23.9 ); multiple aneurysms exist in 20% to 30% of cases. The aneurysms are not present at birth but develop over time because of underlying defects in the vessel media. There is an increased risk for aneurysms in patients with autosomal dominant polycystic kidney disease (Chapter 14) and genetic disorders of extracellular matrix proteins (e.g., Ehler-Danlos syndrome). Overall, roughly 1.3% of aneurysms bleed per year, with the probability of rupture increasing with size. For example, aneurysms larger than 1 cm in diameter have a roughly 50% risk for bleeding per year. In the early period after a subarachnoid hemorrhage, there is an additional risk for ischemic injury from vasospasm of other vessels. Healing and the attendant meningeal fibrosis and scarring sometimes obstruct CSF flow or disrupt CSF resorption, leading to hydrocephalus.
Fig.23.10 ).Beyondtheneckoftheaneurysm,themuscularwallandintimalelasticlaminaareabsent,suchthattheaneurysmsacislinedonlybythickenedhyalinizedintima.Theadventitiacoveringthesaciscontinuouswiththatoftheparentartery.Ruptureusuallyoccursattheapexofthesac,releasingbloodintothesubarachnoidspace,thesubstanceofthebrain,orboth.
In addition to saccular aneurysms, atherosclerotic, mycotic, traumatic, and dissecting aneurysms also occur intracranially. The last three types (like saccular aneurysms) most often are found in the anterior circulation, whereas atherosclerotic aneurysms frequently are fusiform and most commonly involve the basilar artery. Nonsaccular aneurysms usually manifest as cerebral infarction due to vascular occlusion instead of subarachnoid hemorrhage.
Vascular malformations of the brain are classified into four principal types based on the nature of the abnormal vessels: arteriovenous malformations (AVMs), cavernous http://ebooksmedicine.net
Fig. 23.9 Commonsitesofsaccularaneurysms.Anterior cerebral artery Internal carotid artery Posterior cerebral artery Posterior communicating artery Anterior communicating artery Middle cerebral artery Basilar artery 20%34%40%4% malformations, capillary telangiectasias, and venous angiomas. AVMs, the most common of these, affect males twice as frequently as females and most commonly manifest between 10 and 30 years of age with seizures, an intra-cerebral hemorrhage, or a subarachnoid hemorrhage. In the newborn period, large AVMs may lead to high-output congestive heart failure because of blood shunting from arteries to veins. The risk for bleeding makes AVM the most dangerous type of vascular malformation. Multiple AVMs can be seen in the setting of hereditary hemorrhagic telangiectasia, an autosomal dominant condition often associated with mutations affecting the TGFβ pathway.
MORPHOLOGYAVMs mayinvolvesubarachnoidvesselsextendingintobrainparenchymaoroccurexclusivelywithinthebrain.Ongrossinspection,theyresembleatanglednetworkofwormlikevascularchannels(
Fig.23.11 ).Microscopicexaminationshowsenlargedbloodvesselsseparatedbygliotictissue,oftenwithevidenceofprevioushemorrhage.Somevesselscanberecognizedasarterieswithduplicatedandfragmentedinternalelasticlamina,whileothersshowmarkedthickeningorpartialreplacementofthemediabyhyalinizedconnectivetissue.