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Posterior fossa cystic lesions — magnetic resonance imaging manifestations
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Brain & Development 1995; 17:418-24
Original article
Posterior fossa cystic lesions - magnetic resonance imaging manifestations En-Chow Tan *, Takuji Takagi, Kunio Karasawa Department of Neurosurgery, Nagoya City Higashi General Hospital, 1-2-23, Wakamizu, Chikusa-Ku, 464 Nagoya, Japan
Received 26 August 1994; accepted 6 June 1995
Cystic lesions of the posterior fossa remain a controversial subject as to clinical classification and diagnosis, especially for those in combination with other intracranial abnormalities. During the period of November 1985 to J u n e 1991, 16 patients with cystic lesions of the posterior fossa were retrospectively reviewed on neuroradiological evaluation with M R images. The patients were 9 males and 7 females aged from 5 days to 15 years old (medium 1 year old). They were classified into three groups as Dandy-Walker cyst (group A, 5 patients), mega cisterna m a g n a (group B, 4) and retrocerebellar pouch or cyst (group C, 7). Nine patients had associated intracranial anomalies, holoprosencephaly in 3 (group A, 3), dysgenesis of the corpus callosum in 4 (group A, 2; group C, 2), and occipital meningoceles in 2 (group C, 2). Hydrocephalus or ventricular enlargement was found in 10 patients (group A, 5; group B, 2; group C, 3). Surgical treatment (cyst-peritoneal shunt or cystectomy) was performed for 10 patients with clinical symptoms due to cyst. Postoperatively, clinical symptoms due to increased intracranial pressure or hydrocephalus improved in 7 patients. For long-term results, the cyst was reduced in 1 of 5 patients of group A and 2 of 5 of group C. Most of our patients have a poor prognosis because of the associated intracranial anomalies or atrophy of the cerebellum. Muitiplanar M R images may provide sufficient evidence for the diagnosis of posterior fossa cysts, especially in the case of rotation or upward displacement of the cerebellar vermis. However, the cyst membrane, and the communication of fluid between the cyst and the cistern, cannot be demonstrated on MR images. Further investigation on these subjects and indications for surgical intervention is highly necessary. Keywords: Dandy-Walker cyst; Dysgenesis of the cerebellum; Magnetic resonance imaging; Mega cisterna magna; Posterior fossa cyst; Retrocerebellar cyst
1. I N T R O D U C T I O N The posterior fossa cystic lesion is one of the benign lesions in infants and children [1]. Recent surgical results have indicated that the cyst-peritoneal shunt may be a simple and effective method for treating intracranial cysts [2-4]. However, the classification or definitive diagnosis of a posterior fossa cyst is not yet unified because of the complex relationships between the cyst, the tentorium, the hindbrain and the roof of the fourth ventricle. The radiological manifestations on computerized tomography (CT), ventriculography, computerized tomographic cisternography (CTC) or angiography have been used as classifi-
* Corresponding author. Fax: (81) (52) 721o1308. 0387-7604/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0387-7604(95)00067-4
cation criteria for posterior fossa cysts [5-9]. However, it remains controversial to classify the cysts on the basis of the results of neuroradioiogical investigations. It has been indicated that rotation of the lower vermis may give a false impression of vermis dysgenesis on CT scans, which can be minimized by magnetic resonance imaging (MRI) [6]. The present study is an attempt to classify the posterior fossa cystic lesions observed on MRI with a modification of Raybaud's classification [8].
2. CLINICAL MATERIALS AND M E T H O D S Patient population From November 1985 to June 1991, 16 patients with suspected posterior fossa cysts on CT scans or diagnosed as
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having posterior fossa cysts in childhood underwent M R I examination. The patients were 9 males and 7 females aged from 5 days to 15 years old (medium 1 year old).
Classification of the cysts The classification used in the present study is a modification of that of Raybaud [8] which is based on the presence of malformation of the cerebellar vermis. Our classification is mainly based on the findings with M R I , and C T C is performed when the classification cannot be defined well by M R I only.
3. R E S U L T S
Clinical information Table 1 is a summary of the clinical findings in the 16 patients. The clinical symptoms were an enlarged head in 5 patients, neonatal distress in 3, decreased agility in 2, acute increased intracranial pressure in 3, mental retardation in 4, and headache in 3. Nine patients had associated intracranial anomalies, holoprosencephaly in 3 (semilobar, 3), dysgenesis of the corpus callosum in 4, and occipital meningocele in 2. Hydrocephalus or ventricular enlargement was found in 10 patients, three of w h o m were diagnosed by ultrasonography or M R I in the prenatal period.
Classification of posterior fossa cyst In Raybaud's original classification, cystic lesions were classified into Dandy-Walker malformation, Dandy-Walker variant and retrocerebellar cyst. In the present study mega
cisterna magna represented another distinctive group. Our patients were divided into three groups: as Dandy-Walker cyst, m e g a cisterna magna and retrocerebellar arachnoid cyst or pouch groups. No patient with a Dandy-Walker variant was found in our series.
Dandy-Walker cyst. Five patients were classified as having Dandy-Walker cyst. In this group, M R I showed cystic dilatation of the fourth ventricle and dysgenesis of the cerebellar vermis. Fig. 1 shows the M R I findings in case 1, in which cystic dilatation of the fourth ventricle with distension of the roof of the fourth ventricle, and dysgenesis of the cerebellar vermis were clearly visualized. However, the tentorium or torcula was not elevated in the presence of the cyst. The other 4 patients had elevation of the tentorium and torcula. The roof of the fourth ventricle could be visualized in cases 1 and 5 ( Figs. 1 and 2, right), and could not be found in severe dysgenesis of cerebellar vermis and hemispheres (cases 2, 3 and 4) (Fig. 2, left). T h r e e patients had holoprosencephaly, semilobar type (cases 2, 3 and 4). M e g a c i s t e r n a m a g n a . This group comprised four patients. The distinction of mega cisterna magna from a retrocerebellar arachnoid cyst in our patients was based on enlargement of the outlet of the fourth ventricle in the cisterna magna (Fig. 3). It would be difficult to differentiate between them if the cisterna magna extended upwards or the arachnoid cyst was enlarged downwards. In such cases, mega cisterna magna was defined as without a mass effect but with marked cerebellar atrophy or with having a communication with the other cisterns on CTC. T h e r e were 2 patients with these findings in this group.
Table 1 Clinical summary of 16 patients with posterior fossa cysts Case
Age
Dandy-Walker cyst 1 5 days 2 6 days 3 9 days
Sex
Symptoms
Hydrocephalus
Associated intracranial anomalies
Operations
Reduction of cyst size
F F M
+ + +
aqueduct atresia holoprosencephaly holoprosencephaly
V-P and C-P shunt V-P shunt V-P and C-P shunt
+ -
+
holoprosencephaly
V-P shunt
-
+
corpus callosum hypoplasia
V-P and C-P shunt
-
4
19 days
M
5
14 years
M
neonatal distress decreased agility neonatal distress enlarged head neonatal distress enlarged head enlarged head
F M M F
decreased agility vomiting enlarged head mental retardation
+ +
cerebral atrophy supratentorial arachnoid cyst -
V-P shunt -
no change no change no change no change
Retrocerebellar arachnoid cyst or pouch 10 1 year F enlarged head 11 2.8 years M mental retardation 12 3 years F mental retardation 13 10 years M headache 14 10 years M headache 15 15 years F headache
+ +
occipital meningocele
V-P shunt and ectomy ectomy ectomy V-P shunt and ectomy
+ pouch
16
+
V-P shunt and ectomy
-
Mega cisterna magna 6 2 months 7 2 months 8 1.8 years 9 4 years
15 years
M
mental retardation
occipital meningocele corpus callusom hypoplasia corpus callosum hypoplasia cerebellar dysgenesis
pouch +
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One patient (case 9) had ventricular enlargement and enlargement of the cisterna magna (Fig. 4). A coronal MR image (Fig. 4, right) showed that the medial aspects of both tonsils were displaced laterally, which may indicate that the pressure in the fourth ventricle may be high enough to displace the tonsilla. Although this patient was classified as having mega cisterna magna, enlargement of the cistema magna due to cystic dilatation of the fourth ventricle as a persistent Blake's cyst could not be defined because it was impossible to identify the tela choroidea on MRI.
Retrocerebeilar arachnoid cyst or pouch. Seven patients
Fig. 1. MRI in case 1, Dandy-Walker cyst. Cystic dilatation of the fourth ventricle and dysgenesis of the cerebellar vermis can be seen, however, the torcula and the tentorium are not elevated. The roof of the fourth ventricle can be visualized.
were classified as having a retrocerebellar arachnoid cyst or pouch. The distinction between an arachnoid cyst and a pouch depended on whether communication exists or not between the cystic fluid and the surrounding CSF spaces [8]. In the present study, an arachnoid cyst is defined as exhibiting compression on the underlying cerebellum, while a pouch is diagnosed as in the midline and exhibiting no compression. Each type may extend supratentorially through the defect in the posterior part of the tentorium. Two patients were diagnosed as pouch (case 11, 13) and the other 5 as cyst. CTC was performed in one of the pouch patients and it showed free communication with the surrounding cisterns. In 3 patients with arachnoid cyst, CTC showed absence of communication between cyst and surrounding cisterns in 1 and delayed clearance of contrast medium in 2. CTC was not performed in 2 patients whose arachnoid cyst developed and enlarged several years after repair of an occipital meningocele. Two patients had dysgenesis of corpus callosum and the
Fig. 2. Left: Case 3, holoprosencephaly (semilobar type) with Dandy-Walker cyst. The torcula and tentorium are elevated, but the roof of the fourth ventricle cannot be seen. Right: Case 5, Dandy-Walker cyst. The torcula and the tentorium are elevated. The roof of the fourth ventricle is visualized (arrows).
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Fig. 3. Left: Retrocerebellar cyst. The cisterna magna is not enlarged. Right: Mega cisterna magna. The cisterna magna is enlarged and extends to the tentorium.
other 2 had dysgenesis of the cerebellum. Five patients had the cyst in the midline portion, 1 exhibited left predominance (case 10) and 1 right predominance (case 12).
The definitive diagnosis of an arachnoid cyst with associated infratentorial anomalies was difficult on M R images only. Fig. 5 shows M R I of case 16, who had hypoplasia of
Fig. 4. MRI in case 9 (mega cisterna magna with hydrocephalus). The fourth ventricle and the cisterna magna are enlarged. On coronal MRI, the medial aspects of the tonsilla are displaced laterally.
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corpus callosum and dysgenesis of the cerebellar vermis and hemispheres. The dorsal parts of the right cerebellar hemispheres were greatly displaced anteriorly by the cystic lesion. It had been assumed to be a Dandy-Walker variant or an
arachnoid cyst on M R I ; however, CTC showed the absence of communication between the cyst and the fourth ventricle or perimedullary cisterns (Fig. 5). Retrocerebellar arachnoid cyst was diagnosed by means of CTC.
Fig. 5. MRI and CTC in case 16 with a retrocerebellar cyst. Upper row: MRI showing severe dysgenesis of vermis (sagittal view, right), and compressed cerebellar hemisphere especially in the right side (transverse view, left). These findings cannot differentiate arachnoid cyst from Dandy-Walker variant. Lower row: CTC showing absence of communication between the cyst and the fourth ventricle (left) or the perimedullary cisterns (right). Arachnoid cyst is diagnosed.
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Surgical treatment and results Because of the association of intracranial anomalies in most of our patients with Dandy-Walker cyst, ventriculo-peritoneal and cyst-peritoneal shunts were performed in 3, and a ventriculo-peritoneal shunt in another 2. In patients diagnosed as having mega cisterna magna, surgical intervention was not performed, except that a ventriculo-peritoneal shunt was performed in 1 patient with acute onset of hydrocephalus. In another patient with associated supratentorial arachnoid cyst the operation was not performed because the cyst was small. In patients diagnosed as having a retrocerebellar arachnoid cyst or pouch, surgical intervention was performed in those with symptoms, disturbance of CSF circulation or progressive enlargement of the cyst with a space-occupying lesion. A ventriculo-peritoneal shunt was performed in 3 patients with hydrocephalus, and 5 received excision of the inner and outer cyst wall. Postoperatively, clinical symptoms due to increased intracranial pressure or hydrocephalus improved in 7 patients. As a long-term result (follow-up period 2.3-5.2 years), the cyst size is reduced in 1 patient of group A (case 1) and in 2 of group C (cases 10 and 15). In group A 1 patient (case 1) had a moderate impairment of development, 1 (case 5) had a moderate impairment of intelligence and the other 3 were bed-ridden and had poor development. In group C 1 patient (case 10) had a normal development and intelligence, 1 (case 15) had a mild impairment of intelligence, 2 (cases 12 and 16) had mental retardation as preoperative state and 1 (case 14) died of drug anaphylaxis. The poor results are mainly caused by the associated supratentorial anomalies.
4. D I S C U S S I O N The pathogenesis of posterior fossa cystic lesions has been extensively reviewed as to the embryological development of the posterior fossa. It is suggested that cystic malformation of the posterior fossa is due to varying degrees of abnormalities in the development of the cerebellar vermis, tela choroidea, retrocerebellar and retromedullary space, and dura mater [8]. Several classifications have been proposed for differentiating posterior fossa cystic lesions based on the structural relationship, developmental anomalies or the communication between the cysts and the surrounding CSF space [5,6,8,9], but none of them has been unified or clearly defines the lesion. Raybaud [8] suggested that hydrocephalus, a raised tentorium or dehiscence in the posterior tentorium is not specific for a diagnosis, but partial agenesis of the vermis can be used to differentiate the rhomboschisis group of abnormalities (Dandy-Walker cyst or Dandy-Walker variant) from isolated expansion of the tela choroidea. In some cases upward displacement of the cerebellar vermis observed on axial CT may be misinterpreted as dysgenesis of the lower vermis [6]. Multiplanar MRI may minimize the misinterpretation, however, clear distinction of radiological manifestations remains impossible [6]. In our 5 patients with Dandy-Walker malformation the dysgenesis of the cerebellar vermis varies and the roof of the fourth ventricle can be identified in only 2 patients. In severe cases the roof of the fourth ventricle is difficult to visualize as
423
also found in another report [10]. Three of our patients with Dandy-Walker cyst have associated holoprosencephaly, which is rarely found but more frequent in Dandy-Walker malformation or variant than in retrocerebellar cyst [8]. Dysgenesis of the vermis may also be found in patients with arachnoid cysts [8], and differentiation from DandyWalker variant is difficult by MRI only. In case 16 with arachnoid cyst the roof of the fourth ventricle was not clearly defined. Detection of communication between a cyst and the surrounding cisterns or fourth ventricle remains difficult. It is also difficult to demonstrate the membrane present between a cyst and the fourth ventricle [6]. The cyst fluid intensities of the CSF flow effect on MRI may show whether there is a communication or not between a cyst and the surrounding CSF space [11]. In the present study we did not intend to make a clear distinction by MRI whether the cyst has a communication with the surrounding cisterns or not. In that case CTC was performed and showed the absence of communication between the cyst and the fourth ventricle or the cisterns, and thus an arachnoid cyst was diagnosed. In our limited cases, Dandy-Walker variants, persistent Blake's cyst, or other cystic lesions such as intraventricular cyst or glioependymal cyst were not found. Each disease entity may have its own distinctive findings on neuroimaging, however, a precise diagnosis is not possible for Dandy-Walker variant and a persistent Blake's cyst, and an auxiliary examination such as angiography is necessary [5]. It is suggested that the glia tissue identified on the histopathological examination of the cyst membrane favors a diagnosis of persistent Blake's cyst [12], but recent evidence shows that glia tissue may be identified in the wall of an arachnoid cyst [13]. Mega cisterna magna is an incidental finding due to a benign developmental anomaly [14]. In most cases, atrophy of the cerebellum is found. Our cases diagnosed as having mega cisterna magna were not suggested to undergo surgery, except for hydrocephalus in one case. Cisterna magna may be enlarged in hydrocephalus patients such as in our case 9, the enlarged cisterna magna cannot be well demarcated due to the persistent Blake's pouch or due to the pressure increase in the ventricle due to hydrocephalus. Further investigation of the progressive enlargement of the pouch from the outlet of the fourth ventricle is necessary. The prognosis of patients with Dandy-Walker cyst after surgery depends on re-establishment of posterior fossa architecture [15] and the presence of other associated major anomalies [16]. For patients with Dandy-Walker cyst, Maria et al. [16] had reduced the cyst size in 3 of 9 patients by cyst-peritoneal shunt with or without ventriculo-peritoneal shunt, and that in 2 of 6 patients by cystectomy or cyst fenestration with or without ventriculo-peritoneal shunt. In another 2 studies cyst-peritoneal shunt reduced the cyst size in most posterior fossa cyst patients [2,4]. Early placement of cyst-peritoneal shunt in the posterior fossa cyst is suggested to improve the neurological development [15,17]. Furthermore, Arai and Sato [5] suggested radical resection of the cyst wall and a cyst-peritoneal shunt to minimize the risk of occlusion of the shunt by the remaining external wall of the cyst. They achieved a reduction of cyst size and re-expansion of the cerebellum in 10 of 17 patients. In our patients postoperative reduction of cyst size was found in 1 patient with Dandy-Walker cyst by cyst-peritoneal shunt and 2 of 5
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patients with arachnoid cyst developed later after repairing the occipital meningocele by cystectomy. In the other patients the failure of re-expansion of the cerebellum is probably due to atrophy of the cerebellum. Further studies on the effect of cyst-peritoneal shunt on the outcome of posterior fossa cyst and the factors affecting re-expansion of cerebellum are necessary. REFERENCES
1. Harwood-Nach DC, Fitz CR. Intracranial cyst. In: HarwoodNach DC, Fitz CR, eds. Neuroradiology in infants and children. St Louis: Mosby, 1976: 965-97. 2. Harsh IV GR, Edwards MSB, Wilson CB. Intracranial arachnoid cyst in children. J Neurosurg 1986; 64: 835-42. 3. Sato K, Shimoji T, Yaguchi K, Sumie H, Kuru Y, Ishii S. Middle fossa arachnoid cyst. Child's Brain 1983; 10: 301-16. 4. Stein CS. Intracranial developmental cysts in children: treatment by cystoperitoneal shunting. Neurosurgery 1981; 8: 647-50. 5. Arai H, Sato K. Posterior fossa cysts: clinical, neuroradiological and surgical features. Child's Nerv Syst 1991; 7: 156-64. 6. Barkovich AJ, Kjos BO, Norman D, Edwards MS. Revised classification of posterior fossa cysts and cystlike malformations based on the results of multiplanar MR imaging. AJNR 1989; 10: 977-88. 7. Mori K, Hayashi T, Handa H. Radiological manifestations of infratentorial retrocerebellar cysts. Neuroradiology 1977; 13: 201-7.
8. Raybaud C. Cystic malformations of the posterior fossa. Abnormalities associated with the development of the roof of the fourth ventricle and adjacent meningeal structures. J Neuroradiol 1982; 9: 103-33, 9. Yokota A, Kajiwara H, Matsuoka S, Matsukado Y, Choux M. Classification of dorsal midline cystic cavities in the posterior fossa (in Japanese). Shoni no Noshinkei (Tokyo) 1987; 12: 167-77. 10. Hanigan WC, Wright R, Wright S. Magnetic resonance imaging of the Dandy-Walker malformation. Pediatr Neurosci 1985-1986; 12: 151-6. 11. Huk WJ. Practical aspects of blood and CSF flow. In: Huk WJ, Gademann G, Friedmann G, eds. Magnetic resonance imaging of central nervous system diseases. Berlin: Springer-Verlag, 1990: 123-32. 12. Gilles FH, Rockett FX. Infantile hydrocephalus: Retrocerebellar 'arachnoidal' cyst. J Pediatr 1971: 79: 436-43. 13. Hirano A, Hirano M. Benign cystic lesions in the central nervous system. Light and electron microscopic observations of cyst walls. Child's Nerv Syst 1988; 4: 325-33. 14. Adam R, Greenberg JO. The mega cisterna magna. J Neurosurg 1978; 48: 190-2. 15. Golden JA, Rorke LB, Bruce DA. Dandy-Walker syndrome and associated anomalies. Pediatr Neurosci 1987; 13: 38-44. 16. Maria BL, Zinreich SJ, Carson BC, Rosenbaum AE, Freeman JM. Dandy-Walker syndrome revisited. Pediatr Neurosci 1987; 13: 45-51. 17. James HE, Kaiser G, Schut L, Bruce DA. Problems of diagnosis and treatment in Dandy-Walker syrrdrome. Child's Brain 1979; 5: 24-30.
Brain & Development 1995; 17:418-24
Original article
Posterior fossa cystic lesions - magnetic resonance imaging manifestations En-Chow Tan *, Takuji Takagi, Kunio Karasawa Department of Neurosurgery, Nagoya City Higashi General Hospital, 1-2-23, Wakamizu, Chikusa-Ku, 464 Nagoya, Japan
Received 26 August 1994; accepted 6 June 1995
Cystic lesions of the posterior fossa remain a controversial subject as to clinical classification and diagnosis, especially for those in combination with other intracranial abnormalities. During the period of November 1985 to J u n e 1991, 16 patients with cystic lesions of the posterior fossa were retrospectively reviewed on neuroradiological evaluation with M R images. The patients were 9 males and 7 females aged from 5 days to 15 years old (medium 1 year old). They were classified into three groups as Dandy-Walker cyst (group A, 5 patients), mega cisterna m a g n a (group B, 4) and retrocerebellar pouch or cyst (group C, 7). Nine patients had associated intracranial anomalies, holoprosencephaly in 3 (group A, 3), dysgenesis of the corpus callosum in 4 (group A, 2; group C, 2), and occipital meningoceles in 2 (group C, 2). Hydrocephalus or ventricular enlargement was found in 10 patients (group A, 5; group B, 2; group C, 3). Surgical treatment (cyst-peritoneal shunt or cystectomy) was performed for 10 patients with clinical symptoms due to cyst. Postoperatively, clinical symptoms due to increased intracranial pressure or hydrocephalus improved in 7 patients. For long-term results, the cyst was reduced in 1 of 5 patients of group A and 2 of 5 of group C. Most of our patients have a poor prognosis because of the associated intracranial anomalies or atrophy of the cerebellum. Muitiplanar M R images may provide sufficient evidence for the diagnosis of posterior fossa cysts, especially in the case of rotation or upward displacement of the cerebellar vermis. However, the cyst membrane, and the communication of fluid between the cyst and the cistern, cannot be demonstrated on MR images. Further investigation on these subjects and indications for surgical intervention is highly necessary. Keywords: Dandy-Walker cyst; Dysgenesis of the cerebellum; Magnetic resonance imaging; Mega cisterna magna; Posterior fossa cyst; Retrocerebellar cyst
1. I N T R O D U C T I O N The posterior fossa cystic lesion is one of the benign lesions in infants and children [1]. Recent surgical results have indicated that the cyst-peritoneal shunt may be a simple and effective method for treating intracranial cysts [2-4]. However, the classification or definitive diagnosis of a posterior fossa cyst is not yet unified because of the complex relationships between the cyst, the tentorium, the hindbrain and the roof of the fourth ventricle. The radiological manifestations on computerized tomography (CT), ventriculography, computerized tomographic cisternography (CTC) or angiography have been used as classifi-
* Corresponding author. Fax: (81) (52) 721o1308. 0387-7604/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0387-7604(95)00067-4
cation criteria for posterior fossa cysts [5-9]. However, it remains controversial to classify the cysts on the basis of the results of neuroradioiogical investigations. It has been indicated that rotation of the lower vermis may give a false impression of vermis dysgenesis on CT scans, which can be minimized by magnetic resonance imaging (MRI) [6]. The present study is an attempt to classify the posterior fossa cystic lesions observed on MRI with a modification of Raybaud's classification [8].
2. CLINICAL MATERIALS AND M E T H O D S Patient population From November 1985 to June 1991, 16 patients with suspected posterior fossa cysts on CT scans or diagnosed as
419
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having posterior fossa cysts in childhood underwent M R I examination. The patients were 9 males and 7 females aged from 5 days to 15 years old (medium 1 year old).
Classification of the cysts The classification used in the present study is a modification of that of Raybaud [8] which is based on the presence of malformation of the cerebellar vermis. Our classification is mainly based on the findings with M R I , and C T C is performed when the classification cannot be defined well by M R I only.
3. R E S U L T S
Clinical information Table 1 is a summary of the clinical findings in the 16 patients. The clinical symptoms were an enlarged head in 5 patients, neonatal distress in 3, decreased agility in 2, acute increased intracranial pressure in 3, mental retardation in 4, and headache in 3. Nine patients had associated intracranial anomalies, holoprosencephaly in 3 (semilobar, 3), dysgenesis of the corpus callosum in 4, and occipital meningocele in 2. Hydrocephalus or ventricular enlargement was found in 10 patients, three of w h o m were diagnosed by ultrasonography or M R I in the prenatal period.
Classification of posterior fossa cyst In Raybaud's original classification, cystic lesions were classified into Dandy-Walker malformation, Dandy-Walker variant and retrocerebellar cyst. In the present study mega
cisterna magna represented another distinctive group. Our patients were divided into three groups: as Dandy-Walker cyst, m e g a cisterna magna and retrocerebellar arachnoid cyst or pouch groups. No patient with a Dandy-Walker variant was found in our series.
Dandy-Walker cyst. Five patients were classified as having Dandy-Walker cyst. In this group, M R I showed cystic dilatation of the fourth ventricle and dysgenesis of the cerebellar vermis. Fig. 1 shows the M R I findings in case 1, in which cystic dilatation of the fourth ventricle with distension of the roof of the fourth ventricle, and dysgenesis of the cerebellar vermis were clearly visualized. However, the tentorium or torcula was not elevated in the presence of the cyst. The other 4 patients had elevation of the tentorium and torcula. The roof of the fourth ventricle could be visualized in cases 1 and 5 ( Figs. 1 and 2, right), and could not be found in severe dysgenesis of cerebellar vermis and hemispheres (cases 2, 3 and 4) (Fig. 2, left). T h r e e patients had holoprosencephaly, semilobar type (cases 2, 3 and 4). M e g a c i s t e r n a m a g n a . This group comprised four patients. The distinction of mega cisterna magna from a retrocerebellar arachnoid cyst in our patients was based on enlargement of the outlet of the fourth ventricle in the cisterna magna (Fig. 3). It would be difficult to differentiate between them if the cisterna magna extended upwards or the arachnoid cyst was enlarged downwards. In such cases, mega cisterna magna was defined as without a mass effect but with marked cerebellar atrophy or with having a communication with the other cisterns on CTC. T h e r e were 2 patients with these findings in this group.
Table 1 Clinical summary of 16 patients with posterior fossa cysts Case
Age
Dandy-Walker cyst 1 5 days 2 6 days 3 9 days
Sex
Symptoms
Hydrocephalus
Associated intracranial anomalies
Operations
Reduction of cyst size
F F M
+ + +
aqueduct atresia holoprosencephaly holoprosencephaly
V-P and C-P shunt V-P shunt V-P and C-P shunt
+ -
+
holoprosencephaly
V-P shunt
-
+
corpus callosum hypoplasia
V-P and C-P shunt
-
4
19 days
M
5
14 years
M
neonatal distress decreased agility neonatal distress enlarged head neonatal distress enlarged head enlarged head
F M M F
decreased agility vomiting enlarged head mental retardation
+ +
cerebral atrophy supratentorial arachnoid cyst -
V-P shunt -
no change no change no change no change
Retrocerebellar arachnoid cyst or pouch 10 1 year F enlarged head 11 2.8 years M mental retardation 12 3 years F mental retardation 13 10 years M headache 14 10 years M headache 15 15 years F headache
+ +
occipital meningocele
V-P shunt and ectomy ectomy ectomy V-P shunt and ectomy
+ pouch
16
+
V-P shunt and ectomy
-
Mega cisterna magna 6 2 months 7 2 months 8 1.8 years 9 4 years
15 years
M
mental retardation
occipital meningocele corpus callusom hypoplasia corpus callosum hypoplasia cerebellar dysgenesis
pouch +
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One patient (case 9) had ventricular enlargement and enlargement of the cisterna magna (Fig. 4). A coronal MR image (Fig. 4, right) showed that the medial aspects of both tonsils were displaced laterally, which may indicate that the pressure in the fourth ventricle may be high enough to displace the tonsilla. Although this patient was classified as having mega cisterna magna, enlargement of the cistema magna due to cystic dilatation of the fourth ventricle as a persistent Blake's cyst could not be defined because it was impossible to identify the tela choroidea on MRI.
Retrocerebeilar arachnoid cyst or pouch. Seven patients
Fig. 1. MRI in case 1, Dandy-Walker cyst. Cystic dilatation of the fourth ventricle and dysgenesis of the cerebellar vermis can be seen, however, the torcula and the tentorium are not elevated. The roof of the fourth ventricle can be visualized.
were classified as having a retrocerebellar arachnoid cyst or pouch. The distinction between an arachnoid cyst and a pouch depended on whether communication exists or not between the cystic fluid and the surrounding CSF spaces [8]. In the present study, an arachnoid cyst is defined as exhibiting compression on the underlying cerebellum, while a pouch is diagnosed as in the midline and exhibiting no compression. Each type may extend supratentorially through the defect in the posterior part of the tentorium. Two patients were diagnosed as pouch (case 11, 13) and the other 5 as cyst. CTC was performed in one of the pouch patients and it showed free communication with the surrounding cisterns. In 3 patients with arachnoid cyst, CTC showed absence of communication between cyst and surrounding cisterns in 1 and delayed clearance of contrast medium in 2. CTC was not performed in 2 patients whose arachnoid cyst developed and enlarged several years after repair of an occipital meningocele. Two patients had dysgenesis of corpus callosum and the
Fig. 2. Left: Case 3, holoprosencephaly (semilobar type) with Dandy-Walker cyst. The torcula and tentorium are elevated, but the roof of the fourth ventricle cannot be seen. Right: Case 5, Dandy-Walker cyst. The torcula and the tentorium are elevated. The roof of the fourth ventricle is visualized (arrows).
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Fig. 3. Left: Retrocerebellar cyst. The cisterna magna is not enlarged. Right: Mega cisterna magna. The cisterna magna is enlarged and extends to the tentorium.
other 2 had dysgenesis of the cerebellum. Five patients had the cyst in the midline portion, 1 exhibited left predominance (case 10) and 1 right predominance (case 12).
The definitive diagnosis of an arachnoid cyst with associated infratentorial anomalies was difficult on M R images only. Fig. 5 shows M R I of case 16, who had hypoplasia of
Fig. 4. MRI in case 9 (mega cisterna magna with hydrocephalus). The fourth ventricle and the cisterna magna are enlarged. On coronal MRI, the medial aspects of the tonsilla are displaced laterally.
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corpus callosum and dysgenesis of the cerebellar vermis and hemispheres. The dorsal parts of the right cerebellar hemispheres were greatly displaced anteriorly by the cystic lesion. It had been assumed to be a Dandy-Walker variant or an
arachnoid cyst on M R I ; however, CTC showed the absence of communication between the cyst and the fourth ventricle or perimedullary cisterns (Fig. 5). Retrocerebellar arachnoid cyst was diagnosed by means of CTC.
Fig. 5. MRI and CTC in case 16 with a retrocerebellar cyst. Upper row: MRI showing severe dysgenesis of vermis (sagittal view, right), and compressed cerebellar hemisphere especially in the right side (transverse view, left). These findings cannot differentiate arachnoid cyst from Dandy-Walker variant. Lower row: CTC showing absence of communication between the cyst and the fourth ventricle (left) or the perimedullary cisterns (right). Arachnoid cyst is diagnosed.
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Surgical treatment and results Because of the association of intracranial anomalies in most of our patients with Dandy-Walker cyst, ventriculo-peritoneal and cyst-peritoneal shunts were performed in 3, and a ventriculo-peritoneal shunt in another 2. In patients diagnosed as having mega cisterna magna, surgical intervention was not performed, except that a ventriculo-peritoneal shunt was performed in 1 patient with acute onset of hydrocephalus. In another patient with associated supratentorial arachnoid cyst the operation was not performed because the cyst was small. In patients diagnosed as having a retrocerebellar arachnoid cyst or pouch, surgical intervention was performed in those with symptoms, disturbance of CSF circulation or progressive enlargement of the cyst with a space-occupying lesion. A ventriculo-peritoneal shunt was performed in 3 patients with hydrocephalus, and 5 received excision of the inner and outer cyst wall. Postoperatively, clinical symptoms due to increased intracranial pressure or hydrocephalus improved in 7 patients. As a long-term result (follow-up period 2.3-5.2 years), the cyst size is reduced in 1 patient of group A (case 1) and in 2 of group C (cases 10 and 15). In group A 1 patient (case 1) had a moderate impairment of development, 1 (case 5) had a moderate impairment of intelligence and the other 3 were bed-ridden and had poor development. In group C 1 patient (case 10) had a normal development and intelligence, 1 (case 15) had a mild impairment of intelligence, 2 (cases 12 and 16) had mental retardation as preoperative state and 1 (case 14) died of drug anaphylaxis. The poor results are mainly caused by the associated supratentorial anomalies.
4. D I S C U S S I O N The pathogenesis of posterior fossa cystic lesions has been extensively reviewed as to the embryological development of the posterior fossa. It is suggested that cystic malformation of the posterior fossa is due to varying degrees of abnormalities in the development of the cerebellar vermis, tela choroidea, retrocerebellar and retromedullary space, and dura mater [8]. Several classifications have been proposed for differentiating posterior fossa cystic lesions based on the structural relationship, developmental anomalies or the communication between the cysts and the surrounding CSF space [5,6,8,9], but none of them has been unified or clearly defines the lesion. Raybaud [8] suggested that hydrocephalus, a raised tentorium or dehiscence in the posterior tentorium is not specific for a diagnosis, but partial agenesis of the vermis can be used to differentiate the rhomboschisis group of abnormalities (Dandy-Walker cyst or Dandy-Walker variant) from isolated expansion of the tela choroidea. In some cases upward displacement of the cerebellar vermis observed on axial CT may be misinterpreted as dysgenesis of the lower vermis [6]. Multiplanar MRI may minimize the misinterpretation, however, clear distinction of radiological manifestations remains impossible [6]. In our 5 patients with Dandy-Walker malformation the dysgenesis of the cerebellar vermis varies and the roof of the fourth ventricle can be identified in only 2 patients. In severe cases the roof of the fourth ventricle is difficult to visualize as
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also found in another report [10]. Three of our patients with Dandy-Walker cyst have associated holoprosencephaly, which is rarely found but more frequent in Dandy-Walker malformation or variant than in retrocerebellar cyst [8]. Dysgenesis of the vermis may also be found in patients with arachnoid cysts [8], and differentiation from DandyWalker variant is difficult by MRI only. In case 16 with arachnoid cyst the roof of the fourth ventricle was not clearly defined. Detection of communication between a cyst and the surrounding cisterns or fourth ventricle remains difficult. It is also difficult to demonstrate the membrane present between a cyst and the fourth ventricle [6]. The cyst fluid intensities of the CSF flow effect on MRI may show whether there is a communication or not between a cyst and the surrounding CSF space [11]. In the present study we did not intend to make a clear distinction by MRI whether the cyst has a communication with the surrounding cisterns or not. In that case CTC was performed and showed the absence of communication between the cyst and the fourth ventricle or the cisterns, and thus an arachnoid cyst was diagnosed. In our limited cases, Dandy-Walker variants, persistent Blake's cyst, or other cystic lesions such as intraventricular cyst or glioependymal cyst were not found. Each disease entity may have its own distinctive findings on neuroimaging, however, a precise diagnosis is not possible for Dandy-Walker variant and a persistent Blake's cyst, and an auxiliary examination such as angiography is necessary [5]. It is suggested that the glia tissue identified on the histopathological examination of the cyst membrane favors a diagnosis of persistent Blake's cyst [12], but recent evidence shows that glia tissue may be identified in the wall of an arachnoid cyst [13]. Mega cisterna magna is an incidental finding due to a benign developmental anomaly [14]. In most cases, atrophy of the cerebellum is found. Our cases diagnosed as having mega cisterna magna were not suggested to undergo surgery, except for hydrocephalus in one case. Cisterna magna may be enlarged in hydrocephalus patients such as in our case 9, the enlarged cisterna magna cannot be well demarcated due to the persistent Blake's pouch or due to the pressure increase in the ventricle due to hydrocephalus. Further investigation of the progressive enlargement of the pouch from the outlet of the fourth ventricle is necessary. The prognosis of patients with Dandy-Walker cyst after surgery depends on re-establishment of posterior fossa architecture [15] and the presence of other associated major anomalies [16]. For patients with Dandy-Walker cyst, Maria et al. [16] had reduced the cyst size in 3 of 9 patients by cyst-peritoneal shunt with or without ventriculo-peritoneal shunt, and that in 2 of 6 patients by cystectomy or cyst fenestration with or without ventriculo-peritoneal shunt. In another 2 studies cyst-peritoneal shunt reduced the cyst size in most posterior fossa cyst patients [2,4]. Early placement of cyst-peritoneal shunt in the posterior fossa cyst is suggested to improve the neurological development [15,17]. Furthermore, Arai and Sato [5] suggested radical resection of the cyst wall and a cyst-peritoneal shunt to minimize the risk of occlusion of the shunt by the remaining external wall of the cyst. They achieved a reduction of cyst size and re-expansion of the cerebellum in 10 of 17 patients. In our patients postoperative reduction of cyst size was found in 1 patient with Dandy-Walker cyst by cyst-peritoneal shunt and 2 of 5
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patients with arachnoid cyst developed later after repairing the occipital meningocele by cystectomy. In the other patients the failure of re-expansion of the cerebellum is probably due to atrophy of the cerebellum. Further studies on the effect of cyst-peritoneal shunt on the outcome of posterior fossa cyst and the factors affecting re-expansion of cerebellum are necessary. REFERENCES
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