Pruritus is the sensation that provokes a desire to scratch. Pruritus as a medical complaint is 40% as common as low back pain. Elderly Asian men are most significantly affected, with 20% of all health care visits in Asian men over the age of 65 involving the complaint of itch. The quality of life of a patient with chronic pruritus is the same as a patient undergoing hemodialysis. Evidence suggests that increased interleukin-31 (IL-31) signaling through the IL-31 receptor on epithelial cells and keratinocytes is associated with itch, especially in allergic skin disease. It also appears that Il-4 and Il-13 (via binding to the Il-4R) lower the itch threshold of sensory neurons. Janus kinase signaling (which occurs downstream from the Il-4R) is also important in the pathophysiology of itch.
Dry skin is the first cause of itch that should be sought, since it is common and easily treated. The next step in evaluation is deciding whether a primary skin lesion with associated pruritus is present or absent. Examples of primary cutaneous pruritic diseases include scabies, atopic dermatitis, insect bites, pediculosis, contact dermatitis, drug reactions, urticaria, psoriasis, lichen planus, and fiberglass dermatitis, all of which have recognizable morphologies. The treatment of an underlying primary skin condition usually results in control of the associated pruritus.
Persistent pruritus not explained by cutaneous disease or association with a primary skin eruption should prompt a staged workup for systemic causes. Common causes of pruritus associated with systemic diseases include endocrine disorders (eg, hypothyroidism, hyperthyroidism, or hyperparathyroidism), psychiatric disturbances, lymphoma, leukemia, and other internal malignant disorders, iron deficiency anemia, HIV, hypercalcemia, cholestasis, and some neurologic disorders. Calcium channel blockers can cause pruritus with or without eczema, even years after they have been started, and it may take up to 1 year for pruritus to resolve after the calcium channel blocker has been stopped.
The treatment of chronic pruritus can be frustrating. Most cases of pruritus are not mediated by histamine, hence the poor response of many patients to antihistamines. Emollients for dry skin are listed in Table 6–2. Emollient creams (preferred over lotions) should be generously applied from neck to toe immediately after towel drying and again one more time per day. Neuropathic pruritus responds to neurally acting agents, such as gabapentin (starting at 300 mg orally at around 4 PM and a second dose of 600 mg orally at bedtime) or pregabalin (150 mg orally daily). Combinations of antihistamines, sinequan, gabapentin, pregabalin, mirtazapine, and opioid antagonists can be attempted in refractory cases. In cancer-associated and other forms of pruritus, aprepitant 80 mg orally daily for several days can be dramatically effective. Pruritus in conjunction with uremia and hemodialysis and to a lesser degree the pruritus of liver disease may be helped by phototherapy with ultraviolet B or PUVA. Gabapentin or mirtazapine may relieve the pruritus of chronic kidney disease. Il-31 blockade (nemolizumab), Il-4 blockade (dupilumab), and ...