Purpose Differentiation between PCOM aneurysm and infundibulum is a common clinical question and often difficult to resolve on CT angiography or MRA. PCOM infundibula are also known to constitute a “false positive” finding of aneurysm on CTA of subarachnoid hemorrhage patients, even leading to unnecessary craniotomy. It is commonly believed that, by demonstrating a vessel arising from the apex of the lesion, DSA easily differentiates infundibulum from aneurysm. However, significant difficulty can be encountered if the vessel arising from the apex fails to opacify in an antegrade fashion. In such cases, a PCOM infundibulum can be misdiagnosed as an aneurysm. We describe an “Empty apex sign” on DSA as a faint and variable filling defect at the apex of such abnormalities that can resolve the lesion as infundibulum even if one is unable to see the vessel arising from the apex. This filling defect stems from cyclical inflow of un-opacified, reversed blood flow in the PCOM, where it contacts the infundibular dilatation.

Materials and Methods Over a 2 years period, we prospectively identified seven patients with eight lesions in the communicating segments of ICA that appeared to be small aneurysms on initial 2D-DSA but were proved to be infundibuli with further angiographic workup. All studies were performed in an Artis Zee® biplane angiographic suite (Siemens, Erlangen, Germany) using standard neurointerventional techniques and OmnipaqueTM 300 (GE Healthcare, Little Chalfont, UK) contrast material. In all selected cases, there was difficulty in determining the nature of the PCOM vascular abnormality to be either infundibulum or aneurysm using standard 2D-DSA, besides the use multiple views. Our protocol was then to attempt to identify the nature of the PCOM segment lesion using 3D-DSA acquisition with shaded-surface display reconstructions performed using a Siemens Syngo® LEONARDO (Siemens, Erlangen, Germany) workstation. Next step was evaluation of these lesions using source and maximum intensity projection images off the 3D-DSA sequence. If that was not conclusive, a vertebral artery injection with carotid compression was performed, in an attempt to fill the PCOM associated with possible infundibulum and demonstrate a connection site with the ICA abnormality (at its apex).

Results There were four lesions on the right and four on the left. Average size was 2.7 mm. Four of the 8 (50%) were somewhat conical and 50% were rounded. Three lesions (37.5%) were resolved with the help of 3D-DSA reconstructions and source images. The remaining five were diagnosed with vertebral artery angiogram, performed simultaneously with ICA compression on the side of the infundibulum. All lesions demonstrated what we now call “empty apex sign” on 2D-DSA.

Conclusion When the PCOM is relatively small in size and a hemodynamic equality exists between the anterior and posterior circulations, the PCOM can fail to opacify on the carotid injection. In presence of an infundibulum, lack of filling of an associated small PCOM can be misinterpreted as an aneurysm. Using case examples from this series, we demonstrate the usefulness of the “empty apex sign” on 2D-DSA that suggests that the lesion in question is an infundibulum.

Competing interests None.