Children's Hospital Colorado

Arachnoid Cysts

Arachnoid cysts are located in the back part of the brain. The cyst tends to be filled with the same type of liquid that surrounds the spinal cord and brain. 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. The left side of the brain is more commonly affected than the right, though arachnoid cysts in general tend to be rare.

Most cases of arachnoid cysts are detected at greater than 20 weeks of pregnancy. The precise cause for the development of arachnoid cyst is unknown. Most arachnoid cysts do not change significantly during the pregnancy. However, some can put pressure on other brain structures causing extra fluid to accumulate in the brain. The term for this is ventriculomegaly or hydrocephalus. Unless significant hydrocephalus develops, the pregnancy management likely will not change if an arachnoid cyst is discovered.

 

For Patients

There are two types of arachnoid cysts.  One type is the result of abnormal development of a brain structure called the leptomeninges. This type is referred to as congenital. The other type is considered acquired, meaning that it is the result of bleeding, trauma or infection. Other abnormalities in the back of the brain can resemble an arachnoid cyst. A Dandy-Walker malformation and a mega cisterna magna are the two most common abnormalities that can appear in the same area and be confused for an arachnoid cyst. The brain and spine are normally bathed in fluid called cerebrospinal fluid (CSF). Sometimes, an arachnoid cyst can compress the channels or openings that allow the CSF to flow through the brain and spine, similar to a kink in a garden hose. Excess fluid then builds up in the brain resulting in a condition called ventriculomegaly or hydrocephalus. Hydrocephalus literally means water on the brain.

As a parent, learning your baby may have an arachnoid cyst can be frightening. But the Colorado Fetal Care Center (CFCC) is at the forefront of treatment and care for this condition. Our fetal care specialist at the Colorado Fetal Care Center will help your family through this journey to a healthy delivery.

An arachnoid cyst is generally first observed through abnormalities visible during an ultrasound in the second trimester or later. On ultrasound, the cyst usually appears as a clear mass within the brain. The mass will have smooth, thin walls. An important distinction is that the mass does not connect with a brain structures called the ventricles. The ventricles are part of the system that distributes CSF as described above. If the mass is affecting the flow of cerebrospinal fluid, then excess fluid may collect, particularly in the lateral ventricles. The lateral ventricles usually measure less than 10 mm in width. If the lateral ventricles are 10-20 mm in width, the term for these enlarged lateral ventricles is ventriculomegaly. If the lateral ventricles measure greater than 20 mm in width, the term hydrocephalus is used. 

If the arachnoid cyst was caused by bleeding in the fetal brain, it might be more difficult to visualize the arachnoid cyst. Structures towards the back and base of the brain (cerebellar vermis, cerebellar hemispheres, and brainstem) should appear normal except if the arachnoid cyst is compressing these structures. Two-thirds of the arachnoid cysts detected during pregnancy will be located above brain structures called the brainstem and cerebellum.

A large mass might potentially put pressure on the flow of cerebrospinal fluid. This compression can result in enlarged ventricles or hydrocephalus, essentially extra fluid accumulating in the brain as described above. This extra fluid can be seen by ultrasound and measured. Associated hydrocephalus is present in more than 30% of cases. Depending on the amount, this extra fluid might compress surrounding brain tissue. Abnormalities outside of the brain are not commonly seen, though it is possible. Rare cases of arachnoid cyst have been associated with a serious heart abnormality (Tetralogy of Fallot) and a genetic condition called neurofibromatosis type 1, which uniquely affects the skin, bones, eyes and central nervous system (brain and spine).

There are treatment options for an Arachnoid Cyst and the Colorado Fetal Care Center is on the forefront of developing new treatment options.

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 to rule out other abnormalities. Sometimes, a prenatal MRI may be necessary.  If other abnormalities are present, an amniocentesis may be recommended to look for a chromosomal or genetic abnormality. If the arachnoid cyst is the result of a chromosomal abnormality, the most common findings are portions of chromosome 14 or 16 might be damaged, a third chromosome 18 might be present or a complete extra set of chromosomes might exist (a condition known as triploidy). Genetic counseling may also become part of the management.  Ultrasounds every 2-4 weeks might be performed to monitor whether the cyst is become larger and causing ventriculomegaly or hydrocephalus. If hydrocephalus develops or other abnormalities are present, a cesarean section may be recommended.

Newborns who have been diagnosed with an arachnoid cyst during pregnancy 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 an assessment as to whether the skull appears uneven (asymmetric). The newborn should be observed for any changes in head circumference. In newborns in whom an arachnoid cyst is detected after birth, the overwhelming presenting symptom is a larger head circumference. 

Seizures may occasionally occur in infants with arachnoid cyst. Increased pressure in the brain is rarely a presenting clinical sign.

The operative procedures for treatment of arachnoid cysts consist of removing the cyst or allowing it to drain. Small cysts without symptoms do not require any intervention. The most common treatment consists of a cystoperitoneal shunt, which involves placement of a thin tube to drain the cyst into the abdominal cavity. Most neurosurgeons prefer to treat the patient initially with observation, as the avoidance of a shunt is the primary goal in management.

What are Arachnoid Cysts?
Arachnoid cysts are rare central nervous system malformations that represent 1% of neonatal intracranial masses.  They are most commonly diagnosed during pregnancy as a simple, clear area within the fetal head that does not connect with a brain structures called the ventricular system.

Does an Arachnoid Cyst become larger?
Most arachnoid cysts remain stable during pregnancy, but some may cause ventriculomegaly or hydrocephalus by the mass compressing the flow of cerebrospinal fluid as the pregnancy progresses.

How will this diagnosis affect my pregnancy?
Management of pregnancy is generally not altered with an arachnoid cyst unless significant hydrocephalus is present. Careful head imaging in the newborn period is required to confirm the diagnosis and exclude associated abnormalities.

What is the prognosis?
Prognosis depends on the presence of other malformations, whether the cyst was caused by bleeding into the brain, rate of cyst growth, and progression of ventriculomegaly or hydrocephalus.

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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