What is a Challenging Clot? A DELPHI Consensus Statement from the CLOTS 7.0 Summit

Johanna M. Ospel*, Mahmood Mirza, Frédéric Clarençon, Adnan Siddiqui, Karen Doyle, Arturo Consoli, Maxim Mokin, Teresa Ullberg, Osama Zaidat, Romain Bourcier, Zsolt Kulcsar, Matthew J. Gounis, David S. Liebeskind, Jens Fiehler, Ana Paula Narata, Marc Ribo, Tudor Jovin, Nobuyuki Sakai, Ansaar Rai, Ray McCarthyFranziska Dorn, Tommy Andersson, Charles B.L.M. Majoie, Ricardo Hanel, Ashutosh Jadhav, Christian Riedel, Angel Chamorro, Waleed Brinjikji, Vincent Costalat, Simon F. DeMeyer, Raul G. Nogueira, Christophe Cognard, Joan Montaner, Thomas W. Leung, Carlos Molina, Heleen van Beusekom, Antoni Davalos, John Weisel, Rene Chapot, Markus Möhlenbruch, Patrick Brouwer

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

1 Citation (Scopus)


Background: Predicting a challenging clot when performing mechanical thrombectomy in acute stroke can be difficult. One reason for this difficulty is a lack of agreement on how to precisely define these clots. We explored the opinions of stroke thrombectomy and clot research experts regarding challenging clots, defined as difficult to recanalize clots by endovascular approaches, and clot/patient features that may be indicative of such clots. Methods: A modified DELPHI technique was used before and during the CLOTS 7.0 Summit, which included experts in thrombectomy and clot research from different specialties. The first round included open-ended questions and the second and final rounds each consisted of 30 closed-ended questions, 29 on various clinical and clot features, and 1 on number of passes before switching techniques. Consensus was defined as agreement ≥ 50%. Features with consensus and rated ≥ 3 out of 4 on the certainty scale were included in the definition of a challenging clot. Results: Three DELPHI rounds were performed. Panelists achieved consensus on 16/30 questions, of which 8 were rated 3 or 4 on the certainty scale, namely white-colored clots (mean certainty score 3.1), calcified clots under histology (3.7) and imaging (3.7), stiff clots (3.0), sticky/adherent clots (3.1), hard clots (3.1), difficult to pass clots (3.1) and clots that are resistant to pulling (3.0). Most panelists considered switching endovascular treatment (EVT) techniques after 2–3 unsuccessful attempts. Conclusion: This DELPHI consensus identified 8 distinct features of a challenging clot. The varying degree of certainty amongst the panelists emphasizes the need for more pragmatic studies to enable accurate a priori identification of such occlusions prior to EVT.

Original languageEnglish
Pages (from-to)1007-1016
Number of pages10
JournalClinical Neuroradiology
Issue number4
Early online date7 Jun 2023
Publication statusPublished - Dec 2023

Bibliographical note

Cerenovus Inc. funded the CLOTS 7.0 Summit.

Publisher Copyright:
© 2023, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany.


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