Endovascular Access

In contrast with open surgical treatment of aneurysms, in which the focus is on the regional intracranial vascular anatomy, during endovascular treatment of aneurysms consideration must also be given to peripheral vascular anatomy, aortic arch, and great vessel anatomy in addition to the intracranial vascular anatomy. Gaining endovascular access to the arteries of the brain requires first accessing the peripheral vascular system, usually by a transfemoral route (although transbrachial or transradial routes may be used), navigating the aortic arch, and selecting the desired carotid or vertebral artery. Increased tortuosity of the great vessels, which is common with increasing age, or aberrant branching patterns from the arch can make it difficult to pass a catheter or can decrease its stability during the treatment.

Consideration must also be given to the course, orientation, and disease state of the cervical portion of the carotid and vertebral arteries. A high-grade cervical stenosis, or excessive tortuosity, may limit the ability to access intracranial lesions or may increase the risk of complications from the endovascular procedure, and strategies for treating or navigating these obstacles should be planned ahead of time, based on preoperative imaging studies. The final step of endovascular access is navigation of the intracranial vessels, and, again, review of preoperative imaging is important, because individual variability is high. The use of biplane angiography and biplane roadmapping is the standard of care in the endovascular treatment of cerebral aneurysms. Having 2 separate views of the regional arterial anatomy and the aneurysm increases the safety of the procedure by reducing the risk of parent artery or aneurysm perforation during aneurysm access and coil delivery.

In general, aneurysm treatment is performed by the most direct route available, usually via the ipsilateral carotid or vertebral artery. However, it is important to assess the patency of the circle of Willis, and give consideration to alternate access routes, based on the shape and orientation of the aneurysm, and in case of emergency, such as loss of access from the ipsilateral side during treatment.

Aneurysm and Parent Artery Anatomy

As described by Rhoton, aneurysms tend to occur at branch points of the parent artery, usually where the vessel takes a curve, and point in the direction blood would have flowed if the branch or curve did not occur. Almost 80% of aneurysms occur in the anterior circulation, with the remaining 20% in the posterior circulation. The location and direction of the aneurysm with respect to the parent artery has important implications for the strategy used to access the inside of the lesion. For example, aneurysms arising at major bifurcations, such as the carotid T or basilar Y are usually entered with a straight or 45° angled microcatheter, whereas those occurring at smaller branch bifurcations, such as the posterior communicating or ophthalmic artery, may require a 90° or J-shaped microcatheter for access. For aneurysms of the anterior circulation, attention should be paid to whether the carotid siphon has an open or closed configuration, because the increased tortuosity of the latter can make catheter tracking more difficult.

For the most common type of intracranial aneurysm, the saccular aneurysm, numerous morphologic and geometric descriptors have been used. The most basic anatomic assessment is size, measured as the maximum diameter dimension of the aneurysm dome, and is important because increasing size is correlated with an increased risk of rupture according to data from the International Study of Unruptured Intracranial Aneurysms (ISUIA). The shape of the aneurysm is also important, because the presence of irregularities or a daughter sac was associated with increased risk of rupture in the Japanese Unruptured Cerebral Aneurysm Study (UCAS). Other geometric assessments based on ratios, such as the aspect ratio (dome height to neck width) and size ratio (aneurysm size to parent artery size) have also been positively associated with rupture risk.

With regard to the suitability of aneurysms for endovascular treatment, alone or with adjuvants, the determination of the aneurysm as wide necked is important. The traditional definition of a wide-necked aneurysm is a neck width greater than 4 mm or a dome to neck ratio of less than 2, and was based on the likelihood of successful coiling in the early era of endovascular treatment. With the advent of coils with complex or three-dimensional (3D) shapes, it was suggested that aneurysms with dome to neck ratios down to 1.5 could be routinely treated with coiling alone. In the modern era of adjunctive endovascular techniques, especially balloon remodeling and stent assistance, the assessment of morphology can help determined which aneurysms will require an adjunct. A retrospective review found that aneurysms with a dome to neck ratio of greater than 1.6, and an aspect ratio greater than 1.6, rarely required adjuncts for coiling, whereas those with dome to neck and aspect ratios of less than 1.2 almost always required an adjunctive technique. Aneurysms in the middle range, from 1.2 to 1.6, were equally divided between needing adjunctive techniques and not. A recent large national registry study from Japan found that adjunctive techniques were applied in 54.8% of procedures for the endovascular treatment of unruptured aneurysms, highlighting their common use for managing lesions that would previously not have been able to be treated by endovascular therapy.

Another important anatomic consideration is the presence of branch arteries in proximity to the aneurysm. Some of the most common branch vessels to consider include the ophthalmic, posterior communicating, anterior choroidal, posterior inferior cerebellar, large lenticulostriate perforators, and the recurrent artery of Heubner. The precise origin of these small vessels can be difficult to appreciate using conventional imaging techniques, but 3D rotational angiography can provide exquisite detail of their relation to the aneurysm neck. If the vessels arise from the parent artery, treatment of the aneurysm with conventional techniques and adjuncts is usually safe. If the branch arises from the base of the neck of the aneurysm, coiling may still be possible, although great care must be taken to preserve the origin of the vessel, and a neck remnant may be left to achieve this end. If the branch arises from higher up the neck or from the dome, coiling may present too high a risk for vessel occlusion with ischemic complication, or may leave too large a remnant for effective treatment of the aneurysm.

Endovascular Access

In contrast with open surgical treatment of aneurysms, in which the focus is on the regional intracranial vascular anatomy, during endovascular treatment of aneurysms consideration must also be given to peripheral vascular anatomy, aortic arch, and great vessel anatomy in addition to the intracranial vascular anatomy. Gaining endovascular access to the arteries of the brain requires first accessing the peripheral vascular system, usually by a transfemoral route (although transbrachial or transradial routes may be used), navigating the aortic arch, and selecting the desired carotid or vertebral artery. Increased tortuosity of the great vessels, which is common with increasing age, or aberrant branching patterns from the arch can make it difficult to pass a catheter or can decrease its stability during the treatment.

Consideration must also be given to the course, orientation, and disease state of the cervical portion of the carotid and vertebral arteries. A high-grade cervical stenosis, or excessive tortuosity, may limit the ability to access intracranial lesions or may increase the risk of complications from the endovascular procedure, and strategies for treating or navigating these obstacles should be planned ahead of time, based on preoperative imaging studies. The final step of endovascular access is navigation of the intracranial vessels, and, again, review of preoperative imaging is important, because individual variability is high. The use of biplane angiography and biplane roadmapping is the standard of care in the endovascular treatment of cerebral aneurysms. Having 2 separate views of the regional arterial anatomy and the aneurysm increases the safety of the procedure by reducing the risk of parent artery or aneurysm perforation during aneurysm access and coil delivery.

In general, aneurysm treatment is performed by the most direct route available, usually via the ipsilateral carotid or vertebral artery. However, it is important to assess the patency of the circle of Willis, and give consideration to alternate access routes, based on the shape and orientation of the aneurysm, and in case of emergency, such as loss of access from the ipsilateral side during treatment.

Aneurysm and Parent Artery Anatomy

As described by Rhoton, aneurysms tend to occur at branch points of the parent artery, usually where the vessel takes a curve, and point in the direction blood would have flowed if the branch or curve did not occur. Almost 80% of aneurysms occur in the anterior circulation, with the remaining 20% in the posterior circulation. The location and direction of the aneurysm with respect to the parent artery has important implications for the strategy used to access the inside of the lesion. For example, aneurysms arising at major bifurcations, such as the carotid T or basilar Y are usually entered with a straight or 45° angled microcatheter, whereas those occurring at smaller branch bifurcations, such as the posterior communicating or ophthalmic artery, may require a 90° or J-shaped microcatheter for access. For aneurysms of the anterior circulation, attention should be paid to whether the carotid siphon has an open or closed configuration, because the increased tortuosity of the latter can make catheter tracking more difficult.

For the most common type of intracranial aneurysm, the saccular aneurysm, numerous morphologic and geometric descriptors have been used. The most basic anatomic assessment is size, measured as the maximum diameter dimension of the aneurysm dome, and is important because increasing size is correlated with an increased risk of rupture according to data from the International Study of Unruptured Intracranial Aneurysms (ISUIA). The shape of the aneurysm is also important, because the presence of irregularities or a daughter sac was associated with increased risk of rupture in the Japanese Unruptured Cerebral Aneurysm Study (UCAS). Other geometric assessments based on ratios, such as the aspect ratio (dome height to neck width) and size ratio (aneurysm size to parent artery size) have also been positively associated with rupture risk.

With regard to the suitability of aneurysms for endovascular treatment, alone or with adjuvants, the determination of the aneurysm as wide necked is important. The traditional definition of a wide-necked aneurysm is a neck width greater than 4 mm or a dome to neck ratio of less than 2, and was based on the likelihood of successful coiling in the early era of endovascular treatment. With the advent of coils with complex or three-dimensional (3D) shapes, it was suggested that aneurysms with dome to neck ratios down to 1.5 could be routinely treated with coiling alone. In the modern era of adjunctive endovascular techniques, especially balloon remodeling and stent assistance, the assessment of morphology can help determined which aneurysms will require an adjunct. A retrospective review found that aneurysms with a dome to neck ratio of greater than 1.6, and an aspect ratio greater than 1.6, rarely required adjuncts for coiling, whereas those with dome to neck and aspect ratios of less than 1.2 almost always required an adjunctive technique. Aneurysms in the middle range, from 1.2 to 1.6, were equally divided between needing adjunctive techniques and not. A recent large national registry study from Japan found that adjunctive techniques were applied in 54.8% of procedures for the endovascular treatment of unruptured aneurysms, highlighting their common use for managing lesions that would previously not have been able to be treated by endovascular therapy.

Another important anatomic consideration is the presence of branch arteries in proximity to the aneurysm. Some of the most common branch vessels to consider include the ophthalmic, posterior communicating, anterior choroidal, posterior inferior cerebellar, large lenticulostriate perforators, and the recurrent artery of Heubner. The precise origin of these small vessels can be difficult to appreciate using conventional imaging techniques, but 3D rotational angiography can provide exquisite detail of their relation to the aneurysm neck. If the vessels arise from the parent artery, treatment of the aneurysm with conventional techniques and adjuncts is usually safe. If the branch arises from the base of the neck of the aneurysm, coiling may still be possible, although great care must be taken to preserve the origin of the vessel, and a neck remnant may be left to achieve this end. If the branch arises from higher up the neck or from the dome, coiling may present too high a risk for vessel occlusion with ischemic complication, or may leave too large a remnant for effective treatment of the aneurysm.