Posterior fossa arachnoid cysts

B ritish Jour nal of N eurosurger y 1999;13(1):10± 17

ORIG INAL ART ICLE

Posterior fossa arachnoid cysts P. ERD INC Ë LER, M . Y. K AYNAR, H. BOZK US & N. C Ë IPLAK D epartm ent of N eurosurger y, Istanbul University, Istanbul, Turkey

Abstract Arachnoid cysts of the posterior fossa are rare lesions that are considered to be m ostly congenital in origin. In this article, we retrospectively review 12 patients who underwent surgical treatment for their symptomatic posterior fossa arachnoid cysts. The m ost common presenting symptoms were gait disturbances and headache. The diagnosis was established on computed tomography or m agnetic resonance imaging. Surgery consisted of cyst wall excision with fenestration in nine cases and shunting procedures in three cases. In all cases except one who died, the postsurgical follow-up neuroradiological investigations showed that the cysts had decreased in size, the cerebellum had re-expanded, and if there was preoperative hydrocephalus, the ventricular size was decreased. The follow-up period ranged from 1 to 11 years. All surviving cases are free of symptoms and no arachnoid cysts recurred. The classi® cation, pathophysiology, differential diagnosis and surgical treatment of infratentorial arachnoid cysts are discussed and the relevant literature is reviewed. Key words: Arachnoid cysts, classi® cation, differential diagnosis, posterior fossa cysts, surgical treatm ent.

Introduction Intracranial arachnoid cysts are rare central nervous system lesions which account for only 1% of all intracranial m asses. O ne-quarter to one-third of all arach1± 3 noid cysts involve the posterior fossa. Posterior fossa arachnoid cysts produce a range of clinical presen tations. T he clinical sym ptom s result from hydrocephalus and com pression of the underlying structures. D ifferentiation of the posterior fossa arachnoid cysts from other cystic and/or cyst like lesions is essential for proper treatm ent planning. Treatm ent of sym ptom atic cysts consists of cyst wall excision w ith fen estration or cystoperiton eal (C P) shu nting. Through out the literature controversies exist about the classi® cation, pathophysiology, differential diagnosis and treatm ent of arachnoid cysts. In this report, we retrospectively review 12 patients with sym ptom atic posterior fossa arachnoid cysts w ho underwent surgical treatm ent in our department, and discuss the classi® cation, pathophysiology, differential diagnosis and surgical treatm ent of these malform ations.

of CerrahpasË a M edical School. T hese cysts represen ted 13 % o f 93 in tracran ia l arach n o id cy st s diagnosed during the study period. T here were six m ales and six fem ales, whose ages at the tim e of clinical presentation ranged from 2 m onths to 59 years. C linical inform ation was obtained by review of the patient records, operative reports and follow-up data were obtained via records of outpatient clinic visits. All patients had at least one com puted tom ography (CT ) or m agnetic resonance im ag ing (M RI) at 6± 12 m onths postoperatively and were followed-up clinically for 1± 11 years. R esults C linical presentation T he presen ting com plain ts were instability w hen walking and headache in most of the cases. N eurological ® ndings were related to the posterior fossa in m ost of the cases. M acrocephaly was a com m on ® ndings in the infant group. The duration of sym ptom s before diagnosis was usually shor t. Case 11 was in com a at adm ission to the hospital.Table I sum m arizes the clinical pictures.

Patients and M ethods Between Janu ar y 1983 an d D ecem ber 1996, 12 patien ts w ith infratentorial arachnoid cysts, were treated surgically in the N eurosurger y D epartment

N euroradiological ® ndings and locations T he diagnosis of arachnoid cyst was m ade by CT in seven cases, by M RI in two cases, and by CT and

Correspondence to: Pam ir ErdincË ler M .D., PK 19, 34312 , K.M . PasË a, Istanbul, Turkey. Tel: (90)21252 94590 . Fax: (90)2122 732477 . E-m ail: oym akas@ superonline.com . Received for publication 28th October 1997 . Accepted 28th April 1998. 0268-869 7/99/010010 ± 08 $9.5 0 ½

The Neurosurgical Foundation

Posterior fossa arachnoid cysts

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TABLE I. Clinical features, outcome of surgery, and follow-up periods of 12 patients with infratentorial arachnoid cysts No.

Age (year)/sex

Complaints and duration

Neurological ® ndings

Cyst location & hydrocephalus

Surgery

1

26/F

H eadache, nausea, vom iting (1 m onth)

papilloedem a

Retrocerebellar, m ild hydrocephalus

Cyst excision

2

31/F

G ait ataxia, nystagmus

Retrocerebellar

Cyst excision

3

38/F

Instability when walking, vertigo (1 year) H eadache, amnesia (5 years)

Papilloedema

Supracerebellar, m ild hydrocephalus

Cyst excision

4

59/M

G ait ataxia, left V, VIII th nerve palsy

Left CPA

Cyst excision

5

26/M

G ait ataxia, nystagmus, dysmetria


6

53/M

Left VIIIth nerve palsy, gait ataxia, left dysm etria

7

33/F

H eadache, vertigo, instability when walking, hypoacusia in left ear, hypaesthesia in left face (3 m onths) H eadache, instability when walking, m uscle wast’ ing (3 weeks) Instability when walking, nausea, headache (3 m onths) T innitus in right ear (4 years)

8

36/M

9

6 months/M

H eadache, instability when walking (6 m onths) M acrocephaly

10

7 months/M

11

1/F

12

2 months /F

Outcome

Follow-up period

Cyst and ventricular sizes decreased, no complaints Cyst size decreased, no complaints Ventricular size decreased, sm all residual cyst, no complaints Sm all residual cyst at the CPA, no com plaints

6 years

Cyst excision

Cyst and ventricular sizes decreased, no complaints

Lost to followup 3 years after the surgery

Right intracerebellar, m ild hydrocephalus Right CPA

Cyst excision

9 years

G ait ataxia, papilloedem a


Cyst excision

Bulging fontanel

Retrocerebellar, severe hydrocephalus

Y connected VP and CP shunt

M acrocephaly

Bulging fontanel

Supracerebellar, severe hydrocephalus

VP and CP shunt

Cyst and ventricular sizes decreased, no complaints Cyst disappeared, no complaints Cyst and ventricular sizes deceased, no complaints Cyst and ventricular sizes decreased, no complaints Ventricular size decreased, cyst disappeared, no complaints

M acrocephaly, vomiting, drowsiness, convulsions, dysphagia (1 week) M acrocephaly

Impaired consciousness, bulging fontanel, sunset eyes


EVD + Cyst excision

Ð

Died after surgery

Sunset eyes, bulging fontanel


VP and CP shunt

Cyst and ventricular sizes decreased, no complaints

3 years

Hypoacusia in right

Cyst excision

11 years

5 years

7 years

7 years

2 years

1 years

1,5 years

*Abbreviations: EVD: external ventricular drainage; CPA: cerebello-pontine angle.

M R I in three cases. T he location w as re trocerebellar in seven , su pracere bellar in tw o, in the cerebellopontine an gle (C PA) in two an d in tracerebellar in one. Arachn oid cysts appeared on C T as u n calci® ed , low -d en sity, extra-a xial m asse s w ith regu lar bo rd er s th at d id n o t en h an ce w ith th e adm inistration of con trast m ediu m (Fig. 1). T he four th ventricle an d ve rm is were visu alized by axial

C T or M R I in all cases except C ase 10, w here the C T was in itially m isinterpreted as a com m un icating hydrocephalus (Fig. 2A). A C T 3 m onths after insertion o f a ven tricu lo periton eal (VP ) shun t revealed an isolated cyst at the ten torial incisura and bilateral subd ural haem atom as. M RI showed a huge cyst ® lling the posterior fossa (F ig. 2B,C ). T he cyst was d ia gn o sed a s a su p ra cer eb ellar ar ach n o id cy st

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P. ErdincË ler et al. Surgical treatment and ou tcome Surgery consisted of cyst wall excision with fenestration into the fourth ventricle or surrounding subarachnoid cisterns in nine cases and shunting procedures in three cases. In these nine cases, the cyst wall histologically resem bled the arachnoid in structure. A VP shunt was ® rst inserted in case 10 and 3 m onths later a CP shun t was done (Fig. 2D ). In case 9, a Y connected VP and CP shunt was inserted at the sam e tim e because of the non-com m unicating nature of the cyst (Fig. 3B). C ase 11 presented with abrupt neurological deterioration before surgery. An external ventricular drainage (EVD ) was inserted urgently, thereafter the posterior fossa arachn oid cyst was excised. However, the patient died on the fourth postoperative day. A VP shunt was inserted at 2 m onths of age in case 12 as the ® rst treatm ent (Fig. 4a). The child showed norm al psychom otor developm ent until 3 years of age. M RI perform ed at this tim e revealed severe com pression of the cerebellum and the brain stem by the retrocerebellar arachnoid cyst (Fig. 4B). A CP shunt was then inserted. 1 m onth later, m oderate re-expansion of the cerebellum was seen on control M RI (Fig. 4C ). In all surviving cases, control C T or M RI scans showed that the cysts had decreased in size, the cerebellum s re-expanded, and if there was preoperative hydrocephalus, the ventricular size had decreased (Fig. 5). There was no case in the cystectomy group w ho needed VP shunting after treatm ent.T he bilateral subdural haem atom as and the huge supracerebellar arachnoid cyst resolved com pletely after cystoperiton eal shun tin g in case 10 (Fig. 2D ). T he clinical follow -up periods ranged from 1 to 11 years. All cases are free of sym ptom s and no arachnoid cyst recurred (Table I). D iscussion

F IG . 1. (A) Preoperative C T scan with contrast of case 6 shows an intracerebellar arachnoid cyst compressing the 4th ventricle. (B ) Six m onths after cyst wall excision, on postcontrast CT scans, the cyst has almost entirely disappeared and the fourth ventricle is now well visualized.

T he posterior fossa is the second m ost comm on location of arac hnoid cysts after the m idd le cran ial fossa. 2± 4 In paediatric series, the incidence of posterior fossa cysts among intracranial arachnoid cysts is given 3 ,5 between 20 and 30% . O ur 13% incidence probably re¯ ects the adult preponderance in our practice. C linical presentation and pathophysiology

com pressing the fourth ven tricle. In one case, m etrizam ide CT ventriculography w as perform ed in order to iden tify any com m u nication betw een the cyst an d four th ven tricle (Fig. 3A ). In no ne of the cases reviewed d id plain skull radiog raphs, C T or M RI stu d ies su g gest an elevatio n o f th e to rcu la an enlargem en t of the posterior fossa o r localiz ed thin nin g of the occipital bon e. O n M RI, the contents of arachn oid cysts had the sam e signal characteristics as cerebrospin al ¯ uid (C S F ) on both T 1-weighted an d T 2-weighted im ag es. N o peripheral oedem a was seen even in T 2-weighted im ages.

It is not know n how exactly arachnoid cysts grow, but w hen they give rise to sym ptom s, these are usually caused by lo cal com pression of the su rround ing nervous structures and/or obstructive hydro6 ,7 cephalus. In the light of the follow-up data of our case 12, we know that the retrocerebellar arachnoid cyst gradually com pressed the cerebellum and the brain stem during 3 years of follow-up dem onstrating the gradual increase in the size of the cyst despite of a functionin g V P shunt. T hree theories exist for explaining the ® lling of the cysts by C SF: T he ball8 valve m echanism , active secretion of C SF through


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F IG . 2. Case 10. (A) Axial CT scan shows massive dilatation of the third and lateral ventricles. The arachnoid cyst lies at the tentorial incisura. (B) Three months after insertion of a VP shunt, T1-weighted axial M RI scan shows bilateral subdural haematom as, and an enlarged arachnoid cyst at the tentorial incisura. (C) T1-weighted mid-saggital M RI scan shows the supracerebellar arachnoid cyst ® lling the posterior fossa. Note that despite the size of the cyst, the poterior fossa is not enlarged and the torcula is normally positioned. (D) CT scan 1 year after CP shunting, dem onstrates the disappearance of the bilateral subdural hematomas and arachnoid cyst.

the walls of the cysts 9 and ® lling of the cyst by osm otic 10 gradient. The anatom ical com m unication m ay also 2 be interm ittent. Som etim es, a m inor head injury can provoke the 2 onset of sym ptom s. G alassi et al. suggested that the clinical picture is one of long duration with an episodic course in adults. Som e com plaints, such as headache or vertigo can have a long histor y re¯ ecting a chronic increased intracranial pressure, but the duration of signs and sym ptom s before diagnosis, such as gait ataxia or cranial nerve palsy was usually short in our cases. If the cysts are associated with other central nervous system developm ental abnorm alities or if a history of a traum atic delivery or an in¯ am m atory process is presen t in the aetiology, psychom otor 2 ,1 1 retardation is usually seen. We did not detect any in¯ am m atory or traum atic aetiology in the history of our patients, nor any association of any other severe central nervous system developm ental abnorm alities. N one of our patients had psychom otor retardation.

As suggested by Barkovich et al., 7 posterior fossa CSF collections can result from abnorm al developm ent of the cerebellum or fourth ventricle or from form ation of arachnoid cysts arising as a result of anom alous splitting and duplication of the endom eninx during neural tube folding as in true arachnoid 4 ,1 2 ,1 3 cyst at other sites in the brain. Because of the hydrocephalus which is frequently associated with infratentorial arachnoid cysts som e authors have ascribed the pathogenesis either to defec14 tive C SF absorption m echanism s or to a develop1 5 ,1 6 m ental block of the cisterns rostral to the cyst, rather than m erely to m echanical factors. In all of our four paediatric cases m acrocephaly was the alerting sign of a posterior fossa arachnoid cyst, though in the m ajority of adult cases the presenting complaints were related to the posterior fossa. In ® ve adult cases w ith associated m ild hydrocephalus, a decrease in ventricular size followed sim ple partial cyst excision. Thus, it seems that in adult cases m echanical obstruction of

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P. ErdincË ler et al.

F IG . 3. C ase 9. (A) A non-comm unicating retrocerebellar posterior fossa arachnoid cyst is seen on axial CT scans after intraventricular injection of contrast via the bregmatic fontanel. (B) A reduction in cyst and ventricular size is seen after insertion of a Y connected ventriculo-cysto-peritoneal shunt.

the C SF circulation pathways by the cyst m ay be the m ain cause of hydrocephalus. All of the paediatric cases presented with a rapid progression of hydrocephalus. All of them but the one who died following cyst excision, were treated with VP and C P shunting.

Classi® cation M a ny p ro p o sed cla ssi® catio n sy stem s exist fo r 17 posterior fossa cysts. Little et al. grouped infratentorial arachnoid cysts according to their location. A topographic classi® cation was proposed by Vaquero

F IG . 4. Case 12. (A) T1-weighted midsagittal MRI scan at 2 m onths of age shows retrocerebellar arachnoid cyst and marked non-communicating hydrocephalus. (B) 3 years after insertion of a VP shunt, the ventricular size was decreased, but the cerebellum and the brain stem were severely compressed by the enlarged cyst. There was a moderate herniation of the cerebellar tonsils below the foramen magnum (arrow). (C) One month after insertion of a CP shunt, the compression of the cerebellum and the brain stem has disappeared. Note that the posterior fossa is not enlarged and the torcular herophilli is normally positioned.


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developm ental anom alies of the posterior fossa. 7 We are in ag reem ent with them . We think that a true arachnoid cyst has a congenital origin, but is not associated with other developm ental anom alies of the cerebrum and cerebellum . Based on clinical and neuroradiological features, they m ust be classi® ed as a separate entity. 1 1

Differential diagnosis

F IG . 5. C ase 8. (A) Preoperative T1-weighted m id-sagittal M RI scan shows a retrocerebellar arachnoid cyst com pressing the fourth ventricle. There was no elevation of the torcula and no enlargement of the posterior fossa. (B) After cyst wall excision and fenestration into the fourth ventricle, the c yst ha s d ecreased in size. T he co m m u n ication established between the cyst and the fourth ventricle is seen (arrow). 18

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et al. Arai and Sato classi® ed all posterior fossa cysts depending on the location and extension. We included our two cases of supracerebellar cyst in this study, because they were m ainly localized in the posterior fossa w ith extension superiorly and they were approached operatively through the posterior fossa. In recent reports, great im portance is attributed to the form ation of the ¯ oor of the fourth ventricle and 7 ,1 1 7 verm is. Barkovich et al. de® ne posterior fossa arachnoid cysts as all discrete posterior fossa CSF collections that are clearly separate from the fourth ven tricle and vallecu la. T hey categoriz e D an d y± Walker m alform ation, D an dy± Walker variant and m ega-cisterna m agna as a separate continuum of

T he differential diagnosis of posterior fossa arachnoid cysts includes any cystic lesion of the posterior fossa, such as cerebellar cystic astrocytom as, cystic haem angioblastom a, hydatid cysts, abscesses, epidermoid or derm oid tumours.There are also a few reports in the literature in which an epithelial, glioependym al or choroidal cyst was diagnosed histologically. 1 1 ,1 9 ,2 0 T he characteristic appearance of an u ncalci® ed, low-density, extraaxial m ass with regular borders that d o no t en hance w ith ad m inistration o f con trast m edium m ake the differential diagnosis from neoplastic or infectious disorders sim ple in m ost cases. Epiderm oid tum ours are slightly hyperintense on proton density-weighted M RI im ages, encase and engulf arteries and cranial nerves, whereas arachnoid 21 cysts displace adjacent structures. T he m ain problem exists in differentiating arachnoid cysts from the D andy± Walker m alform ation, verm ian -cerebellar hypoplasia and m ega cistern a m agna. 2 2 In our study, the key features for the distinction were visualization of the fourth ventricle and verm is on C T or M RI, demonstration of the torcula and its sinuses norm ally positioned, and a norm alsized posterior fosa. In all cases, the posterior fossa was norm al-sized, the torcula was normally positioned and the fourth ventricle and verm is were seen on CT or M RI except case 10. As previously described this case was ® rst m isdiagn osed as a com m unicating hydrocephalus, although preoperative C T were highly su gg estive o f a po ster io r fo ssa c yst. T h e n o n com m unicating nature of the cyst and the significance of a norm ally positioned torcula were appreciated when M RI was available. The fourth ventricle and brain stem were severely com pressed by the cyst, but the verm is was intact. T he signi® cance of the position of the torcula and the size of the posterior fossa are not yet clearly de® ned in the differential diagnosis of posterior fossa cysts. It is well known that in the D andy± Walker com plex, there is a large posterior fossa with eleva1 tion of the torcula and its sinuses. H owever, there are also m any repor ts w hich d escribe the sam e m orphological characteristics w ith posterior fossa arachnoid cysts. 2 ,1 8 ,2 3 N evertheless, in none of the six cases of posterior fossa arachnoid cyst reported 7 by Barkovich et al., was there an enlarged posterior fossa or an elevated torcula. We think that a norm alsized posterior fossa with a n orm ally positioned torcula are the key features of this differentiation

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P. ErdincË ler et al.

even in infants. O ur four paediatric cases under 1 year of age accorded with this obser vation. Angiographical evidence of arteriovenous dissociation over the surface of the brain is proposed for differentiating these cysts from the D andy± Walker 11 malform ation and a m ega cisterna m agna. However, the developm ent of M RI m ay obviate the need for 24 angiography. T he w id e variatio n in th e ap pearan ce o f the cerebellar verm is seen on sagittal M RI, the expansion and invagination of the arachnoid wall, m arked com pression capacity of the cyst to the surrounding structures can m ake the diagnosis of an arachnoid cyst difficult in som e cases. Two cases reported by 25 Wolpert et al. exem plify the difficulties in achieving a ® nal diagnosis even after surgery. In their cases, the preoperative radiological ® ndings of the posterior fossa anatom ical str uctures and arachn oid cysts were com pletely changed after surger y. T his can be due to the growth capacity of both cerebrum and cerebellum at an early age, but also can re¯ ect potential invagination of arachnoid walls into the subarachnoid spaces and fourth ventricle. We think that distinction between a com m unicating and non-com m unicating cyst is not always necessary. In cases of mega-cisterna m agna, if the posterior fossa is norm ally sized, and if there is no elevation of the tentorium and the torcula, ® lling of the cyst by contrast m akes a clear distinction between an arachnoid cyst and a m ega cisterna m agna. 1 1 ,2 6 We had no cases in this study that needed differentiation from a m ega cisterna magna. In a single case in w hich we perform ed CT ventriculography, we decided to insert a Y con n ected ve n tricu lo -cy sto p er ito n eal sh u n t because of the non-com municating nature of the cyst. If CT cisternography reveals early or late ® lling of the cyst by contrast, it shows a com m unication from the subarachnoid space to the cyst, but does not give any inform ation if there is a com m unication from the cyst to the subarachnoid space.We believe that, except for som e selected cases, knowing the existence of any com m unication between the cyst and the subarachnoid spaces is n ot really helpful and reliable for diagnosis and surgical treatm ent. Surgical treatment The debate w hich exists over the surgical treatm ent of supratentorial arachnoid cysts by shunting procedures or cystectomy exists also for the posterior 3 fossa. We preferred in m ost of our cases excision of the cyst wall alone so as to avoid shunt dependence. In all of the cases where we perform ed cyst excision, successful results were obtained with a single operation and no recurrence was observed. O n the other hand, in our three cases that were treated only by shunting procedures, the results were also successful. O ur m ortality in case 11 was related to the preoperative poor neurological status rather than a com plication of surgery. Our results were satisfactory w ith both surgical treatm ent m odalities.

C onclusions We suggest that, as a rule, if there are no developm ental anom alies associated w ith a posterior fossa arachnoid cyst, and if the sym ptom atology is only of a space occupying lesion in the posterior fossa, and if after surgery, the cyst decreases in size, the cerebellum and verm is re-exp and and relief of sym ptom s is provided then it is questionable whether these cases have a congenital origin, or whether they are really developmental anom alies. Elevation of the torcula and its sinuses, an enlarged posterior fossa and clear developmental anom alies of the rhombencephalic roof are features that suggest a less favourable prognosis w ith surgical treatm ent than for a true arachnoid cyst. M echan ical obstr uction at the level o f fou rth ventricle or aquaduct of Sylvius is the m ajor underlying cause of hydrocephalus in adult patients with posterior fossa arachnoid cysts and this is preventable with the restoration of C SF circulation pathways by cyst excision w ithout the need for shunting. O n the other hand, in paediatric patients, the hydrocephalus is m ore severe and successful results can be obtained with shunting procedures alone.

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22 Kollias SS, Ball WS Jr, Prenger EC. Cystic malformations of the posterior fossa: differential diagnosis clari® ed through embrylogic analysis. Radiographics 1993; 13:1211 ± 31. 23 La Torre E, Fortuna A, Occhipinti E. Angiographic differentiation between Dandy± Walker cyst and arachnoid cyst of the posterior fossa in newborn infants and children. J Neurosurg 1973;38:298± 308. 24 Rock JP, Zimm erman R, Bell OW, Fraser RAR. Arachnoid cysts of the posterior fossa. N eurosurger y 1986; 18:176± 9. 25 Wolpert SM , Scott RM , R unge VM , K wan ESK. Difficulties in diagnosing congenital posterior fossa fluid collections after shunting procedures. A JN R 1987; 8:653± 6. 26 Hough DR, M engel M B, Malik AH. M ega cisterna m agna: diagnosis using metrizam ide computed tomographic cisternography. Neurosurger y 1982;11 ;260± 2.