Written By: Ravish Patel

Brain aneurysms are outpouchings of blood vessels in the brain. These balloon-like outpouchings can grow over time and eventually rupture, resulting in a subarachnoid hemorrhage, a serious form of brain bleed. One of the options for prophylactic treatment of a high-risk aneurysm is endovascular coiling, a minimally invasive technique that fills the aneurysm with small, flexible coils which effectively seal it off, reducing its risk of rupture significantly. The development of this technique was the result of a conglomeration of efforts from various pioneers since the 1960s (Figure 1).

Figure 1: A timeline of some of the advancements involved in the evolution of endovascular coiling

The brain’s vasculature is tortuous, making it a notoriously hazardous highway to navigate with a catheter. In 1964, Luessenhop and Velasquez reported the first catheterization of brain vessels using an early glass chamber apparatus, surgically connected to the external carotid artery. This milestone revealed that the brain’s blood vessels could be catheterized, but they still didn’t have a way of navigating the treacherous turns of the brain’s vessels safely. To address this problem, Frei and colleagues designed a catheter with a silicone rubber tip that housed a micromagnet. By manipulating external magnetic fields, they were able to steer the catheter around more effectively and safely. Various blood flow-dependent catheters were also designed around this time.

In 1967, Yodh and colleagues used the system developed by Frei to design a type of catheter that could include a detachable tip. The tip was tethered to the rest of the catheter by paraffin wax, which was embedded with copper wire. An external electric current could be applied to the copper wire, causing the wax to heat up, resulting in the jettisoning of the tip of the catheter. They predicted three potential uses for the catheters: (1) to block feeding vessels of certain vascular malformations with congealable plastic, (2) to thrombose certain aneurysms by injection of a congealable plastic or detaching the tip of the catheter into the aneurysm and then injecting iron particles that would stick to the magnet in the tip and fill the aneurysm, and (3) for very selective injection of chemotherapeutic drugs in high concentrations for the treatment of a tumor.

With the realization of our intimate connection with the cerebral vasculature and equipped with the ability to deploy various tools within catheters, it wasn’t long before new tools were developed to try to treat brain aneurysms. One of the early nominees was a detachable balloon by Montgomery et al. They developed a balloon made of latex surrounded by a carbon steel cylinder that could be inflated and detached from the catheter by injecting an albumin-containing solution into the balloon and then heating the carbon steel cylinder using an external radiofrequency induction coil. When the temperature of the cylinder reached 55°C, the albumin coagulated and sealed the opening of the balloon. Injection of a little bit of additional albumin solution expanded the delivery microcatheter and detached the balloon.

While the navigation systems that relied upon external magnetic fields and blood flow were substantial, there was still room for improvement before widespread adoption. This improvement came when Engelson, a biomechanical engineer, had an idea to modify and improve the existing microcatheter. He attached a portion to the tip of the catheter that was flexible enough to be molded but stiff enough to hold its shape when navigating the vasculature. Thus, the “tracker” microcatheter came to be. This new catheter allowed for advancement of the microcatheter over a microguidewire that made it easier to steer through the vasculature without relying heavily on blood flow or an external magnetic field. This solution opened the door to widespread use of intracranial navigation with a safe and simple system.

With newfound increased ability to navigate the vasculature, several people tried to use different styles of detachable balloons to occlude aneurysms in the brain. However, another challenge presented. Balloons did not adopt the shape of the aneurysm; rather, they forced the aneurysm to stretch out to the shape of the balloon within. This caused more stress on the already weak walls of the aneurysm, increasing their risk of rupture. Additionally, the balloons, which were filled with a solidifying agent, acted as conduits that transmitted energy from the systolic pulse of blood flow to the aneurysm’s walls, further increasing their risk of rupture. The high rates of morbidity and mortality from this solution beckoned for a new solution, one that wasn’t as unforgiving as the balloon in terms of rigidity but one that could still effectively occlude the aneurysm.

Figure 2: An aneurysm can be occluded by deploying numerous platinum coils, which fill the aneurysm but do not place much pressure on the thin walls.

In 1988, Hilal reported the first use of pushable coils for packing of an aneurysm (Figure 1). Initial iterations of the coil were too stiff and therefore they did not adequately conform to the shape of the aneurysm. Additionally, they were also detachable but not retrievable, which posed a problem when coils drifted down an incorrect artery. The original design was improved upon to produce smaller, more flexible, and retrievable platinum coils. These new coils could be retrieved and redeployed if their positioning was unsatisfactory. Once the positioning was satisfactory, a positive direct electrical current could be applied to the delivery wire which then electrolytically dissolved the delivery wire just proximal to the platinum coil, detaching it into the aneurysm. Numerous coils could be deployed using this method until the aneurysm was completely and tightly packed. They conformed to the shape of the aneurysm and did not exert as much pressure on the fragile walls of the aneurysm as the inflatable balloons. Thus, endovascular coiling was born, a minimally invasive technique to treat brain aneurysms that resulted from incremental improvements in design that will likely continue to undergo incremental improvements in the future.1

Sources:

1. Guglielmi G. History of endovascular endosaccular occlusion of brain aneurysms: 1965-1990. Interv Neuroradiol. 2007;13(3):217-224. doi:10.1177/159101990701300301

Figure 1 created with the help of canva.com

Figure 2 created with the help of Danielle Lanier