We have been hearing a lot lately about deep tissue injury and its hypothesized association with the "purple heel." The concept of the purple heel as a sentinel or a harbinger of deep tissue injury is not a straightforward clinical issue, however. In fact, a current Cochrane assessment and a review of the literature provide no specific articles characterizing the purple heel as a distinct entity or a syndrome.
Nonetheless, those of us working at the bedside know empirically that an area of skin manifesting a change in color, especially in areas traditionally at risk such as skin over bony prominences, means impending, evolving, and significant pathology that may be related to deep tissue injury (a downward spiral of ischemia, reperfusion, infarction, necrosis, and liquefaction). In the forefoot, there are homologues that are categorized as purple toe syndrome and blue toe syndrome.
Where, then, does the purple heel best fit in? Is it a syndrome as well? Can we say with certainty that the purple heel is exclusively related to deep tissue injury (pressure)? Or is it somehow related to purple toe/blue toe syndrome, which has a strong relationship to significant atherosclerotic disease, diabetes, and several other conditions?
Toes of a Different Color
As the name suggests, purple toe/blue toe syndrome is characterized by the purple or blue color of the toes. It may occur bilaterally, and it generally is marked by skin necrosis and ischemic gangrene resulting from arterial occlusion by emboli, thrombi, or injury. The patient may also have intense pain in the affected toe or toes. The most common cause of purple toe/blue toe syndrome is atherosclerotic plaques from large proximal arterial vessels, specifically the aorta. Vascular surgery and anticoagulation-especially with warfarin (Coumadin)-are often the triggers.
On the surface, the purple heel seems to be a different entity, more in line with deep tissue injury. However, the risk factors for the purple heel and the purple toe/blue toe are not that dissimilar, with the exception of the pressure component found in the purple heel. Sudden and significant changes in skin color are an ominous visual clue that ulceration is imminent and that ischemia and necrosis are progressing, similar to what occurs in purple toe/blue toe syndrome.
Beneath the Skin
Alterations in skin color usually tell us about the degree of deep tissue injury. The spectrum of colors we see ranges from red, the early harbinger of injury (ischemia); to purple, signaling infarction; to black, meaning tissue necrosis.
By traditional clinical inspection, we can easily observe changing colors in the skin of persons with lightly pigmented skin by utilizing ambient white light. For individuals with darkly pigmented skin, however, color changes are not as readily apparent with ambient light. Investigators are currently exploring ways to manipulate our ability to look under the skin for signals of deep tissue injury by making alterations in the light spectrum (near-infrared scanning).
The anatomic characteristics of the hindfoot-a relatively strict blood supply and skin devoid of melanocytes-are what make it possible for us to observe the changes in color (ie, the purple heel). The tough, thick skin on the heel and the plantar aspect of the foot is actually almost translucent.
In addition, the calcaneous-which literally means "hard rock"-is a bony prominence with only a small bursa as protection against pressure; it has a concomitant poor blood supply. Additionally, we have ample evidence that the bone/muscle interface is more sensitive to ischemia and reperfusion injury than the skin, and what we see in the skin is the "tip of the iceberg."
Related Syndromes?
The etiologic and pathophysiologic relationships, risk factors, and clinical presentations that lead to purple/blue toe syndrome and the purple heel syndrome are not straightforward. We should consider, therefore, that the causality of these syndromes may be related, and we should consider their coexistence and similar etiologic factors in the differential diagnosis, especially in patients who present with purple heel syndrome.
Selected References