneuploidies  most isolated cases are   of no consequence. If there  is the impression of a V  shaped cleft in the cerebellum in the axial  plane and the median   plane  reveals   a vermis  that is intact  with  a cyst in the posterior fossa  the diagnosis  is most likely  a Blake’s  pouch cyst represents herniation of the inferior aspect of the fourth  ventricle  through  the foramen of Magendie into the vallecula and retovermian cistern.

The difficulty in using  one term to describe these entities is that the prognosis for each can be  different Strictly describe  marked cystic dilatation of the fourth ventricle which may  fill  much of the posterior fossa; hypogenesis   ors agenesis of the cerebellar   vermis  and superior   displacement of the  tentorium  and lateral sinuses.

 

What changes in brain happens in Dandy walker malformations??      The cistern magna is compressed and reduced to a virtual space between  the dilated  fourth  ventricle  and the duramater . The cerebellar hemispheres are often   anterolaterally displaced. Hydrocephalus   although frequently seen in cases of Dandy Walker malformation is not necessary for the diagnosis.

In which plane the brain should be imaged?? Sonographically the median   plane is the most important   one for the diagnosis. The cystic fourth ventricle extends superiorly displacing the cerebellar vermis and  elevates the tentorium  cerebelli and the oracular Herophili above   its normal position . Dandy Walker   malformation is frequently associated with other neural defects mostly  with other midline anomalies  such as ACG  and holoprosencephaly.

What  are the  associated intracranial or Sytemic abnormalities?? Other deformities include encephaloceles polycystic kidneys cardiovascular   defects and facial clefting. The most frequent associations. Even if isolated it has been generally considered to have a poor prognosis. However  recent experience with MRI  in infants   suggests that those cases in which the vermis  appears  hypoplastic have a subnormal intelligence in 85%  of cases  versus   only 15%  of those in which the  vermis  appears to be intact.

 

Role of MRI?    Some   investigators that  the cerebellar vermis  was considered  normal  with MRI  if the was possible  to  document  three main  anatomic landmarks in the median  plane the fastigium point that corresponds to the posterior apex  of the fourth ventricle  and the two  main fissures. A  similar  approach  has also been suggested  for antenatal studies  Both MRI   and multiplanar possibly transvaginal  ultrasound  can be used. However  it is expected  that the small size   of the vermis and the difficulty in obtaining  an  exact  median  plane will at times represent  major  limiting  factors  particularly in early gestation.

Some   investigators in 1976  introduced  the term  Dandy  Walker  variant to describe  an out pouching  of an  ependymal lined  cyst  from the fourth  ventricle associated  with dysplasia  of the vermis. A hypoplastic  superiorly   rotated  vermis is the  landmark of vermain  hypoplasia  previously    referred  to as Dandy walker  variant  a condition that is frequently  associated with   other intracranial anomalies  and carries a very high risk of abnormal  neurodevelopment. Distinction between an intact  and an hypoplastic vermis  is far from simple. From approximately midgestation under normal conditions it is usually possible with either  sonography or MRI  to demonstrate he fastigium of the fourth  ventricle  and the two main fissures in the median plane. With  hypoplasia  of the vermis  these landmarks cannot be  identified.  Nomograms of the normal  size of the vermis  throughout gestation are also available.

Most investigators however agree that that  isolated megacisterna magna  and Blake  ‘s pouch cyst represent  the bulk of the posterior fossa anomalies  that are diagnosed antenatally. Although  the data thus far  are limited   and the neurologic risk cannot be  predicted precisely  it would  seem that in the majority of these cases the prognosis is good . It should be noted  that  there are some authors   who believe   that both  conditions   may represent a Blake’s  pouch cyst.

The greatest diagnostic challenge is probably  represented by Foubert syndrome  an anomaly with autosomal recessive  transmission  that is featured  by absence or hypoplasia of the vermis  which is not rotated . In a pregnancy at specific risk  the condition can be suspected  when a communication is seen between the fourth venricel and a cistern magna of  normal size  I in the transverse plane.  In the  absence of the vermis the two  hemispheres impinge on eh midline  and therefore  the median plane is of little use. However  at times  it may demonstrate   a fourth  ventricle of slightly  however this  finding has a very low predictive value  and most cases  we have  seen were  found  to be normal at birth.

Caution is warranted  :---Caution is warranted  while making the diagnosis of a minor variety of the Dandy walker  continuum the   Dandy walker  variant or inferior vermian  agenesis . Development of the cerebellar  vermis is incomplete before  20 weeks  gestation and thus a defect and abnormality of the vermis may be suspected if imaged too early Even in cases diagnosed after 20 weeks  there is evidence that isolated inferior vermian hypoplasia is over diagnosed prenatally even with the use of MRI  Thus  postnatal confirmation is warranted In a study correlating the Dandy Walker malformation compiled and postmortem  pathology Some researchers have  warned   that  two sonographic variables in the transverse  axial plane correlated  well with the pathologic  diagnosis th severity  of the vermain defect  and the shape  of the gap between  the  hemispheres in transverse   cerebellar images , a trapezoidal  gap with a broad  dorsal base and relatively straight sides   corresponded to hypoplasia  of aplasia of the vermis at autopsy  . However  a keyhole shaped gap with convex sides was  not associated with macroscopic or microscopic  cerebellar   pathology .

In general meticulous  scanning with a multiplanar  approach is recommended  in cases with a suspicion  of a posterior fossa anomaly . The poor  correlation that has been demonstrated in several  studies between  antenatal diagnosis  and autopsy  findings is probably related  to the use of only  axial  planes , which  can be extremely misleading . In the axial plane entities that are clinically  different such as Dandy Walker   malformation vermian hypoplaisa and Blake’s pouch cyst may be difficult to distinguish from one another  Because the diagnosis of midline anomalies  depends on section planes  of the fetal head that are sometimes  difficult to obtain we have found that three dimensional ultrasound is frequently  of considerable  help Multiplanar  slicing of a volume obtained from axial scans usually provide images of diagnostic quality.