Arachnoid cysts are non-cancerous, fluid-filled sacs that develop within the central nervous system (the brain and spinal cord). They are the most common type of brain cyst and are benign. These cysts form between the brain (or spinal cord) and the arachnoid membrane, one of the three protective layers covering the central nervous system. The sacs contain cerebrospinal fluid (CSF), the clear liquid that normally surrounds and cushions the brain and spinal cord. Most individuals with an arachnoid cyst never experience symptoms, as the cysts often remain small and stable.
Understanding How Arachnoid Cysts Form
Most arachnoid cysts are classified as primary or congenital, meaning they are present at birth. They are believed to arise from a developmental anomaly during the early weeks of gestation. This anomaly causes a splitting or duplication of the arachnoid membrane, which traps cerebrospinal fluid (CSF) within the newly formed sac. The exact cause of this congenital malformation is not fully understood, but it is a structural deviation. Primary cysts are significantly more common and can occur anywhere along the brain or spinal cord, often found in the middle cranial fossa.
Secondary or acquired arachnoid cysts are much rarer, developing after birth due to external factors. These may result from trauma, post-surgical complications, or infections like meningitis. In all cases, the cyst wall prevents the trapped CSF from draining and circulating naturally, allowing fluid to accumulate and potentially cause the cyst to grow.
Symptoms Based on Location and Size
The majority of arachnoid cysts are asymptomatic and are discovered incidentally during imaging for unrelated medical reasons. Symptoms emerge only if the cyst grows large enough to exert pressure on surrounding tissue or obstruct the normal flow of CSF. The symptoms depend entirely on the cyst’s size and specific location.
Cysts located on the brain, particularly those causing increased intracranial pressure, can lead to chronic headaches, nausea, and vomiting. Larger cysts may compress functional areas of the brain, resulting in neurological issues such as seizures, balance problems, or disturbances in vision and hearing. In infants, a cyst may cause hydrocephalus (a buildup of fluid in the brain), leading to an enlarged head circumference or developmental delays.
When a cyst is situated along the spinal cord, it places pressure on the nerve roots or the spinal cord itself. This can manifest as back pain, muscle weakness, or tingling or numbness in the arms or legs.
Confirmation Through Diagnostic Imaging
Confirmation of an arachnoid cyst relies primarily on specialized neuroimaging techniques. Magnetic Resonance Imaging (MRI) is the preferred method for diagnosis and detailed evaluation. MRI scans are effective because the fluid within an uncomplicated arachnoid cyst appears identical to normal CSF, confirming the fluid’s composition and distinguishing the sac from solid tumors.
A Computed Tomography (CT) scan can also identify the cyst, particularly in emergency situations or when MRI is unavailable. Imaging provides precise details on the cyst’s size, anatomical location, and its relationship to nearby structures. This information is crucial for determining the necessity and type of treatment.
Treatment Approaches and Long-Term Monitoring
Management depends heavily on whether the cyst is causing symptoms and the degree of pressure it exerts. For small, stable, and asymptomatic cysts, the standard approach is observation. This involves routine neurological examinations and periodic follow-up imaging (typically MRI) to monitor for changes in size or symptom progression.
Intervention is reserved for symptomatic cysts causing debilitating problems or those showing significant growth or pressure. The goal of surgical treatment is to relieve pressure by draining the trapped fluid. One common technique is fenestration, which involves creating a small opening in the cyst wall. Fenestration allows the cyst fluid to drain into the surrounding subarachnoid space, where it is naturally reabsorbed into the CSF circulation. This is often performed using a minimally invasive neuroendoscopic technique.
Another option is the placement of a shunt, a permanent drainage system that diverts cyst fluid away from the brain or spine, often into the abdominal cavity for absorption. Endoscopic fenestration is frequently favored due to its lower dependence on implanted hardware compared to shunting. The prognosis for most people is positive, and successful treatment often results in significant improvement in neurological symptoms. Long-term monitoring remains important after intervention to ensure the cyst does not recur.