- Email: [email protected]
Upward herniation of the posterior fossa cyst in the shunted child
Surg Neurol 1987;28:215-20
215
Upward Herniation of the Posterior Fossa Cyst in the Shunted Child A r t h u r N . W a n g , M . D . , a n d B e n j a m i n S. C a r s o n , M . D . Department of Neurosurgery, The Johns Hopkins Hospital, Baltimore, Maryland
Wang AN, Carson BS. Upward herniation of the posterior fossa cyst in the shunted child. Surg Neurol 1987;28:215-20.
Seventy-two years after Dandy and Blackfan's initial description of the Dandy-Walker syndrome, controversies still exist regarding the precise methods of treatment for this disorder. One potential complication of simple lateral ventricular shunting is upward herniation of the posterior fossa cyst, which at times can result in precipitous clinical deterioration. We present three examples of posterior fossa cysts with upward herniation and review the pertinent literature. We advocate contrast studies to determine whether aqueductal communication exists, before shunting is performed, as well as close follow-up of the shunted child. KEYWORDS: Dandy-Walker syndrome; Posterior fossa cyst; Herniation; Shunt; Hydrocephalus
The Dandy-Walker syndrome consists of an abnormally developed roof of the fourth ventricle, cystic dilation of the fourth ventricle, hypoplasia of the vermis and medial cerebellar hemispheres, and elevation of the tentorium cerebeUi and torcular. Dandy and Blackfan [3] described the entity of hydrocephalus, posterior fossa cyst, and hypoplasia of the cerebellar vermis in 1914. Dandy [2] and Taggart and Walker [13] concluded that the condition was due to atresia of the foramina of Magendie and Luschka. Controversy still exists as to its true pathogenesis. Over 300 cases of the Dandy-Walker syndrome have been reported [8]. Specific treatment of posterior fossa cysts remains controversial. Whether to shunt the lateral ventricle, the posterior fossa cyst, or both is debatable. We feel that one factor that should influence this decision is the potential for upward herniation of the posterior fossa cyst which can occur with simple lateral ventricular shunting. We present three
Address reprint requests to: Arthur N. Wang, M.D., Department of Neurosurgery, Johns Hopkins Hospital, 600 North Wolfe Street, Meyer 7-109, Baltimore, Maryland 21205.
© 1987 by Elsevier Science Publishing Co., Inc.
such cases to emphasize the importance of this phenomenon. Case R e p o r t s
Case 1 A 7-year-old white boy presented with a 1-year history of intermittent episodes of bifrontal headache, nausea, vomiting, and lethargy, which were becoming more frequent. He was diagnosed prenatally as having hydrocephalus and was delivered by cesarean section with a head size of 55 cm at birth. A lateral ventriculoperitoneal (VP) shunt was placed 4 days after birth at another hospital. Preoperative computed tomography (CT) scan showed marked hydrocephalus as well as a posterior fossa cystic mass with supratentorial extension (Figure 1). He subsequently had eight shunt revisions over the years, all involving the lateral VP shunt. He was transferred to us for evaluation of possible recurrent shunt malfunction with the above symptoms. On examination, his markedly dolichocephalic head measured 59 cm in circumference. His neurologic examination was notable for poor attention, 1-year developmental delay, lower extremity hyperreflexia, and bilateral upgoing toes. Aspiration of the right parietal VP shunt bulb yielded clear cerebrospinal fluid (CSF) with excellent dynamics and normal pressures at both ventricular and peritoneal ends. CT scan and magnetic resonance imaging (MRI) (Figure 2A-C) showed the lateral ventricles to be draped over a large cystic mass which seemed to correspond to the posterior fossa cystic mass seen 7 years before. However, it was now much larger and had herniated upward nearly to the cranial vertex. Metrizamide (2 mL of iodine, 170 mg/mL) was injected into the lateral ventricular shunt (while compressing the peritoneal end) to outline the ventricular system (Figure 3). This study showed the massive posterior fossa cystic structure extending supratentorially and compressing the lateral ventricles in a superolateral direction with splaying apart of the occipital horns. The ventricles were not in communication with the large cystic structure. A left parietooccipital cystoperitoneal (CP) shunt was therefore placed 0090 5019/87/$3.50
216
Surg Neurol 1987;28:215-20
Figure 1. Case I: Initial C T scan in the first week of life before shunting. Hydrocephalus and a posterior fossa cystic lesion extending supratentorially are seen.
Wang and Carson
B
without complications. A postoperative CT scan (Figure 4) showed that the cyst decreased in size and the lateral ventricles were less compressed. Because of the presence of septa within the cyst, we wanted to determine whether there were any loculations within the cyst. Metrizamide injected into the CP shunt (Figure 4) circu-
A
Figure 2. (A-C)Magnetic resonance images at 7 years of age. The cystic mass has enlarged and herniated upwards further after lateral ventricular shunting.
Upward Herniation of Posterior Fossa Cyst
Surg Neurol 1987;28:215-20
217
Figure 5. Case 2." C T sagittal reconstruction before CP shunting. A posterior fossa (yst with supratentorial extension fi~ seen. lated throughout the cyst, however. The patient has had no further symptomatic episodes and is back in school.
Case 2 Figure 3. Metrizamide stu@ of lateral ventricle. There is no communication between the cyst and the lateral ventricles.
Figure 4. Metrizamide study of cyst after CP shunt placement. The cyst £ smaller, the lateral ventricles are less compressed, and there is good communh'ation within the ~Tstdespite the presence of septa.
An 8-month-old white boy was transferred from another hospital with possible shunt obstruction, a few days of poor feeding and irritability. H e had undergone a right
Figure 6. Metrizamide study of lateral ventrk~le. The posterior fossa ~yst and lateral ventricles do not communicate.
218
Surg Neurol 1987;28:215-20
Figure 7. Case 3: A large posterior fossa cystic structure with a rim of cerebellar tissue surrounding it is seen.
Wang and Carson
Figure 9. The posterior fossa cyst has enlarged and herniated further supratentorially after placement of the second VP shunt. Metrizamide injected into the VP shunt does not enter the cyst.
VP shunt shortly after birth for congenital hydrocephalus. This shunt was indeed found to be obstructed at the ventricular end and was revised without difficulty. A posterior fossa cyst was noted on the preoperative CT scan and did not change in size before his discharge.
Figure 8. A posterior fossa cyst exists and the asymmetric lateral t,entricles are not in communication. A single VP shunt had been placed,
Figure 10. The posterior fossa cyst has been shunted and the ventricular system assumes a more normal appearance.
Upward Herniation of Posterior Fossa Cyst
H e returned a few days after discharge with a 1-day history of lethargy, irritability, limitation of upward gaze, and poor feeding. The VP shunt functioned well but CT scan demonstrated an enlarged posterior fossa cyst that projected upward through the tentorial hiatus (Figure 5). Metrizamide was injected into the VP shunt and the study showed no communication with the cyst (Figure 6). A posterior fossa CP shunt was then placed with resolution of the patient's symptoms. H e later required an emergency CP shunt revision when it became occluded with membranous septal tissue and he presented in extremis with lethargy, bradycardia, sun-setting, and opisthotonus. After revision, he was alert, interactive, and ate well. H e was discharged without complications.
Case .~ A 26-week gestational premature white newborn boy had a stormy neonatal course with grade 4 intraventricular hemorrhage, bronchopulmonary dysplasia, and Escherichia coli sepsis. At 1 month of age, a right VP shunt was placed for panventricular hydrocephalus associated with spells of apnea and bradycardia. Three months later, a left VP shunt was placed because the left lateral ventricle had enlarged and was not in communication with the right. Also noted was a large posterior fossa cyst bulging up through the tentorium at the quadrigeminal plate (Figures 7 and 8). Shortly after VP shunting, he had spells of listlessness, apnea, and bradycardia. A C T scan showed enlargement of the cyst and it did not communicate with the lateral ventricles (Figure 9). A left CP shunt was inserted after posterior fossa craniectomy. The cyst thereafter reduced in size (Figure 10). H e subsequently required two more lateral VP shunt revisions but has been doing well since then, and at 1 year of age he is doing well.
Discussion Anatomic and radiographic studies have previously shown that posterior fossa cysts may bulge through the incisura into the supratentorial compartment [ 1,10,12,14] or can herniate inferiorly through the foramen magnum into the subarachnoid space of the upper cervical spine [ 10,15 ]. In the study of Raimondi et al [12] four of eight cases of Dandy-Walker syndrome had supratentorial extension of the encysted fourth ventricle with anterosuperior displacement of the cerebellar vermis, ventral displacement of the pons and medulla, lateral recess distention, and stretching of the 9th and 10th cranial nerves. The remaining hypoplastic superior vermis may be displaced superiorly by the cyst and herniate into the quadrigeminal cistern thereby compressing the aqueduct [12]. The aqueduct may also be incorporated into the fourth
Surg Neurol 1987;28:215-20
219
ventricle cyst when the superior vermis is severely hypoplastic [ 15 ]. The clinical presentation of upward herniation of an enlarging posterior fossa cyst can vary as our three cases demonstrate. The signs are those of brainstem compression or of shunt malfunction. They include stupor, cardiopulmonary instability, nystagmus, lower cranial nerve problems, slurred or explosive speech, ataxia, dysmetria, headache, vomiting, and feeding difficulty. On the other hand, the cyst enlargement may go unobserved, and it may be first noted only on routine follow-up CT scan. The time frame for clinical deterioration also varies; the most acute course seems to occur in those cases in which a functioning CP shunt fails. Although tapping a shunt can provide important diagnostic information, it can be misleading in a patient with a Dandy-Walker malformation and a lateral ventricular shunt. If aspiration of fluid from the lateral ventricle is not possible, it should not be assumed that the problem is failure of the lateral ventricular shunt. Rather, the Dandy-Walker cyst may have herniated supratentorially and caused collapse of the lateral ventricles. This possibility can be readily determined by CT with or without metrizamide injection into the lateral ventricular shunt to determine patency. In four of Raimondi's cases, signs of a posterior fossa mass became obvious only after isolated lateral ventricle shunt placement when the cyst and ventricle did not communicate [12]. In two of the cases, lateral ventricular shunting resulted in hypothalamic insufficiency. In the study by Carmel et al [ 1], in one of six patients with Dandy-Walker syndrome who received shunts, placement of a lateral VP shunt resulted in aqueductal obstruction necessitating CP shunt placement. This shunt required revision after subsequent upward herniation of the cyst with development of bradycardia and unresponsiveness. Raimondi et al recommended combined lateral ventricle and cyst shunting, whereas Carmel et al advocated combined shunting only if inadequate communication exists between cyst and lateral ventricle. Carmel et al stressed that children with combined shunts were at particularly high risk if one shunt should fail because of development of a gradient in compartmental pressures resulting in upward or downward herniation of neural structures. Some authors feel that this problem of upward herniation is insignificant. Progressive posterior fossa cyst enlargment or herniation after shunting on pneumoencephalography was absent in Fischer's report of 27 patients with Dandy-Walker syndrome seen between 1947 and 1972 [4]. Also, Udvarhelyi and Epstein [14] noted no severe clinical deterioration with upward herniation of these cysts. Hirsch et al [8] noted that only one of 12 patients with Dandy-Walker malformations
220
Surg Neurol 1987;28:215-20
who underwent lateral ventricular shunting had complications later due to aqueductal stenosis and cyst enlargement. Whereas the overall incidence of this complication may indeed be low, we feel it is an important consideration and strongly recommend determination of whether communication exists between the cyst and lateral ventricle preoperatively. This can easily be done by a lateral ventricular puncture through an open fontanelle and injection of metrizamide or iodohexol. If communication does not exist, combined VP and CP shunting are performed. If communication does exist, only CP shunting is performed to maintain the flow of CSF in the physiologic direction and possibly maintain patency of the aqueduct. Close follow-up is required inasmuch as aqueductal stenosis may develop subsequently. If communication between the posterior fossa cyst and lateral ventricle does exist, it is important not to simply place a lateral VP shunt, for fear of causing aqueductal occlusion. Several cases of "double compartment hydrocephalus" have been described wherein supratentorial hydrocephalus is treated with lateral ventricular shunting; over a period of time this causes an acquired aqueductal stenosis or occlusion with subsequent infratentorial hydrocephalus or a trapped fourth ventricle [5]. The fourth ventricle "balloons" so that the aqueduct is forced upward resulting in a thin membranous veil occlusion between the third and fourth ventricles. In addition, a communicating hydrocephalus can be converted to a noncommunicating one after prolonged lateral ventricular shunting as the dilated aqueduct collapses and becomes forked and stenotic [6,7,9,16]. We advocate shunting of the posterior fossa cyst only, not the lateral ventricle, after determining that communication exists between them. The failure rate of cyst shunts may be lower than for lateral ventricle shunts because the cyst cannot collapse as completely against the catheter as the lateral ventricle can. This is so because of the large posterior fossa and associated cerebellar dysgenesis [ 11 ]. In addition, there is little problem with the choroid plexus obstructing the catheter. Carmel et al [ 1] noted a high complication rate for posterior fossa cyst shunts with the development of subgaleal CSF leaks in four of six cases and an asymptomatic posterior fossa hematoma. The failure rate for cyst shunting is increased if there are septa within the cyst, which can occlude the catheter or cause loculations. Fischer [4] speculated that the absence of upward herniation of Dandy-Walker cysts in his series may be due to the high number of patients who underwent pos-
Wang and Carson
terior fossa exploration and membrane excision. However, the mean rate of failure of surgery with membrane excision, as calculated from a different series, is about 75% [8]. Nevertheless, Udvarhelyi and Epstein [14] advocate it for children over 2 years of age (the development of a functioning subarachnoid space before this age is unlikely). Membrane excision should be reserved as an alternative much less preferable to shunting, and attempted perhaps only if shunting fails in the presence of occluding membranes.
References 1. Carmel P, Antunes JL, Hilal SK, Gold AP. Dandy-Walker syndrome: clinicopathological features. Re-evaluation of modes of treatment. Surg Neurol 1977;8:132-8. 2. Dandy WE. The diagnosis and treatment of hydrocephalus due to occlusions of the foramen of Magendie and Luschka. Surg Gynecol Obstet 1921;32:113-24. 3. Dandy WE, Blackfan KD. Internal hydrocephalus. An experimental, clinical, and pathological study. Am J Dis Child 1914;8: 406-82. 4. Fischer E. Dandy-Walker syndrome: an evaluation of surgical treatment. J Neurosurg 1973;39:615-21. 5. Foltz EL, Defeo DR. Double compartment hydrocephalus: a new clinical entity. Neurosurgery 1980;7:551-9. 6. Foltz EL, Shurtleff DB. Conversion of communicating hydrocephalus to stenosis or occlusion of the aqueduct during ventricular shunt. J Neurosurg 1966;24:520-9. 7. Hawkins JC lIl, Hoffman HJ, Humphreys RP. Isolated fourth ventricle as a complication of ventricular shunting: report of three cases. J Neurosurg 1978;49:910-3. 8. Hirsch JF, Pierre-Kahn A, Renier D, Sainte-Rose C, HoppeHirsch E. The Dandy-Walker malformation. J Neurosurg 1984;61: 515-22. 9. Lourie H, Shende MC, Krawchenko J, Stewart DH Jr. Trapped fourth ventricle: a report of two unusual cases. Neurosurgery 1980;7:279-82. 10. Matson DD. Prenatal obstruction of fourth ventricle. AmJ Roentgenol Radiat Therapy Nucl Med 1956;76:499-506. 11. McLaurin RL. Dandy-Walker syndrome. In: Wilkins RH, Rengachary SS, eds. Neurosurgery. Vol. 3. New York: McGraw-Hill, 1985:2153-6. 12. Raimondi AJ, Samuelson G, Yarzagaray L, Norton T. Atresia of the foramina of Luschka and Magendie: the Dandy-Walker cyst. J Neurosurg 1969;31:202-16. 13. Taggart JK Jr, Walker AE. Congenital atresia of the foramens of Luschka and Magendie. Arch Neurol Psychiatry 1942;48:583-612. 14. Udvarhelyi GB, Epstein MH. The so-called Dandy-Walker syndrome: analysis of twelve operated cases. Child's Brain 1975;1: 158-82. 15. Wolpert SM, Hailer JS, Rabe EF. The value of angiography in the Dandy-Walker syndrome and posterior fossa extra-axial cysts. Am J Roentgenol Radiat Therapy Nucl Med 1970;109:261-72. 16. Zimmerman RA, Bilaniuk LT, Gallo E. CT of the trapped fourth ventricle. AJR 1978;130:503-6.
215
Upward Herniation of the Posterior Fossa Cyst in the Shunted Child A r t h u r N . W a n g , M . D . , a n d B e n j a m i n S. C a r s o n , M . D . Department of Neurosurgery, The Johns Hopkins Hospital, Baltimore, Maryland
Wang AN, Carson BS. Upward herniation of the posterior fossa cyst in the shunted child. Surg Neurol 1987;28:215-20.
Seventy-two years after Dandy and Blackfan's initial description of the Dandy-Walker syndrome, controversies still exist regarding the precise methods of treatment for this disorder. One potential complication of simple lateral ventricular shunting is upward herniation of the posterior fossa cyst, which at times can result in precipitous clinical deterioration. We present three examples of posterior fossa cysts with upward herniation and review the pertinent literature. We advocate contrast studies to determine whether aqueductal communication exists, before shunting is performed, as well as close follow-up of the shunted child. KEYWORDS: Dandy-Walker syndrome; Posterior fossa cyst; Herniation; Shunt; Hydrocephalus
The Dandy-Walker syndrome consists of an abnormally developed roof of the fourth ventricle, cystic dilation of the fourth ventricle, hypoplasia of the vermis and medial cerebellar hemispheres, and elevation of the tentorium cerebeUi and torcular. Dandy and Blackfan [3] described the entity of hydrocephalus, posterior fossa cyst, and hypoplasia of the cerebellar vermis in 1914. Dandy [2] and Taggart and Walker [13] concluded that the condition was due to atresia of the foramina of Magendie and Luschka. Controversy still exists as to its true pathogenesis. Over 300 cases of the Dandy-Walker syndrome have been reported [8]. Specific treatment of posterior fossa cysts remains controversial. Whether to shunt the lateral ventricle, the posterior fossa cyst, or both is debatable. We feel that one factor that should influence this decision is the potential for upward herniation of the posterior fossa cyst which can occur with simple lateral ventricular shunting. We present three
Address reprint requests to: Arthur N. Wang, M.D., Department of Neurosurgery, Johns Hopkins Hospital, 600 North Wolfe Street, Meyer 7-109, Baltimore, Maryland 21205.
© 1987 by Elsevier Science Publishing Co., Inc.
such cases to emphasize the importance of this phenomenon. Case R e p o r t s
Case 1 A 7-year-old white boy presented with a 1-year history of intermittent episodes of bifrontal headache, nausea, vomiting, and lethargy, which were becoming more frequent. He was diagnosed prenatally as having hydrocephalus and was delivered by cesarean section with a head size of 55 cm at birth. A lateral ventriculoperitoneal (VP) shunt was placed 4 days after birth at another hospital. Preoperative computed tomography (CT) scan showed marked hydrocephalus as well as a posterior fossa cystic mass with supratentorial extension (Figure 1). He subsequently had eight shunt revisions over the years, all involving the lateral VP shunt. He was transferred to us for evaluation of possible recurrent shunt malfunction with the above symptoms. On examination, his markedly dolichocephalic head measured 59 cm in circumference. His neurologic examination was notable for poor attention, 1-year developmental delay, lower extremity hyperreflexia, and bilateral upgoing toes. Aspiration of the right parietal VP shunt bulb yielded clear cerebrospinal fluid (CSF) with excellent dynamics and normal pressures at both ventricular and peritoneal ends. CT scan and magnetic resonance imaging (MRI) (Figure 2A-C) showed the lateral ventricles to be draped over a large cystic mass which seemed to correspond to the posterior fossa cystic mass seen 7 years before. However, it was now much larger and had herniated upward nearly to the cranial vertex. Metrizamide (2 mL of iodine, 170 mg/mL) was injected into the lateral ventricular shunt (while compressing the peritoneal end) to outline the ventricular system (Figure 3). This study showed the massive posterior fossa cystic structure extending supratentorially and compressing the lateral ventricles in a superolateral direction with splaying apart of the occipital horns. The ventricles were not in communication with the large cystic structure. A left parietooccipital cystoperitoneal (CP) shunt was therefore placed 0090 5019/87/$3.50
216
Surg Neurol 1987;28:215-20
Figure 1. Case I: Initial C T scan in the first week of life before shunting. Hydrocephalus and a posterior fossa cystic lesion extending supratentorially are seen.
Wang and Carson
B
without complications. A postoperative CT scan (Figure 4) showed that the cyst decreased in size and the lateral ventricles were less compressed. Because of the presence of septa within the cyst, we wanted to determine whether there were any loculations within the cyst. Metrizamide injected into the CP shunt (Figure 4) circu-
A
Figure 2. (A-C)Magnetic resonance images at 7 years of age. The cystic mass has enlarged and herniated upwards further after lateral ventricular shunting.
Upward Herniation of Posterior Fossa Cyst
Surg Neurol 1987;28:215-20
217
Figure 5. Case 2." C T sagittal reconstruction before CP shunting. A posterior fossa (yst with supratentorial extension fi~ seen. lated throughout the cyst, however. The patient has had no further symptomatic episodes and is back in school.
Case 2 Figure 3. Metrizamide stu@ of lateral ventricle. There is no communication between the cyst and the lateral ventricles.
Figure 4. Metrizamide study of cyst after CP shunt placement. The cyst £ smaller, the lateral ventricles are less compressed, and there is good communh'ation within the ~Tstdespite the presence of septa.
An 8-month-old white boy was transferred from another hospital with possible shunt obstruction, a few days of poor feeding and irritability. H e had undergone a right
Figure 6. Metrizamide study of lateral ventrk~le. The posterior fossa ~yst and lateral ventricles do not communicate.
218
Surg Neurol 1987;28:215-20
Figure 7. Case 3: A large posterior fossa cystic structure with a rim of cerebellar tissue surrounding it is seen.
Wang and Carson
Figure 9. The posterior fossa cyst has enlarged and herniated further supratentorially after placement of the second VP shunt. Metrizamide injected into the VP shunt does not enter the cyst.
VP shunt shortly after birth for congenital hydrocephalus. This shunt was indeed found to be obstructed at the ventricular end and was revised without difficulty. A posterior fossa cyst was noted on the preoperative CT scan and did not change in size before his discharge.
Figure 8. A posterior fossa cyst exists and the asymmetric lateral t,entricles are not in communication. A single VP shunt had been placed,
Figure 10. The posterior fossa cyst has been shunted and the ventricular system assumes a more normal appearance.
Upward Herniation of Posterior Fossa Cyst
H e returned a few days after discharge with a 1-day history of lethargy, irritability, limitation of upward gaze, and poor feeding. The VP shunt functioned well but CT scan demonstrated an enlarged posterior fossa cyst that projected upward through the tentorial hiatus (Figure 5). Metrizamide was injected into the VP shunt and the study showed no communication with the cyst (Figure 6). A posterior fossa CP shunt was then placed with resolution of the patient's symptoms. H e later required an emergency CP shunt revision when it became occluded with membranous septal tissue and he presented in extremis with lethargy, bradycardia, sun-setting, and opisthotonus. After revision, he was alert, interactive, and ate well. H e was discharged without complications.
Case .~ A 26-week gestational premature white newborn boy had a stormy neonatal course with grade 4 intraventricular hemorrhage, bronchopulmonary dysplasia, and Escherichia coli sepsis. At 1 month of age, a right VP shunt was placed for panventricular hydrocephalus associated with spells of apnea and bradycardia. Three months later, a left VP shunt was placed because the left lateral ventricle had enlarged and was not in communication with the right. Also noted was a large posterior fossa cyst bulging up through the tentorium at the quadrigeminal plate (Figures 7 and 8). Shortly after VP shunting, he had spells of listlessness, apnea, and bradycardia. A C T scan showed enlargement of the cyst and it did not communicate with the lateral ventricles (Figure 9). A left CP shunt was inserted after posterior fossa craniectomy. The cyst thereafter reduced in size (Figure 10). H e subsequently required two more lateral VP shunt revisions but has been doing well since then, and at 1 year of age he is doing well.
Discussion Anatomic and radiographic studies have previously shown that posterior fossa cysts may bulge through the incisura into the supratentorial compartment [ 1,10,12,14] or can herniate inferiorly through the foramen magnum into the subarachnoid space of the upper cervical spine [ 10,15 ]. In the study of Raimondi et al [12] four of eight cases of Dandy-Walker syndrome had supratentorial extension of the encysted fourth ventricle with anterosuperior displacement of the cerebellar vermis, ventral displacement of the pons and medulla, lateral recess distention, and stretching of the 9th and 10th cranial nerves. The remaining hypoplastic superior vermis may be displaced superiorly by the cyst and herniate into the quadrigeminal cistern thereby compressing the aqueduct [12]. The aqueduct may also be incorporated into the fourth
Surg Neurol 1987;28:215-20
219
ventricle cyst when the superior vermis is severely hypoplastic [ 15 ]. The clinical presentation of upward herniation of an enlarging posterior fossa cyst can vary as our three cases demonstrate. The signs are those of brainstem compression or of shunt malfunction. They include stupor, cardiopulmonary instability, nystagmus, lower cranial nerve problems, slurred or explosive speech, ataxia, dysmetria, headache, vomiting, and feeding difficulty. On the other hand, the cyst enlargement may go unobserved, and it may be first noted only on routine follow-up CT scan. The time frame for clinical deterioration also varies; the most acute course seems to occur in those cases in which a functioning CP shunt fails. Although tapping a shunt can provide important diagnostic information, it can be misleading in a patient with a Dandy-Walker malformation and a lateral ventricular shunt. If aspiration of fluid from the lateral ventricle is not possible, it should not be assumed that the problem is failure of the lateral ventricular shunt. Rather, the Dandy-Walker cyst may have herniated supratentorially and caused collapse of the lateral ventricles. This possibility can be readily determined by CT with or without metrizamide injection into the lateral ventricular shunt to determine patency. In four of Raimondi's cases, signs of a posterior fossa mass became obvious only after isolated lateral ventricle shunt placement when the cyst and ventricle did not communicate [12]. In two of the cases, lateral ventricular shunting resulted in hypothalamic insufficiency. In the study by Carmel et al [ 1], in one of six patients with Dandy-Walker syndrome who received shunts, placement of a lateral VP shunt resulted in aqueductal obstruction necessitating CP shunt placement. This shunt required revision after subsequent upward herniation of the cyst with development of bradycardia and unresponsiveness. Raimondi et al recommended combined lateral ventricle and cyst shunting, whereas Carmel et al advocated combined shunting only if inadequate communication exists between cyst and lateral ventricle. Carmel et al stressed that children with combined shunts were at particularly high risk if one shunt should fail because of development of a gradient in compartmental pressures resulting in upward or downward herniation of neural structures. Some authors feel that this problem of upward herniation is insignificant. Progressive posterior fossa cyst enlargment or herniation after shunting on pneumoencephalography was absent in Fischer's report of 27 patients with Dandy-Walker syndrome seen between 1947 and 1972 [4]. Also, Udvarhelyi and Epstein [14] noted no severe clinical deterioration with upward herniation of these cysts. Hirsch et al [8] noted that only one of 12 patients with Dandy-Walker malformations
220
Surg Neurol 1987;28:215-20
who underwent lateral ventricular shunting had complications later due to aqueductal stenosis and cyst enlargement. Whereas the overall incidence of this complication may indeed be low, we feel it is an important consideration and strongly recommend determination of whether communication exists between the cyst and lateral ventricle preoperatively. This can easily be done by a lateral ventricular puncture through an open fontanelle and injection of metrizamide or iodohexol. If communication does not exist, combined VP and CP shunting are performed. If communication does exist, only CP shunting is performed to maintain the flow of CSF in the physiologic direction and possibly maintain patency of the aqueduct. Close follow-up is required inasmuch as aqueductal stenosis may develop subsequently. If communication between the posterior fossa cyst and lateral ventricle does exist, it is important not to simply place a lateral VP shunt, for fear of causing aqueductal occlusion. Several cases of "double compartment hydrocephalus" have been described wherein supratentorial hydrocephalus is treated with lateral ventricular shunting; over a period of time this causes an acquired aqueductal stenosis or occlusion with subsequent infratentorial hydrocephalus or a trapped fourth ventricle [5]. The fourth ventricle "balloons" so that the aqueduct is forced upward resulting in a thin membranous veil occlusion between the third and fourth ventricles. In addition, a communicating hydrocephalus can be converted to a noncommunicating one after prolonged lateral ventricular shunting as the dilated aqueduct collapses and becomes forked and stenotic [6,7,9,16]. We advocate shunting of the posterior fossa cyst only, not the lateral ventricle, after determining that communication exists between them. The failure rate of cyst shunts may be lower than for lateral ventricle shunts because the cyst cannot collapse as completely against the catheter as the lateral ventricle can. This is so because of the large posterior fossa and associated cerebellar dysgenesis [ 11 ]. In addition, there is little problem with the choroid plexus obstructing the catheter. Carmel et al [ 1] noted a high complication rate for posterior fossa cyst shunts with the development of subgaleal CSF leaks in four of six cases and an asymptomatic posterior fossa hematoma. The failure rate for cyst shunting is increased if there are septa within the cyst, which can occlude the catheter or cause loculations. Fischer [4] speculated that the absence of upward herniation of Dandy-Walker cysts in his series may be due to the high number of patients who underwent pos-
Wang and Carson
terior fossa exploration and membrane excision. However, the mean rate of failure of surgery with membrane excision, as calculated from a different series, is about 75% [8]. Nevertheless, Udvarhelyi and Epstein [14] advocate it for children over 2 years of age (the development of a functioning subarachnoid space before this age is unlikely). Membrane excision should be reserved as an alternative much less preferable to shunting, and attempted perhaps only if shunting fails in the presence of occluding membranes.
References 1. Carmel P, Antunes JL, Hilal SK, Gold AP. Dandy-Walker syndrome: clinicopathological features. Re-evaluation of modes of treatment. Surg Neurol 1977;8:132-8. 2. Dandy WE. The diagnosis and treatment of hydrocephalus due to occlusions of the foramen of Magendie and Luschka. Surg Gynecol Obstet 1921;32:113-24. 3. Dandy WE, Blackfan KD. Internal hydrocephalus. An experimental, clinical, and pathological study. Am J Dis Child 1914;8: 406-82. 4. Fischer E. Dandy-Walker syndrome: an evaluation of surgical treatment. J Neurosurg 1973;39:615-21. 5. Foltz EL, Defeo DR. Double compartment hydrocephalus: a new clinical entity. Neurosurgery 1980;7:551-9. 6. Foltz EL, Shurtleff DB. Conversion of communicating hydrocephalus to stenosis or occlusion of the aqueduct during ventricular shunt. J Neurosurg 1966;24:520-9. 7. Hawkins JC lIl, Hoffman HJ, Humphreys RP. Isolated fourth ventricle as a complication of ventricular shunting: report of three cases. J Neurosurg 1978;49:910-3. 8. Hirsch JF, Pierre-Kahn A, Renier D, Sainte-Rose C, HoppeHirsch E. The Dandy-Walker malformation. J Neurosurg 1984;61: 515-22. 9. Lourie H, Shende MC, Krawchenko J, Stewart DH Jr. Trapped fourth ventricle: a report of two unusual cases. Neurosurgery 1980;7:279-82. 10. Matson DD. Prenatal obstruction of fourth ventricle. AmJ Roentgenol Radiat Therapy Nucl Med 1956;76:499-506. 11. McLaurin RL. Dandy-Walker syndrome. In: Wilkins RH, Rengachary SS, eds. Neurosurgery. Vol. 3. New York: McGraw-Hill, 1985:2153-6. 12. Raimondi AJ, Samuelson G, Yarzagaray L, Norton T. Atresia of the foramina of Luschka and Magendie: the Dandy-Walker cyst. J Neurosurg 1969;31:202-16. 13. Taggart JK Jr, Walker AE. Congenital atresia of the foramens of Luschka and Magendie. Arch Neurol Psychiatry 1942;48:583-612. 14. Udvarhelyi GB, Epstein MH. The so-called Dandy-Walker syndrome: analysis of twelve operated cases. Child's Brain 1975;1: 158-82. 15. Wolpert SM, Hailer JS, Rabe EF. The value of angiography in the Dandy-Walker syndrome and posterior fossa extra-axial cysts. Am J Roentgenol Radiat Therapy Nucl Med 1970;109:261-72. 16. Zimmerman RA, Bilaniuk LT, Gallo E. CT of the trapped fourth ventricle. AJR 1978;130:503-6.