Cardiovascular Radiology


Case 2:
click for bigger picture An 18 year old male presented with history of recurrent headaches. A 3D CT angiography has been performed following a routine plain and contrast CT head.
What is the diagnosis ?



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Study shows aneurysm of the right middle cerebral artery.
Discussion :

Saccular berry aneurysms account for 70-90% of all aneurysms (1). They present as dilatation of an artery connected to the vessel by a wide or a narrow neck. They often arise at the bifurcation of an artery, although they are not always associated with vascular branching. Most saccular aneurysms are found in patients between the ages of 40-60 years; male predominance is present before 40 years of age, after 40 years, there is a strong preponderance in females.

Etiology

of saccular aneurysms is controversial. Bremer postulates that saccular aneurysms arise from the stumps of vessels that are lost as the fetal cerebrovascular network is reduced to the adult system of main arterial trunks (2), while Stehbens argues for a purely degenerative pathogenesis (3). Familial occurrence of intra cranial aneurysms has been sporadically reported in literature. Cerebral aneurysms have been reported with several vascular and pathologic lesions, such as persistent embolic vessels, AVMs, coarctation of the aorta, polycystic kidney disease, fibromuscular dysplasia, moya-moya disease, Ehlers-Danlos syndrome, pseudoxanthoma elasticum, Marfan's syndrome, and brain tumours.

Multiple cerebral aneurysms are not uncommon, and cerebral aneurysms may grow; the critical size for rupture has been estimated as 4 mm by Crompton, 6-15 mm by Crawford, and 5 mm by McCormick.
Probability of rupture increases with increase in intra-aneurysmal pressure, increase in size, decrease in the minimal wall thickness and decrease in the strength of the structural components. Histologic appearance of saccular berry aneurysms has been studied in great detail, the study of the aneurysm sac shows a splitting of the elastic lamina in the sac and a total absence of elastic at the site of rupture.
Other causes of aneurysms besides berry aneurysms include, arteriosclerotic aneurysms (common in basilar artery), bacterial aneurysms (in peripheral branches of major intracranial arteries following septic emboli from bacterial endocarditis/ septicaemia), inflammatory origin (polyarteritis nodosa, Wegener's granulomatosis, giant cell arteritis), traumatic (true / false type), and neoplastic aneurysms (associated with primary tumours such as atrial myxomas, chorioepitheliomas - due to tumour emboli producing cerebral branch occlusion and then further invasion of vessel wall by tumour cells). Giant aneurysms are classified as such when their diameter is larger than 25 mm.

Various modalities have been used to detect aneurysms and their complications such as CECT, DSA, conventional angiography, MRA and 3D CT Angiography (CTA) (4) .

CTA is usually able to demonstrate aneurysms with a diameter of 1 mm because of its higher spatial resolution. 3D pictures obtained by CTA are fundamentally different from the 2D projection images of MRA and DSA, thus CTA provides the useful anatomical presentation of the aneurysmal lesion, such as the shape of the neck and the dome, direction of aneurysmal growth, and adjacent arterial branches. Certain studies have found CTA to be superior to MRA in the diagnosis of cerebral aneurysms. MIP images are usually able to depict calcification within the arterial wall and this alerts the surgeon to possible intra-operative risk, especially while clipping the aneurysm. Ordinal image processing helps in excellent visualisation of the intra cranial vasculature inspite of subarachnoid hemorrhage. CTA does have several disadvantages, firstly overlap with the skull base bone can constitute a major problem, especially in the detection of infraclinoidal aneurysms. Thus it is necessary during post-image processing to exclude the clinoidal processes, lesser sphenoid wing and so on. Also, early enhancement of the cavernous can obscure the cavernous portion of the ICA, with the result that intra cavernous anuerysms may be overlooked, but by means of a large dose infusion of contrast and elevation of the threshold over 250 HU in SSR images, aneurysms within or close to the cavernous sinus can be clearly visible. Other disadvantages include difficulty in detecting small vessels like perforating arteries, hypoplastic PCoA, and anterior choroidal arteries, and failure to give information about blood flow characteristics (5).


Contribution : Dr. Kiron Master, Detroit, USA.