Children's Hospital Colorado

Arachnoid Cysts

Arachnoid cysts are found in the back of a baby's brain during ultrasound later in pregnancy (usually after 20 weeks). These cysts are filled with the same type of liquid that surrounds the spinal cord and brain and can affect cerebrospinal fluid levels in the baby's nervous system.

As a parent, learning your baby may have an arachnoid cyst raises a lot of questions. At the Colorado Fetal Care Center, we are at the forefront of treatment and care for babies with this condition. With the help of our team of fetal care specialists, we can diagnose, treat and deliver your baby safely.

For patients

There are two types of arachnoid cysts: congenital and acquired. Congenital arachnoid cysts are the result of abnormal development of a brain structure called leptomeninges. Acquired congenital arachnoid cysts are the result of bleeding, trauma or infection.

An arachnoid cyst is usually a single or simple cyst rather than a collection of multiple cysts. It can be round, oval or crescent moon-shaped. These cysts usually do not grow quickly, remaining stable during pregnancy. Arachnoid cysts tend to account for only 1% of neonatal intracranial masses and mostly affect the left side of the brain. The precise of arachnoid cysts is unknown.

Arachnoid cysts are generally discovered through abnormalities noted during an ultrasound in the second trimester of pregnancy. On ultrasound, the cyst usually appears as a clear mass with smooth, thin walls within the brain.

The brain and spine of a developing baby are normally bathed in fluid called cerebrospinal fluid (CSF). Sometimes, an arachnoid cyst can compress the channels or openings that allow this fluid to flow through the brain and spine, similar to a kink in a garden hose.

Excess fluid then builds up in the baby's brain, resulting in conditions like ventriculomegaly (enlarged ventricles) or hydrocephalus (excess fluid on the brain). Associated hydrocephalus is present in more than 30% of cases of arachnoid cysts but, unless it is significant, the pregnancy likely will not be affected.

Abnormalities outside of the brain are not commonly seen, though they may be possible. Rare cases of arachnoid cysts have been associated with Tetralogy of Fallot, a heart condition, and neurofibromatosis type 1, which uniquely affects the skin, bones, eyes, brain and spine. Seizures may also occur in infants with an arachnoid cyst.

If the cyst was caused by bleeding, trauma or infection in the brain, that brings its own set of complications. That's why diagnosis of an arachnoid cyst and its underlying causes are critical for timely and effective treatment.

How are arachnoid cysts diagnosed?

Most cases of arachnoid cysts are detected after 20 weeks of pregnancy during ultrasound. On ultrasound, an arachnoid cyst may look like a clear, round, oval or crescent moon-shaped mass in the back of the brain. When an arachnoid cyst is detected within the fetal brain, the patient will be given a complete anatomic sonographic fetal diagnosis to rule out other abnormalities.

Sometimes, a prenatal MRI may be necessary, as well. If other abnormalities are present, amniocentesis may be recommended to look for a chromosomal or genetic abnormality. If the arachnoid cyst is the result of a chromosomal abnormality, genetic counseling may also become part of the mother and baby's treatment plan.

Based on the measurement of cerebrospinal fluid in the brain, as well as the size of a baby's ventricles, an arachnoid cyst may be suspected. If the volume of fluid is high, a baby with a cyst may also be diagnosed with hydrocephalus.

There are a number of treatment options for arachnoid cysts offered at the Colorado Fetal Care Center.

Prenatal monitoring

Ultrasounds every 2-4 weeks will be used to monitor the cyst's size and evaluate for risk of ventriculomegaly or hydrocephalus.


If hydrocephalus develops or other abnormalities are present, a cesarean section may be recommended.

Post-birth monitoring

Depending on the location of the cyst, CT or MRI scanning will be used to confirm the presence of the cyst after birth. The newborn should also be observed for any changes in head circumference and symmetry.


After birth, standard treatment for problematic arachnoid cysts consists of removing the cyst or allowing it to drain. The most common treatment consists of a cystoperitoneal shunt, where a thin tube is placed to drain the cyst fluid into the abdominal cavity. Most neurosurgeons prefer to treat the patient initially with observation to prevent the need for a shunt.

For healthcare professionals

Arachnoid cysts represent collections of cerebrospinal fluid enclosed within layers of pia arachnoid that do not communicate with the intracranial ventricular system. Arachnoid cysts are unilocular, round, oval or crescent-like in shape. They initially communicate with the subarachnoid space and have the potential to grow due to this continued communication. Fluid accumulates as a result of a ball-valve mechanism. Choroid plexus-like tissue can be present within the cyst wall. This ectopic tissue secretes cerebrospinal fluid, resulting in progressive distention of the cyst. Two types of arachnoid cysts exist: the congenital type, which is considered to be the result of maldevelopment of the leptomeninges, and the acquired type, which is the result of hemorrhage, trauma or infection. The main differential diagnosis for a posterior fossa arachnoid cyst is between mega cisterna magna and Dandy-Walker malformation. Most arachnoid cysts remain stable antenatally, but some may cause hydrocephalus by their mass effect as the pregnancy progresses. Management of pregnancy is generally not altered with an arachnoid cyst unless significant hydrocephalus is present. Careful head imaging in the neonatal period is required to confirm the diagnosis and exclude associated abnormalities.

Sonographic diagnosis of arachnoid cyst relies on the finding of a sonolucent mass with smooth and thin walls within the brain. This cyst does not communicate with the lateral ventricles but may be associated with hydrocephalus due to a mass effect that obstructs the flow of cerebrospinal fluid.

In this condition, the cerebellar vermis is normal in size. The cyst is usually completely echolucent with the same signal intensity as CSF. Occasionally, hemorrhage into an arachnoid cyst may obscure its simple appearance. These fluid-filled masses must be distinguished on prenatal sonographic evaluation from the fourth ventricle and vallecula. The cerebellar vermis, hemispheres and brainstem are usually normal in this condition except when the cyst compresses these structures. Two-thirds of prenatally detected cases are supratentorial in location whereas one-third are located within the posterior fossa. In contrast, most postnatally detected cases are located within the posterior fossa. Five percent of supratentorial interhemispheric arachnoid cysts are associated with agenesis of the corpus callosum. Only rarely are extracranial abnormalities detected. Sonographic distinction between the retrocerebellar arachnoid cysts and the Dandy-Walker cyst may be difficult. Both malformations may be associated with hydrocephalus.

In arachnoid cysts, the underlying cerebellar hemispheres and vermis are normal, although they are displaced and compressed anteriorly. In contrast, Dandy-Walker cysts are associated with a hypoplastic cerebellum vermis which may be more evident on fetal MRI than ultrasound.

The incidence of arachnoid cysts is unknown. They are rare in prenatal life. Arachnoid cysts represent 1% of all space occupying masses in childhood. Arachnoid cysts are more common in males than in females. The left side of the brain is more commonly affected than the right. Most cases of arachnoid cysts are detected at greater than 20 weeks of gestation. The precise cause for the development of arachnoid cyst is unknown. Arachnoid cysts are thought to be a developmental anomaly of the subarachnoid cistern. Mega cisterna magna can be confused with arachnoid cyst but there is no mass effect on the cerebellar hemispheres nor hydrocephalus in the setting of mega cisterna magna. Arachnoid cysts in the midline that lie wholly posterior to the cerebellum may be difficult to distinguish from the mega cisterna magna. Dandy-Walker malformation can also be mistaken for an arachnoid cyst but it is associated with a well-defined posterior fossa cyst that communicates with the fourth ventricle. Anterior extension of an arachnoid cyst to the cerebellar hemispheres effectively rules out the Dandy-Walker malformation. An additional consideration in the differential diagnosis of arachnoid cyst is a variant of normal in which fluid in Blake's pouch is mistaken for a cyst.

When an arachnoid cyst is detected within the fetal brain, the patient should be referred to a center capable of complete anatomic sonographic fetal diagnosis. If differential diagnosis of a posterior fossa arachnoid cyst from Dandy-Walker malformation is difficult, prenatal MRI may be necessary. Serial sonographic examination should be performed to monitor potential enlargement of the cyst and subsequent development of ventriculomegaly. It is important to seek evidence of associated malformations, as arachnoid cysts may be part of a multiple malformation syndrome. Rare cases of arachnoid cyst have been associated with tetralogy of Fallot and neurofibromatosis type 1. 

Serious consideration should be given to performing amniocentesis to obtain a fetal karyotype, particularly if other anomalies are present. Chromosomal abnormalities described in association with fetal arachnoid cyst include deletion of the distal long arms of chromosome 14 and chromosome16, trisomy 18 and triploidy. Prospective parents of a fetus demonstrated to have an arachnoid cyst should be offered the opportunity to consult with a pediatric neurologist, neurosurgeon and medical geneticist. The presence of an isolated arachnoid cyst is not an indication for cesarean section. However, because of the potential for development of ventriculomegaly, the patient should be followed by serial sonograms to determine if the fetal head size is small enough for a vaginal delivery. An additional consideration in the determination of route of delivery is the risk of trauma and hemorrhage within the cyst.

Newborn babies who have been diagnosed with an arachnoid cyst antenatally should have an evaluation with consultants in genetics and neurology. Depending on the location of the cyst, CT or MRI scanning is indicated to confirm the presence of the cyst. A careful measurement should be made of the newborn’s head circumference and the presence of asymmetry of the calvarium should be noted. The newborn should be observed for any changes in head circumference, as the potential exists for expansion of the cyst by hemorrhage due to trauma at delivery through the rupture of bridging veins. Clinical characteristics at birth will depend on the size of the cyst. Associated hydrocephalus is present in 30% to 100% of cases. In newborns in whom an arachnoid cyst is detected postnatally, the overwhelming presenting symptom is macrocephaly. Seizures may occasionally occur in infants with arachnoid cyst. Increased intracranial pressure is rarely a presenting clinical sign.

The operative procedures for treatment of arachnoid cysts consist of cyst excision, fenestration or drainage into an adjacent cistern, abdominal cavity or atrium. Small asymptomatic cysts do not require any intervention. Surgical excision of the cyst is indicated in the rare cases in which increased intracranial pressure is detected. Craniotomy, however, may result in severe complications. The most common treatment consists of a cystoperitoneal shunt which results in a high rate of regression of the cyst and rarely produces recurrence or complications. These shunts may need revision due to blockage or growth of the patient. Most neurosurgeons prefer to treat the patient initially with expectant management, as the avoidance of a shunt is the primary goal in management. Additionally, endoscopic cyst fenestration may also be used as a less invasive alternative to open surgical management. 

The major complications in the long-term outcome for affected patients include hydrocephalus, seizures and neurologic abnormalities. Most cases of arachnoid cyst appear to be sporadic. Rare cases of arachnoid cyst are associated with neurofibromatosis type 1 or other multiple congenital anomaly disorders due to single gene mutations. If a prenatal karyotype analysis is performed and an unbalanced chromosome abnormality is detected, parental chromosome analysis is strongly suggested. Occasional families have been described with recurrence of arachnoid cysts suggesting the possible existence of heritable factors that predispose individuals to the formation of arachnoid cysts.

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