Can Perineural (Tarlov) Cysts Increase in Size?
Yes, perineural (Tarlov) cysts can increase in size, though most remain stable over time. Based on the available evidence, approximately 17% of Tarlov cysts demonstrate growth, with an average annual growth rate of 2.9% in the anteroposterior dimension, 4.3% in the craniocaudal dimension, and 1.4% in the transverse dimension 1.
Natural History and Growth Patterns
The majority of Tarlov cysts remain stable. In a natural history study following 42 cysts across 28 patients over a mean of 4.7 years, only 7 cysts (17%) across 5 patients demonstrated measurable growth 1. Importantly, none of the cysts decreased in size between successive imaging studies 1, with one notable exception of spontaneous regression documented in a single case report 2.
Growth Mechanism
The growth of Tarlov cysts occurs through a ball-valve mechanism where cerebrospinal fluid (CSF) flows into the cyst but cannot easily exit, leading to progressive dilation under hydrostatic and pulsatile forces 1. This mechanism explains why:
- Cysts can enlarge over time through net CSF inflow 3
- Growth accelerates when cysts extend into areas of minimal resistance (such as the intrapelvic cavity) 4
- Giant Tarlov cysts demonstrate increasing growth velocity over time, with annual growth rates of 7.7% in one dimension and 6.2% in another 4
Clinical Predictors of Growth
Symptoms of CSF hypotension, particularly positional headaches, are positive predictors of prior cyst growth (odds ratio = 10, p = 0.02) 1. This finding suggests that cysts with active CSF dynamics are more likely to enlarge.
Symptom Development During Growth
Growth has direct clinical consequences:
- 2 of 5 patients who were initially asymptomatic developed symptoms during the period of cyst growth 1
- 2 others experienced worsening of existing symptoms during documented cyst growth 1
- Large cysts can cause progressive painful radiculopathy, muscle atrophy, weakness, and cauda equina syndrome 5
Management Implications
Asymptomatic Tarlov cysts do not require treatment but should be documented 3, 5. However, the potential for growth necessitates:
Surveillance Strategy
- Serial MRI imaging for symptomatic cysts or those with concerning features
- Documentation of even incidental findings in case symptoms develop later 5
- Early surgical intervention may be considered for giant cysts to prevent rapid growth in the intrapelvic cavity and reduce complications 4
Surgical Considerations
When cysts become symptomatic or demonstrate progressive growth:
- Cyst-wrapping surgery appears to offer the best long-term outcomes by permanently reducing cyst size and reconstructing nerve root sleeves 5
- This approach curtails ongoing axonal injury and may promote nerve regeneration 5
- Surgery should be considered before cysts erode bone or extend as intrapelvic masses 5
Key Clinical Pitfalls
Do not assume all Tarlov cysts are static anatomic variants. While most remain stable, a significant minority (17%) will grow, and this growth correlates with symptom development. The presence of positional headaches should raise suspicion for an actively growing cyst requiring closer monitoring.
Women are disproportionately affected (90% of symptomatic patients) 5, and gender bias has been a documented concern in managing these patients 6. Symptoms should not be dismissed, particularly in women with connective tissue disorders like Ehlers-Danlos syndrome, which convey considerable risk 5.