Tarlov cysts are fluid-filled sacs that develop along the spinal nerve roots, representing a distinct form of perineural cyst. They are often discovered incidentally during magnetic resonance imaging (MRI) scans. When these cysts enlarge, they can press on surrounding nerves and bone. The current understanding of their cause centers on a combination of congenital predisposition and mechanical factors that drive their formation and expansion.
Anatomical Location and Structure
Tarlov cysts are most frequently observed in the sacral region of the spine, the fused, triangle-shaped bone at the base of the vertebral column. They predominantly form around the sacral nerve roots, specifically from the first to the fourth sacral segments (S1 to S4). These cysts are dilations of the nerve root sheath itself, occurring near the dorsal root ganglion.
The sacs are filled with cerebrospinal fluid (CSF), the clear liquid that cushions the brain and spinal cord. Structurally, the cyst wall is a continuation of the nerve root sheath. A defining characteristic is that spinal nerve fibers are often found embedded within the cyst wall or traversing the cyst cavity.
This involvement of neural elements distinguishes Tarlov cysts from other types of spinal cysts, such as meningeal diverticula, which do not contain nerve fibers. The cysts are situated within the perineural space, which lies between the endoneurium and the perineurium, two layers of the connective tissue covering the nerve root.
Pathophysiological Mechanism of Development
The core mechanism explaining how Tarlov cysts develop and grow is the “ball-valve” or “check-valve” mechanism. This theory posits that a tiny, one-way connection exists between the main subarachnoid space, which contains the CSF, and the interior of the cyst.
This valve-like opening allows CSF to be pushed into the nerve root dilation but restricts the fluid from flowing back out efficiently. The pulsatile nature of CSF flow, which changes with every heartbeat and breath, plays a significant role, acting like a pump that forces fluid into the cyst.
Because the fluid cannot escape easily, this constant influx creates a progressive increase in hydrostatic pressure inside the sac. This sustained internal pressure causes the cyst to slowly inflate and enlarge, much like a balloon being gradually filled. The continuous expansion ultimately leads to compression of the surrounding nerve roots, which is the source of symptoms.
The structural abnormality that creates the valve is believed to involve a circular scarring or proliferation of the arachnoid matter around the opening of the cyst. This structural feature ensures the pressure differential is maintained, leading to the gradual, irreversible expansion of the lesion.
Potential Initiating and Aggravating Factors
While the “ball-valve” mechanism describes internal growth, external or internal events can initiate cyst formation or cause a pre-existing, asymptomatic cyst to rapidly enlarge and become symptomatic. The underlying predisposition is thought to be congenital, suggesting some individuals are born with a weakness in the nerve root sheath.
Traumatic events are frequently cited as potential triggers. Significant physical trauma, such as a direct fall, a motor vehicle accident, or intense straining associated with childbirth, can cause localized inflammation or damage to the nerve sheath. This localized injury may disrupt nerve root integrity, potentially creating or worsening the one-way valve connection to the subarachnoid space.
Changes in cerebrospinal fluid pressure also serve as a strong aggravating factor. Activities that temporarily increase intraspinal pressure, such as heavy lifting, coughing, or prolonged straining, can force more CSF into the cyst via the valve mechanism. This increase can lead to a sudden, noticeable enlargement and the onset or worsening of symptoms.
Furthermore, systemic conditions that affect connective tissue integrity may predispose an individual to developing or enlarging Tarlov cysts. Disorders like Ehlers-Danlos syndrome and Marfan syndrome involve generalized connective tissue abnormalities and are associated with dural ectasia, a ballooning of the protective sac around the spinal cord. This underlying tissue weakness makes the nerve root sheaths more prone to dilation and cyst formation. Inflammatory processes, such as arachnoiditis (inflammation of the arachnoid membrane), are also considered potential contributors by causing localized scarring that could facilitate the check-valve effect.