A thoracic arachnoid cyst is a benign, fluid-filled sac located within the spinal canal, specifically in the upper and middle back region. These lesions form from a localized abnormality in the arachnoid mater, one of the protective membranes surrounding the spinal cord. The cyst is a pocket of cerebrospinal fluid (CSF) that has become trapped, causing an expansion that can eventually press on the neural structures. The presence of a thoracic arachnoid cyst often goes unnoticed, as many remain small and cause no symptoms throughout a person’s lifetime.
Understanding Thoracic Arachnoid Cysts
The spinal cord and brain are protected by three layers of tissue called the meninges: the dura mater, arachnoid mater, and pia mater. The dura mater is the tough, outermost layer, while the pia mater adheres directly to the spinal cord’s surface. The arachnoid mater lies between these two layers and helps contain the cerebrospinal fluid (CSF).
A thoracic arachnoid cyst forms when the arachnoid membrane splits or duplicates, creating a pouch where cerebrospinal fluid accumulates over time. The fluid within the cyst is identical to the surrounding CSF, which circulates to cushion and nourish the central nervous system. This cyst may be classified as primary (congenital), meaning it is present from birth, likely due to an error in the development of the meningeal layers.
Secondary (acquired) cysts develop later in life due to external factors that cause inflammation or scarring of the arachnoid membrane. These acquired causes can include trauma, prior spinal surgery, or infections such as meningitis, which lead to localized adhesions that trap the CSF. The majority of spinal arachnoid cysts are located in the thoracic region, positioned most often on the dorsal side of the spinal cord.
Manifestation of Neurological Symptoms
A thoracic arachnoid cyst causes symptoms when its growing size creates a mass effect, physically compressing the adjacent spinal cord and nerve roots. Because the thoracic spine has a narrower canal diameter than other spinal regions, even a small increase in cyst size can quickly translate into significant neurological problems. Symptoms typically begin subtly and may wax and wane, often progressing slowly under prolonged pressure.
One of the most common complaints is radicular pain, which presents as a sharp, shooting, or burning sensation that follows the path of compressed nerve roots around the chest or into the abdomen. Motor deficits develop due to compression of the descending pathways, leading to progressive weakness in the lower extremities. This weakness can manifest as a spasticity or stiffness in the legs and a noticeable disturbance in gait.
Sensory changes are also frequent, including paresthesia, which patients describe as a feeling of numbness, tingling, or a prickly sensation in the legs or trunk. The physical examination may reveal signs consistent with myelopathy, such as hyperactive reflexes below the level of the compression. In more severe cases, the pressure can affect the pathways controlling the pelvic organs, resulting in sphincter dysfunction. This can include difficulty with bladder control, such as urinary urgency or retention, and bowel control issues, signaling an urgent need for intervention.
Diagnostic Imaging and Evaluation
The diagnostic process begins with a detailed neurological examination to identify the specific pattern and level of sensory and motor deficits. This clinical evaluation helps determine the likely location of spinal cord compression before any imaging is performed. The symptoms can mimic other spinal conditions, so imaging is necessary to definitively identify the cyst and rule out other potential causes.
Magnetic Resonance Imaging (MRI) is the preferred method for confirming the presence of a spinal arachnoid cyst. MRI uses strong magnetic fields and radio waves to produce detailed images of soft tissues, clearly distinguishing the fluid-filled cyst from the surrounding spinal cord and bone. On T2-weighted MRI sequences, the cyst fluid appears bright, similar to the cerebrospinal fluid, allowing for precise localization and assessment of spinal cord displacement.
In certain situations, CT myelography may be utilized, which involves injecting a contrast agent into the subarachnoid space before performing a computed tomography scan. This procedure is useful for determining if the cyst communicates with the surrounding cerebrospinal fluid space, as the contrast agent will flow into the cyst if a connection exists. This detail provides valuable information for surgical planning, especially when the MRI findings are inconclusive.
Treatment and Management Strategies
The management of a thoracic arachnoid cyst is guided primarily by the presence and severity of neurological symptoms. Cysts that are discovered incidentally during imaging for an unrelated condition and cause no symptoms are typically managed with observation. These asymptomatic cysts are monitored with periodic neurological checks and follow-up MRI scans to ensure they do not enlarge or begin to cause compression.
When the cyst causes pain, progressive weakness, or sphincter dysfunction, surgical intervention is generally recommended to relieve pressure on the spinal cord. The goal of surgery is spinal cord decompression and restoration of normal cerebrospinal fluid flow dynamics. One common surgical technique is fenestration, where a small opening is created in the cyst wall to allow the trapped fluid to drain into the surrounding subarachnoid space.
Another approach is the placement of a shunt, a small tube inserted into the cyst to divert the fluid to another body cavity, such as the abdominal space, where it can be safely absorbed. Complete surgical excision of the cyst wall may be performed, particularly with small or easily accessible cysts. While fenestration is often favored as a primary treatment, shunting may be reserved for recurrent cysts or those where fenestration has failed to resolve the compression.