Differentials
Annular lichen planus
SIGNS / SYMPTOMS
Flat-topped, shiny, violaceous plaques with irregular white streaks called Wickham striae that may also be seen in mouth and gums.[33]
INVESTIGATIONS
Skin biopsy: liquefaction degeneration of the basal layer, band-like mononuclear infiltrate of the epidermis, and Civatte bodies at dermoepidermal junction.
Dermatophyte infections
SIGNS / SYMPTOMS
Typically unilateral lesions on the fingers associated with an intense pruritus.
Worsened by the application of a topical corticosteroid.
INVESTIGATIONS
Skin scrapings sent for mycology show positive result for dermatophytic infection.
Mycosis fungoides
SIGNS / SYMPTOMS
Generalized or solitary patches or plaques. Can be nonspecific in appearance. Very itchy. Long natural history.[16]
INVESTIGATIONS
Skin biopsy: band-like and patchy infiltrate of atypical lymphocytes (mycosis cells) and positive T-cell rearrangement studies.
Insect bites
SIGNS / SYMPTOMS
Can resemble a papule.
Patient may recall insect bite or have relevant travel history.
INVESTIGATIONS
Skin biopsy: dermal infiltrate predominantly eosinophilic.
Rheumatoid nodule
SIGNS / SYMPTOMS
Tends to occur over bony prominences.
Presence of classic features of rheumatoid arthritis, such as hand deformity.
INVESTIGATIONS
Rheumatoid factor: usually positive if nodules present.
Cutaneous sarcoidosis
SIGNS / SYMPTOMS
May be indistinguishable clinically, as can present in many ways.[34]
INVESTIGATIONS
Chest x-ray: bihilar lymphadenopathy.
Serum calcium and angiotensin-converting enzyme levels elevated.
Skin biopsy: large islands of epithelioid cells, few giant cells, and occasionally fibrinoid necrosis.
Perforating collagen disorders
SIGNS / SYMPTOMS
Rare. Start as very small lesions and grow over a few weeks into papules.
Intensely pruritic.
Have a keratinous plug.[35]
INVESTIGATIONS
Serum BUN, creatinine, LFTs, and serum glucose levels may be elevated, as renal failure and diabetes can be associated with the perforating collagen disorders.
Skin biopsy: transepidermal elimination of collagen.
Soft-tissue sarcoma
SIGNS / SYMPTOMS
May be indistinguishable clinically.
Typically a solitary lump that may have been present for some time.
Gradually increasing in size.
INVESTIGATIONS
Core needle biopsy confirms diagnosis.
Necrobiosis lipoidica
SIGNS / SYMPTOMS
Difficult to distinguish clinically, but patient is likely to have diabetes mellitus (type 1 or 2). Telangiectasia visible centrally.
Tender to palpation.
INVESTIGATIONS
Skin biopsy: inflammatory reaction around destroyed collagen, characteristic appearance.
Erythema migrans
SIGNS / SYMPTOMS
Typical bull's-eye appearance, enlarging area of erythema with central clearing. May have joint pain, fever, and fatigue suggestive of Lyme disease.
INVESTIGATIONS
Blood test for antibodies to Borrelia burgdorferi but may not be present in the first week after infection.
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