Introduction

Along with polymyositis and inclusion body myositis, dermatomyositis is classified as one of the idiopathic inflammatory myopathies. Dermatomyositis is a disease of presumed autoimmune pathogenesis that presents with a symmetric, proximal, extensor inflammatory myopathy and a characteristic cutaneous eruption. Previously, polymyositis was thought to represent the same disease process but limited to the muscles; however, there is now evidence that the pathogenetic mechanisms of polymyositis and dermatomyositis differ significantly. In polymyositis, clonally expanded, autoreactive CD8 ⁺ T cells invade myocytes expressing MHC class I antigens and cause necrosis via the perforin pathway. In contrast, there are data to support the theory that autoantigens activate a humoral immune process in dermatomyositis, in which complement is deposited in capillaries causing capillary necrosis and ischemia . Because an appreciation of these pathogenetic differences occurred relatively recently, previous studies often pooled cases of dermatomyositis and polymyositis, making interpretation of their data challenging.

Dermatomyositis is characterized by a bimodal age distribution, with both adult and juvenile forms, and up to one-quarter of patients in the adult group having an associated malignancy. Patients with juvenile dermatomyositis do not have an increased risk of malignancy, but do have an increased incidence of calcinosis cutis and associated small vessel vasculitis. There is a subset of patients in whom the cutaneous manifestations of dermatomyositis exist without objective evidence of muscle inflammation, referred to as amyopathic dermatomyositis, and formerly known as dermatomyositis sine myositis. For dermatologists, establishing the diagnosis of dermatomyositis is important because it is a serious, treatable, multisystem disease that has a different prognosis and treatment approach from lupus erythematosus (LE). The malignancy association in adults provides the dermatologist with an opportunity to assist medical colleagues in detecting the tumor at an early stage because the skin disease often precedes the onset of symptoms related to the malignancy.

History

Polymyositis and dermatomyositis have been recognized as clinical entities for more than 100 years. Although the first adult patients with dermatomyositis and malignancy were described early in the twentieth century, a causal association was not suggested until the 1940s. A major step forward in the understanding of these disorders occurred in 1975, when Bohan and Peter proposed generally accepted clinical criteria for their diagnosis . Over the past 15 years, it has become more widely accepted that a subset of patients has skin-limited disease (i.e. amyopathic dermatomyositis). For this reason, Sontheimer proposed a revised classification of the idiopathic inflammatory myopathies that includes amyopathic forms of dermatomyositis, with muscle involvement no longer required for a definitive diagnosis ( Table 42.1 ) . Based upon recent epidemiologic studies, an estimated 20% of dermatomyositis patients have clinically amyopathic disease .

Epidemiology

Dermatomyositis is a relatively rare disease that occurs throughout the world. In adults, dermatomyositis affects women two to three times more often than men. The incidence of dermatomyositis ranges from 2 to 9 per million amongst various populations and seems to be increasing, although this may simply be a result of increased detection, as well as better diagnosis and reporting. Table 42.2 summarizes the findings in four recent studies .

Pathogenesis

Dermatomyositis is believed to result from an immune-mediated process triggered by “outside” factors (e.g. malignancy, drugs, infectious agents) in genetically predisposed individuals. Table 42.3 illustrates some of the evidence that supports this theory . Serum antinuclear autoantibodies are often present, as are other myositis-specific autoantibodies, as summarized in Table 42.4 . The antisynthetase antibodies are directed against cytoplasmic antigens; therefore the antinuclear antibody test may be negative. Patients with antisynthetase antibodies often have overlap syndromes. The term antisynthetase syndrome has been coined to refer to patients with these autoantibodies, fever, erosive polyarthritis, “mechanic’s hands”, Raynaud phenomenon, and interstitial lung disease .

Clinical Features

Cutaneous Disease

The features that are classically considered pathognomonic for dermatomyositis include the heliotrope sign and Gottron papules ( Fig. 42.1 ). The former is characterized by a pink–violet color, primarily of the eyelids and periorbital skin, and there may be associated edema ( Fig. 42.2 ). The heliotrope sign can be quite subtle, with only mild erythema of the eyelids, and it may wax and wane in intensity. Some patients will have more widespread facial erythema or midfacial erythema that tends to involve the nasolabial folds ( Fig. 42.3 ).

Cutaneous and systemic findings in dermatomyositis.

A Cutaneous. B Systemic. Nail-fold changes include dilated capillary loops, capillary dropout, cuticular hypertrophy, and ragged cuticles.

Dermatomyositis – eyelid edema and heliotrope sign.

A Inflammation of the upper eyelid can be more subtle in darkly pigmented skin; note involvement of the lateral nasal root and the cheeks. The eyebrow alopecia is due to chemotherapy. B The characteristic pink–violet color is seen with involvement of the hairline, lower forehead, upper eyelids and cheeks; the edema is striking and involves the nasal root as well as the eyelids.

B, Courtesy, Jean L Bolognia, MD.

Dermatomyositis – marked facial involvement.

Note the violaceous hue. Misdiagnoses include psoriasis, contact dermatitis, and if there is significant facial involvement as in this patient, acute cutaneous lupus erythematosus.

The cutaneous lesions of dermatomyositis are accentuated on extensor surfaces, including the elbows, knees, metacarpophalangeal joints, and both the proximal and distal interphalangeal joints (knuckles; Fig. 42.4A ). When papules on the knuckles develop a secondary lichenoid quality, they are termed Gottron papules ( Fig. 42.4B,C ); involvement of the elbows and/or knees is known as Gottron sign ( Fig. 42.5 ).

Dermatomyositis – Gottron papules.

A Only a few Gottron papules are present, but there is definite accentuation of the pink–violet inflammation over the metacarpophalangeal (MCP) joints and proximal and distal interphalangeal (PIP, DIP) joints as well as the proximal nail folds. Note the ragged cuticles. B The flat-topped (lichenoid) papules overlying the DIP, PIP, and MCP joints (knuckles) are subtle and were misdiagnosed as verrucae vulgares in this child. C More obvious disease in which the multiple pink–violet lichenoid papules are coalescing and there is some involvement of the interphalangeal skin.

A, Courtesy, Kalman Watsky, MD; B, Courtesy, Julie V Schaffer, MD.

Dermatomyositis – Gottron sign.

Pink–violet papules ( A ) and thin pink plaques ( B ) of the elbows and knee ( C ). Some of the papules on the elbow are flat-topped (lichenoid) and others have white scale. The presence of plaques on the elbows and knees can lead to the misdiagnosis of psoriasis.

B,C, Courtesy, Julie V Schaffer, MD.

A critical diagnostic feature of the cutaneous eruption of dermatomyositis is poikiloderma. This feature may occur both in patients with dermatomyositis, where it is frequently characterized by a pink–violet color, and in patients with lupus erythematosus, where the poikiloderma is a more red color. However, in patients with skin phototypes I and II, the hue of the poikiloderma of dermatomyositis may be pink–red rather than truly violet. Photodistributed poikiloderma is very characteristic of dermatomyositis, often involving the upper chest ( V-neck sign ) and the upper back ( shawl sign ). Poikiloderma can also affect photoprotected areas, such as the lateral thigh, referred to as the holster sign . If the clinician misses the poikiloderma, the eruption of dermatomyositis may occasionally be misdiagnosed as psoriasis because there may be lesions on the elbows and knees as well-defined plaques with fine silvery scale.

The cutaneous lesions of dermatomyositis are often intensely pruritic ( Fig. 42.6 ), and this can significantly affect patients’ quality of life . Of note, pruritus is a feature that can occasionally help to distinguish dermatomyositis from LE.

Dermatomyositis – involvement of the upper back.

The pink–violet plaques, some with associated scale, were very pruritic, as evidenced by multiple excoriations. Linear streaks of erythema are also seen.

Courtesy, Jean L Bolognia, MD.

An additional clinical clue to the diagnosis of dermatomyositis is nail-fold changes. Cuticular dystrophy (i.e. “ragged” cuticles and cuticular hypertrophy) is quite characteristic, as are nail-fold telangiectasias in which dilated capillary loops alternate with capillary dropout ( Fig. 42.7 ). If the photodistribution and nail-fold changes are missed, the eruption may be misdiagnosed as other conditions characterized by poikiloderma, such as cutaneous T-cell lymphoma. Often, the dermatologist correctly notices the photodistribution but considers the diagnosis of photodrug eruption or lupus erythematosus rather than dermatomyositis.

Dermatomyositis – cuticular dystrophy and nail-fold telangiectasias.

The cuticles are “ragged” and within the proximal nail fold, dilated capillary loops alternate with vessel dropout (inset). Atrophy, telangiectasias and hypopigmentation are present on the fingers.

Courtesy, Julie V Schaffer, MD.

Calcinosis cutis is more prevalent in juvenile dermatomyositis, affecting 25–70% of pediatric patients , and it is usually associated with a delay in diagnosis, a delay in the institution of systemic corticosteroids, and/or advanced, therapy-resistant disease. In addition to the skin, calcium deposits may develop within the deep fascia and intramuscular connective tissue. Calcinosis cutis presents as hard, irregular papules or nodules that occasionally drain a chalky material (see Ch. 50 ). Lesions favor sites of trauma, such as the elbows and knees ( Fig. 42.8 ), but can occur anywhere and may be painful and interfere with function.

Juvenile dermatomyositis – calcinosis cutis.

The nodules favor the extensor surface of the knees and result in a decrease in joint mobility.

Additional cutaneous manifestations range from poikiloderma and scaling of the scalp (often accompanied by non-scarring alopecia) to centripetal flagellate erythema ( Fig. 42.9A ) to erosions and ulcerations ( Fig. 42.9B ; Table 42.5 ) . Wong-type dermatomyositis is a variant seen more commonly in Asians in which the cutaneous findings mimic pityriasis rubra pilaris . Lastly, because of the frequency of overlap syndromes, it is important to look for the skin signs of other autoimmune connective tissue diseases in patients with dermatomyositis.

Dermatomyositis – less common presentations.

A Flagellate erythema of the posterior trunk. B Cutaneous erosions and ulcerations can also occur.

Table 42.5

Cutaneous manifestations of dermatomyositis.

See Fig. 42.1 . PRP, pityriasis rubra pilaris.

CUTANEOUS MANIFESTATIONS OF DERMATOMYOSITIS
Common	Uncommon
Heliotrope sign ( Fig. 42.2 ) Eyelid edema ( Fig. 42.2 ) Gottron papules ( Fig. 42.4 ) Gottron sign ( Fig. 42.5 ) Photodistributed poikiloderma (includes facial erythema, shawl sign, V-neck sign) ( Figs 42.3 & 42.6) Scalp poikiloderma and scaling (see eFig. 13.2 ) Non-scarring alopecia Nail-fold changes (includes ragged cuticles, cuticular hypertrophy, nail-fold telangiectasias * ) ( Fig. 42.7 ) Holster sign (poikiloderma of the lateral thighs) Psoriasiform lesions Calcinosis cutis (especially in juvenile dermatomyositis) ( Fig. 42.8 )	Cutaneous erosions or ulcerations ( Fig. 42.9B ) Flagellate erythema ( Fig. 42.9A ) Tender palmar papules Hyperkeratotic palmar papules Exfoliative erythroderma Cutaneous eruption that mimics PRP (Wong variant) Vesiculobullous lesions Pustular eruption of the elbows and knees Ovoid palatal patch Gingival telangiectasias Panniculitis Lipoatrophy (especially in juvenile dermatomyositis) Small vessel vasculitis (especially in juvenile dermatomyositis)

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