Near-infrared spectroscopy (NIRS) is increasingly displaying potential for wound management. Offering several case studies, these authors explain NIRS technology and explore its usage in the wound clinic.
Near-infrared spectroscopy (NIRS) is a mature technology with broad applications, including in agriculture, neuroscience, analytical chemistry, and increasingly in wound care.
The history of NIRS starts in 1800, when Frederick William Herschel performed diffraction experiments using a prism and thermometers. He observed, to his surprise, that the temperatures on the thermometers were above ambient temperature. This is the first noted indication of energy beyond the “visible” spectrum of light.1 It was not until the 1970s that it was determined that oxygen saturation may be reliably monitored using NIRS.2 Since that time, use of this technology for medical purposes has been greatly expanded.
The use of NIRS technology in humans is based on the different near-infrared light absorption properties of hemoglobin in its oxygenated and deoxygenated forms. This is also distinct from myoglobin absorption characteristics, and may be correlated to tissue (venous) oxygen saturation.3 It is this phenomenon that allows the evaluation of chronic tissue perfusion in the chronic wound. We present a review article that discusses application of this technology to wound management.
A Closer Look at the NIRS Device
In our wound practice, the NIRS device of choice is the SnapshotNIR® (Kent Imaging) (see Figure 2).4 It is a portable handheld unit, weighing only 2 kg. Like a conventional digital camera, it functions in a “point-and-shoot" fashion. Image acquisition is nearly instantaneous, does not involve IV contrast, and may be reviewed on the touch-screen interface. The operating system is Windows®-based.
The device analyzes six NIR wavelengths between 600 and 1,000 nm to calculate oxygen saturation, relative oxyhemoglobin level, and relative deoxyhemoglobin level. If performing a sterile procedure, the device may be covered with an optional one-time-use sterile drape.
The ability to use NIRS technology in clinical practice requires understanding of the physical principles of spectroscopy, understanding of the specific disease pathophysiology, and clinical experience in visualizing and treating clinical wounds. Proficiency in imaging interpretation requires many “retina-miles,” i.e., a review of a significant number of images. With this experience, clinicians will increasingly find themselves able to interpret NIRS images and use this information to augment their clinical decision-making. However, it cannot be overstated that in the authors’ opinion, this technology is an adjunct and not a replacement for astute clinical acumen.
Pressure ulcerations/injuries are commonly treated in the modern wound clinic. Pressure injury is classified by the National Pressure Injury Advisory Panel (NPIAP) stage system. Stage I pressure injury is nonblanching erythema; stage II is partial thickness injury; stage III is full-thickness injury without bone, tendon, or muscle exposed; and stage IV is full-thickness injury with bone, tendon, or muscle exposed.5
Furthermore, two additional phenomena, suspected deep tissue injury and unstageable/unclassified injury, are also recognized. Stage I injury is postulated to occur in a “top-down” fashion. Superficial shear and friction result in an inflammatory response.6 This type of pressure injury is less likely to deteriorate.
In contrast, the other stages of pressure injury develop via a “bottom-up” mechanism.7 There is sustained compression of the tissue at the muscle-bone interface, leading to rapid development of deep tissue damage. This more serious mechanism results in wounds that are more likely to progress and deteriorate. This initially may not result in skin-level changes.8 The tissue compression results in occlusion of blood vessels, occlusion of lymph vessels, tissue ischemia, reperfusion injury due to free radicals, and direct cellular deformation.9–12
Case Study 1: Chronic Left Heel Pressure Injury
Let us apply these concepts to a specific patient (see Figure 3). Figure 3a demonstrates a chronic left heel pressure injury that has failed to heal since August 2017. The patient’s treatment included an advanced biologic product and proper offloading. At this visit, the wound had deteriorated. There was a new area of maceration to the periwound, and there was increased slough and discoloration noted at one aspect of the wound.
Our clinical questions at this point were, “Is re-injury due to pressure the etiology of the deterioration of this wound?” and “If so, what form of pressure?” Again, this is pertinent because the mechanism of injury allows us to predict severity. Let us use what we know about pathophysiology to come to a clinical conclusion. The wound, based on the patient’s history and its current appearance, is most likely due to pressure. However, what is the mechanism? If the wound is due to a “top-down" mechanism, we would expect a “bright-red" signal, which indicates robust oxygen saturation due to an acute inflammatory state. On the other hand, a “bottom-up” etiology would likely result in relatively ischemic “blue” tissue due to the mechanism previously described.
The latter is in fact what we see in this image: relatively ischemic “blue” tissue. If we look further at this same wound, there is another region of interest at the plantar aspect of the periwound. We see skin level changes with contouring. Based on the clinical appearance, our differential diagnosis includes moisture-associated skin disease (MASD) and pressure-related intact discoloration of the skin (PRIDAS). PRIDAS includes Stage I pressure injury and suspected deep tissue injury (sDTI). MASD and Stage I are “top-down” phenomena, so we would expect a relatively hyperemic “bright red” insult. SDTI would appear as a “bottom-up” insult as previously described. This area appears blue under NIRS spectroscopy, which raises concern for another area of “bottom-up” insult.
Our clinical decision was therefore to reassess and optimize the patient’s offloading regimen. We also counseled the patient on the expected deterioration of this wound due to the suspected more severe “bottom-up” insult.
We have demonstrated in the first case that NIRS is a useful adjunct for wound diagnosis. The next case will demonstrate that it is also helpful in the selection of which advanced treatment modality to apply in indicated patients. This is quite beneficial for several reasons. Advanced wound modalities are expensive; in the third-party payer system, pre-authorization acquisition is usually required. These modalities may be labor intensive to apply and usually not within the scope of practice of the wound nurse to apply. Thus, they require the provider’s time. There is usually a finite number of applications that are approved or indicated. Therefore, thoughtful choice of the correct product for a given patient is important.
Case Study 2: Chronic Ischial Pressure Ulceration in a C5-C7 Partial Quadriplegic
The patient in this next example is a 51-year-old woman who is a C5-C7 partial quadriplegic. She is nevertheless is otherwise quite healthy and active but has suffered with a left ischial pressure ulceration for 9 months (see Figure 3). She has a history of prior pressure ulcerations that healed uneventfully with offloading and standard-of-care moist wound healing. This ulceration was treated unsuccessfully at an affiliated wound care center, so the patient was referred to our clinic. The patient was treated with a whole blood autograft that functions by recreating the acute phase of wound healing in the chronic wound.13
After the permitted applications were completed, the patient was transitioned to standard-of-care therapy. Unfortunately, the patient was unable to completely heal the wound, and we wanted to again apply an advanced product after optimizing nutrition and offloading, and ascertaining that the wound healing was not delayed due to infection.
Should we reapply the whole-blood autograft? For this therapy to be most effective, the wound would be expected to demonstrate certain specific characteristics. Most specifically, it would be expected to be in a hypo-inflammatory state due to, among other reasons, matrix metalloproteinase (MMP) and neutrophil elastase over-expression and biofilm exuberance.14
Clinical Expectations and Observations
The NIRS imaging would be expected to be “cool.” The opposite was demonstrated upon NIRS assessment. The wound is in fact quite warm. This is congruent with our clinical assessment of the wound, which does not demonstrate evidence of heavy bioburden.
While this information does not elucidate which product to use, it gives us some indication of what type of product would be suboptimal. We thus elected to proceed with a more standard gentian violet/methylene blue (GV/MB) foam dressing and collagen extracellular matrix (ECM) combo. In these cases, we see that NIRS data in conjunction with clinical evaluation may guide clinical decision-making.
Naz Wahab, MD, FAAFP, FAPWCA, is a board-certified family practitioner who has been practicing wound care and hyperbaric medicine for more than 15 years in the Las Vegas Valley. Currently, she is an active committee member for an advocate for regulatory and legislative outreach for Wound Care healthcare professionals. Dr. Wahab participates in multiple educational committees for wound healing. Dr. Wahab is also the lead clinical researcher and developer for international biotech companies. Her ongoing interests now include research and development of wound care devices and product development.
Mateusz A. Lapucha, MD, CWS, is a wound care and hyperbaric specialist serving the southern Nevada region. He completed his Internship in General Surgery in New Orleans before proceeding to Residency in Las Vegas. His research interests include near-infrared spectroscopy and skin substitutes.
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