Definition

Arachnoid cysts are rare extraparenchymal congenital malformations. They are benign with a good prognosis if located near the Sylvian fissure and when they appear late in pregnancy.

Arachnoid cysts are usually discovered at the end of the second trimester or during the third trimester. Diagnosis in the first trimester is possible, although exceptional and challenging. We report a case of a fetus with an arachnoid cyst discovered in the first trimester. Our initial diagnosis was encephalocele or acrania.

Case report

We present the case of a 25-year-old patient, G1P0, scanned at 13 weeks GA. Her medical history was noncontributive. On sonography we found a live singleton pregnancy. Although nuchal translucency was normal, the fetus seemed to have acrania and a large cephalocele of 15 x 12 mm (images 1, 2, 3, and 4). The parents were informed of the anomaly and chose interruption of pregnancy. An amniocentesis was performed just before. The karyotype was normal (46, XX), as was the AFP result. Pathophysiologic analysis was performed. The diagnosis was not cephalocele; the entity was an interhemispheric arachnoid cyst distending the cranium.

Images 1, 2. (1) Pregnancy at 13 weeks GA with "cephalocele.†(2) Pregnancy at 13 weeks GA with abnormal skull, with "acrania and cephalocele."

Images 3, 4. 3D views of fetus.

Images 5, 6. Postmortem view of the fetus.

History

In 1984, Barth et al [2] researched world literature and found only four cases of interhemispheric arachnoid cysts described up to that time. In 1996, Hassan et al [8] described two cases of arachnoid cysts in the central fissure, one at 17 weeks GA and the second at 18 weeks GA.  In both cases, an interruption of pregnancy was performed. In the first case, isolated ventriculomegaly was found; in the second, absence of the aqueduct of Sylvius was noted.

Prevalence

Cerebral cysts (including arachnoid cysts) constitute 1% of intracranial masses in children. Two-thirds of these are subtentorial, one-third supratentorial. They are most often isolated and appear sporadically. In 2001, Pierre-Kahn et al [11] reported  a series of 54 fetuses with arachnoid cysts all diagnosed after 20 weeks GA. 55% of these cases were diagnosed between 20 and 30 weeks GA. Most arachnoid cysts are located in the Sylvian fissure. The literature shows that arachnoid cysts of the tentorial fissure or in the interhemispheric fissure are extremely rare.

Etiology

Arachnoid cysts may be primary or acquired; primary arachnoid cysts are a  consequence of abnormal brain development. Causes of acquired arachnoid cysts are:

• Hemorrhage;

• Infections such as cytomegalovirus; or

• Trauma from ischemia or other damage due to the fragility of the arachnoid membrane.

Pathogenesis

Histology of arachnoid cysts has the following distribution:

• 90% are simply pure arachnoid membrane, located in the subarachnoid spaces. They have a peripheral location, usually in the Sylvian fissure. They most often remain latent.

• 10% of arachnoid cysts are neuroepithelial, usually located in the subarachnoid space, but rarely in the ventricles or brain parenchyma. Their location is most often the median fissure, simulating an early hydrocephalus as was the case with our patient. The arachnoid membrane is particularly sensitive to trauma or inflammation, either of which can cause arachnoid cyst formation.

Developmental etiology of arachnoid cysts is not well known. It is likely that the appearance of arachnoid cysts involves compression of the aqueduct of Sylvius resulting in ventriculomegaly which then compromises cerebral parenchyma. Therefore the size, date of appearance, and the location of arachnoid cysts influence diagnosis and prognosis of the affected fetus; first-trimester appearance of an arachnoid cyst usually yields a dire prognosis.

Sonographic findings

The first indication of an arachnoid cyst is often seen at the subtentorial level, where a hypoechogenic area, will be seen, avascular on Doppler interrogation. Arachnoid cysts may deform adjacent cerebral structures, as happened in our case. Their size can compress brain parenchyma, impeding circulation of cerebrospinal fluid in the aqueduct of Sylvius. It can cause ventriculomegaly, resulting in hydrocephalus with extreme dilatation or effacement of the third and fourth ventricles. The posterior fossa remains normal unless an arachnoid cyst is located in the cisterna magna.

Implications for targeted examinations

Arachnoid cysts only rarely appear in the first trimester; theoretically, they are an isolated, sporadic phenomenon. Nevertheless, sonography and amniocentesis are important to rule out other anomalies. Amniocentesis should be performed prophylactially upon discovery of an arachnoid cyst, but fetal karyotype in an isolated case of arachnoid cyst is normal. Interhemispheric arachnoid cysts are frequently associated with partial or total agenesis of the corpus callosum. This diagnosis is not possible in the first trimester, however, as the corpus callosum does not develop fully until 18 weeks GA. Small arachnoid cysts are often asymptomatic; but large cysts display symptoms involving cerebral parenchymal compression and then hydrocephalus, as in our case.

Differential diagnosis

• Acrania;

• Encephalocele;

• Holoprosencephaly;

• Agenesis of the corpus callosum with hydrocephalus;

• Enormous primary ventriculomegaly;

• Vein of Galen aneurysm (usually undiscovered until the second trimester, though Doppler interrogation can assist in early diagnosis);

• Porencephaly (which appears in the third trimester);

• Septum pellucidum cysts (which also appear in the third trimester).

During the first trimester, brain compression by a large arachnoid cyst may lead to hydrocephalus, which may regress, as in the case of Elbers, Furness, et al [6].

Prognosis

Isolated arachnoid cysts appearing in the third trimester have a good prognosis. However, early-appearing arachnoid cysts have a grave prognosis, and interruption of pregnancy is often proposed.

Conclusion

Although early-appearing arachnoid cysts carry a serious prognosis, the choice to interrupt pregnancy should be well considered, since rarely interhemispheric arachnoid cysts may regress as in the case cited above by Elbers [6]. Sonographers and clinicians should add arachnoid cysts to their differential diagnosis of intracranial cystic masses, especially since discovery of arachnoid cysts is increasingly possible at 11 to 14 weeks GA. Education about arachnoid cysts will help parents make an early and informed choice about pregnancy in case of discovery.

References