What Causes Retention Cysts in the Brain?
Retention cysts in the brain are caused by obstruction of normal cerebrospinal fluid (CSF) drainage pathways or ductal systems, leading to secondary dilatation and fluid accumulation behind the obstruction. 1
Primary Mechanism of Formation
Obstructive processes are the fundamental cause of retention cysts in the brain:
- Mass lesions (tumors, abscesses, or other space-occupying lesions) can block normal CSF flow or drainage pathways, causing secondary dilatation of structures upstream from the obstruction 1
- Inflammatory processes can lead to scarring and obstruction of normal drainage routes 1
- The dilated structures acquire fluid accumulation, creating cyst-like appearances on imaging 1
Key Distinguishing Features from True Neoplastic Cysts
Retention cysts differ fundamentally from true neoplastic cysts:
- Lack of florid papillary elements and maintain round, open lumina rather than irregular borders 1
- Peripheral location relative to any obstructing mass, rather than being the primary pathology 1
- Often retain focal cuboidal lining rather than the mucinous epithelium seen in neoplastic cysts 1
- No intrinsic neoplastic transformation - they are reactive rather than proliferative 1
Specific Obstructive Causes
Several pathologic processes can create the obstruction leading to retention cysts:
- Neuroendocrine tumors arising on duct walls (particularly from serotonin cells) can cause significant ductal dilatation mimicking other cystic lesions 1
- Small carcinomas can lead to marked dilatation of upstream structures, with the obstructing tumor potentially being missed on imaging 1
- Inflammatory arachnoiditis from infectious processes can obstruct CSF pathways 1
Important Clinical Pitfall
The obstructing lesion causing the retention cyst can be easily overlooked, particularly when small carcinomas cause marked upstream dilatation that dominates the imaging appearance 1. Always search for an underlying obstructive process when retention cysts are identified, as treating the cyst alone without addressing the underlying cause will not resolve the problem.
Differential Considerations
When evaluating cystic brain lesions, retention cysts must be distinguished from:
- Arachnoid cysts - developmental CSF collections that are not caused by obstruction 2, 3
- Parasitic cysts (neurocysticercosis, echinococcosis) - infectious causes with viable organisms 1
- Neoplastic cysts - primary tumoral processes with intrinsic growth potential 1, 4
The location (commonly peripheral to a mass), lack of papillary features, and identification of an obstructive process help confirm retention cysts as secondary phenomena rather than primary pathology 1.