Exercise: Intracranial Vascular Abnormalities

EXERCISE 12-6. INTRACRANIAL VASCULAR ABNORMALITIES

12-14.  In Case 12-14, what is the reason for the abnormality on the CT scan (Figure 12-34 A–C)?

A.         Cerebral aneurysm

B.         Arteriovenous malformation

C.         Head trauma

D.         Carotid dissection

12-15. In Case 12-15, what is the reason for the abnormality on the CT scan (Figure 12-35 A-C)?

A.         Cerebral aneurysm

B.         Arteriovenous malformation

C.         Head trauma

D.         Carotid dissection

E.         Vasculitis

Radiologic Findings

12-14. In this case, the CT scan (Figure 12-34 A) shows exten-sive subarachnoid hemorrhage filling the basal cis-terns, more pronounced on the right (black arrows), with extension of hemorrhage into the interhemi-spheric and Sylvian fissures. Enlargement of the tem-poral horns (white arrows) is indicative of early hydrocephalus. An oblique craniocaudal view from a CT angiogram (Figure 12-34 B) shows a 4-mm saccu-lar aneurysm arising from the right paraclinoid inter-nal carotid artery (arrow), most likely posterior communicating artery origin. A lateral view taken from a right common carotid catheter angiogram (Figure 12-34 C) confirms a posterior communicating artery origin aneurysm (black arrow) with upward distortion of the normal anterior cerebral artery con-figuration secondary to hydrocephalus (black arrow-heads). (A is the correct answer to Question 12-14.)

12-15.  In this case, the noncontrast CT scan (Figure 12-35 A) shows a lobulated, hyperdense mass (black arrows) centered in the medial right occipital lobe with ef-facement of the right occipital horn. Accompanying axial T2 MRI (Figure 12-35 B) reveals a nidus of low signal vascular “flow-voids” (white arrowheads) in the occipital lobe with a more prominent draining vein extending into the quadrigeminal plate cistern (white arrow). A subsequent catheter angiogram (Figure 12-35 C) of the right vertebral artery (curved arrow) confirms a high-flow vascular lesion with a tangle of vessels (double black arrows) and early ve-nous opacification (arrowhead) characteristic of an arteriovenous malformation. (B is the correct answer to Question 12-15.)

Discussion

Cerebrovascular disorders (strokes) were discussed in Exer- cise 12-2, which dealt mainly with cerebral infarction second- ary to atherosclerosis. For information on other causes of other  common  vascular  conditions  affecting  the  CNS: aneurysms and vascular malformationsbeen thought to develop at congenitally weak areas of a blood vessel wall. Recent evidence, however, has questioned this view, and many now believe that saccular aneurysms are probably acquired lesions from abnormal hemodynamic stresses that damage the arterial wall.

Intracranial aneurysms are usually asymptomatic until they rupture, at which time the patient typically presents with a severe headache resulting from subarachnoid hemor-rhage. The vast majority of nontraumatic SAHs occur as a result of aneurysm rupture. CT is very good at demonstrating SAH. Patients usually undergo CT angiography whenever nontraumatic SAH is detected, and occasionally cerebral ar-teriography.

Common locations for intracranial aneurysms include the anterior communicating artery, the internal carotid ar-tery at the origin of the posterior communicating artery, and the middle cerebral artery trifurcation. Posterior fossa aneurysms are less common; they make up only around 10% of all intracranial aneurysms and typically arise from the basilar artery tip.

Vascular malformations can be divided into four major types: true arteriovenous malformations (as demonstrated in Case 12-15), cavernous hemangiomas, venous angiomas, and capillary telangiectasias. AVMs are congenital lesions consist-ing of a tangle of abnormal blood vessels, usually within the brain parenchyma, that are fed by enlarged cerebral arteries and drained by dilated, tortuous veins. Because there is no normal intervening brain parenchyma for the blood to flow through, blood is rapidly shunted from the arterial to the ve-nous side. This shunting is dramatically demonstrated on cerebral arteriography. Patients with AVMs usually present with intracranial hemorrhage or seizures. MR imaging or contrast-enhanced CT can demonstrate the tortuous vascu-lar channels of most AVMs, although cerebral arteriography is the definitive study in this condition.

The other intracranial vascular malformations have very characteristic appearances on MR imaging, although they are frequently invisible on cerebral arteriography. Patients with these “low-pressure” malformations can present with headaches, seizures, or, rarely, intracranial hemorrhage. Many of these lesions, however, are incidentally discovered on MR scans performed for other reasons.