Monitoring the feline diabetic with a continuous glucose monitor

Feline diabetics can be challenging to manage. We know that monitoring response to insulin and avoiding hyper- and hypoglycaemia are vital to successful control of clinical signs. However, blood glucose curves can be influenced by stress in the veterinary clinic (Sparkes et al., 2015), and taking multiple blood samples can cause stress and negative associations. Alternatives include blood glucose measurement at home by willing owners (Hazuchova et al., 2017), yet even if this is successful, there can be day-to-day variation in the curves that would alter treatment decisions (Alt et al., 2007). Continuous glucose monitors (CGM) are used with increasing frequency in small animal practice to monitor both canine and feline diabetic patients, seeming to offer a solution to the problem of interpreting single blood glucose curves and avoiding serial venepuncture. The devices are generally well tolerated and easy to place, and recent research has shown that they can play an important role in feline diabetic management. This article will discuss this evidence, describe the procedure of placement, discuss complications and explain when such a device can be most useful for the diabetic feline patient.

About the CGM system

The most commonly used CGM system currently is the “FreeStyle Libre” (Abbott) which consists of a small sensor disc placed on the animal (Figure 1) which measures interstitial glucose, and either a mobile phone application or a reader device which stores the data from the sensor disc. As well as providing a glucose reading, the phone or reader will have an arrow showing if the interstitial glucose is on the way up or down. The device, unlike previous systems, does not require calibration with blood glucose and can take readings for 14 days, although may not function for this long in cats. The sensor disc can store up to eight hours of data which is then easily transferred to the phone or reader device.

Accuracy of the FreeStyle Libre has been assessed in humans comparing favourably to capillary blood glucose, and three recent studies in cats have shown good agreement between interstitial and blood glucose readings (Deiting and Mischke, 2021; Del Baldo et al., 2021; Shea and Hess, 2021). However, the time lag for equilibration between blood and interstitium must be considered when discrepancies are present between results. Del Baldo et al. (2021) showed that during rapid changes in blood glucose there can be marked differences between the two compartments, and this should be considered when interpreting results. Deiting  and Mischke(2021) also commented on certain readings that differed greatly between interstitial and blood glucose, particularly in certain patients.

All non-plausible results should be checked by measuring blood glucose and trends should be analysed, taking into account the limitations of the device. The system is designed for humans with diabetes, so has preset parameters for that species

All non-plausible results should be checked by measuring blood glucose and trends should be analysed, taking into account the limitations of the device. The system is designed for humans with diabetes, so has preset parameters for that species. The “Libre 2” system allows for an alarm to be set when interstitial glucose falls above or below certain parameters, although this can be turned off.

       

Application of the device

The vast majority of sensors can be placed in conscious patients, but pushing the sensor on requires a little pressure so, for some feline patients and when inexperienced at placing sensors, sedation or anxiolysis with gabapentin or butorphanol may be helpful. If diabetic patients are sedated for other procedures (eg imaging), the opportunity should be taken to place a sensor. Each sensor comes with an applicator device (Figure 2) that is required to place the sensor disc.

The device can be placed in any location and studies have used different sites. The dorsal neck can be used in cats (Shea and Hess, 2021), or the dorsal thorax caudal to the scapula (Figure 3; Deiting and Mischke, 2021; Shoelson et al., 2021). It is important to clip the area closely, clean to remove dirt (wipes are supplied with the sensor) and allow to dry completely before application.

Although the sensor disc has adhesive, the author adds “dots” of tissue glue (cyanoacrylate) around the edge of the adhesive area or the edges can “lift” with time (Figure 4); the same strategy was adopted in recent studies (Del Baldo et al., 2021; Shea and Hess, 2021). In one study of 34 diabetic cats, the authors did not use additional adhesive but secured the edges with sutures, which they report was well tolerated (Deiting and Mischke, 2021). In the author’s experience this is not required.

Patient interference is uncommon and in general the sensor does not need to be covered. In cats, dressings and t-shirts cause stress and may limit normal behaviour and affect readings. If the sensor is placed on the neck, some clinicians will cover it with a loose dressing or fabric collar (Del Baldo et al., 2021). A soft Buster collar, if anything, is used in cats in the author’s clinic.

Interpretation of data

The sensor should be scanned every eight hours to upload the data to a phone or reader device. Results can be sent from clients to the clinic using the application, or the reader connected to a computer to upload the data once required software is installed. Figure 6 shows an example of a report which has various features including daily graphs, average daily glucose and proportion of time interstitial glucose is within certain parameters (again, preset for humans). The graphs tend to be the most useful of the information obtained.

Indications for use of a CGM

The system can be used in clinic on newly diagnosed diabetics to look at response to insulin and duration of effect, after which the patient is discharged with the sensor in place. Cats with diabetic ketoacidosis (DKA), unstable diabetics (eg persistent clinical signs, persistent hyperglycaemia or hypoglycaemia episodes, suspicion of Somogyi overswing) or those feline patients approaching remission may also benefit.

CGM could be used in septic or critical patients where hypoglycaemia is a concern, or patients on insulin or glucose continuous rate infusions. However, these indications have not been studied in cats at this time and the time lag between blood and interstitial glucose must be taken into account, which is particularly evident during rapid changes in blood glucose (Del Baldo et al., 2021). Validation of results with measurement of blood glucose is recommended in such patients before adjusting treatment protocols.

The author has found [a CGM] most useful in cats approaching remission, in patients with DKA and in animals where stress in the clinic is highly likely to influence glucose curve interpretation and decisions on insulin dose

The author has found it most useful in cats approaching remission, in patients with DKA and in animals where stress in the clinic is highly likely to influence glucose curve interpretation and decisions on insulin dose. Figure 6 shows data from a FreeStyle Libre device on a cat entering diabetic remission.

Complications and limitations

A recent study examined the complications of the FreeStyle Libre device in 20 cats (Shoelson et al., 2021). A total of 33 sensors were placed, most of which were placed over the dorsolateral thorax. The most common complications were early sensor detachment (15 percent) and other complications related to the skin site where the sensor attached, mainly minor skin irritation, but two cats had erosion or abscess. The authors of this study felt that additional cyanoacrylate glue applied to the skin-facing surface of the sensor could have contributed to the skin trauma, particularly if the sensor is then prematurely removed. It seems sensible to be cautious with the amount of glue used. Shea and Hess (2021) used cyanoacrylate glue in small amounts and found minimal skin irritation.

Hair may be slow to regrow in the area of sensor placement (Figure 7), and in pointed cats the area may grow back fur of a different colour.

Deiting and Mischke (2021) noted that a shortened duration of sensor function was common in their patients with premature sensor detachment occurring, leading to an average runtime of 8.3 days. Additionally, in their study of cats discharged with sensors, Shea and Hess (2021) found the median time of sensor activity was seven days and in 80 percent of placements sensor failure or displacement was reported before the end of the 13-day study period. This high number of sensor failures could be related to the positioning (all on the dorsal neck), suggesting the lower detachment rate in the Shoelson study (15 percent) could relate to the placement on the lateral thorax (Shoelson et al., 2021).

The sensors are designed for human skin, and in human patients various dressings and bands are available to secure the sensors. Cats are highly mobile with thin skin, and in debilitated patients the small catheter can easily become dislodged. Over the days of sensor placement, hair may regrow and loosen the adhesive. Owners should be warned that premature sensor detachment may occur, and the costs of such a complication and sensor replacement taken into account.

The FreeStyle Libre CGM system has an upper interstitial glucose measurement limit of 27.8mmol/l and readings above this level are recorded as “Hi”. This is potentially a limitation in cats with DKA, for example, as they can be extremely hyperglycaemic, and also in poorly controlled cats where that additional information might be useful.  

Conclusions

Data must be interpreted along with clinical parameters indicative of response (weight, water intake, appetite) and limitations of the monitoring system acknowledged

CGM can be a useful tool in the management of diabetic cats. However, data must be interpreted along with clinical parameters indicative of response (weight, water intake, appetite) and limitations of the monitoring system acknowledged. There may be discrepancy between interstitial and blood glucose, particularly during rapid changes in blood glucose when a time lag between the two compartments may occur. The FreeStyle Libre sensor has been studied in feline diabetics recently and is well tolerated with good accuracy, but premature sensor dislodgement is common and duration of sensor function likely to be less than the full 14 days the sensor functions for in humans.