Unruptured (radiopedia)
- Anterior circulation: 90%
- ACA/ACoA complex: 30-40%
- Pericallosal aneurysm
- 2nd most common ACA aneurysm
- 3% of all aneurysm
- Supraclinoid ICA and ICA/PCoA junction: 30%
- MCA (M1/M2 junction) bi/trifurcation: 20-30%
Type | Site | Frequency of all IC aneurysms (%) |
Supraophthalmic aneurysm | Upper surface of ICA at the origin of the ophthalmic artery | 5% |
Hypophyseal artery aneurysm | Medial wall of ICA at the origin of the hypophyseal artery | 1% |
Posterior communicating artery aneurysm | Posterior wall of ICA immediately superolateral to the origin of the PcommA | 25% |
Carotid bifurcation aneurysm | Apex of the terminal ICA bifurcation into ACA and MCA | 5% |
Anterior choroidal artery aneurysm | Posterior wall of the ICA immediately superior to the origin for the anterior choroidal artery | 5% |
- Posterior circulation: 10%
- Basilar tip: 8%
- SCA:
- PICA:
Ruptured (Korja 2016)
- MCA (32%)
- ACA (32%)
- PCOM (14%)
- Pericallosal arteries (5%)
Pericallosal aneurysm
- Pericallosal aneurysm is the second most common aneurysm of the anterior cerebral artery (ACA).
- It originates at the callosomarginal artery from the pericallosal artery.
- Typically found near the anterior portion of the corpus callosum.
- Occurs near the point where the genu of the ACA is most angled.
- Pericallosal aneurysms make up about 3% of all intracranial aneurysms.
- They extend distally into the space between the pericallosal and callosomarginal arteries junction.
PCOM
- The posterior communicating artery originates from the posterior wall of the ICA, curving posteriorly as it ascends to the terminal bifurcation beneath the anterior perforated substance.
- Aneurysms arise near the apex of this curve
- Just above the posterior communicating artery's origin, and point downward and posteriorly.
- CN3 is usually found lateral to the aneurysm's neck, with the posterior communicating artery inferomedial and the anterior choroidal artery superior or superolateral to the neck.
- Aneurysms larger than 4-5 mm may compress the oculomotor nerve, potentially causing ophthalmoplegia.
Superior hypophysial artery aneurysms
- Aneurysms arise just distal to the origin of the superior hypophysial artery.
- They originate from the medial or posterior wall of the internal carotid artery (ICA) where the ICA curves medially.
- These aneurysms are located lateral to the pituitary stalk and point medially under the optic chiasm.
- Medial expansion of an aneurysm can compromise:
- Perforating arteries to the floor of the third ventricle
- Optic nerves
- Optic chiasm
- Pituitary stalk
- Hypophysial vascular supply
Supraopthalmic aneurysm
- Aneurysms typically originate from the superior wall of the carotid artery, just distal to the ophthalmic artery.
- They project upward toward the optic nerve and can be large with a complex, multi-lobulated shape.
- Surgical exposure is challenging due to the variable origin and course of the ophthalmic artery and the dura's multiple folds near the optic foramen and clinoid process.
- Many aneurysms have a wide neck, requiring remodelling techniques.
- Unruptured aneurysms may cause symptoms like headaches or cranial nerve compression.
Anterior choroidal:
- The supraclinoid ICA has a posteriorly convex curve that forms its apex at the level of the anterior choroidal artery.
- This curvature shifts the hemodynamic force distally from the posterior communicating artery to the anterior choroidal artery.
- Anterior choroidal aneurysms typically form just distal, superior, or superolateral to the origin of the anterior choroidal artery and point posterior or posterolaterally.
- These aneurysms are usually well above the CN3.
- Aneurysms from the choroidal segment have more perforating branches around their neck compared to those from the communicating or ophthalmic segment.
- This is because the choroidal segment has a greater number of perforating branches, most of which arise from the posterior wall where the aneurysm's neck is located.
Here is the summary in point form:
- ACA aneurysms typically form near the anterior communicating artery complex, making up about 30% of all intracranial aneurysms.
- They are one of the most common types and often associated with anatomical variants.
- Aneurysms frequently occur when one A1 segment is hypoplastic, and the dominant A1 gives rise to both A2 segments.
- The aneurysm usually arises at the level of the anterior communicating artery where the dominant A1 bifurcates.
- The dome of the aneurysm points away from the dominant segment toward the opposite side.
- Treatment approaches must ensure the patency of the anterior communicating artery and the adjacent recurrent artery of Heubner.
- The AcomA gives rise to small perforating branches that perfuse the fornix, corpus callosum, and septal region.
- Occlusion of the anterior communicating artery may lead to personality disorders, even if both A2 segments are perfused.
- The recurrent artery of Heubner variably arises from the distal A1, proximal A2, or the frontopolar branch of the ACA.
- Occlusion of the recurrent artery of Heubner may cause hemiparesis or aphasia.
MCA bifurcation aneurysm
- 15% of saccular aneurysms originate from the Middle Cerebral Artery (MCA).
- They typically arise at the first major bifurcation or trifurcation of the MCA and point laterally.
- The proximity of the bifurcation affects the number of lenticulostriate branches that may be stretched around the aneurysm's neck.
- Unruptured MCA aneurysms are usually clinically silent.
- Proximal M1 segment aneurysms are rare, tend to point upward, and are associated with the anterior perforated substance.
- Aneurysms may also originate from the temporopolar branch of the M1 segment, typically pointing inferiorly.
- Distal MCA bifurcation aneurysms are rare and often linked to infectious diseases.
Basilar Tip aneurysm
- 15% of saccular aneurysms occur in the vertebrobasilar system.
- 60% of these arise at the basilar bifurcation where the posterior cerebral arteries branch off from the basilar artery.
- Aneurysms at this site change blood flow from vertical to nearly horizontal, projecting upward along the basilar artery's long axis.
- The posterior thalamoperforate arteries (retromammillary arteries) are vital perforators arising from the basilar tip and P1.
- These arteries enter the brain through the posterior perforated substance in the interpeduncular fossa, ascend through the midbrain to the thalamus.
- Occlusion risks include visual loss, paralysis, sensory disturbances, weakness, memory deficits, autonomic and endocrine imbalance, abnormal movements, diplopia, and depression of consciousness.
- Endovascular approaches are preferred for treating basilar apex aneurysms due to lower morbidity compared to surgical approaches.
- This preference is especially true for posterior basilar tip aneurysms, as they affect more vital thalamoperforators when enlarged.
SCA aneurysm
- Basilar artery aneurysms typically occur at the level of the SCA where the upper basilar artery curves.
- The hemodynamic thrust from the basilar artery flow impacts just above the SCA origin, not at the basilar apex.
- SCA aneurysms have a broad connection with the SCA, a large neck, and a challenging neck-to-dome ratio for endovascular therapy.
- Preserving the SCA during endovascular therapy is crucial as it supplies the deep nuclei of the cerebellum.
- Large SCA aneurysms may cause oculomotor nerve palsies by pressing on the nerve in the interpeduncular cistern cranial to the SCA.
Basilar fenestration
- Basilar artery aneurysms are more likely with anomalous basilar system architecture.
- Basilar system anomalous or variant architecture
- Basilar nonfusion (fenestration)
- Asymmetric or caudal fusion of the caudal divisions of the foetal ICA
- Hypoplastic communicating artery
- Foetal (persistent carotid) origin of the PCA.
- Embryology & mech
- Rare proximal non-dissecting basilar artery aneurysms usually occur due to embryologic development failure.
- Single basilar artery forms by the fusion of paired longitudinal neural arteries around the fifth foetal week.
- Failed fusion of these arteries often leads to aneurysms at the proximal portion of the nonfused artery.
- The medial wall base of unfused arteries has structural weaknesses, making them prone to aneurysms.
- The media is absent, the elastic is discontinuous, and the subendothelium is thinned
- Lateral wall are normal
- Secondary triggers like hemodynamic stress can increase the likelihood of arterial aneurysms.
- Treatment
- Surgical treatment of these aneurysms is challenging due to their location and the complexity of surgical approaches.
- Endovascular embolization is an alternative treatment, but it requires preserving both limbs of the unfused basilar artery and managing the broad neck of the aneurysms.
- There is a risk of aneurysm regrowth due to unfavourable hemodynamics at the unfused segment site.
PICA aneurysm
- Vertebral artery aneurysms mostly originate at the PICA, particularly when PICA is at the apex of a superiorly directed curve of the vertebral artery.
- These aneurysms usually point upward and have a wide communication with the PICA.
- The size of the territory supplied by the PICA varies and affects the choice of aneurysm therapy.
- Common anatomic variants of the vertebral artery include unilateral agenesis/hypoplasia, double origin, and extracranial or epidural origin.
- There are inverse relationships among the sizes of the territories supplied by the PICA, AICA, and SCA, leading to variations like the AICA-PICA trunk.
- PICA supply to both cerebellar hemispheres is rare but possible and should be considered before endovascular procedures.
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