What Is An Arachnoid Cyst?

Overview

“Arachnoid cysts“ has become a term that we frequently hear in our daily life. Although it seriously affects a wide population of children, it is mostly discovered unintentionally due to its almost silent course. Yet, our concern about the next generation still drives our curiosity to explore its meaning, causes, course, progression, and possibility of treatment.

Arachnoid mater is the middle layer of the membrane covering the brain, that lies in between the outer dura and the inner pia layer. The name arachnoid is derived from the Latin for spiders, due to its content of a high number of crossing blood vessels that resemble the spider web. Its importance results from its function,  harbouring the needed space for the feeding blood vessels of the brain. This highlights the danger of any restriction of this space by a compressing mass like accumulated blood or a tumour.

A cyst is known medically to be a fluid-filled sac; however, this sac can be formed in the arachnoid by a local separation of its outer and inner layers which should adhere to each other.  In addition, this cyst becomes surrounded by thick collagen cells and arachnoid cells without crossing blood vessels inside these collections.¹

Causes of arachnoid cysts

Although the appearance of arachnoid cysts has been linked to early development, its appearance in childhood has not always been of congenital origin.  Secondary arachnoid cysts may be the result of trauma, starting from the birth traumas through infancy, mechanical traumas and childhood falls. Regarding the congenital causes, the appearance of these cysts may be related to pockets that underlie the arachnoid layer, which are formed during the development of the nervous system of the foetus in the womb.²

Signs and symptoms of arachnoid cysts

One can imagine how the presence of this fluid localisation may be stressful, hindering blood flow in the surrounding blood vessels. Consequently, the denser this cyst and the cloudier the fluid within it, the worse are the pressure effects. However, clear arachnoid cysts are more common, so less symptomatic due to their lower density.³

This stress may irritate the brain and its surrounding layers, resulting in increased pressure inside the skull. Once this cyst increases in size and weight, symptoms of nausea and vomiting may occur due to the reflex pressure and irritation of the vomiting centers in the brain. Headache and pain may occur due to the stretching of the layers covering the brain. Blurred and double vision may happen due to compressing the optic nerves inside the skull. Convulsions may occur due to the irritation of the brain tissue proper, including the sensory-motor centres.⁴

Some loss of the motor  and sensory functions from numbness and pain up to paralysis may occur if one of these arachnoid cysts arise in the spine and compress the nerve root. Loss of autonomic functions, such as the control of urination, resulting in involuntary inability to withhold urine especially in the night, may occur. This condition happens specifically in spina bifida, a failure of fusion of the bone of the spine which may give a space for cysts to form.

Management and treatment for arachnoid cysts

Before the age of 2 years, which is the latest time of solidification of the bone-forming centres of the skull, the presence of such cysts may cause separation in between the skull bones, leading to a large head circumference. In addition to the basic surgical removal of the cyst with its surrounding capsule, a process called fenestration, a shunt tube can be inserted in the fluid-forming pocket to avoid further possibility of the fluid re-collection. Painkillers are prescribed for pain and antiemetics are prescribed for vomiting as symptomatic treatments. Neurosurgical intervention such as trepanning may be required in case of increase in the pressure inside the skull.⁵

FAQs

How common are arachnoid cysts

Arachnoid cysts are among the most common intracranial masses in children. They have a prevalence of  approximately 0.2-2.6% in the general population. They more likelyto occur in the middle cranial fossa.⁶

Who are at risks of arachnoid cysts

Arachnoid cysts affect children at a prevalence rate of 1-3%, carrying a higher possibility of formation during neural development in the womb. It affects those assigned male at birth (AMAB) more than those assigned female at birth (AFAB), either in adults or children.⁷ It may appear as a recurrence after the removal operation as well.⁸

How is arachnoid cyst diagnosed

Several methods of diagnosing arachnoid cysts exist, mostly imaging-based methods. Computerized tomography (CT) scan is one of them. Yet, it is not widely used in children due to its radiation dose. Magnetic resonance imaging (MRI) can be used as well for diagnosis of adults. Nevertheless, an ultrasonography scan can help in diagnosis of arachnoid cysts in children whose age is less than 18 months, due to the incomplete fusion of their skull bones. The clinical picture may play a role in diagnosis, only if the arachnoid cysts became symptomatic.

How can I prevent arachnoid cyst

There is nothing specific to do towards preventing the occurrence of the arachnoid cysts. This is because it is mainly genetic in origin. Yet, a follow up scan for children after a fall, as well as extra care about them to prevent falling may help. One good thing is to try to accommodate a healthy environment for pregnant women, especially during the first 3 months when the foetal nervous system is formed. This can happen by regular supplementation with folic acid, which enhances a good foetal neural development at a dose of 400 micrograms per day according to the NHS guidelines.

When should I call my doctor?

• In case of exposure to trauma, especially in children less than 2 years
• In case of recurrent untreated headaches
• Persistent inability to hold urine for longer times
• Unexplained convulsions, not related to fever
• Nausea and vomiting as well as new sensorimotor symptoms

Summary

Arachnoid cysts appear as a non-symptomatic, rather than a symptomatic  incident. However,early diagnosis and awareness of its existence in a patient may make the medical intervention more potent in stopping its complications.