**6. Diagnosis**

Given that the vast majority of blister aneurysms presents with subarachnoid hemorrhage, the diagnostic evaluation of such a lesion usually starts with a brain CT scan. Hemorrhage is typically lateralized and mainly involves the carotid and suprasellar cisterns as well as the Sylvian fissure. Of note is that, when performed within 24 hours after ictus, CT scans detect subarachnoid blood with a sensitivity of up to 95%, a figure though that quickly drops to less than 50% a week later [31]. This is attributed to the rapid decrease of the hemorrhage density due to dilution by cerebrospinal fluid [32].

The second step in the diagnostic triage of subarachnoid hemorrhage is, for most centers nowadays, a CT angiogram. Blister aneurysms appear, initially at least, as shallow, broad-based lesions, usually less than 2 mm and with a characteristic triangular or thornlike shape (**Figure 1**) [33, 34]. Unfortunately, their small size in combination with their unusual location (i.e. nonbranching arterial sites) and close proximity to the skull base often makes detection of these lesions obscure [35]. Adding to the difficulty, the aneurysmal dome presumably collapses right after rupture, while its parent artery contracts as a reaction to the presence of subarachnoid blood. The end result can be a significant delay in diagnosis and initiation of treatment. Sensitivity of single slice CT angiography in the investigation of intracranial aneurysms smaller than 3 mm has been reported to be 25–64% [36]. Better results with improved image quality and spatial resolution have been achieved with the introduction of multidetector row technology [37]. Blister lesion diagnosis can be also greatly facilitated through the application of a meticulous technique (decreased section thickness, increased pitch, proper bolus timing and elimination of venous contamination) along with appropriate postprocessing of CTA scans (maximum intensity projections, multiplanar reconstructions and volume-rendered 3D images) [33]. Notably, most false-negative CT angiograms, when evaluated in retrospect, do reveal suspicious anomalies that could be well associated with a blister aneurysm. This observation underlines the significance in such cases of a high index of suspicion.

Despite advances in the field of CT angiography, conventional DSA with its excellent spatial resolution remains the gold standard for the detection of cerebral aneurysms, and, as such, it should be performed whenever initial investigations prove to be negative. The appearance of blister lesions on a DSA closely resembles that of their CTA counterparts, but luminar irregularities related to atherosclerosis of adjacent arterial segments may obscure the diagnosis (**Figure 2**) [35]. Multiple

*Reconstructed CTA showing a blister-like aneurysm on the dorsal wall of the internal carotid artery (ICA).*

#### **Figure 2.**

*Internal carotid injection, AP view. A characteristic thornlike blister aneurysm is noted opposite to the anterior choroidal artery origin. Large posterior communicating artery supplying the posterior circulation noted.*

oblique views or even rotational 3D scans significantly increase the sensitivity of the method [38]. Signs of dissection have been reported in up to 89% of blister aneurysm cases and include a false lumen, an intimal flap, a filling defect or contrast pooling [39]. Should the presence of a blister aneurysm be suspected on the basis of DSA findings, the evaluation of collateral flow through the circle of Willis is always advisable in case an occlusion procedure is to be carried out [18, 33]. Cross-compression carotid injections may help demonstrate the anterior communicating artery, while patency, size and collateral potential of its posterior counterpart can be assessed through an Alcock test (vertebral injections with carotid artery compression). For a more detailed study, temporary balloon occlusion will be required [31].

Another important aspect of imaging in cases of blister aneurysms is that, on short-term follow-up angiography after initial presentation, these lesions usually show rapid growth to a saccular configuration [19]. Being at least partially related to lysis of an intra-aneurysmal clot [26], this progression is considered to be a good indicator of a blister lesion even though the only real way to authenticate such a diagnosis is through direct intraoperative inspection [1, 40].
