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

Introduction

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 lite­rature^{2-6, 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.

Discussion

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 space^{7, 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 investi­ga­tors^{12, 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.

(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 patients^{2, 6, 29}. The supratentorial portion was the most common site of arachnoid cyst in the fetus^{4-5, 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 cases^{2, 6, 29, our case}.

Diagnosis

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.

Pathology

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

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 surface^{18, 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 cysts^6,17. 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.

 Prognosis

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 attempted^{17, 28}.

References

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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