Medicine – Essential Features of Dermatomyositis and Polymyositis

Dermatomyositis and polymyositis are inflammatory muscle disorders characterised primarily by immune-mediated inflammation of skeletal muscle, leading to progressive muscle weakness. Dermatomyositis is distinguished by the presence of characteristic cutaneous manifestations, whereas polymyositis has similar muscular features but occurs without the typical skin lesions. Both conditions may be associated with systemic complications and, in some patients, underlying malignancy.

1. Dermatomyositis

Dermatomyositis is an idiopathic inflammatory myopathy associated with characteristic skin changes. The muscle involvement usually affects the proximal muscles of the shoulder and pelvic girdles, while the cutaneous findings provide an important clue to diagnosis. The condition may develop gradually and can interfere with activities such as climbing stairs, rising from a chair, lifting objects, or raising the arms above the head.

2. Polymyositis

Polymyositis is also an inflammatory disorder of skeletal muscle, but unlike dermatomyositis, it occurs without the characteristic cutaneous lesions. Patients mainly present with symmetrical proximal muscle weakness and may experience muscle discomfort or fatigue. The weakness generally develops progressively and affects daily activities that require the use of the shoulder and hip girdle muscles.

Cutaneous Features of Dermatomyositis

1. Heliotrope Rash

A heliotrope rash is a characteristic violaceous or dusky discoloration around the eyelids, often accompanied by periorbital swelling. It is one of the classic skin manifestations of dermatomyositis and may be particularly noticeable over the upper eyelids.

2. Gottron’s Papules

Gottron’s papules are erythematous or violaceous, scaly papules that occur over the extensor surfaces of the joints, especially the knuckles and backs of the hands. They are highly characteristic of dermatomyositis and are useful in distinguishing it from other inflammatory myopathies.

3. Sclerodermatous Changes and Calcification

Some patients may develop sclerodermatous skin changes, with thickening or tightening of the skin. Cutaneous and muscular calcification may also occur, particularly in longstanding disease and more commonly in juvenile forms. These calcium deposits can cause pain, stiffness, and limitation of movement.

4. Raynaud Phenomenon

Raynaud phenomenon may occur, producing episodic vasospasm of the fingers or toes in response to cold or emotional stress. The digits may become pale, blue, and then red as blood flow returns. Its presence may suggest overlap with other connective tissue diseases.

5. Proximal Muscle Weakness

The most important muscular manifestation is symmetrical proximal muscle weakness. Patients may have difficulty standing from a seated position, climbing stairs, combing their hair, or lifting their arms. The weakness generally affects the shoulder and hip girdles more than the distal muscles. In severe cases, involvement of pharyngeal or respiratory muscles may lead to dysphagia or breathing difficulties.

6. Association with Malignancy

Dermatomyositis, particularly in older adults, has an important association with underlying malignancy. The risk is greater in patients over the age of 50, and older teaching sources describe malignancy in around 20% of patients in this age group. Because of this association, age-appropriate cancer screening and investigation for suspicious symptoms are important when dermatomyositis is diagnosed.

Diagnosis

1. Elevated Muscle Enzymes

Laboratory tests often demonstrate increased serum levels of muscle enzymes, reflecting ongoing muscle injury. Creatine kinase is commonly elevated, and other enzymes such as aldolase, AST, ALT, and LDH may also rise.

2. Muscle Biopsy

A muscle biopsy can demonstrate characteristic inflammatory and structural changes within skeletal muscle. The histological pattern can help confirm the presence of an inflammatory myopathy and may also assist in distinguishing between different types.

3. Autoantibodies

Autoantibodies may provide additional diagnostic support. Antinuclear antibodies (ANA) may be positive, and anti-Jo-1 antibodies can be present, particularly in patients with the antisynthetase syndrome. Anti-Jo-1 positivity is also associated with an increased risk of interstitial lung disease, inflammatory arthritis, and other systemic manifestations.

Treatment

Treatment is aimed at reducing muscle inflammation, restoring muscle strength, and preventing complications. Corticosteroids are commonly used as initial therapy, particularly in active inflammatory disease. If the response is inadequate or long-term steroid exposure needs to be reduced, immunosuppressive drugs may be added. Depending on disease severity and organ involvement, agents such as methotrexate, azathioprine, mycophenolate, or other immunomodulatory therapies may be considered. Physical rehabilitation and monitoring for complications such as dysphagia, lung disease, and malignancy are also important parts of management.