Natural history of small incidental intracranial aneurysms: a systematic review and pooled analysis on the influence of follow-up duration and aneurysm location on rupture risk reporting

Background

The rising detection of small unruptured intracranial aneurysms (sUIAs) poses a clinical challenge, requiring careful consideration between the low but real risk of rupture and the potential morbidity of intervention. Reported rupture rates vary widely across studies, influenced by heterogeneity in design, patient selection, aneurysm location, and follow-up duration. This study assessed how aneurysm location and follow-up length affect rupture rates in untreated sUIAs.

Methods

A systematic review and meta-analysis were conducted in line with PRISMA guidelines and registered with PROSPERO (CRD42024601692). Four databases (EMBASE, Ovid MEDLINE, EMCARE, Scopus) were searched for studies from January 2000 onwards reporting longitudinal outcomes for ≥20 untreated sUIAs ≤ 5 mm. The primary outcome was the pooled rupture rate, stratified by location and follow-up duration. Secondary analysis examined aneurysm growth. A random-effects model was used for meta-analysis, with heterogeneity assessed using the I² statistic. Sensitivity analyses evaluated the robustness of findings.

Results

From 10,694 screened records, 28 studies met inclusion criteria, encompassing 10,495 untreated sUIAs ≤ 5 mm. Over a mean follow-up of 38 months, 97 aneurysms ruptured, yielding a pooled rupture rate of 0.8 % (95 % CI, 0.6–1.2). Rupture risk did not significantly differ by location (p = 0.31): 1.1 % for middle cerebral artery, 3.9 % for anterior cerebral artery, and 0.3 % for para-ophthalmic artery aneurysms. Rupture rates remained consistent across follow-up durations (p = 0.53): 0.8 % for <20 months, 0.8 % for 20–40 months, and 1.2 % for >40 months. Although aneurysm growth appeared more frequent with longer follow-up, this was not statistically significant (p = 0.64).

Conclusion

This updated meta-analysis, incorporating novel subgroup analyses by location and follow-up duration, confirms that rupture risk for sUIAs ≤ 5 mm remains low (<1%) over an average 38-month period. However, limited long-term data restrict accurate risk estimation beyond this timeframe, and underreporting of aneurysm location impairs site-specific risk assessment. The trend towards greater aneurysm growth with extended follow-up underscores the importance of continued surveillance.