Arachnoid cysts

The exact underlying cause of arachnoid cysts is unknown. Primary arachnoid cysts, which are congenital (present at birth), are due to developmental abnormalities of the brain and spinal cord during the early fetal period. Secondary arachnoid cysts, which occur more rarely, are associated with head injury, hemorrhage, meningitis, tumors, or a complication of brain surgery. In some cases, arachnoid cysts are part of a genetic syndrome such as Chudley-McCullough syndrome, mucopolysaccharidosis, or Marfan syndrome, where there are many other associated signs and symptoms, related to the specific syndrome.

Most diagnosed cases of arachnoid cysts are sporadic - occurring in people without a family history of arachnoid cysts. However, familial cases of arachnoid cysts have been reported in the medical literature, suggesting that a genetic predisposition may play a role in some people. For example, arachnoid cysts have been reported in at least three unrelated sets of siblings. In familial cases, inheritance may be autosomal recessive.

Arachnoid cysts can also occur secondary to other disorders which may be inherited such as Marfan syndrome.

Even though most arachnoid cysts are congenital (present at birth), they usually do not cause any symptoms throughout an individual's life. Whether symptoms develop depends upon the size and the specific location of the cyst within the brain. Small cysts usually do not cause symptoms, and most remain constant in size. However, some do increase in size and eventually cause symptoms to appear, especially if they press against a cranial nerve, the brain, or the spinal cord.

If the cyst puts increased pressure on structures of the brain, neurological symptoms may develop. These symptoms can include developmental delays, behavioral changes, an inability to control voluntary movements (ataxia), difficulties with balance and walking and cognitive impairment. Weakness or paralysis on one side of the body (hemiparesis) has also been reported. Symptoms often resolve or improve with treatment but if left untreated, progressive growth of an arachnoid cyst can eventually cause permanent, severe neurological damage.

Factors that reportedly seem to influence the prognosis for a fetus with an arachnoid cyst include the size (and whether it is growing), gestational age at the diagnosis, and the location of the cyst. Small cysts are usually asymptomatic. Interhemispheric cysts (those that extend between the 2 hemispheres) are often associated with complete or partial agenesis of the corpus callosum and carry a risk of causing intellectual disability. Brain compression by a large cyst may lead to hydrocephalus and may cause seizures, headache, and neurological signs after birth. The neurological prognosis often depends on the possibility of surgical treatment.

In a study of 54 fetuses with arachnoid cysts diagnosed in the second and third trimesters of pregnancy, a good prognosis was reported in 88% of the cases in terms of behavior, neurological development, and intelligence at 4 years of age. Several pregnancies were terminated because they were associated with other brain abnormalities. Some reports describe complete resolution of the cysts and that cysts rarely progress after birth.

A fetal MRI can help physicians to see and monitor compression of the aqueduct, communication between the cyst and the ventricles, and corpus callosum abnormalities. Because arachnoid cysts have a somewhat uncertain prognosis, knowing what to do when learning about an arachnoid cyst during pregnancy can be difficult. Some individuals may consider termination of pregnancy due to the uncertainty of the prognosis.

Individuals who have learned that an arachnoid cyst has been diagnosed during the pregnancy should consider meeting with a genetic counselor. A genetic counselor can interpret information about the condition, review available options, and provide supportive counseling to individuals and their family members.

Whether to treat all arachnoid cysts, as well as the method of treatment, has been a subject of controversy in the medical literature. The need for treatment depends mostly upon the location and size of the cyst, as well as whether symptoms are present. Some believe that treatment should be reserved for individuals with symptoms, while others believe that even asymptomatic cysts should be treated to avoid future complications. In the past, doctors have placed shunts in the cyst in order to drain the fluid. However, now that specialized techniques and tools allow for minimally invasive surgery, more doctors are now opting to surgically remove the membranes of the cyst or open the cyst so its fluid can drain into the cerebrospinal fluid and be absorbed.