Monozygotic twins not identical with respect to the existence of intracranial aneurysms: a case report

The hypothesis that intracranial aneurysms are inherited is based on published accounts of aneurysms occurring in two or more members of the same family. This hypothesis has been strongly supported by rare cases of intracranial aneurysms in pairs of identical twins. Seven such pairs have been reported to date. In all pairs, both twins had intracranial aneurysms, most of them located at the same site. Only rarely did they appear at exact contralateral locations. In five pairs, both twins suffered from a subarachnoid hemorrhage (SAH). In one case, the asymptomatic twin underwent angiography and was treated before an SAH occurred. We now present the first pair of identical twins. One twin had an SAH and two intracranial aneurysms. The other was asymptomatic and showed no aneurysms with either three-dimensional magnetic resonance angiography or intra-arterial digital subtraction angiography. Based on epidemiologic data, we assume that there must be many unreported cases of identical twins with at least one twin suffering from SAH. Our case indicates that the trait of intracranial aneurysms is not inherited with complete penetrance, which might otherwise be assumed on the basis of all other accounts previously described in the literature. However, as long as the exact means of inheritance of intracranial aneurysms is not understood, we still recommend an angiographic examination of the asymptomatic identical twin in cases where the other sibling had already suffered from an aneurysmal SAH.