Myositis and dermatomyositis are similar conditions that present with muscle weakness; dermatomyositis also presents with dermatological changes and is more likely to be associated with a malignancy (15%).
Symptoms and Signs
Generalised muscle weakness is universal finding.
Skin findings are necessary for a diagnosis of dermatomyositis:
- Gottron's sign: A erythematous scaly rash over the extensor surfaces of the digits and dorsal hand, and less frequently on the extensor surface of the knee and elbow
- Heliotrope rash: An violaceous or erythematous rash of the upper eyelid that may cover the skin of the orbit
- Shawl sign: Erythema of the skin of the posterior neck and upper back. Erythema at other sites is referred to as erythroderma.
- A number of other rashes are possible
Weakness of non-skeletal muscle (eg. oesophageal, cardiac) can also occur.
The exact mechanism of dermatomyositis in relation to malignancies is not well understood but it is thought that autoantibodies may have a role (similar to neurological paraneoplastic syndromes). A number of autoantibodies have been detected in non-malignancy related dermatomyositis, including anti-nuclear antibody and a vast number of other antibodies that are either myositis specific or more generally related to autoimmune disorders. Specific cancer associated myositis antibodies have been described but are not wholly specific or sensitive for malignancy associated myositis.
Adenocarcinomas are the most commonly associated malignancy, arising from the cervix, ovary, lung, stomach, pancreas or bladder. Dermatomyositis is seen with other cancer types with much lower frequency.
Initial treatment is both pharmacological (steroids) and non-pharmacological (exercise, avoidance of sunlight). Cure of the underlying malignancy should improve symptoms. Resistant disease is treated with rituximab or intravenous gammaglobulin.