An arachnoid cyst (AC) is a benign, fluid-filled sac that forms within the membranes covering the brain and spinal cord. These cysts are not tumors and contain cerebrospinal fluid (CSF), the clear fluid that normally cushions the central nervous system. The size of the cyst is the primary factor determining whether medical intervention is required. Defining a “large” arachnoid cyst is essential for clinical evaluation and management.
Understanding Arachnoid Cysts
Arachnoid cysts arise between the arachnoid membrane (the middle layer of the meninges) and the pia mater, which rests directly on the brain and spinal cord. The fluid accumulates due to a developmental anomaly that causes the arachnoid membrane layers to split and form a pocket.
The vast majority of arachnoid cysts are classified as primary or congenital, meaning they are present from birth. They are the most common type of brain cyst and are typically discovered incidentally during imaging for an unrelated issue. Although they can occur anywhere along the brain or spinal cord, the most frequent location is the middle cranial fossa, near the temporal lobe.
These cysts are generally considered stable and often do not grow or cause symptoms throughout a person’s life. However, if a cyst grows or is located in a functionally sensitive area, its mass can begin to displace adjacent neurological structures.
Defining Cyst Size and Classification
The term “large arachnoid cyst” does not correspond to a single, universally defined measurement. Instead, it refers to the cyst’s relative size and its effect on surrounding brain structures. The clinical definition of “large” focuses on the degree of mass effect and displacement of the brain tissue, as a small cyst in a sensitive area can be more significant than a physically larger cyst in a less sensitive location.
The Galassi Classification System is the most common method for classifying cysts in the middle cranial fossa (where 50% to 60% of all ACs occur). This system divides cysts into three types based on size and the extent of brain displacement.
Type I cysts are small with no significant mass effect. Type II cysts are medium-sized, causing some displacement of the temporal lobe. A Type III Galassi cyst is considered “large” or “giant.”
A Type III cyst nearly fills the entire middle cranial fossa, causing significant displacement of the temporal, frontal, and parietal lobes. This extensive mass effect often results in a shift of the brain’s midline structures, which is the primary radiological marker for a potentially concerning cyst.
Clinical Significance of Large Cysts
The importance of a large arachnoid cyst stems from its ability to exert pressure on the surrounding brain tissue, known as mass effect. Its physical presence interferes with normal brain function and fluid dynamics, leading to symptoms that necessitate intervention.
A common consequence is the obstruction of cerebrospinal fluid flow, which can lead to hydrocephalus. This buildup causes increased intracranial pressure (ICP), manifesting as persistent, severe headaches, nausea, and vomiting. In infants, a large cyst can cause the head to enlarge abnormally.
Large cysts can also cause focal neurological deficits by pressing directly on specific brain regions or cranial nerves. Symptoms may include visual disturbances, hearing loss, balance problems, or weakness in the limbs. They are also associated with an increased risk of seizures.
In children, a large AC may interfere with normal brain development, leading to developmental delays. They can also cause endocrine issues, such as early onset of puberty, if located near the pituitary gland. The presence of progressive neurological symptoms indicates that a cyst’s size has become clinically significant.
Management and Follow-Up
The management strategy for an arachnoid cyst is based primarily on two factors: size and the presence of symptoms. For a large, asymptomatic cyst discovered incidentally, the standard approach is watchful observation. This involves regular monitoring with magnetic resonance imaging (MRI) or computed tomography (CT) scans to track the cyst’s size and stability.
If a large cyst is causing progressive or debilitating symptoms, surgical intervention is recommended to relieve pressure on the brain. The goal of surgery is to decompress the fluid and establish a pathway for it to drain into the body’s normal CSF circulation system, effectively eliminating the mass effect.
Surgical Techniques
One common technique is cyst fenestration, which involves creating a small opening in the cyst wall. This procedure is often performed endoscopically, allowing the cyst fluid to flow freely into the surrounding subarachnoid space where it is absorbed naturally.
Another option is the placement of a shunt, a hollow tube inserted into the cyst to drain the fluid into another body cavity, typically the abdomen. Shunting is generally reserved for cysts that have recurred after fenestration or for those in difficult-to-access locations. Regardless of the procedure, long-term prognosis is usually favorable, but regular imaging follow-up is necessary.