The annual risk of an intracranial aneurysm rupturing may be lower than most previous estimates, according to calculations based on data from a large Norwegian population-based study.
Although earlier studies have placed the annual risk of rupture at 0.5%-5%, Dr. Tomm Brostrup Müller of St. Olav’s Hospital/Trondheim (Norway) University Hospital and his colleagues came up with the figure of 0.83%.
Neurologists have long debated the management of unruptured intracranial aneurysms. “Our patients want to know the risk of rupture, and they want to know the risk of treatment,” he said at the annual meeting of the Congress of Neurological Surgeons. “We now have quite good data on the risk of treatment. The controversy is mainly related to the risk of rupture.”
Researchers have found two methods for estimating the risk of rupture. One is to look at the natural history of ruptured aneurysms in which a group of patients is followed over time. This provides some information about the size and location of the aneurysms most likely to rupture, but the data in all these studies are confounded by selection bias, Dr. Müller said.
The second method is to study a large population, dividing the incidence of ruptures by the prevalence of aneurysms. “We have good data on aneurysmal subarachnoid hemorrhage from all over the world,” Dr. Müller said. “However, the incidence of unruptured intracranial aneurysms is another story.” This may explain why previous studies have resulted in a wide range of estimates of risk.
The Norwegian researchers hoped to come up with a more accurate estimate of risk by studying more people for a longer period of time. So they used data from the Nord-Trøndelag Health Study (HUNT), one of the largest population-based studies ever conducted.
All inhabitants of the county of Nord-Trøndelag in central Norway older than 20 years were invited to participate. A total of 95,097 people were followed during 1984-1986 or during 1995-1997. The investigators recorded the number of aneurysmal subarachnoid hemorrhages that occurred in these first two waves of the study.
“To our knowledge, this is the first time that the incidence of unruptured intracranial aneurysms and the prevalence of subarachnoid hemorrhage has been established for one large population cohort,” he said.
The researchers then randomly selected 1,000 participants aged 50-65 years from the third wave of the study (which took place during 2006-2008) and scanned them with magnetic resonance angiography. They found that 19 participants had aneurysms: 17 had one aneurysm each, 1 had two aneurysms and 1 had three aneurysms. The prevalence was therefore 1.9%.
They verified all but two of these aneurysms either intraoperatively or by CT scan or digital subtraction angiography.
The aneurysms measured 2-6 mm in diameter in 13 cases and 7-9 mm in 9 cases.
Clinicians handled these aneurysms according to their practice. They treated two of the patients endovascularly and five surgically. In the patient with three aneurysms, they clipped two and coiled the other. They took a conservative approach with the remaining 11 patients, following up by MRI and CT.
From this survey, the investigators calculated the incidence of aneurysmal subarachnoid hemorrhage in participants aged 50-65 years as 15.7/100,000 person-years. Dividing the incidence by the prevalence (0.000157/0.019) yielded an annual rupture risk of 0.83%.
In the total HUNT population, the incidence of aneurysmal subarachnoid hemorrhage was 10.2/100,000 person years. If the annual rupture risk is the same in this population, that would mean its overall prevalence of unruptured intracranial aneurysms was 1.2%.
Dr. Müller said the study had limitations. People aged 50-65 years may not represent the whole population, though this age group is particularly relevant for the study of aneurysm ruptures. Also, the population-based approach does not allow for analysis of the size and location of aneurysms that rupture.
The investigators reported no relevant disclosures.