This patient has classic dermatologic findings for dermatomyositis (DM).
DM is an inflammatory disorder which generally presents with characteristic skin findings and progressive muscle weakness. Early in training it can be difficult to distinguish DM from other photosensitive eruptions, particularly systemic lupus erythematosus (SLE) both clinically and histologically. A couple of helpful clinical clues that favor DM over SLE are the marked pruritus, the malar erythema that does not spare the nasolabial folds, and rash overlying the knuckles (rather than sparing the knuckles).
The nail fold changes seen in DM differ slightly from other diseases with nail fold changes including SLE and systemic sclerosis (SS). The poikiloderma seen in DM also differs slightly from the poikiloderma seen in SLE. These differences take practice to recognize. See CORE and APPLIED QBank for further detail.
The muscle disease associated with DM can also be difficult to distinguish from other idiopathic inflammatory myopathies (polymyositis and inclusion body myositis) and other connective tissue diseases.
Below is a thorough review of dermatomyositis testable factoids:
Pellagra is caused by a deficiency of Vitamin B3 (nicotinic Acid; niacin). Pellagra is characterized by the 4 D’s: dermatitis, diarrhea, dementia, and death (if untreated). The cutaneous eruption is characteristic and consists of a photosensitive eruption on the face, neck, and upper chest. In addition, patients may have fissuring of the palms and soles, cheilitis, psychosis, abdominal pain, irritability, etc. A pathognomonic sign is Casal’s necklace which is a well-demarcated hyperpigmented linear band along the neck (contrast this to the poikilodermatous eruption of dermatomyositis). B3 deficiency can also demonstrate angular cheilitis, seborrheic-like dermatitis, and perianal dermatitis (similar to zinc deficiency).
Systemic sclerosis (SS), which is the diffuse form of scleroderma, is an autoimmune disorder that is marked by collagen accumulation and small vessel injury that produces thickening of the skin. There are two major subtypes of systemic sclerosis: limited (skin involvement includes only the distal extremities and face) and diffuse (skin involvement includes the distal and proximal extremities, trunk, and the face). In this case, note the absence of sclerodactyly and flexion contractures of the digits that would be expected in SS. Diagnosis of SS is made by clinical symptoms, autoantibodies (specifically anti-centromere and anti-scl70/anti-topoisomerase antibodies) and sometimes via biopsy.
Systemic lupus erythematosus (SLE) requires 4/11 diagnostic criteria be met to establish a diagnosis, 4 of which are cutaneous signs (malar rash, discoid rash, photosensitivity, and oral ulcers). The other diagnostic criteria are arthritis, serositis, renal disorder, neurologic disorder, hematologic disorder, immunologic disorder, and antinuclear antibody). As detailed above, SLE can be a clinical and histologic mimicker of DM. Be able to recognize some of the differing clinical signs which are pictured in this case.
Clinical Pearl: Dermatomyositis is an idiopathic inflammatory myopathy with symmetric proximal extensor muscle weakness and characteristic skin findings. Clinically, it can be difficult to distinguish from other idiopathic inflammatory myopathies (polymyositis and inclusion body myositis) and other connective tissue diseases including SLE. Recognizing clinical features and confirming with biopsies (skin and/or muscle), imaging (MRI or US), and laboratory evaluation including detection of certain antibodies, can lead to early diagnosis and appropriate treatment including appropriate malignancy screening and detection if interstitial lung disease, or other internal organ involvement.