Required case volume to support the hypothesis
Hypothesis
The hypothesis to be tested is that unruptured cerebral aneurysms larger than or equal to 5 mm rupture at an annual rate of 0.5% or more.
To reject the null hypothesis (annual rupture rate of unruptured aneurysms sized 5mm or larger is less than 0.005) with α-error 5% (two sided) and β-error 20%, it is estimated that 1,971 person-years are required according to the one sample binomial distribution if annual rupture rate is 1.0%. If the observed rupture rate is 0.75%, 7,111 person-years are required.
| Observed rupture rate | Case volume (person-year) |
| 0.75% | 7,111 |
| 1.0% | 1,971 |
Since the reported median size of the unruptured cerebral aneurysms was around 5 mm in Japan (references 1, 2), 3,000 cases of untreated unruptured cerebral aneurysms need to be registered in total, assuming 3 years for follow-up and possible drop-out cases. (9,000patient-year, 2,000~4,500patient-years valid follow-ups for the aneurysms larger than or equal to 5 mm.)
Preliminary questionnaire
While we have the data of surgical volume of unruptured intracranial aneurysms in Japan (around 10,000 per year according to the survey by Japan Neurosurgical Society), we did not have the data indicating how many cases were followed without treatment. In 1999, a questionnaire, asking the annual case volume of unruptured cerebral aneurysms, was sent to 1,031 neurosurgical training institutions registered to Japan Neurosurgical Society, and 298 institutions reported back.
In these institutions, total of 1,177 cases with unruptured cerebral aneurysm were followed without management each year. At the same time, 4,530 cases were managed either by craniotomy or endovascular treatment.
Study design
According to the required case volume and annual number of untreated unruptured cerebral aneurysms in the institutions answered the questionnaire (about 1,000 cases per year) as well as the possible drop offs, we decided to have a prospective case data entry for 3 years and follow each patient for 3 years, totaling 9,000 patient-year..
- Nakagawa T, Hashi K. The incidence and treatment of asymptomatic, unruptured cerebral aneurysms. J Neurosurg 1994;80(2):217-23.
- Mizoi K, Yoshimoto T, Nagamine Y, Kayama T, Koshu K. How to treat incidental cerebral aneurysms: a review of 139 consecutive cases. Surg Neurol 1995;44(2):114-20.
