Reviews of the Medtronic Onyx 18 | Which Medical Device

Introduction

Onyx is widely used as a first line liquid embolic agent for the treatment of intracranial arteriovenous malformations (AVMs). It is being increasingly used in the treatment of peripheral AVMs and visceral aneurysms, in EVAR endoleaks and occasionally in gastrointestinal haemorrhage.

Features

Onyx is a non-adhesive liquid polymer. It comprises ethylene vinyl alcohol (EVOH) co-polymer dissolved in DMSO (di-methyl sulfoxide) and suspended tantalum powder to provide contrast for visualisation under fluoroscopy. Onyx is delivered through a DMSO compatible micro-catheter. As the DMSO diffuses away, the EVOH co-polymer precipitates and forms a coherent embolus. Onyx is non-adhesive and the catheter can therefore be left in situ whilst additional injections are made. Three formulations of Onyx are available, 18, 20 and 34, describing the viscosity and with increasing percentage EVOH content, the higher the flow in the lesion the higher viscosity that is needed to prevent it streaming away from the catheter tip during injection. Onyx 500 is used for aneurysm occlusion, it is highly viscous and cohesive, and it is normally only injected with an occlusion balloon temporarily occluding the patent vessel and a trapped micro-catheter.

Some special preparation is required but this is easy. The catheter must be DMSO compatible. It is flushed with 3x 2ml heparinised saline to remove any blood. The catheter dead space is then filled with pure DMSO solvent. The catheter should have its dead space identified on its packing. The selected Onyx is then drawn up and very slowly injected to clear the DMSO within the catheter, typically over 90-120 seconds. Under continuous fluoroscopy the radio-opaque Onyx is observed as it emerges from the micro-catheter. It should initially be injected very, very slowly to allow an occlusive plug to build up around the tip of the catheter. Reflux back around the catheter will occur and when this is seen the injection is paused for 15-90 seconds. The injection is restarted and the liquid will be seen to advance forwards away from the catheter tip through the vessel to be occluded. As Onyx is administered it forms a soft spongy polymer cast, with a semi-liquid centre, and when more Onyx is injected additional material breaks through the outer layer propagating further into the malformation. If proximal reflux does recur, injection can be paused, allowing a plug to form and partly solidify, which then promotes forward flow on resuming injection. When the embolisation is complete the Onyx should be allowed to solidify for 90 seconds then suction applied to the micro-catheter as it is pulled out of the Onyx plug and vessel.

Click the related video tab to view a video demonstrating in-vitro how Onyx works. It was filmed during a discussion/demonstration of Onyx by Dr Andy Platts at the SERF course in Newquay, UK.

In use

We illustrate the properties of Onyx - demonstrating its use in the treatment of three cases.

Case 1 – High flow congenital left forearm vascular malformation. The AVM pedicles were selectively catheterised using an Ev3 Marathon micro-catheter and the Onyx 18 delivered. Extensive propagation of the Onyx through the nidus of the AVM was achieved from an interosseous branch (images 1-6).

Case 2 – Type II endo-leak following EVAR. On angiography the endoleak was seen to arise from an enlarged lower left lumbar artery – this was selectively catheterised via the internal iliac artery using a micro-catheter. The endoleak cavity was obliterated using Onyx 18 (images 7 - 11)

Case 3 – AVM arising from the superficial temporal artery, with filling from multiple collaterals from other external carotid artery branches. A micro-catheter was placed at the point of fistulation and Onyx injected (images 12 - 16)

Cautions.

1. DMSO causes vasospasm if injected rapidly, it should be displaced from the catheter very slowly.
2. DMSO is painful. In peripheral malformation I only use it with the patient anaesthetised and ideally anaesthetise patients for treatment of EVAR Type 2 endoleaks.
3. Plan a point of reflux back around the catheter beyond which you will not allow the Onyx to flow before you start. This should be clearly visualised on fluoroscopy and not obscured by the forwards flowing embolic plug.
4. DMSO smells and so will the patient when they return to the ward. It is quite unpleasant, rather oniony / garlicky and exhaled on the breath. Forewarning relatives is helpful.
5. The Tantalum metal powder in Onyx is inflammable and may ignite with single polar diathermy, this is a real clinical risk when plastic surgeons excise embolised malformations. It is not ignited by bipolar diathermy but a bowl of saline should be close to hand to extinguish any points of combustion during resection and the surgeon should be aware of this beforehand.

Conclusions

The non-adhesive nature and its slow solidification allow Onyx to be injected in a controlled manner. The propagation of the embolic cast is accurately monitored by screening. In our cases complete and solid casting of the AVM nidus, and the endoleak cavity was achieved. Excellent clinical outcomes were obtained in all cases. The non-adhesive nature of Onyx also allowed the catheter to be retrieved; there is a very low rate of catheter retention though this may occur if more than 1.5cm of reflux is achieved.

No conflict declared
Dr Andrew Wigham - Radiology SpR
Dr Andrew Platts - Consultant Interventional Radiologist
Royal Free Hospital, London, UK