Proposed definition of medium vessel occlusion (both A and B have to apply)*
| A) Occlusion in one of the following vessel segments | |
| Anatomical | M2 segment (from the main MCA bifurcation/trifurcation to the circular sulcus of the insula)12 |
| M3 segment (from the circular sulcus of the insula to the external/superior surface of the Sylvian fissure)12 | |
| A2 segment (from the origin of the anterior communicating artery to the origin of the callosomarginal artery)13 | |
| A3 segment (from the origin of the callosomarginal artery to the artery’s posterior turn above the corpus callosum)13 | |
| P2 segment (from the origin of the posterior communicating artery to the point of entrance in the quadrigeminal cistern)14 | |
| P3 segment (segment within the quadrigeminal cistern)14 |
| B) Substantial clinical deficit (one of the following) | |
| Functional | NIHSS ≥ 5 |
| NIHSS < 5 with disabling deficit† |
*Vessel size allows for safe endovascular thrombectomy based on available technology and techniques (typical vessel size in MeVOs ranges from 1 to 3 mm15).
†Particularly important for P2/3 segment occlusions, since the NIHSS score does not accurately capture clinical deficits caused by posterior circulation occlusions.16
MCA, middle cerebral artery; MeVO, medium vessel occlusion ; NIHSS, National Institutes of Health Stroke Scale.