Improving mechanical thrombectomy through neuroanatomy

Stroke outcomes can differ by sex. Michael Chen (Rush University Medical Center, Chicago, USA) comments on how differences in neuroanatomy may play a role.

Recently published Level 1a evidence in 2015 has proven the clinical benefit of mechanical thrombectomy for acute ischaemic strokes involving proximal vessels of the anterior circulation.^1-3 There has been tremendous enthusiasm following these studies and subsequent studies have been focused on liberalising patient selection criteria, whether by elapsed time core infarct size, lower initial stroke severity, distal occlusions and baseline morbidity among others.^4,5

What may be overlooked in these efforts is an honest re-evaluation of the effectiveness of the “one size fits all” technique of thrombectomy as it is currently used. Although it has been shown to yield a statistical benefit over medical management, there still remains a remarkable discrepancy between how favourable the final angiographic images appear and how significant the disability that remains at 90 days.

Although a portion of this discrepancy may be caused by the volume of infarcted brain prior to the procedure, other contributors, such as emboli to distal territory, the sequelae from multiple passes, vasospasm, dissection, interactions with collateral blood flow and/or reperfusion injury that may also contribute to this discrepancy.

One approach to optimising the technique of thrombectomy is to reevaluate the primary disease, particularly those subgroups who fare relatively worse. One basic categorisation would be that of gender. Multiple studies report a gender disparity in post-thrombectomy outcomes with females faring worse than males.^6,7 Furthermore, gender based cerebrovascular anatomic differences have also been described.⁸

Anecdotally, it appears as if women may have smaller arterial diameters than men. The Hagen Poiseuille equation also tells us that the arterial diameter proportionally affects the volumetric vessel flow rate. Hence, there may be value in validating the importance of arterial diameter as an essential determinant of clinical outcomes. In doing so, arterial diameter may be an important piece of information not only in planning the technical approach to thrombectomy (catheter size selection, stenttriever diameter selection, whether to pursue distal occlusions, etc.), but in the design of future devices and treatment approaches.

In the study by Davison et al, 92 consecutive anterior circulation large vessel occlusive stroke patients underwent manual detailed arterial measurements using a standardised approach using catheter angiogram  images.⁹ Clinical outcomes were collected. Significant gender differences in mean arterial diameters were seen at the internal carotid artery just proximal to the bifurcation as well as the origin of the M1 segment of the middle cerebral artery. Caliber size also modestly correlated with clinical outcomes.

These results provide limited evidence that men have larger arterial diameters than women and that arterial diameter may be an important determinant in thrombectomy clinical outcomes. Although the actual measured mean MCA diameter difference was only 0.29mm between genders, according to the Hagen-Poiseuille equation, whereby the volumetric flow rate is proportional to its radius to the fourth power, this would correlate to a 60.7% difference in volumetric flow rate.

Similarly, other anatomic, physiologic and pathologic features may be worthwhile to study, correlate with thrombectomy outcomes, and potentially incorporate into either treatment planning or future designs. Vascular tortuosity, poor cardiac output, hypercoaguable state, collateral flow, degree of peripheral arterial disease, tandem lesions, and vascular dissection are among other variables that deserve to help evolve the current debate on mechanical thrombectomy technique (usually focused on the merits of aspiration vs. stent-triever) to one that incorporates a more sophisticated understanding of the underlying patient-specific comorbidities and pathologic features.

References

1. Berkhemer OA, Fransen PS, Beumer D, et al. A randomized trial of intra-arterial treatment for acute ischemic stroke. N Engl J Med 2015;372(1):11-20. doi: 10.1056/NEJMoa1411587
2. Campbell BC, Mitchell PJ, Kleinig TJ, et al. Endovascular therapy for ischemic str oke with perfusion-imaging selection. N Engl J Med 2015;372(11):1009-18. doi: 10.1056/NEJMoa1414792
3. Goyal M, Demchuk AM, Menon BK, et al. Randomized assessment of rapid endovascular treatment of ischemic stroke. N Engl J Med 2015;372(11):1019-30. doi: 10.1056/NEJMoa1414905
4. Nogueira RG, Jadhav AP, Haussen DC, et al. Thrombectomy 6 to 24 Hours after Stroke with a Mismatch between Deficit and Infar ct. N Engl J Med 2018;378(1):11-21. doi: 10.1056/NEJMoa1706442
5. Albers GW, Marks MP, Kemp S, et al. Thrombectomy for Stroke at 6 to 16 Hours with Selection by Perfusion Imaging. N Engl J Med 2018;378(8):708-18. doi:10.1056/NEJMoa1713973
6. Silva GS, Lima FO, Camar go EC, et al. Gender differences in outcomes after ischemic str oke: role of ischemic lesion volume and intracranial lar ge-artery occlusion. Cerebrovasc Dis 2010;30(5):470-5. doi:10.1159/000317088
7. Gargano JW, Reeves MJ, Paul Cover dell National Acute Stroke Registry Michigan Prototype I. Sex differences in stroke recovery and stroke-specific quality of life: results from a state-wide stroke registry. Stroke 2007;38(9):2541-8. doi: 10.1161/ STROKEAHA.107.485482
8. Ghods AJ, Lopes D, Chen M. Gender differences in cerebral aneurysm location. Front Neurol 2012;3:78. doi: 10.3389/fneur.2012.00078
9. Davison MA, Ouyang B, Keppetipola KM, et al. Arterial diameter and the gender disparity in stroke thrombectomy outcomes. J Neurointerv Surg 2018 doi:10.1136/neurintsurg-2017-013697

Michael Chen is associate professor in the department of Neurological Surgery at Rush University Medical