Cellophane is making a splash in the veterinary surgical literature!

ноябрь 30, 2018

Several new veterinary cellophane and portosystemic shunt studies published

Have you been keeping track of the literature this past few months? If not, there are a plethora of new studies looking at various aspects of cellophane or thin film banding use for attenuation of portosystemic shunts in veterinary patients.

I've taken the liberty of summarizing and analyzing them all here and you will find links to the original articles throughout this blog so you can take the time to read them yourself and make up your own mind.

Dr Strickland et al (Vet Surg, 2018) from the Royal Veterinary College reported on the "Incidence and risk factors for neurological signs after attenuation of single congenital portosystemic shunts in 253 dogs"
using thin film banding. This study looked at dogs over a 15 year period between 2000 and 2015 who had surgical attenuation of their singular, extra-hepatic portosystemic shunts confirmed by intra-op mesenteric porto-venogram. Whilst this paper doesn't describe the surgeons' techniques, I have contacted one of the authors who described some key features of their operations:

In more modern cases, pre-op CT angiograms, intra-op imaging and portal pressures were always performed.
Acute, complete attenuation with suture was performed when tolerated based on portal pressure and clinical signs of portal hypertension. When complete attenuation was not able to be performed, then thin film banding was performed with the vessel attenuated as far as possible without excessive causing portal hypertension. If thin film banding is performed a loose propropylene suture is left near the attenuation site to help ID the area if revision is required.
Thin film was of uncertain provenance and could not be determined to be 'real' cellophane.
Unpublished (as yet) data suggests excellent long term outcomes with ~80% complete occlusion at 3 months and revision surgery very rarely required even if small residual shunting was present, as bile acid levels and clinical signs often improve to a near normal level.

Multivariate analysis showed a statistically significant relationship between these variables and post op neurological signs:

Increasing age
Presence of signs of HE immediately preoperatively

Overall post-op neurological sign rate of 11.1% with no effect of pre-treatment of levitiracetam (Keppra) on the risk of post op neuro signs. This study suggests that early diagnosis and treatment of dogs with portosystemic shunts helps to reduce the risk of post op neurological signs.

This study by Dr Wallace et al looked at the "Gradual attenuation of a congenital extrahepatic portosystemic shunt with a self-retaining polyacrylic acid-silicone device in 6 dogs" and was published in Veterinary Surgery in 2018. This clinical study reported the outcome in 6 dogs who underwent portosystemic shunt attenuation using a novel gradual occluder made from a polyetheretherketone ring and proprietary polyacrylic acid-silicone hygroscopic gel. Like an ameroid, the device works similarly by having the polyacrylic gel slowly swell within a solid ring, which causes slow concentric occlusion of the shunt vessel. In Wallace's study, 4/6 dogs had complete occlusion of the shunt vessel based on post-op CT angiogram and ultrasound at 8 weeks, and no dogs in the study developed acquired shunts. Of the 2/6 dogs which had residual shunting, one had normal serum bile acids despite continued shunting. This study showed that the novel occluder worked as anticipated and had similarly successful results when compared to thin film banding and traditional ameroids, although the case numbers are very small. The polyacrylic acid-silicone device has one substantial advantage over ameroids however, in that it is nearly completely radiolucent and allows for post-op CT angiograms without the presence of artifact.

In a study by Dr Joffe, also published in Veterinary Surgery in 2018, the researchers looked at comparing different means of clipping the thin film in place as well as the effect of 3 vs 4 layers on security of the clips. The study tested the the mechanisms of and pressures at failure of the thin film banding by comparing the combination effects of:

3 vs 4 layered cellophane
25% vs 50% attenuation of cadaveric external jugular veins
Medium vs medium-large clips
Titanium vs polymer clips

The study found that approximately 15% of all clips partially failed (irrespective of size or composition) by slippage and another 3.7% failed by falling off. None of the cellophane broke in any test. None of the medium sized clips, irrespective of their composition or layers of thin film, slipped. Statistically, there was a significant risk for failure when using 3 layers vs 4 and when using larger clips (medium-large). There was no difference between titanium or polymer locking clips based on the statistical models used. It is important to note than only one clip was ever applied to any of the constructs, and all failures in this study occurred when supraphysiological intravascular pressures were applied. It stands to reason that a single, medium sized clip may be enough to adequately attenuate a shunt using thin film. Current clinical recommendations to place 3-4 clips however still remain a valid option.

Dr Joffe's study used the 'sourced from the same cellophane as that reported by Smith et al paper from 2014.' In that study however the researchers used spectroscopy to determine the composition of the thin film, and only one of the 4 sources was determined to be cellulose based. Now, with due credit, Dr Joffe et al did attempt to obtain real cellophane, but their source, Hallmark cards (yes, that Hallmark), is hardly a medical device manufacturer and could easily swap out the substrate of their "Clear Cello Sheet" to a plastic with no notice given to the public. No spectroscopy was performed on any of the thin film used in Dr Joffe's study to confirm whether they used actual cellophane. This could be evidenced by the fact that the same facility in Australia recently also reported a recent case of recanalization of a thin film attenuated shunt in a cat. This report was by Dr Crowley et al in the Journal of Feline Medicine and Surgery Open Reports. In that case, the patient had a 'commercial' roll cellophane (i.e. likely plastic) used in its first procedure, which failed by incompletely attenuating the shunt. The surgeons then obtained the same "Clear Cello Sheet" as reported by Smith et al and Joffe et al and again, the surgery failed to completely attenuate the vessel based on clinical signs, CT angiogram and serum bile acids. Crowley et al contend that the source of cellophane hardly matters clinically, as in their single case, both types of thin film failed. This contention is flawed on several counts:

No spectroscopy was ever performed on either of their thin films, so it is absolutely possible that BOTH their bands were plastic and could account for the recanalization.
Drawing conclusions from a single case and applying it to a whole body of research is foolhardy.

In the end, further research is required, and not all thin films are cellophane. We are partnering with several academic institutions in order to help determine exactly how real, uncoated, cellophane works and what we can do to improve clinical outcomes in our patients.