Dandy-Walker Malformation: Classification and Management

1. Dandy-Walker Malformation:
Classification and Management
By: Dr. Shahnawaz Alam
MCh-Neurosurgery

2. Terminology:
• The Dandy-Walker continuum (DWC) is a
spectrum of anomalies.
• It includes Dandy-Walker malformation (DWM),
vermian hypoplasia (VH), Blake pouch cyst
(BPC), and mega cisterna magna (MCM).
• 2 measurements are important in distinguishing
these entities:
1. The Tegmento-vermian angle (the angle
formed by lines along the anterior surface of
the vermis and the dorsal surface of the
brainstem, normally < 18°)
2. The Fastigium-declive line (a line drawn
from the fastigium—the dorsal "point" of
the 4th ventricle on sagittal images—and the
dorsal most point of the vermis).

3.  DWM is a generalized disorder of mesenchymal development that affects both the
cerebellum and overlying meninges.
 It consists of a large PF with a high-inserting venous sinus confluence, large PF
pia-ependyma-lined cyst extending dorsally from the fourth ventricle, and varying
degrees of vermian and cerebellar hemispheric hypoplasia.
 The fourth ventricle choroid plexus is absent.
 Spennato P, Mirone G, Nastro A, et al.
Hydrocephalus in Dandy-Walker malformation.
Childs Nerv Syst 2011;27(10):1665–1681

5.  Vermian hypoplasia (VH) is part of the DWC spectrum.
 There is superior rotation of the vermis, increased tegmento-vermian angle (18-
45°), and variable cerebellar hypoplasia (diminished vermian volume below the
fastigium-declive line).
 The overall posterior fossa volume is normal in VH.

6.  Blake pouch cyst (BPC) is an ependyma-lined protrusion of the fourth ventricle
through the foramen of Magendie into the retrovermian cistern.
 The fourth ventricle choroid plexus is present but displaced into the superior
cyst wall.
 The tegmento-vermian angle is increased, but the vermis is normal in size and
configuration.
 The fourth ventricle has a "key hole" appearance.

7. • Mega cisterna magna (MCM) is an enlarged retrocerebellar CSF collection (> 10
mm).
 There is no mass effect on the cerebellar hemispheres or vermis.
 The vermis is normal as is the tegmento-vermian angle (< 18°).
 Cerebellar veins and elements of the falx cerebelli can be seen crossing
through the MCS.

9. Etiology:
Embryology
• The anterior membranous area (AMA) of the embryonic fourth ventricle fails to
incorporate properly into the choroid plexus.
 There is delayed opening of the foramen of Magendie, CSF pulsations cause the
nonintegrated anterior membranous area to balloon posteriorly within the PF. This
large CSF-filled cyst does not communicate with the subarachnoid space.
• In the 1990s, Tortori Donati et al. using MRI findings, developed the concept of the
persisting Blake’s pouch.
 Blake’s pouch is a finger-like expansion of the posterior membranous area
(PMA) that normally regresses during embryonic development. It leaves behind a
median opening, Blake’s metapore, which eventually develops into the foramen
of Magendie.

10. • Therefore, the Dandy–Walker malformation and its variant may be thought of as
malformations of the AMA and a persisting Blake’s pouch is a malformation arising
from the PMA.
• Mega cisterna magna has also been postulated to arise from a malformation of the
PMA.
 According to this theory, mega cisterna magna results from a midline cystic
evagination of the tela-choroidea of the fourth ventricle.
 The resulting cyst eventually communicates with the subarachnoid space;
therefore, hydrocephalus does not occur.

11. Etiology and Genetics
• Three main DWM causative genes have been identified: FOXC1 on chromosome
3q24 and the linked ZIC1 and ZIC4 genes on chromosome 6q25.3.
• Each member of the ZIC gene family encodes a highly related zinc-finger
transcription factor that is broadly expressed throughout cerebellar development. Both
ZIC1 and ZIC4 have key roles in the regulation of both cerebellar size and normal
cerebellar foliation.
• In addition, DWM can arise as a result of maternal diabetes, use of warfarin, or fetal
infection by cytomegalovirus, rubella, or Zika virus.
• Known clinical associations with DWM include PHACES, neurocutaneous melanosis
midline anomalies, and trisomy 18.

12. Pathology
Gross Pathology
• The most striking gross findings in DWM are (1) an enlarged PF with (2) upward
displacement of the tentorium and accompanying venous sinuses and (3) cystic dilatation
of the fourth ventricle.
• Vermian abnormalities range from complete absence to varying degrees of hypoplasia. In
DWM, the 4th ventricle choroid plexus is absent.
• DWM is frequently associated with other CNS anomalies. Almost two-thirds of patients
have gyral abnormalities (e.g., pachy- or polymicrogyria and heterotopic GM). Callosal
dysgenesis is common.
Microscopic Features
• The PF cyst in DWM is typically lined by two layers: an outer layer of pia-arachnoid and
an inner layer of ependyma.
• Occasionally, microscopic remnants of cerebellar tissue are present in the cyst wall.

13. Clinical Issues:
Epidemiology and Demographics
• DWM is the most common congenital cerebellar malformation with an estimated
prevalence of 1:5,000 live births.
• There is a slight female predominance (F:M = 1.5-2:1).
Presentation
• The most common presentation of DWM is increased intracranial pressure secondary to
hydrocephalus. Despite the extensive cerebellar abnormalities, cerebellar signs are
relatively uncommon.
Natural History
• Early death is common in classic DWM. If DWM is relatively mild and uncomplicated
by other CNS anomalies, intelligence can be normal and neurologic deficits minimal.

14. Associated Abnormalities
• Agenesis of the corpus callosum appears in most series.
• Aqueductal stenosis can occur and can be of the congenital forked aqueduct type or
can be an acquired, functional stenosis caused by ventrally directed pressure by the
fourth ventricular cyst against the dorsum of the midbrain.
• Occipital encephaloceles and meningoceles may occur in up to 17.5% of cases.
• Systemic, non-CNS anomalies were found in 25% of the cases. They included
syndactyly and polydactyly, cardiac anomalies, hemangiomas, craniofacial
abnormalities, and gastrointestinal and urogenital abnormalities.
• A dorsal interhemispheric cyst may be present.

15. Associated Syndromes:
 Cranio-cerebello-cardiac dysplasia (3C / Ritscher–Schinzel syndrome)
• In this syndrome, the cranial malformation is a large head with an especially tall and
broad forehead; the cerebellar malformation is vermian hypoplasia with a fourth
ventricular cyst.
 PHACE syndrome
• Posterior fossa anomalies are combined with facial hemangiomas, arterial anomalies,
coarctation of the aorta and other cardiac anomalies, and eye abnormalities.
• Dandy–Walker malformation is the most common CNS malformation in the PHACE
syndrome. The facial hemangiomas usually follow trigeminal dermatomes.
 Nearly 10% patients of Neurocutaneous melanosis have had Dandy–Walker
malformations.
 Meckel–Gruber, classically includes an occipital encephalocele, multi-cystic
kidneys, and polydactyly.

16. Other Imaging Findings:
• The straight sinus, sinus confluence, and tentorial apex are elevated above the lambdoid
suture ("lambdoid-torcular inversion").
• The transverse sinuses descend at a steep angle from the torcular herophili toward the
sigmoid sinuses.
• The occipital bone may appear scalloped, focally thinned, and remodeled with all
posterior fossa cysts.
• Retrocerebellar CSF cysts (formerly termed "mega cisterna magna") often
demonstrates partially infolded dura- arachnoid (falx cerebelli) on axial T2 scans.
• The falx cerebelli is usually absent in DWM.
• Generalized obstructive hydrocephalus is present in over 80% of neonates with DWM
at birth.
• If callosal dysgenesis is present, the lateral ventricles are widely separated and may
have unusually prominent occipital horns (colpocephaly).

17.  Nelson et al. proposed that the nature of a posterior fossa cyst can be determined by the
position of the fourth ventricular choroid plexus.
 It is absent in a Dandy–Walker malformation and displaced into the superior wall of a
persisting Blake’s pouch.
 In a developmentally unrelated abnormality, arachnoid cyst of the posterior fossa, the
choroid plexus lies in its normal position in the fourth ventricle, although the fourth
ventricle itself may be displaced.

22. Treatment
• Some Dandy–Walker malformations are asymptomatic and need no treatment.
• However, if there is symptomatic hydrocephalus or brainstem or cerebellar symptoms
from the pressure by the fourth ventricular cyst, then treatment becomes necessary.
• Dandy, Taggart and Walker, and Matson all advocated excising the wall of the fourth
ventricular cyst, thereby presumably re-establishing the pathway of cerebrospinal fluid
flow from the ventricles to the subarachnoid space. For many years this was the only
accepted treatment.
• However, hydrocephalus was so infrequently controlled by cyst wall excision and the
morbidity and mortality were so high that CSF shunting without cyst wall excision
became the procedure of choice for the Dandy–Walker malformation.
• CSF diversion, usually ventriculoperitoneal shunting with or without cyst shunting or
marsupialization, is the standard treatment for DWM-related hydrocephalus.

23. • When planning treatment for a patient with Dandy-Walker malformation, it is essential
to know the status of communication between the third ventricle and the fourth
ventricular cyst.
• Flow voids in the aqueduct on MRI indicate free communication. CT ventriculography
or cisternography with intrathecal contrast is preferred.
• A CT cisternogram can also differentiate between mega cisterna magna and a posterior
fossa arachnoid cyst. Contrast immediately enters the former but will most likely be
delayed in entering the latter, if it enters at all.
• Upward transtentorial herniation of the fourth ventricular cyst is a known complication
of shunting the lateral ventricle without shunting the cyst. This can cause progressive
obtundation, brain stem dysfunction, and even death.

24. • The characteristic “keyhole” sign, indicating upward herniation of the cyst, can be seen
on axial CT and MRI and the “snail” sign can be seen on sagittal MRI. These signs
denote the shape assumed by the posterior fossa cyst as it is squeezed through the
tentorial hiatus.
• Prenatally, the Dandy-Walker malformation can be detected using either transabdominal
or transvaginal ultrasound. The abnormality has been detected by ultrasound as early as
the first trimester.
• Fetus diagnosed before 21-weeks gestation have been found to have significantly worse
prognoses than do those diagnosed later.
• Fetal MRI scanning is now an option when a posterior fossa cyst is discovered on
ultrasound and can provide exceptional detail of both supratentorial and infratentorial
structures.

25. • The surgeon must be cognizant, when planning to insert a catheter into the lateral ventricle
via an occipital approach, that the transverse sinus is typically very high and absolutely
must be avoided.
• A catheter in the fourth ventricular cyst can, if indelicately inserted or if malpositioned,
injure the floor of the fourth ventricle - the medulla or pons.
• A rare complication of cystoperitoneal shunting is brainstem tethering, which can lead to
cranial nerve deficits.
• Posterior fossa subdural hematomas are another complication of shunting posterior fossa
cysts.

26. Treatment strategy for hydrocephalus with cystic dilation of fourth ventricle
CPS, cystoperitoneal shunt; ETV, endoscopic third ventriculostomy; VPS, ventriculoperitoneal shunt.
 Paediatric Neurosurgery: Tricks of the Trade By Alan R. Cohen

27.  Third ventriculostomy (open, stereotactic, and endoscopic) has been used as a
treatment for the hydrocephalus associated with Dandy–Walker malformation.
 The aqueduct must be patent, as must the subarachnoid pathways for this approach
to be successful.
 If third ventriculostomy is contemplated, an MRI, preferably a cine MRI, or a CT
ventriculogram is usually performed.
 When performing a third ventriculostomy, it must be remembered that the anatomy
of the interpeduncular cistern in the Dandy–Walker malformation is usually
significantly distorted. The floor of the third ventricle is nearly vertical and the tip
of the basilar artery is quite high.
 In three patients with aqueductal stenosis and a Dandy–Walker malformation, Mohanty
et al. combined endoscopic third ventriculostomy with cystoventricular stent
placement.
 The stent was placed through an extremely thinned superior vermis.
 The procedure was ultimately successful in two of the three patients; one of these
patients required repositioning of the stent and the third patient eventually required
a VP shunt.

28. Schematic diagram demonstrating the foramen of
Monro and the surrounding structures.
• The trajectory of the endoscope for ETV, with the fenestration between
the mammillary bodies and the infundibular recess.
• The surgeon must keep in mind the proximity of critical vascular
structures to the floor of the third ventricle, including the basilar artery
and perforating arteries in the interpeduncular cistern.
Superior view of the floor of the lateral ventricles
• Note that in the right lateral ventricle, the
thalamostriate vein is to the right of the choroid
plexus.
• Also note that the fornix forms the superior and
anterior boundary of the foramen of Monro.

29.  Endoscopic third ventriculostomy for rare causes of obstructive hydrocephalus by Yvonnee Mondrof et al.

30. VP-shunt Vs CP-shunt
 Fischer and Carmel advocated placing the catheter in a lateral ventricle and not
shunting the cyst unless it later became necessary.
 Kumar at al. found that only 8 of their 28 patients who initially underwent
ventriculoperitoneal (VP) shunting later needed the addition of a catheter into the cyst,
whereas 6 of their 7 patients who initially underwent cystoperitoneal shunting later
needed a ventricular catheter.
 Asai et al. found that only one of their 10 patients who initially underwent
cystoperitoneal shunting later needed a ventricular catheter.
 Some surgeons have suggested that cystoperitoneal shunts have a higher incidence of
malfunction than VP shunts, whereas Sawaya and McLaurin believe that there is a
lower incidence of shunt occlusion because the cyst does not collapse upon
decompression.

31.  Rather than initially shunting only the supratentorial ventricles or only the fourth
ventricular cyst, Raimondi et al. advocated shunting both simultaneously, using two
catheters joined by a Y-connector.
 This prevents pressure differences that could potentially lead to transtentorial
herniation, functional aqueductal stenosis, and supratentorial hydrocephalus or
brainstem distortion and dysfunction.
 Downward transtentorial herniation of the medial aspects of the lateral ventricles
is possible if only the cyst is shunted. Other authors have also advocated shunting
a lateral ventricle and the cyst simultaneously.
 A somewhat different strategy is to drain both the supratentorial ventricles and the cyst
via the same catheter. This has been accomplished under ultrasound guidance by
Cedzich et al. using a specially prepared catheter with many perforations; the catheter
was placed through the lateral ventricle and foramen of Monro into the third ventricle
and then into the cyst.

32. • In recent years, there has been renewed interest in open cyst fenestration.
 Liu et al. Almeida et al. and Villavicencio and colleagues all report good results
using posterior fossa craniectomy and wide fenestration in patients with histories of
multiple shunt revisions.
 One of Villavicencio et al. patients, who did very well after fenestration alone, had
never had a shunt. Villavicencio emphasized the importance of wide fenestration
into the cerebellopontine (CP) angles bilaterally as well as into the cervical
subarachnoid space via a C1 laminectomy.
 Liu’s single patient presented with focal CN deficits from tethering of the brainstem
in association with a pre-existing cystoperitoneal shunt; all deficits improved after
surgical untethering and cyst fenestration.
• Some authors in the past have advocated open cyst fenestration only for patients
older than 3 years because of the presumed better developed subarachnoid space in
older children.

33. • In patients with aqueductal stenosis, one alternative is to combine endoscopic third
ventriculostomy with open cyst fenestration.
• In patients having an occipital meningocele that communicates with the fourth
ventricular cyst, shunting of the cyst can cause diminution or disappearance of the
meningocele.
• It has also been reported that repair of an occipital meningocele associated with the
Dandy-Walker malformation in asymptomatic patients can be precipitate symptomatic
hydrocephalus.
• A Dandy–Walker variant usually needs no treatment unless there is hydrocephalus.
Generally, placement of a lateral ventricular shunt is then performed.
• Shunting of either a lateral ventricle or the cyst has been recommended for persisting
Blake’s pouch. Endoscopic third ventriculostomy and open cyst fenestration can also
be considered.
• Mega cisterna magna needs no treatment.

34. Outcome
• Mortality in the Dandy–Walker malformation once ranged from 25 to 50%; however,
improvements in anaesthesia and intensive care have lowered mortality dramatically.
• More current published mortality figures are in the range of 10 to 14%.
• Morbidity and mortality from shunt infection or malfunction and from cardiorespiratory
arrest due to pressure on the brainstem by the fourth ventricular cyst still occur.
• Intellectual outcome is variable. The incidence of normal intelligence has ranged from
10 to 50%. Overall, 40% of these children have been intellectually normal, 20%
borderline, and 40% severely intellectually compromised.
• Some authors have found no correlation between intellect and CNS anomalies; others
have found the opposite, especially if there is agenesis of the corpus callosum or cortical
dysplasia.
• Seizures, hearing deficits, and visual difficulties do correlate with intellectual
impairment in Dandy–Walker patients.

35.  Gerstzen and Albright found no relationship between cerebellar volume (corrected for
posterior fossa volume) and intellectual or cerebellar function.
 However, Klein et al. showed that extent of dysplasia can predict intellectual outcome.
In patients in whom the vermis had two fissures, three lobes, and a fastigium (based
on prenatal or postnatal MRI), 19 of 21 had a normal intellect.
 In patients with a highly malformed, dysplastic vermis, with a single fissure or no
fissure at all, none had normal intelligence.
 Associated cerebral anomalies, most often agenesis of the corpus callosum, were
always present in the latter group.
• Although some authors have found no relationship between hydrocephalus and
intellectual outcome, certainly untreated hydrocephalus would be strongly expected to
predispose to poor outcome.
• Therefore, to maximize intellectual potential, hydrocephalus should be treated
aggressively, and shunt malfunctions and infections avoided or quickly diagnosed and
treated.

36. • Prematurely born baby with diagnosis of Dandy-Walker malformation.
• ETV was attempted at approximately age 4 weeks with ventricular cystic stent but
did not control the hydrocephalus. Shunt to peritoneum was attached to stent 5
days later.
• (a) T2 sagittal MRI showing large posterior fossa cystic dilation with vermian
hypoplasia. (b) T1 axial view of same patient showing the membrane fenestrated
endoscopically. (c) Post- operative NCCT appearance.

37. References:
• Youmans and Winn neurological surgery 7th edition
• Ramamurthi & Tandon's textbook of neurosurgery 3rd edition
• Osborn Brain Imaging ; 2nd edition
• Principles and Practice of Paediatric Neurosurgery 2nd edition by A. Leland Albright
• Paediatric Neurosurgery: Tricks of the Trade By Alan R. Cohen
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