Blood sugar management redefined? Understanding the utility of a ‘Smart’ Continuous Glucose Monitor (CGM)

The young woman sitting across the table was perplexed and angry. “Why is my diabetes uncontrolled doctor? My fasting sugar value is 110 mg/dl, and after breakfast it never exceeds 170 mg/dl. I take all my medicines regularly. Why is my HbA1c (three-month average) invariably around eight per cent? How do I reach my target of seven per cent?”

After some discussion I managed to convince her to attach a continuous glucose monitor to her body, so that we could unravel the mystery of her high HbA1c. We had the answer next week. The monitor readings showed that her sugar began to rise late in the evening and returned to normalcy only by early morning. The conventional method of testing fasting and post prandial blood sugar was missing several hours of high blood glucose. A modification of her evening snack and adjustment of medication ensured that we were able to rectify her problem soon.

Many years ago, the legendary Elliott Joslin said that the aim of treating diabetes is to keep the blood glucose as close to normal as possible. Our blood glucose level is a widely varying parameter. However, most of the time we rely on a fasting value or a few values taken over a period of days or weeks, along with an HbA1c (a three month average of our blood glucose) to decide the course of treatment. Such estimates are at best crude as they do not factor in variations in our blood glucose within each day. The common way of obtaining blood glucose values during the day is by finger-prick testing using glucose meters. While these devices have greatly enhanced our ability to monitor and manage diabetes, they involve finger pricks, which can be uncomfortable. There is also a limit to the number of times one can check. There have been numerous attempts, therefore, to develop technology that will be able to monitor glucose on a continuous basis. This approach has met with success in recent years, and continuous glucose monitoring systems (CGMs) are now widely used.

WHY CONTINUOUS GLUCOSE MONITORS (CGMs)

What is the principle of the commonly available CGMs? Most methods measure the glucose level in the interstitial fluid (the fluid between our cells) rather than the blood. Usually it follows the blood level by about 15-20 minutes. A very fine cannula with a sensor is inserted beneath our skin so that it is in contact with the fluid. The sensor measures glucose level in the fluid every few minutes and then transmits the reading by a transmitter to a small external device or phone. Most such devices require once or twice a day calibration with a finger stick blood glucose value measured on your glucose meter. The sensor life is seven to 14 days, at which point it has to be replaced by a fresh one.

In the past few years, CGMs have become popular in India. One of the most popular devices is flash libre, where the sensor is to be attached on the upper arm. It comes with a hand-held reader which displays glucose values when held near the sensor. In addition, it also shows the trend of glucose values. This can be helpful in preventing low glucose reactions. The data can be downloaded periodically.

A device from Medtronic can display blood glucose values on your smartphone in a continuous manner and has alarms for low and high sugars. Parents of children with diabetes find this very useful when the child is in school and can have a peaceful night sleep without the fear of low sugars. The system can also integrate with an insulin pump and help in automatic adjustment of insulin doses.

A very popular device worldwide, which is not easily available in India, comes from Dexcom, the newer versions of which do not require calibration and can integrate with insulin pumps. All these features of CGMs make them a very important component of managing Type 1, Type 2 and gestational diabetes.

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A major advance in continuous glucose monitoring has been the development of an implantable fluorescence-based glucose sensor that works for six months. Called the Eversense E3 Continuous Glucose Monitoring (CGM) System, it gives real-time blood sugar readings every five minutes. However, it does require a small skin incision by the doctor to implant the sensor. The transmitter is worn externally and is visible. It requires charging for 10 minutes daily. The data currently does not connect with other diabetes devices like insulin pumps.

WILL WE GET NON-INVASIVE DEVICES?

The currently available devices are minimally invasive but not truly non-invasive. Truly non-invasive methods under development involve no prick at all. Some of these use spectroscopy, a technique that identifies chemicals based on the interaction of molecules with electromagnetic radiation. Infrared (or near infrared) technology which involves passing radiation through the forearm or finger has been under study for decades.

An interesting concept is the use of tear glucose for estimating glucose levels. Glucose level in tears can be used as surrogate markers for blood glucose levels. ‘Smart” contact lenses, which have a glucose sensor integrated in them have been developed. However, significant logistical challenges remain. For example, electrochemical sensors may get heated; enzymatic glucose sensors may result in production of hydrogen peroxide which is an eye irritant.

WHAT ARE CHALLENGES?

While technological developments in the field of glucose monitoring are a paradigm shift in diabetes management and will continue to become sleeker, more accurate and more economical over a period of time, technology also has some challenges. In the current scenario, most of these devices are not available through public healthcare or insurance, which makes them expensive for the majority of Indians. There can be inaccuracies at the lower and higher end of glucose values, hence it is recommended that we cross check these with a glucometer before treatment decisions are taken.