To the Editor:
I have read the research letter entitled “Unruptured intracranial aneurysms: incidence of rupture and risk factors” by Ishibashi et al with great interest.1 I felt the need to clarify a few points regarding the aneurysm size as one of the well-known risk factors. The authors emphasized on the results of the International Study on Unruptured Intracranial Aneurysms (ISUIA) reporting a rupture rate of approximately 0.1% per year for aneurysm <7 mm in diameter.1,2 Ishibashi et al reported an overall incidence of 1.4%.1 Since their study was carried out in Japan, the authors attributed this discrepancy to racial difference and cited an article indicating Japanese and Finnish descendents might have higher potential for aneurysm rupture.3 I would like to get back to this point later on.
In the USUIA study authors categorized rupture rates for aneurysms in the following manner: <7 mm, 7 to 12 mm, 13 to 24 mm, ≥25 mm.2 Ishibashi et al categorized aneurysm size into four: as small (<4.9 mm), medium size/wide neck (5 to 9.9 mm), large (<10 mm) and giant (>25), which makes it harder for the reader to compare both studies. Also authors did not mention on the very small sized aneurysms.1 It is getting to be widely accepted that the term “very small intracranial aneurysm” encompasses the aneurysms that are less than 3 mm in diameter. With the advent of newer microcatheters and coils, endovascular treatment of these aneurysms have become feasible.4 This also brings new questions about offering treatment to unruptured very small aneurysms and safety of the treatment when compared with larger sized aneurysms, answers to which may be sought by a metaanalysis not only covering recent published very small aneurysm reports but also picking up relevant data from earlier larger series.
It is worthwhile to note that in the study group of Ishibashi et al (please refer to Table 2 in the original article), 6 of the 19 ruptured aneurysms were reported to be 3 mm in size that makes a percentage of 31.5%. As a working neuroradiologist dealing with diagnostic and interventional neuroradiology, this percentage is roughly in accordance with the rate of very small sized aneurysms encountered among all ruptured aneurysms in a regional hospital located in central northeastern Turkey (unpublished personal experience). This is higher than the percentage of very small sized aneurysms recently published. Van Rooij et al reported an incidence of 15.4% (149/966) in the study performed in the Netherlands. Even though the authors did not mention on patient demographics, the study group can largely be considered as Dutch population.5 To the best of my knowledge, there is no cumulative study giving results of rupture rates of aneurysms and ratio of very small sized aneurysm in Turkish population. I am also unsure that our personal experience in our regional hospital should reflect a nationwide picture. However, at this point another question arises, other than high annual rupture rate of aneurysm in some societies, are small sized aneurysms more prone to rupture in some particular populations? Although speaking of modern Turkish population, it should be considered as a mixed genetic pool from different sources (highly mediterranean); high rupture rate in countries like Japan, Finland, and Turkey, may be partly explained by similar genes, given that the languages spoken in these countries can be classified under the ural-altaic language family. Based on these clues, future goals may be to conduct studies focusing on HLA DR loci especially in subarachnoid hemorrhage patients with very small sized aneurysms, results of which may provide new tools helpful in the clinical decision making of treat or not to treat.
Ishibashi T, Murayama Y, Urashima M, Saguchi T, Ebara M, Arakawa H, Irie K, Takao H, Abe T. Unruptured intracranial aneurysms: incidence of rupture and risk factors. Stroke. 2009; 40: 313–316.
Wiebers DO, Whisnant JP, Huston J III, Meissner I, Brown RD Jr, Piepgras DG, Forbes GS, Thielen K, Nichols D, O'Fallon WM, Peacock J, Jaeger L, Kassell NF, Kongable-Beckman GL, Torner JC; International Study of Unruptured Intracranial Aneurysms Investigators. Unruptured intracranial aneurysms: natural history, clinical outcome, and risks of surgical and endovascular treatment. Lancet. 2003; 362: 103–110.
Rooij WJ, Keeren GJ, Peluso JP, Sluzewski M. Clinical and angiographic results of coiling of 196 very small (≤3 mm) intracranial aneurysms. AJNR Am J Neuroradiol. 2009 Jan 8. [Epub ahead of print].