Intracranial aneurysm hemodynamics [Neurosurgery Wiki]

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====== Intracranial aneurysm hemodynamics ======

Intracranial aneurysm hemodynamics is a critical area of study in neurosurgery and vascular neurology, focusing on the behavior of blood flow within and around aneurysms in the cerebral vasculature. Understanding the hemodynamics of intracranial aneurysms is essential for assessing their rupture risk, guiding treatment decisions, and developing advanced therapeutic approaches. Below is an overview of key aspects:

1. Hemodynamic Parameters
Wall Shear Stress (WSS):

WSS is the tangential force exerted by blood flow on the vessel wall.
Low WSS is often associated with aneurysm initiation and growth due to endothelial dysfunction.
High WSS is linked to rupture risk as it can weaken the aneurysm wall.
Oscillatory Shear Index (OSI):

OSI measures the directional change of WSS during the cardiac cycle.
High OSI regions may promote endothelial cell damage and inflammatory responses, contributing to aneurysm instability.
Flow Velocity and Patterns:

Disturbed or turbulent flow within the aneurysm sac may indicate higher rupture risk.
Stable and laminar flow patterns are generally associated with lower rupture risk.
Pressure Distribution:

Elevated intraluminal pressure, particularly at the dome of the aneurysm, can contribute to wall weakening.
2. Flow Models and Analysis Techniques
Computational Fluid Dynamics (CFD):

CFD simulations are widely used to model blood flow within aneurysms.
They require patient-specific imaging data (e.g., CT angiography or MR angiography) to create 3D models of the aneurysm and its parent vessel.
4D Flow MRI:

A non-invasive imaging technique that provides time-resolved, three-dimensional velocity data.
Useful for assessing flow patterns and quantifying hemodynamic metrics in vivo.
In Vitro and In Vivo Studies:

In vitro models of aneurysms allow controlled experiments to study hemodynamic behavior.
In vivo studies provide data on real physiological conditions but are more complex.
3. Hemodynamics and Aneurysm Pathophysiology
Aneurysm Formation:

Abnormal hemodynamic forces (e.g., high wall shear stress gradients) can lead to vascular remodeling and the formation of aneurysms.
Growth:

Sustained low WSS may trigger endothelial dysfunction and chronic inflammation, promoting aneurysm enlargement.
Rupture:

High WSS and stress concentration, especially at the dome, can lead to rupture.
The interplay of hemodynamic forces with biological factors like inflammation, extracellular matrix degradation, and apoptosis is critical.
4. Clinical Implications
Risk Assessment:

Hemodynamic features, combined with morphological parameters (size, aspect ratio, neck width), help in evaluating the risk of aneurysm rupture.
Treatment Planning:

Endovascular treatments like coiling or flow diversion aim to alter hemodynamics within the aneurysm.
Flow diverters redirect blood flow away from the aneurysm sac, promoting thrombosis and reducing rupture risk.
Post-Treatment Monitoring:

Understanding hemodynamics is essential for evaluating the success of interventions and the likelihood of aneurysm recurrence.
5. Emerging Research
AI and Machine Learning:

Machine learning models are being developed to predict aneurysm behavior using hemodynamic data.
Personalized Medicine:

Patient-specific hemodynamic analyses are driving personalized therapeutic strategies.
Advanced Imaging:

Techniques like ultra-high-resolution 7T MRI are enhancing our ability to study aneurysm hemodynamics and wall pathology.

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see also [[Unruptured intracranial aneurysm rupture risk]].

For a given [[intracranial aneurysm morphology]], Cebral et al. 
((Cebral JR, Castro MA, Appanaboyina S, Putman CM, Millan D, Frangi AF. Efficient pipeline for image-based patient-specific analysis of cerebral aneurysm hemodynamics: Technique and sensitivity. IEEE Trans Med Imaging. 2005;24:457–467.)) 
showed that [[intracranial aneurysm hemodynamics]] do not vary significantly with physiological variations of [[flow rate]], [[blood pressure]], and [[waveform]]. Therefore, suitable parameters characterizing IA [[geometry]] can capture the characteristic hemodynamics and potentially predict [[rupture risk]]. Several past studies have investigated such parameters.

Geometric or [[intracranial aneurysm]] hemodynamics considerations favor identification of rupture status; however, retrospective identification of the rupture site remains a challenge for both engineers and clinicians. A more precise understanding of the hemodynamic factors involved in [[aneurysm wall]] pathology is likely required for [[computational fluid dynamics]] to add value to current clinical decision-making regarding rupture risk
((Janiga G, Berg P, Sugiyama S, Kono K, Steinman DA. The computational fluid
dynamics rupture challenge 2013-phase I: prediction of rupture status in
intracranial aneurysms. AJNR Am J Neuroradiol. 2015 Mar;36(3):530-6. doi:
10.3174/ajnr.A4157. Epub 2014 Dec 11. PubMed PMID: 25500315.)).

It is known that [[hemodynamics]] play an important role in the [[intracranial aneurysm pathogenesis]]
((Cebral JR, Raschi M (2013) Suggested connections between risk
factors of intracranial aneurysms: a review. Ann Biomed Eng
41:1366–1383.)).

[[Hemodynamic]] [[parameter]]s play a significant role in the development of [[intracranial aneurysm]]s. Parameters such as [[wall shear stress]] (WSS) or velocity could change in time and may contribute to aneurysm growth and rupture. However, the hemodynamic changes at the rupture location remain unclear because it is difficult to obtain data prior to rupture
((Sejkorová A, Dennis KD, Švihlová H, Petr O, Lanzino G, Hejčl A,
Dragomir-Daescu D. Hemodynamic changes in a middle cerebral artery aneurysm at
follow-up times before and after its rupture: a case report and a review of the
literature. Neurosurg Rev. 2016 Nov 24. [Epub ahead of print] PubMed PMID:
27882440.
)).

===== Experimental in vitro studies =====

Variability in long-term [[endovascular treatment]] outcomes for [[intracranial aneurysm]]s has prompted [[question]]s regarding the effects of these treatments on [[aneurysm hemodynamics]]. [[Endovascular technique]]s disrupt aneurysmal [[blood flow]] and [[shear]], but their influence on intra-aneurysmal pressure remains unclear. A better understanding of aneurysm pressure effects may aid in predicting outcomes and guiding treatment decisions.

Medium and large aneurysm models with intramural pressure taps on the [[dome]] and [[parent artery]] were designed and 3D-printed with vessel-like physical properties from UV-cured materials. The models were connected to a comprehensive flow system consisting of a pulsatile pump and a viscosity-matched blood analog. The system provided physiological pressure and flow control. Real-time pressures were recorded in the aneurysm dome and parent artery during initial placement of coils, stents, flow diverters, and temporary balloons under simulated surgical conditions. Coiling, stent-assisted coiling, and flow diverter placement were performed in both aneurysm sizes. Temporary balloon placement was performed in a large aneurysm model.

Coiling resulted in 24-30% packing density and diminished intra-aneurysmal flow. Flow diverter placement reduced intra-aneurysmal flow with near complete flow interruption after placement of three consecutive devices across the aneurysm neck. Compared to untreated controls, real-time pressure measurements during coiling and flow diversion showed minimal changes (< 5%) in intra-aneurysmal pressures. Temporary balloon occlusion blocked the parent artery, increasing the pressure proximal to the site of occlusion (by 9%), and reducing the pressure distally (by 14%). This maneuver also dampened intra-aneurysmal pressure to the average distal vessel pressure measurement. Positive control aneurysm models were 3D-printed with a sealed, "healed" neck. These controls verified a sealed neck eliminates intra-aneurysmal pressure.

Findings quantified minimal changes in [[intra-aneurysmal pressure]] during and immediately post-coiling and [[flow diversion]]. Intra-aneurysmal [[flow disruption]] alone has negligible impact on intra-aneurysmal pressures
((Becker TA, Lewis KL, Berns HF, Robertson SE, Clark WE, Wells JC, Alnajrani MK, Rapoport C, Barhouse P, Ramirez-Velandia F, Filo J, Young M, Muram S, Granstein JH, Ogilvy CS. Aneurysm dome and vessel pressure measurements with coiling, stent assisted coiling and flow diversion. Acta Neurochir (Wien). 2025 Jan 9;167(1):8. doi: 10.1007/s00701-024-06392-5. PMID: 39789382.)).
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This study makes a significant contribution by quantifying the minimal impact of endovascular techniques on intra-aneurysmal pressures. However, its findings must be interpreted within the context of its limitations. Future research should focus on integrating additional hemodynamic parameters, long-term outcomes, and in vivo studies to bridge the gap between experimental results and clinical application.