Closed loops and eyes. What are the potential links?

As many will be aware, the NHS is undertaking an England based population pilot of hybrid closed loop systems, to try and determine what are the best outcomes and see if the technology is up to scratch.

It’s being undertaken at centres across England, where patients are being signed up to use one of three systems available in the UK.

• Medtronic 780G
• CamAPS-FX
• Tandem T:Slim X2 with Contorl-IQ

(I know the image includes Omnipod 5, but that’s because I’m just re-using something)

There have been numerous statements on Social Media from those participating in the pilot, talking about the significant reductions in average glucose levels that they’re seeing, and discussions with various clinicians suggests that many of the participants are those who struggle the most to manage their diabetes, in the hope that this will prove out the technology at multiple levels.

There is, however, one question that use of two of these three systems poses.

Both the Medtronic and Tandem offerings have restrictive targets, with Tandem offering a target of 110mg/dl (6.1 mmol/l) and Medtronic 100mg/dl (5.5mmol/l), whereas CamAPS offers a wide range (from around 4mmol/l to 11mmol/l). Now, why might that be of concern?

According to the 2019-2020 National Diabetes Audit, around about 41% of people with type 1 in the UK have an HbA1c of >70 mmol/mol (or 8.5% in old money). That’s an estimated average glucose level of 11mmol/l (198 mg/dl). That’s quite a big difference from the offerings we’re talking about here.

And as soon as you put someone on a commercial APS system with a target like this, it’s going to do its damnedest to ensure that it maintains glucose levels at these numbers.

The question for me here (and others) is “How fast will someone’s glucose levels drop once they start using a commercial system?”.

Why is this important?

Well, if the population that is using these systems fall into those with higher HbA1c levels, we know that a rapid drop in HbA1c can cause significant issues to the eyes, as outlined here. In fact, it stipulates that a >2% reduction in HbA1c is a major risk factor.

What could this mean for those participating in the trials? I assume that this is something that teams are well aware of and have been monitoring as the new users embark on their journeys, and that the significant drops in average glucose levels we’ve seen reported in social media by some participants are accompanied by regular eye checks to ensure there are no major side effects.

But it raises a bigger question on technologies like this. Where these systems may well work for those who have previously struggled to maintain lower glucose levels, why don’t they allow for a slower reduction in average glucose levels?

We know that the lower limit was imposed to meet FDA concerns about hypos, and the costs of trials, but surely higher limits should therefore not be an issue?

Wouldn’t it make sense to set these systems up to limit the possibility of known effects of rapid glucose level drops from the word “go” rather than wait for epidemiological data and wider population trials to see if anything happens? Or is that an issue with the initial clinical trials that were undertaken where the participants all started with relatively low values for HbA1c?

Either way, it certainly raises the prospect of an additional piece of information relating to the outcomes of the NHS England closed loop pilot. One that certain manufacturers should perhaps be watching carefully…