**3.2 Spectral Doppler analysis of the orbital (retro-bulbar) vessels in NA-AION**

In contrast, the patients with NA-AION present the following characteristics in acute stage, and at 1 week of evolution:


Severe ICA stenosis (≥70% of vessel diameter) combined with an insufficient Willis polygon led to diminish PSV in ipsilateral OA [9–14, 41].

*An Integrated Approach to the Role of Neurosonology in the Diagnosis of Giant Cell Arteritis DOI: http://dx.doi.org/10.5772/intechopen.96379*

In 1 month, CDI examinations of orbital blood vessels reveal that blood flow normalization is reached. The exceptions are the cases with severe ipsilateral ICA stenosis/occlusion [9–14, 41].

In conclusion, in NA-AION, blood velocities and RI in PCAs are preserved. Similar results were obtained in other studies [9–14, 41].

Fluorescein angiogram and CDI of the orbital vessels data support the histopathological evidence of involvement of the entire trunk of the PCAs in the A-AION (impaired optic disc and choroidal perfusion in the patients with A-AION). On the other hand, in the NA-AION, the impaired flow to the optic nerve head (ONH) is distal to the PCAs themselves, possibly at the level of the para-optic branches (only 1/3 of the flow of the PCAs) [36–40].

These branches supply the ONH directly (impaired optic disc perfusion, with relatively conservation of the choroidal perfusion) [36–40].

Extremely delayed or absent filling of the choroid has been depicted as a fluorescein angiogram characteristic of arteritic AION and has been suggested as one useful factor by which A-AION can be differentiated from NA-AION [36–40].
