Arachnoid cysts



1. What is an arachnoid cyst?

Arachnoid cyst is a loculated cavity within the arachnoid membrane. True arachnoid cysts are congenital. However, besides them, secondary arachnoid cysts could develop following intracerebral haemorrhage or reactive leptomeningeal cysts.

The arachnoid membrane is the middle layer that covers the brain and spine. {Brain and spine are covered by three layers: They are:

  • pia mater

  • arachnoid membrane

  • Dura


Arachnoid is a semi-transparent membrane. The arachnoid cyst is formed within the arachnoid layer.


{The outermost layer is dura mater. In the head. it lies just below the skull. It has a consistency of a tough sheath. The middle layer is the arachnoid membrane. It is a thin translucent membrane. The innermost layer is the pia mater. It is tightly adherent to the brain. Cerebrospinal fluid flows between the arachnoid membrane and the pia mater.}

2. How common are arachnoid cysts?


The prevalence of arachnoid cyst in children is around 2.6%. {a study from Michigan. The study was based on consecutive MRI imaging). This study was based on consecutive children having MRI scans of their head. As such the study population was biased i.e. there was some need for an MRI of the head of these children and might not represent the general child  population. Therefore the prevalence of 2.6 reported in the study is likely to be an over estimate. 


Multiple arachnoid cysts are seen in 5.8% of the children with arachnoid cysts. In one of the patients up to 4 arachnoids cysts were noted (0.3%). It occurs in both gender but it is more common in boys (male:Female::18:10). Intracranial arachnoid cysts occur more commonly on the left side than the right side (M:F::45:27). 

3. Why do arachnoid cysts form?

The pathogenesis of arachnoid cysts is controversial. During embryological development the pia mater and arachnoid membrane arise from the menix primitiva (perimedullary mesh). The arachnoid cysts are thought to arise due to abnormality of the menix primitiva or morel likely to incomplete separation of pia mater and arachnoid membrane.

4. Why do arachnoid cysts enlarge?  

Most cysts do not expand. There is no evidence that fluid is secreted from the cyst wall. It is likely that arachnoid cysts enlarge through flow of CSF into the cyst through an opening which acts as a one-way valve system (slit valve or ball-and-socket valve) 


4. Is arachnoid cyst genetic?

There is an increased risk of arachnoid cysts with the following conditions:

  • Mafan’s syndrome

  • Neurofibromatosis

  • Glutamic aciduria type I

  • Autosomal dominant polycystic kidney disease

  • Tuberous sclerosis

It has also been reported that increased risk of arachnoid cysts in children born out of consanguineous relationships.


In familial arachnoid cysts, autosomal recessive inheritance has been linked to chromosome 6q.


4. Do arachnoid cysts cause problems?

5. What are the symptoms related to arachnoid cysts?

6. If my child has headaches and a arachnoid cyst on the MRI of  head, is the headache due to the arachnoid cyst?
7. Do arachnoid cystsneed operating?

8. What would be the purpose of an operation for arachnoid cysts?

9. What surgical optiosn are there for managing arahnoid cysts?
10. What are the possible complications of surgery for arachnoid cysts?

11. Do patiens who have had surgery, need follow-up?


1. McComb JG (2018) Arachnoid Cysts. Part Ii Surg Aspects 239–251.

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