Acanthosis nigricans: Dermatomyositis,Scleroderma:

Acanthosis nigricans presents as thickened, hyperpigmented skin predominantly of the flexures (Fig. 23.24). It can appear warty or velvety when advanced. In early life it is seen in obese individuals who have very high levels of insulin owing to insulin resistance (and this is sometimes termed 'pseudo-acanthosis nigricans'). In older people it normally reflects an underlying malignancy (especially gastrointestinal tumours). Rarely it is associated with hyperandrogenism in females.
Treatment
Topical or oral retinoids (0.5 mg/kg/day) may help (p. 1338) and weight loss is advised in the obese. Any underlying malignancy should be treated.

Dermatomyositis:
The rash is distinctive. Facial erythema and a magenta-coloured rash around the eyes with associated oedema are often present. Bluish red nodules or plaques may be present over the knuckles and extensor surfaces. The nail folds are frequently ragged with dilated capillaries. The diagnosis is made from the clinical appearance, muscle biopsy, EMG and a serum creatine phosphokinase. Skin biopsy is not diagnostic.
There is a childhood form which usually occurs before the age of 10 and which eventually resolves. This type is often associated with calcinosis in the skin and can cause significant long-term functional problems with weak muscles and contractures. Life-threatening bowel infarction can also occur in the childhood form. The adult form usually occurs after the age of 40. Some cases are associated with an underlying malignancy, whereas others appear to reflect a 'connective tissue disease'. This latter group may overlap with scleroderma and lupus erythematosus.
Treatment
Skin disease may respond to hydroxychloroquine (200 mg twice daily) as well as immunosuppressants, e.g. azathioprine or ciclosporin.

Scleroderma
The term scleroderma refers to a thickening or hardening of the skin owing to abnormal dermal collagen. It is not a diagnostic entity in itself. Systemic sclerosis and morphoea both show sclerodermatous changes but are separate conditions.
Systemic sclerosis (often called scleroderma) has cutaneous and systemic features and is discussed fully on page 577.
Morphoea is confined to the skin and usually presents in children or young adults. It is commoner in females and the cause is unknown. Lesions are usually on the trunk and appear as bluish red plaques which progress to induration and then central white atrophy. A linear variant exists in childhood which is more severe as it can cause atrophy of underlying deep tissues and thus can cause unequal limb growth or scarring alopecia.
Rarely, sclerodermatous skin changes may be seen in chronic Lyme disease (acrodermatitis chronica atrophicans), chronic graft-versus-host disease, polyvinyl chloride disease, eosinophilic myalgia syndrome (due to tryptophan therapy) and bleomycin therapy.