Many patients complain of pruritus but have either no discernible rash or a rash that is initiated by scratching.

One of the more common causes of pruritus is dry skin, particularly in the elderly and during winter. It is worsened with bathing and/or using harsh soaps. Patients may have no visible skin changes, or they may develop excoriated, eczematous patches, which can become impetiginized.

When no visible changes are present, an attempt should be made to elicit dermatographism, the appearance of a hive from stroking of the skin (Fig. 15-2). Dermatographism is more common in patients with urticaria.

Patients without obvious skin diseases should also be evaluated for systemic disease. Common causes are renal disease (uremia and hemodialysis-related), hepatobiliary disease (primary biliary cirrhosis and cholestatic problems), thyroid disease (both hypothyroidism and hyperthyroidism), diabetes mellitus, hematologic disease (polycythemia vera), malignancy (lymphoma), and human immunodeficiency virus (HIV) infection.

Drugs associated with pruritus include antibiotics, opiates, bleomycin, angiotensin-converting enzyme inhibitors, diuretics, sulfonylureas, estrogens, antithyroid agents, and anticoagulants.

Two conditions initiated by constant scratching are prurigo nodularis and lichen simplex chronicus. Large, firm, hyperpigmented nodules of prurigo nodularis often develop in crops on the arms and legs (Fig. 8-1). Lichen simplex chronicus involves intense itching and habitual scratching the skin of a particular area. Lichenification is the hallmark sign, but the lesions are also notably red and scaling. Common sites are the lower legs, neck, wrists, and ankles (Fig. 8-2).