Although most PPD can be drug induced, DK has shown the greatest tendency for pruritic erythematous plaques following drug usage including but not limited to amlodipine, aspirin, acetaminophen, thiamine, interferon alfa, chlordiazepoxide, and isotretinoin. Additionally, DK has been associated with a contact allergy to clothing dyes and rubber.4 On histology, epidermal spongiosis may be seen, correlating with the eczematoid clinical findings. Spontaneous remission also is more common compared to the other PPDs. Treatment consists of topical corticosteroids and antihistamines.16
Lichen Aureus—Lichen aureus was first observed by the dermatologist R.H. Martin in 1958.17 It is clinically characterized by closely aggregated purpuric papules with a distinctive golden-brown color more often localized to the lower extremities and sometimes in a dermatomal distribution. Lichen aureus affects males and females equally, and similar to Majocchi purpura can be seen in children.4 Histopathologic examination reveals a prominent lichenoid plus superficial and deep perivascular lymphocytic infiltrate, extravasated erythrocytes, papillary dermal edema, hemosiderophages, and an unaffected epidermis. In rare cases, perineural infiltrates may be seen. Topical steroids usually are ineffective in lichen aureus treatment, but responses to psoralen plus UVA therapy also have been noted.17
Differential Diagnosis
COVID-19–Related Cutaneous Changes—Because COVID-19–related pathology is now a common differential diagnosis for many cutaneous eruptions, one must be mindful of the possibility for patients to have PPD, cutaneous changes from underlying COVID-19, or both.18 The microvascular changes from COVID-19 infection can be variable.19 Besides the presence of erythema along a distal digit, manifestations can include reticulated dusky erythema mimicking livedoid vasculopathy or inflammatory purpura.19
Retiform Purpura—Retiform purpura may occur in the setting of microvascular occlusion and can represent the pattern of underlying dermal vasculature. It is nonblanching and typically stellate or branching.20 The microvascular occlusion may be a result of hypercoagulability or may be secondary to cutaneous vasculitis, resulting in thrombosis and subsequent vascular occlusion.21 There are many reasons for hypercoagulability in retiform purpura, including disseminated intravascular coagulation in the setting of COVID-19 infection.22 The treatment of retiform purpura is aimed at alleviating the underlying cause and providing symptomatic relief. Conversely, the PPDs generally are benign and require minimal workup.
Leukocytoclastic Vasculitis—The hallmark of leukocytoclastic vasculitis is palpable purpura, often appearing as nonblanchable papules, typically in a dependent distribution such as the lower extremities (Figure 3). Although it primarily affects children, Henoch-Schönlein purpura is a type of leukocytoclastic vasculitis with lesions potentially similar in appearance to those of PPD.23 Palpable purpura may be painful and may ulcerate but rarely is pruritic. Leukocytoclastic vasculitis represents perivascular infiltrates composed of neutrophils, lymphocytes, and occasionally eosinophils, along with karyorrhexis, luminal fibrin, and fibrinoid degeneration of blood vessel walls, often resulting from immune complex deposition. Leukocytoclastic vasculitis may affect blood vessels of any size and requires further clinical and laboratory evaluation for infection (including COVID-19), hypercoagulability, autoimmune disease, or medication-related reactions.24
FIGURE 3. Palpable purpura of the lower extremities with nonblanching, dusky, erythematous papules in a patient with leukocytoclastic vasculitis.
Stasis Dermatitis—Stasis dermatitis, a chronic inflammatory condition stemming from retrograde venous flow due to incompetent venous valves, mimics PPD. Stasis dermatitis initially appears as demarcated erythematous plaques, fissures, and scaling of the lower legs bilaterally, usually involving the medial malleolus.25 With time, the affected region develops overlying brawny hyperpigmentation and fibrosis (Figure 4). Pruritus or pain are common features, while fissures and superficial erosions may heal and recur, leading to lichenification.
FIGURE 4. Stasis dermatitis with hyperpigmentation, induration, and edema of the legs.
Although both commonly appear on the lower extremities, duplex ultrasonography may be helpful to distinguish PPDs from stasis dermatitis since the latter occurs in the context of chronic venous insufficiency, varicose veins, soft tissue edema, and lymphedema.25 Additionally, pruritus, lichenification, and edema often are not seen in most PPD variants, although stasis dermatitis and PPD may occur in tandem. Conservative treatment involves elevation of the extremities, compression, and topical steroids for symptomatic relief.
Cellulitis—The key characteristics of cellulitis are redness, swelling, warmth, tenderness, fever, and leukocytosis. A history of trauma, such as a prior break in the skin, and pain in the affected area suggest cellulitis. Several skin conditions present similarly to cellulitis, including PPD, and thus approximately 30% of cases are misdiagnosed.26 Cellulitis rarely presents in a bilateral or diffusely scattered pattern as seen in PPDs. Rather, it is unilateral with smooth indistinct borders. Variables suggestive of cellulitis include immunosuppression, rapid progression, and previous occurrences. Hyperpigmented plaques or thickening of the skin are more indicative of a chronic process such as stasis dermatitis or lipodermatosclerosis rather than acute cellulitis. Purpura is not a typical finding in most cases of soft tissue cellulitis. Treatment may be case specific depending on severity, presence or absence of sepsis, findings on blood cultures, or other pathologic evaluation. Antibiotics are directed to the causative organism, typically Streptococcus and Staphylococcus species, although coverage against various gram-negative organisms may be indicated.27
Caution With Teledermatology
COVID-19 has established the value of telemedicine in providing access to health care services for at-risk or underserved individuals. The PPDs are benign, often asymptomatic, and potentially identifiable with teledermatology alone; however, they also can easily be mistaken for COVID-19–related eruptions, vasculitis, other types of purpura, stasis dermatitis, or other complications of lower extremity stasis and lymphedema, especially in an aging population. If tissue biopsy is required, as in the workup of vasculitis, the efficacy of telemedicine becomes more questionable. It is important to delineate the potentially confusing PPDs from other potentially dangerous or life-threatening inflammatory dermatoses.28