What are the characteristics of skin and muscle involvement in patients with dermatomyositis? What are the diagnostic criteria?

Author: Wu Donghai, Chief Physician of China-Japan Friendship Hospital

Reviewer: Zhang Zhuoli, Chief Physician, Peking University First Hospital

Dermatomyositis is an autoimmune disease that affects multiple systems. The cause is currently unknown and it manifests as varying degrees of involvement of the skin, muscles, and internal organs.

Dermatomyositis is uncommon and has a low incidence rate. There are usually two peak periods of onset: children and adolescents aged 5-15 years, and middle-aged and elderly people aged 45-65 years. In addition, the incidence rate in women is higher than that in men, with a male-to-female ratio of about 1:2.

1. Typical rash characteristics of dermatomyositis

Specific skin manifestations of dermatomyositis include the following:

1. Gottron's papules, which appear as red or purple papules on the extensor side of the metacarpophalangeal joints and interphalangeal joints, sometimes covered with scales, and are often distributed symmetrically.

Figure 1 Original copyright image, no permission to reprint

2. Gottron's sign: the rash is symmetrically distributed on the skin surface of bony protrusions, such as the metacarpophalangeal joints, interphalangeal joints, elbows, knees, ankles and heels, appearing as symmetrical, confluent erythema or purpura, often accompanied by skin atrophy, capillary dilation and pigmentation or hypopigmentation.

3. Heliotrope sign, manifested as a red rash on the periorbital skin or upper eyelids, which occurs symmetrically on both sides or unilaterally, sometimes accompanied by eyelid edema, which may be very obvious.

Figure 2 Original copyright image, no permission to reprint

Other skin manifestations with higher diagnostic value include:

Shawl sign: erythema or purplish red spots on the upper back, like a shawl; V-sign: erythema or purplish red spots on the front of the neck and upper chest, generally in a V shape, may show pigmentation, capillary dilation or atrophy; holster sign: rash on the outer thigh, the shape and position are similar to a pistol holster. This rash usually appears as erythema or purplish red spots; nail fold changes: rough, erythematous nail folds, and dilated nail fold capillaries; head skin lesions: scalp erythema, plaques, skin atrophy, may be covered with scales, often accompanied by non-scarring alopecia.

Rare skin manifestations include vesicular bullous lesions, skin necrosis, skin ulcers, skin vasculitis, skin calcification, "mechanic's hands", whiplash erythema, follicular hyperkeratosis, sebaceous membrane inflammation, erythroderma, etc. In addition, dermatomyositis lesions may be accompanied by varying degrees of itching.

2. Characteristics of muscle involvement in dermatomyositis

The muscle groups most commonly affected by dermatomyositis are the proximal muscles of the limbs, shoulder girdle muscles, neck and throat muscles, which are manifested by progressive symmetrical muscle weakness, pain and tenderness. In the early stages of the disease, the affected muscles may become swollen.

When the proximal muscles of the limbs are affected, the upper limbs may be affected, which may make it difficult to raise the hands, such as combing hair or brushing teeth, and when the proximal muscles of the lower limbs are affected, it may be difficult to squat, stand up or climb stairs. In severe cases, patients are bedridden and have completely lost their autonomous movements.

Involvement of the neck muscles results in the head being tilted back, making it difficult to lift the head when lying flat; involvement of the throat muscles can cause difficulty in speaking, hoarseness, and nasal voice, and food can easily choke into the lungs and trachea during meals, resulting in difficulty in swallowing.

Typical dermatomyositis is characterized by the presence of both skin and muscle symptoms. Skin lesions and myositis may occur simultaneously or one after the other, but the severity of the two is often inconsistent. Amyopathic dermatomyositis has the skin involvement manifestations of dermatomyositis, but does not involve or only slightly involves the muscles, so patients usually do not show symptoms of muscle weakness.

In addition to affecting the skin and muscles, dermatomyositis can also affect the internal organs. The most important organs affected are the lungs, as well as the heart, digestive tract, and kidneys.

3. Diagnosis of dermatomyositis

Dermatomyositis is an idiopathic inflammatory myopathy. The diagnostic criteria are constantly changing. So far, the most commonly used is the Bohan/Peter criteria proposed in 1975. The criteria include five diagnostic items and three judgment criteria.

Diagnostic entries include:

1. Muscle weakness: Symmetrical weakness of the proximal muscles of the limbs and the flexor muscles of the neck, lasting for weeks to months, with or without dysphagia and respiratory muscle involvement.

2. Abnormal muscle biopsy: muscle fiber necrosis, cell phagocytosis, regeneration, basophilic degeneration, enlarged nuclear membrane, prominent nucleolus, atrophy of perifascial structures, muscle fiber size variation, and inflammatory exudation.

3. Elevated serum muscle enzymes: such as creatine kinase, aldolase, alanine aminotransferase, aspartate aminotransferase and lactate dehydrogenase.

4. Electromyography shows myogenic damage: showing the electromyographic triad, including short polyphasic motor unit potentials; muscle fiber fibrillation, positive sharp waves, and insertion irritation; odd, high-frequency repetitive discharges.

5. Skin manifestations: purple-red spots on the eyelids with periorbital edema; erythema and scaling on the back of the hands, especially on the metacarpophalangeal joints and proximal interphalangeal joints; skin involvement of the knees, elbows, medial ankles, face, neck, and upper body.

Based on these diagnostic items, the criteria for dermatomyositis are further refined into three levels: suspected, probable, and confirmed. Suspected dermatomyositis requires the fifth skin manifestation plus any one of the other four items; probable dermatomyositis requires the fifth plus any two of the other four items; and confirmed dermatomyositis requires the fifth plus any three of the other four items.