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1991-01-08-17 Supratentorial arachnoid cyst © Kwon www.thefetus.net/
Supratentorial arachnoid cyst

Tae-Hee Kwon, MD*, Philippe Jeanty, MD, PhD

Synonyms: None.

Prevalence: 1% of intracranial masses in newborns.

Definition: Accumulation of fluid, between two membranes, which may communicate with the subarachnoid space.

Etiology: Abnormality of arachnoid development. May be post‑infarction or post‑traumatic in children.

Pathogenesis: Either subarachnoid (between the arachnoid and the pia) or intra‑arachnoid between the inner and outer layer of the arachnoid.
Associated anomalies: Hydro­ce­phalus.
Differential diagnosis: see Table 2.
Prognosis: Good.
Recurrence risk: Not increased.
Management: Standard obstetrical care.

MESH Brain‑Diseases‑diagnosis; ‑phy­sio­pathology; Cysts‑diagno­sis ‑physio­pa­tho­logy; Arachnoid‑; Sub­arachnoid‑Space BDE 3002 ICD9 742.9‑348 CDC 742.420

* Address correspondence to Tae‑Hee Kwon, MD, Visiting Fellow, Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, 21st and Garland Ave, Nashville, TN 37232‑2675. Ph 615‑322‑0999 Fax 615‑322‑3764


Ultrasound can be utilized to de­monstrate a variety of cystic brain lesions in the fetus1. We present a new case of prenatal diagnosis of isolated supratentorial intra­cra­nial arachnoid cyst, with no evi­dence of hydrocephalus. Only a few cases of this anomaly have been reported in the prenatal lite­rature2‑6, 23, 26, 29.

Case report

A 26‑year‑old white woman, gravida 2, para 2, alive 1, was referred for ultrasound scan for elevated alpha‑fetoprotein. A single fetus was observed at 30 weeks of gestation as dated by BPD, femur and humerus length. A round cystic structure was observed in the region of the right quadrigeminal plate of the ambient cistern, measuring 2 x 3 x 2 cm in diameter (fig. 1).



Fig. 1: Top left: A round cystic structure is seen in the right quadrigerminal plate of the ambient cistern. It measures 2x3x2cm. The mass appears to compress laterally the occipital horn of the lateral ventricle. Top right: Absence of Doppler signal. Bottom left: Longitudinal view of the cyst. Bottom right: Minimal increase in size on repeat examination.

The mass appeared to compress laterally the occipital horn of the lateral ven­tricle but did not produce hydro­cephalus. The other intracranial structures were consi­dered normal. A repeat sono­gram was done 4 weeks later. The cyst had minimally enlarged.

The patient went into sponta­neous labor at 36 weeks and de­livered vaginally a female infant weighing 3,145g. The head circum­ference and neurologic exam­ination were within normal limits. An MRI of the infant"s head was obtained on the day of admission. The signal intensity of the mass corresponded to cerebro­spinal fluid (CSF) on all pulse sequences (fig. 2).


Fig. 2: An MRI of the infant"s head. The signal intensity of the mass corresponds to cerebral spinal fluid on all pulse sequences.

The infant was well, and an elective cystoperitoneal shunting was performed on day 15.


Intracranial arachnoid cyst exists between the brain subst­ance and dura and contains clear CSF‑like fluid. Its precise location between pia and arachnoid, with­in layers of the arachnoid, or between ara­chnoid and dura is often not easily discernible at operation or autopsy7.

An arachnoid cyst is a collection of fluid that may exist separately as a loculated accumulation between two membranes or may com­municate with the subarachnoid space. They represent 1% of all intra­cranial masses8. Their incidence in neonates has been re­ported as rare9. Intracranial ara­chnoid cysts may be primary (congenital) or secondary (acqui­red).

Congenital types are believed to be formed by maldevelopment of the lepto­meninges and do not freely communicate with subara­chnoid spa­ce.

Acquired types are formed as the result of hemorrhage, trauma, and infection and often communicate with subara­chnoid space7, 10. Arachnoid cysts have the potential to grow as the result of some communication with the subarachnoid space. The accumulation of fluid is believed to result from a ball valve mecha­nism11. Furthermore, a choroid plexus‑like tissue within the cyst wall, which secretes CSF and thus contributes to a progressive distension of the lesion, has been reported by several investi­ga­tors12, 13. It contains clear cerebro­spinal fluid and has been diagnosed prenatally by ultra­sound14.

In our review of the literature, we have identified eight other cases of perinatal intracranial arachnoid cyst. With the addition of our case, we have attempted to identify the common features of these nine cases (Table 1).

Table 1: Location of arachnoid cysts in fetuses and infants.


Presenting symptoms


Location/Size (cm)

Sonographic features


Outcome & symptoms

Meizer et al5


22, 38

Rt. Supra-tentorial
1st exam: 3x3,
2nd exam: 4x5

Displacement of choroid plexus of lateral ventricle, displacement of midline, no hydrocephalus


NSVD, no surgery, no symptoms

Diakoumaris et al2


32, 35

1st exam: 3.5,
2nd exam: 4.3

Distorted and displaced 3rd ventricle, developed hydrocephalus


Induced vaginal delivery, CP shunt, seizure at 3rd day

Sanjoh et al4


29, 31

Rt. Supra-tentorial 1st exam: 2x2,
2nd exam: enlarged

Displaced right lateral ventricle, no hydrocephalus


NSVD, convulsion at birth

Roach et al6


14 weeks postpartum

Posterior fossa NA

Markedly dilated lateral and 3rd ventricle, hydrocephalus


NSVD, CVP shunt, macrocephaly

Sauerbrei et al23



Left cerebral hemisphere, NA

Displaced the left lateral ventricle, no hydrocephalus


Cesarean section for fetal distress. Tetralogy of Fallot

McGahan et al26



Temporo-occipital, NA

Compression of the surrounding brain with normal size ventricles



Sandler et al29

small for date


Middle fossa, NA

Mild enlargement of both lateral ventricles


NSVD, CP shunt

Kwon and Jeanty

elevated alpha-fetoprotein

30, 34

Right. supratentorial 1st  exam: 3x3, 2nd  exam: mildly enlarged

Lateral displacement of the lateral ventricle, no hydrocephalus


NSVD,CP shunt, No symptoms

(1) Gestational age at detection and follow-up (in weeks); NA: not measured; CVP shunt = cystoventriculoperitoneal shunt; CP shunt = cystoperitoneal shunt; NSVD = normal spontaneous vaginal delivery. 

Arachnoid cysts were diagnosed in fetuses during routine examination, except for one each in fetuses scan­ned for macrocephaly, polyhydramnios, small for date and eleva­ted alpha‑feto­protein. These cysts demon­strated a slow growth in four cases 2, 4‑5, our case. The fetal age at the time of presentation was close to 30 weeks, with the exception of one case at 22 weeks5 and another case 14 weeks after birth. Hydrocephalus was present in three of nine patients2, 6, 29. The supratentorial portion was the most common site of arachnoid cyst in the fetus4‑5, 23, 26, our case. In one case each they were located in the suprasellar region2, the posterior fossa6 and in the middle fossa29. In contrast, in children7 the most common location is the posterior fossa (42%), the middle fossa (22%), over the cerebral convexity (14%), the suprasellar region (12%) and posterior to the third ventricle (10%) (fig. 3). Well‑defined anechoic cysts dis­placing the adjacent ventricle were noted on all cases by ultra­sound. Cystoventriculo­pe­ri­toneal shunt and cysto­pe­ri­to­ne­al shunt were performed in four cases2, 6, 29, our case.


Ultrasound examination of arachnoid cysts demonstrates a well‑defined anechoic lesion with adjacent mass effect. The primary manifestation of an arachnoid cyst is a localized fluid collection occasionally causing hydro­ce­phalus. The cyst can obstruct the foramen of Monro, displace the aque­duct posteriorly, and block the basal cisterns. Hydrocephalus and macrocephaly are the most common presentations in the neo­natal period15. Later, the “midline syndrome” can develop, consisting of headaches, vomiting, bila­teral papilledema, hyperreflexia and ataxia. Subdural hygromas may develop when rupture of the outer membrane of the cyst occurs and fluid leaks into the subdural compartments15.


The meninges are portions of the dura mater, arachnoid, and pia mater. The arachnoid consists of two layers of fibrous and elastic tissue with low or flattened cuboidal mesothelium. The space between the pia mater and the inner layer of the arachnoid is filled by CSF and called the “subarachnoid space”16. Arachnoid cysts have been found anywhere in the CNS, including the spinal canal. The most frequent locations are the surface of the cerebral hemispheres in the sites of the major fissures (sylvian, rolandic, and interhemispheric), the region of sella turcica, the anterior fossa, and the middle fossa. Less frequently, they are seen in the posterior fossa17.

Differential diagnosis

The differential diagnosis from other cystic lesions (Table 2) may be difficult16.

Table 2: Differential diagnoses of arachnoid cysts

Differential Diagnosis


Porencephaly18, 19

ventriculomegaly, shift in midline, communicates with the ventricles, corresponds to a vascular distribution

Dandy-Walker malformation6,17

anomaly of the cerebella vermis

Large third ventricle with aqueduct stenosis20

oval with tapered edges posteriorly

Dysgenesis of the corpus callosum21

enlarged and high third ventricle with large ventricular atria

Astrocytoma, Anaplastic astrocytoma, Cystic ependymoma, Hemangioma, Cholesteatoma22,23

contrast enhanced in contrast CT scan or MRI, definite diagnosis may not be possible. Anaplastic astrocytoma presents with a multicystic mass.

Cystic CNS lesion in neonatal isoimmune thrombocytopenia24

multicystic mass

Caput succedaneum25

no calvarial defect, occipital mass

Vein of Galen aneurysm26

midline occipital lesion with characteristic Doppler flow

Subdural hygroma, Subdural hematoma as the intial manifestation of hemophilia in a newborn, Subdural empyema27 ,

a straight or flat medial border and a convex border as is expected in an extracerebral fluid

Por­ence­phaly is often associated with ventriculomegaly, communicates with the ventricles and follows a vascular distribution. Brain tumors are usually solid or of mixed echogenicity and are rarely completely cystic23. They are located inside the brain substance, whereas arachnoid cysts lie between the skull and brain surface18, 19.

Posterior fossa arachnoid cysts should be differentiated from Dandy‑Walker malformation. The main criterion in these cases is the integrity of the cerebellar vermis in arachnoid cysts6,17. Suprasellar ara­chnoid cysts are rounded and should be differentiated from a large third ventricle. The dilated third ventricle appears oval with tapered edges posteriorly when aqueductal stenosis is present20. An arachnoid cyst in the midline should be differentiated from dysgenesis of corpus callosum with an associated interhemispheric cyst21. In cases of corpus callosal dysgenesis, the enlarged third ventricle is high in location at the level of the lateral ventricles, and the ventricular atria are prominent20.

Arachnoid cyst deeply situated in relation to the basal cisterns or between the cerebral hemispheres may be difficult to differentiate from other lesions such as cystic astrocytoma or he­mangioma. Other cystic lesions includes ana­plastic astrocytoma, cystic ependymoma23, schizencephaly, cystic CNS lesions in neonatal isoimmu­ne throm­bo­cy­topenia24, caput suc­ce­daneum, septum pellucidum fenestration, and ependymal cyst. Unfortunately, there are few distinguishing factors: anaplastic astrocytoma and cystic CNS lesions in neonatal isoimmune thrombocytopenia are multicystic. An occipital mass without calvarial defect, could also represent caput succedaneum25. A vein of Galen aneurysm, is a midline occipital lesion with characteristic Doppler flow26.

Supratentorial arachnoid cysts must be differentiated from subdural hygroma or subdural hematoma as the initial manifestation of hemophilia in a newborn. These usually have a straight or flat medial border and a convex lateral border, as is expected in an extracerebral effusion, while ara­chnoid cysts have biconcave or semi­circular configuration in ultra­sound27.


Insufficient data are available regarding the prognosis of cases diagnosed either antenatally or in the newborn period. In many cases, arachnoid cysts are asymp­tomatic, but they may cause epi­lepsy, mild motor or sensory abnormalities, or hydro­ce­pha­lus18. Depending on the location and extent of the lesion, these cysts can be resected18 or shunted.

Obstetrical management

When the lesion strongly suggests an arachnoid cyst, the patient should be counselled on the rather benign prognosis of the lesion28. However, if the lesion is discovered before 24 weeks and when a more serious lesion (e.g. porencephaly or intracranial tumors) cannot be excluded, termination of pregnancy should be discussed with the parents because the prognosis is largely unknown. In the third trimester, when hydrocephalus is not pre­sent, there is no reason to modify the mode and time of delivery. In the presence of hydro­cephalus with normal skull dimensions, there is no evidence that a cesa­rean section could improve the outcome, and we believe that a vaginal delivery should be attempted17, 28.


1. Hidalgo H, Bowie J, Rosenberg ER, et al: In utero sonographic diagnosis of fetal cerebral anomalies. AJR 139: 143‑148, 1982.

2. Diakoumakis EE, Weinberg B, Molin J: Prenatal sonographic diagnosis of suprasellar arachnoid cyst. J Ultrasound Med 5:529‑530,1986.

3. Chervenak FA, Berkovitz RL, Romero R, et al: The diagnosis of fetal hydrocephalus. Am J Obstet Gynecol 147:703‑716,1983.

4 Sanjoh N, Oda T, Boku M, et al:A case of prenatal ultrasonographic diagnosis of intracranial arachnoid cyst. Acta Obst Gynae (Japan) 40:255‑258,1988.

5 Meizner I, Barki Y, Tadmor R, et al: In utero ultrasonic detection of fetal arachnoid cyst. JCU 16: 506‑509,1988.

6 Roach ES, Laster DW,Sumner TE, et al: Posterior possa arachnoid cyst demonstrated by ultrasound JCU 10:88‑89,1982.

7 Harwood DC, Fitz CR. Neuroradiology in infants and children. CV Mosby, St. Louis, 1976.

8. Robinson RG: Congenital cysts of the brain: Arachnoid malformations. Prog Neurol Surg 4:133,1971.

9. Russell DS, Rubinstein LJ. Pathology of tumors of the nervous system, 5th ed. Williams & Wilkins, Baltimore, 1989.

10. Oliver LC: Primary arachnoid cysts: Report of two cases. Br Med J 1: 1147‑1150, 1958.

11. Williams B, Guthkelch AN: Why do central arachnoid pouches expand? J Neurol Neurosurg Psychiatry 37: 1085,1974.

12. Lewis AJ: Infantile hydrocephalus caused by arachnoid cyst: Case report. J Neurosurg 19:431‑433, 1962.

 13. Koto A, Horoupian DS, Schulman K: Choroidal epithelial cyst. J Neurosurg 47:955‑958, 1977.

14. McGahan JP, Phillips HE: Ultrasonic evaluation of the size of the trigone of the fetal ventricle. J Ultrasound Med 2: 315, 1983.

15. Weinberg P, Flom R: Intracranial subarachnoid cysts. Radiology 106: 329‑33, 1973.

16. Pilu G, Rizzo N, Orsini LF, et al: Antenatal detection of cerebral anomalies. Ultrasound Med Biol 12: 319, 1986.

17. Romero R, Pilu G, Jeanty P, et al. Prenatal diagnosis of congenital anomalies. Appleton & Lange, Norwalk, Connecticut, 1988.

18. Shaw CM, Alvord EC: Congenital arachnoid cysts and their differential diagnosis. In: Vinken PJ, Bruyn GW(eds): Handbook of clinical neurology. Amsterdam, Elsevier/North Holland Biomedial Press, 1977, Vol 30, pp75‑135.

19. Vinztzileos AM, Hovick TJ, Escoto DT, et al: Congenital midline porencephaly. Am J Perinatol 4: 125‑128,1987.

20. Comstock CH, Culp D, Gonzalez J, et al: Agenesis of the corpus callosum in the fetus: Its evaluation and significance. J Ultrasound Med 4: 613, 1985.

21. Spirit BA, Oliphant M, Gordon LP: Fetal central nervous system abnormalities. Radiol Clin North Am 28: 59‑73,1990.

22. Banna M: Arachnoid cysts on computed tomography. AJR 127: 979‑ 982, 1976.

23. Sauerbrei EE, Cooperberg PL: Cystic tumors of the fetal and neonatal cerebrum: Ultrasound and computed tomographic evaluation. Radiology 147:689‑692,1983.

24. Lester III RB, Sty JR: Prenatal diagnosis of cystic CNS lesions in neonatal isoimmune thrombocytopenia. J Ultrasound Med 6: 479‑481, 1987.

25. Schwimer SR, Lebovic J: In utero sonographic demonstration of caput succedaneum. J Ultrasound Med 5: 711‑712, 1986.

26. McGahan JP, Ellis W, Lindfors KK, et al: Congenital CSF containing intracranial abnormalities: A sonographic classification. JCU 16:531‑544, 1988.

27. Ghidini A, Vergani P, Sirtori M, et al: Prenatal diagnosis of subdural hygroma. J Ultrasound Med 7:463‑465, 1988.

28. Nyberg DA, Pretorius DH: Cerebral malformations. in Ultrasound of fetal anomalies. Nyberg DA, Pretorius DH, Mahony B (eds.) Year Book, Chicago, pp83‑145, 1990.

29. Sandler MA, Madrazo BL, Riga PM et al: Ultrasound case of the day: bilateral arachnoid cyst, diagnosed in utero. Radiographics. 1988 8:358‑61

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