Discussion
The results of this study suggest that a telemedicine mat can be used to detect inflammation in patients at-risk for DFU recurrence who have only a single foot available for monitoring. While the most commonly used protocol relies on contralateral temperature differences and thus cannot be used in patients with proximal LEA or those being treated for a wound to one foot, the approach presented herein relies on comparisons among ipsilateral foot temperatures and between foot temperature and ambient temperature.
We completed a secondary analysis of existing data to derive this approach for unilateral once-daily remote temperature monitoring. Although classification accuracy with temperature asymmetry is better than the unilateral monitoring approach presented herein, our approach currently represents the only alternative for remote temperature monitoring of a single foot. At ITR setting 2, the sensitivity is 91% and the lead time is 41 days in our data.
Overall, the burden to the patient, caregivers, and providers is expected to be low, especially relative to the potential benefits of once-daily temperature monitoring in these high-risk populations. At ITR setting 2, 4.2 alerts per participant-year are expected. As noted by Crisologo and Lavery in a recent translational medicine review,40 ‘the potential to arrest re-ulceration is worth the perceived inconvenience to the patient’ associated with increased diligence. Additionally, a recent investigation41 suggests that the false-positive rate reported by Frykberg and colleagues may be artificially elevated due to a lack of meaningful clinical intervention on detection of inflammation. This research reports that in a commercial setting, only 1.4 alerts/patient-year were observed, compared with 3.1 alerts/patient-year reported by Frykberg and colleagues. Thus, it is possible that the alert burden associated with unilateral foot temperature monitoring in a commercial setting is similarly lower than what has been reported in this present effort.
Despite concerted limb salvage efforts, incidence of DFU and LEA remains alarmingly high in high-risk cohorts.4 17 42 Promisingly, some of the more advanced and efficacious recommendations for DFU prevention, such as daily remote foot temperature monitoring, are now finding more widespread use.43 44 These preventive foot care practices represent an enormous opportunity to improve outcomes and reduce resource utilization. Unfortunately, one of the most effective preventive foot care practices, daily remote temperature monitoring, was previously limited in use to patients with two limbs available for measurement, excluding patients with proximal LEA or a wounded foot that is bandaged or casted.
Contralateral complications are common in people in remission from previous diabetic foot complications.24–31 In fact, more than 25% of all LEAs are re-amputations.45 A large number of all LEA suffered by patients with diabetes in many regions qualify as ‘proximal’ (Syme ankle disarticulation or more proximal) and result in loss of the entire plantar surface of one of the feet.46 47
Those being treated for a wound are also at elevated risk for diabetes-related complications to the sound foot. One potentially underappreciated aspect of DFU is that they are likely to recur at anatomical locations distinct from the primary occurrence. Major risk factors, which include peripheral neuropathy and peripheral arterial disease, affect the entirety of both extremities. It is thus crucial that the provision of care for the patient reflect the patient’s elevated risk in both limbs. Orneholm and colleagues48 reported that only 19% of DFU recur at the same location, with 43% occurring at another ipsilateral location and 38% occurring to the contralateral foot. Perhaps surprisingly, given the increased clinical attention during treatment, a recent peer-reviewed abstract49 suggests high incidence (0.41 DFU/ulcer-year) for those being treated for a previous unhealed DFU. High-quality preventive care is thus essential to increase ulcer-free days in this population, but because treatment of DFU often precludes assessment of temperature to the wounded foot due to bandages or accommodative footwear that cannot or should not be removed, these populations cannot benefit from traditional approaches to remote temperature monitoring.
An additional potential benefit of using the study device to monitor the sound foot in patients with a wounded foot is that the patient is able to establish a preventive routine before healing. Research strongly suggests that recurrence is most likely in the first months after healing,1 and beginning a routine of once-daily use of the study device during healing ensures the patient is monitored throughout the critical post-healing period.
Finally, this approach and its extensions may find use in other patient populations. For example, patients who develop bilateral DFU may not present with large temperature asymmetry and thus the traditional asymmetry monitoring approach would not detect any early warning signs of DFU. In the future, more sophisticated models of patient risk may incorporate insights from ITR and ATD to more accurately predict DFU.
This research has limitations that should be considered when interpreting it. Several of these limitations are inherited from the study that served as the source of our data.17 40 We are also limited by data availability, which is inherent to any secondary analysis of existing data. The data from the prior study were not chosen for our aim and therefore may be suboptimal for our purposes. For example, other temperatures, such as those from the dorsal foot or leg, were not available and may be more useful as a comparator for remote temperature monitoring than ITR and ATD.
Another important limitation related to data availability is that the study’s participants did not correspond to those for whom this model was built. Specifically, the study excluded patients with proximal LEA and unhealed wounds. Instead, we approximated the data that would be available from those with proximal LEA by considering each foot in the study independently. These patients may have precipitating risk factors not appropriately represented in our cohort (such as a higher prevalence of peripheral arterial disease). Alternatively, chronic changes in physiology may manifest as a result of these conditions that otherwise differentiates these patients from the prior study’s cohort.
Nonetheless, although it has not been thoroughly studied, there is no reason to suspect that having suffered a proximal LEA or being treated for a wound results in an altered inflammatory response to repetitive microtrauma in the contralateral foot. Furthermore, prior research17 has validated the use of remote temperature monitoring in patients who, while having not suffered proximal LEA, have nonetheless lost the vast majority of the plantar surface of one or both feet (eg, Chopart amputation). Related research50 suggests that remote temperature monitoring is perhaps more accurate in this subcohort of patients.
This study opens several avenues for future research. A prospective study in patients with tailored inclusion and exclusion criteria would allow independent validation of the monitoring approach presented herein without the limitations associated with a secondary analysis of existing data. Such a study could eliminate any remaining questions regarding a potentially altered inflammatory response in patients with high-level amputation history and patients being treated for a wound to one foot.
In addition to appropriate inclusion and exclusion criteria, such a prospective study could also improve the characterization of outcomes. Frykberg and colleagues considered only DFU as outcomes. However, other inflammatory foot conditions such as Charcot neuroarthropathy, pre-ulcerative lesions such as callus and blister, and foot infections may also be detected by the study device, which is Food and Drug Administration (FDA) cleared for the indication of ‘periodic evaluation of the temperature over the soles of the feet for signs of inflammation’. In this study, inflammation associated with any of these outcomes, all of which are clinically relevant, would have been deemed false positives due to this limitation in the study design. Any future prospective study should attempt to characterize the etiology of inflammation detected by the study device and report a false-positive rate reflecting whether detected inflammation was corroborated by clinically relevant findings on exam such as pre-ulcerative lesion or infection.
Furthermore, there is potential clinical value in assessing the progression and healing of DFU with remote temperature monitoring.51 Presumably, the same approaches that are used for identifying emergent inflammation and impending DFU may equally effective in monitoring the resolution of inflammation as DFU heal. However, additional research is warranted to validate accuracy and, if necessary, develop tailored approaches for monitoring healing. Remote temperature monitoring of wounds is complicated by the need to protect against contamination and infection. Currently, use of the study device to monitor a foot with an open wound is contraindicated, although future product development may eliminate this restriction while ensuring patient safety in accordance with FDA guidelines.
Finally, we attempted to maintain consistency with previous research to provide continuity and context for practitioners. Thus, we have chosen to extend existing approaches based on keypoints and simple point-to-point temperature comparisons. However, more sophisticated approaches may be employed in the future to build and validate higher accuracy models for predicting DFU in patients with a history of proximal LEA. While these models may prove more accurate, they will come at the expense of intelligibility to the practitioner, who will lose the ability to interpret and reason about the prediction. These models will likely also require specialized software to implement properly.
In summary, we have developed an empirical approach to remote temperature monitoring for one foot which was found to predict 91% of impending non-acute plantar foot ulcers on average 41 days before clinical presentation with a false-positive rate of 54% in our data. Given the high incidence of subsequent diabetic foot complications in patients with a history of proximal LEA and patients being treated for a wound, practice of daily temperature monitoring in these populations has the potential to significantly reduce morbidity, mortality, and resource utilization.