A subarachnoid cyst, often referred to as an arachnoid cyst, is a benign, fluid-filled sac located within the protective layers surrounding the brain or spinal cord. These cysts form in the subarachnoid space, the area situated between the inner membranes covering the central nervous system. The fluid contained within the cyst is clear, colorless, and closely resembles cerebrospinal fluid (CSF). While they are the most common type of brain cyst, they are relatively rare overall and are not tumors. The majority of these formations do not cause symptoms, and many individuals remain unaware of their existence.
Defining Subarachnoid Cysts and Their Origin
The anatomical location of a subarachnoid cyst is defined by the three layers of meninges, the membranes that shield the brain and spinal cord. The cyst develops in the subarachnoid space, which lies between the arachnoid and pia mater and normally contains circulating cerebrospinal fluid. The fluid inside the sac is trapped CSF, and the cyst itself is lined by arachnoidal cells and collagen.
These cysts are categorized into two types based on their time of formation. The most common type is the primary, or congenital, subarachnoid cyst, which is present at birth. These are believed to arise from a developmental anomaly during the early formation of the brain and spinal cord, often involving a splitting or tearing of the arachnoid membrane. They are generally considered developmental malformations.
Secondary, or acquired, subarachnoid cysts are significantly less common and develop later in life. These cysts are typically the result of external factors that cause inflammation or trauma to the nervous system layers. Causes include head or spinal cord injuries, infections like meningitis, or complications following brain surgery. Secondary cysts form when these events lead to arachnoid adhesions, causing CSF to accumulate in a localized pocket.
Recognizing Manifestations of Cysts
Most subarachnoid cysts are asymptomatic and are often discovered incidentally during imaging for an unrelated condition. When manifestations occur, they are directly related to the cyst’s size and specific location, which dictates which surrounding neural structures are being compressed. Symptoms arise because the enlarging cyst exerts a mass effect on adjacent brain tissue or obstructs the normal flow of cerebrospinal fluid.
The most common symptoms for adults and older children include persistent headaches, nausea, and vomiting, often indicative of increased pressure within the skull. Depending on the location, individuals may also experience focal neurological deficits such as:
- Seizures
- Vision or hearing problems
- Dizziness
- Difficulties with balance and walking
If the cyst is located near the spinal cord, it can compress nerve roots, leading to back pain, muscle weakness, or tingling and numbness in the limbs.
In infants and younger children, symptoms present differently due to the flexibility of their skulls. Visible signs include an abnormal enlargement of the head (macrocephaly) or a bulging of the skull bone. Developmental delays are also frequent, as are symptoms related to hydrocephalus, which is the excessive accumulation of CSF that occurs when the cyst blocks fluid circulation pathways.
Diagnostic Imaging and Monitoring
Diagnosis typically begins when symptoms arise or when a cyst is found unexpectedly during a scan for another health issue. Imaging technology is necessary to confirm the cyst’s presence, determine its exact location, and assess its relationship to nearby brain or spinal structures. Magnetic Resonance Imaging (MRI) is the most comprehensive diagnostic tool for this purpose.
MRI provides detailed soft-tissue images that clearly show the cyst’s size and define its borders. A key feature is that the fluid within the cyst exhibits signal intensity identical to the surrounding cerebrospinal fluid, which helps distinguish it from other types of lesions, such as tumors. Computed Tomography (CT) scans are also used, particularly in emergency situations or when MRI is not accessible, as they are faster and can show the cyst’s fluid density.
For cysts that are asymptomatic and stable, the management approach is observation rather than immediate treatment. This monitoring involves regular follow-up imaging, typically with MRI, to track the cyst’s size and ensure it is not growing or causing a mass effect on neural tissue. This surveillance strategy is adopted because most small, benign cysts remain stable throughout a person’s life.
Management and Surgical Interventions
Management for subarachnoid cysts is determined by whether the cyst is causing symptoms or showing signs of enlargement or complications. For asymptomatic or stable cysts, the course of action is observation and regular monitoring. This approach is favored because surgery carries inherent risks, and many cysts never grow large enough to cause a problem.
If a cyst is symptomatic, growing rapidly, or causing complications like hydrocephalus, active intervention is usually recommended to relieve pressure. The main goal of any surgical procedure is to drain the trapped fluid and restore the normal flow of CSF. There are two primary surgical techniques used to accomplish this: fenestration and shunting.
Fenestration
Fenestration involves creating an opening in the cyst wall to allow the fluid to drain into the surrounding subarachnoid space or the brain’s ventricular system. This procedure is increasingly performed using minimally invasive neuro-endoscopic techniques, guiding a small camera and instruments through a tiny hole in the skull. Endoscopic fenestration is often the preferred method because it is less invasive than traditional craniotomy-based fenestration and can eliminate the need for a permanent implant.
Shunting
Shunting involves placing a permanent, small tube (shunt) into the cyst to divert the fluid to another part of the body, such as the abdominal cavity, where it can be safely absorbed. This is often referred to as a cystoperitoneal shunt. While shunting is a relatively simple operation with a high success rate for immediate pressure relief, it can lead to complications such as infection, blockage, or shunt dependence. The choice between fenestration and shunting is made based on the cyst’s specific size, location, and the patient’s overall health profile.