A thoracic arachnoid cyst is a fluid-filled sac that develops in relation to the arachnoid membrane, one of the thin layers surrounding the spinal cord.
These cysts are uncommon, but when they occur in the spine, the thoracic region is one of the most common locations. Many arachnoid cysts are discovered incidentally and never cause symptoms. Larger cysts can compress the spinal cord and interfere with normal neurologic function.
Treatment depends on the size of the cyst, its effect on the spinal cord, and whether symptoms are progressing.
What Is a Spinal Arachnoid Cyst?
The spinal cord is surrounded by several protective layers called the meninges. One of these layers is the arachnoid membrane.
A spinal arachnoid cyst forms when cerebrospinal fluid becomes trapped within or around this membrane, creating a fluid-filled sac.
The cyst may be located:
- Inside the dura but outside the spinal cord
- Outside the dura
- Along the back or side of the spinal canal
Most thoracic arachnoid cysts are located behind the spinal cord.
As the cyst enlarges, it may push the spinal cord forward and reduce the amount of space available within the spinal canal.
What Causes a Thoracic Arachnoid Cyst?
Some arachnoid cysts are congenital, meaning they likely develop from an abnormality present from birth.
Others may be acquired later in life.
Possible causes include:
- Previous spinal surgery
- Trauma
- Infection
- Inflammation
- Bleeding around the spinal cord
- Arachnoid scarring
- Previous spinal procedures
In many patients, no clear cause can be identified.
Symptoms of a Thoracic Arachnoid Cyst
Small arachnoid cysts may cause no symptoms.
When symptoms occur, they usually result from compression of the spinal cord or disruption of normal cerebrospinal fluid flow.
Possible symptoms include:
- Mid-back pain
- Leg weakness
- Leg numbness or tingling
- Heaviness or stiffness in the legs
- Difficulty walking
- Poor balance
- Loss of coordination
- Abnormal sensation around the chest or abdomen
More advanced spinal cord compression may cause:
- Frequent falls
- Increasing leg stiffness
- Progressive weakness
- Bowel or bladder dysfunction
Symptoms often develop gradually.
Thoracic Myelopathy
When an arachnoid cyst compresses the thoracic spinal cord, it may cause thoracic myelopathy.
Myelopathy means that the spinal cord is not functioning normally.
Signs may include:
- Difficulty walking
- A stiff or unsteady gait
- Leg weakness
- Increased muscle tone
- Abnormal reflexes
- Numbness below the level of the cyst
- Loss of fine control of the legs
Because the thoracic spinal cord controls pathways traveling to both legs, symptoms may affect both sides of the body.
Progressive myelopathy should be evaluated promptly.
Why Can Symptoms Fluctuate?
Some patients with spinal arachnoid cysts describe symptoms that vary over time.
This may occur because the cyst communicates with the surrounding cerebrospinal fluid space.
Changes in pressure within the spinal canal can occasionally alter the degree of spinal cord compression.
Symptoms may therefore vary with:
- Position
- Activity
- Straining
- Changes in cerebrospinal fluid pressure
Not every patient experiences these fluctuations.
How Is a Thoracic Arachnoid Cyst Diagnosed?
MRI is usually the most important imaging study.
MRI
MRI can demonstrate:
- The arachnoid cyst
- Displacement of the spinal cord
- Degree of spinal cord compression
- Abnormal spinal cord signal
- Associated syringomyelia
- Other abnormalities within the spinal canal
Because the fluid inside an arachnoid cyst is similar to normal cerebrospinal fluid, some cysts can be difficult to define precisely on routine MRI.
CT Myelography
A CT myelogram may be helpful when MRI does not clearly demonstrate the anatomy of the cyst.
During myelography, contrast material is placed into the cerebrospinal fluid space.
The study can help determine:
- Whether the cyst communicates with the surrounding fluid space
- Where cerebrospinal fluid flow is blocked
- The exact relationship between the cyst and spinal cord
CT myelography may also be useful for surgical planning.
Arachnoid Cyst Versus Arachnoid Web
A spinal arachnoid web is a thin band of arachnoid tissue that can also compress or deform the spinal cord.
Arachnoid webs occur most commonly in the thoracic spine and may look similar to arachnoid cysts on imaging.
Unlike a true cyst, an arachnoid web does not necessarily form a complete fluid-filled sac.
Both conditions can interfere with cerebrospinal fluid flow and cause spinal cord compression.
MRI and CT myelography can help distinguish between them.
The Scalpel Sign
A characteristic imaging appearance known as the scalpel sign may be seen with thoracic arachnoid webs and occasionally related arachnoid abnormalities.
The spinal cord becomes displaced forward, creating a contour on sagittal imaging that resembles the blade of a scalpel.
This finding can suggest the presence of an arachnoid web or other dorsal arachnoid abnormality.
Arachnoid Cysts and Syringomyelia
A thoracic arachnoid cyst can interfere with normal circulation of cerebrospinal fluid around the spinal cord.
In some patients, this abnormal flow contributes to formation of a syrinx, or fluid-filled cavity within the spinal cord.
This condition is called syringomyelia.
When both conditions are present, treatment is usually directed toward correcting the obstruction around the spinal cord rather than simply draining the syrinx.
Restoring more normal cerebrospinal fluid flow may allow the syrinx to decrease in size over time.
Do All Thoracic Arachnoid Cysts Need Treatment?
No.
Many arachnoid cysts are found incidentally during MRI performed for another reason.
Observation may be appropriate when:
- The cyst is small
- There is minimal spinal cord compression
- There are no neurologic symptoms
- Symptoms are mild and stable
- Imaging remains unchanged
Follow-up may include periodic neurologic examinations and repeat MRI.
When Is Surgery Considered?
Surgery is more likely to be recommended when the cyst causes significant spinal cord compression or progressive neurologic dysfunction.
Possible reasons for surgery include:
- Progressive difficulty walking
- Increasing leg weakness
- Worsening balance
- Progressive numbness
- Significant thoracic myelopathy
- Spinal cord signal changes
- Progressive cyst enlargement
- Associated syringomyelia
- Bowel or bladder dysfunction related to spinal cord compression
The goal of surgery is to relieve pressure on the spinal cord and restore normal cerebrospinal fluid circulation.
Surgical Treatment
Surgery typically involves exposing the affected portion of the spinal canal through a posterior approach.
A thoracic laminectomy or laminoplasty may be performed to reach the cyst.
Once the dura is opened, the surgeon carefully identifies the cyst and its relationship to the spinal cord.
Treatment may include:
- Cyst fenestration
- Partial cyst wall removal
- Complete cyst excision when safely possible
- Opening the cyst into the normal cerebrospinal fluid space
- Removal of associated arachnoid webs
- Lysis of arachnoid adhesions
The exact technique depends on the anatomy of the cyst.
Cyst Fenestration
Fenestration means creating an opening in the cyst wall.
This allows fluid inside the cyst to communicate more freely with the surrounding cerebrospinal fluid space.
Fenestration may relieve pressure on the spinal cord without requiring complete removal of the entire cyst wall.
In many cases, this is sufficient to restore normal spinal cord position and cerebrospinal fluid flow.
Cyst Excision
When the cyst wall can be safely separated from the surrounding tissues, part or all of it may be removed.
Complete excision may reduce the risk of recurrence, but aggressive removal is not always appropriate if the cyst wall is tightly attached to:
- The spinal cord
- Nerve roots
- Blood vessels
- Dura
The priority is to relieve spinal cord compression without causing neurologic injury.
Arachnoid Scarring
Some patients have more extensive arachnoiditis, or scarring of the membranes surrounding the spinal cord.
This can make surgery more complex.
Extensive scar tissue may:
- Tether the spinal cord
- Block cerebrospinal fluid flow
- Create multiple fluid pockets
- Increase the risk of cyst recurrence
In these cases, surgery may involve careful release of adhesions and reconstruction of the cerebrospinal fluid space.
Is Spinal Fusion Necessary?
Most patients undergoing surgery for a thoracic arachnoid cyst do not automatically require spinal fusion.
Fusion may be considered when:
- A large amount of bone must be removed
- Significant facet joint removal is necessary
- Preexisting instability is present
- Spinal deformity exists
- Multiple prior surgeries have weakened the spine
The need for fusion is determined individually.
Risks of Surgery
Because surgery is performed around the spinal cord, potential risks include:
- Neurologic injury
- Weakness
- Numbness
- Dural tear
- Cerebrospinal fluid leak
- Infection
- Bleeding
- Spinal instability
- Persistent symptoms
- Recurrence of the cyst
- Additional arachnoid scarring
The degree of risk depends on the location of the cyst, the amount of scarring, and the severity of spinal cord compression.
Recovery After Surgery
Recovery varies depending on how long the spinal cord was compressed and how severe the neurologic symptoms were before treatment.
Patients are often encouraged to begin walking relatively early after surgery.
Physical therapy may focus on:
- Walking
- Balance
- Leg strength
- Coordination
- Endurance
Neurologic improvement may occur gradually over several months.
Patients with long-standing severe myelopathy may have some permanent symptoms even after successful decompression.
Can a Thoracic Arachnoid Cyst Come Back?
Recurrence is possible.
The risk depends on:
- Type of cyst
- Whether the cyst communicates with the cerebrospinal fluid space
- Amount of cyst wall removed
- Presence of arachnoid scarring
- Previous infection or surgery
Follow-up MRI may be recommended after surgery, particularly if symptoms recur.
When Should Symptoms Be Evaluated Promptly?
Prompt evaluation is appropriate when symptoms suggest worsening spinal cord compression.
Concerning symptoms include:
- Progressive leg weakness
- Increasing difficulty walking
- Worsening balance
- Frequent falls
- Increasing numbness in both legs
- Rapid loss of coordination
- New bowel or bladder dysfunction
These symptoms may indicate significant thoracic spinal cord dysfunction.
Outlook
Thoracic arachnoid cysts are uncommon and many never cause symptoms.
When a cyst significantly compresses the spinal cord, however, it can cause progressive thoracic myelopathy.
Treatment should be based on:
- Severity of spinal cord compression
- Neurologic symptoms
- Progression over time
- Presence of syringomyelia
- Cerebrospinal fluid flow abnormalities
- Overall health
Small, asymptomatic cysts can often be monitored.
Symptomatic cysts causing progressive spinal cord dysfunction can frequently be treated with surgical decompression, cyst fenestration, or removal of the abnormal arachnoid tissue.
The primary goal is to relieve pressure on the spinal cord and preserve neurologic function.
Related Topics
- Syringomyelia
- Spinal Arachnoid Web
- Thoracic Myelopathy
- Intradural Spinal Tumors
- Thoracic Spinal Cord Compression
- Thoracic Laminectomy
- Thoracic Laminoplasty
- Spinal Cord Surgery