In order to pin down the correct diagnosis, the specific symptoms, findings on physical exam, and risk factors all need to be taken into consideration.
COMMON DERMATOLOGIC CAUSES
Psoriasis. Up to 80% of patients with psoriasis report pruritus that is cyclical, with nocturnal exacerbations that interrupt sleep. Pruritus is often more generalized and not restricted to areas of psoriatic plaques.
Atopic Dermatitis: or eczema, is characterized by pruritus. This is a chronic, relapsing inflammatory skin disease that often occurs in patients with a personal or family history of asthma or allergic rhinitis. Skin findings include excoriation (an injury to a surface of the body caused by trauma, such as scratching), exuding fluid, lichenification (thickening and hardening of the skin, with exaggeration of its normalmarkings) and pigment changes.
Contact Dermatitis. a type of rash caused by direct skin exposure to an irritant. A very common skin condition, with a lifetime prevalence of 30%. It causes severe itchiness, and can be induced by an allergen (like dust mites that hide in beddings and mattresses) or more commonly by an irritant (perfume, cosmetics, certain fabrics like nylon etc.)
Dermatophytes. A type of fungus that causes localized pruritus and a rash that is clear in the center and has scales around it. Tinea pedis (athlete's foot) usually occurs between the toes with dry, cracking skin and white areas of maceration. Tinea infections can occur at several other sites, including the scalp, trunk, and groin.
Scabies. pruritus is caused by deposition of mite eggs on the surface of the skin. The pruritus is often severe and worsens at night. The lesions start as small, red papules that is often excoriated.
Lice. (Pediculosis Body lice are typically found in patients with poor hygiene, whereas pubic lice are sexually transmitted.
Lichen Simplex Chronicus a localized disorder characterized by itching that leads to thickened, violaceous patches which are intensely pruritic; the patient will continue to scratch, perpetuatin the itch-scratch-itch cycle.
Urticaria: also known as hives, is a common disorder. It affects up to 25 % of the population. The lesions are typically intensely pruritic, they look like an elevated wheel that is reddish in color and has well-defined borders. Lesions may merge together and wax and wane over several hours.
COMMON SYSTEMIC CAUSES
Pruritus in the absence of a primary dermatologic etiology may be indicative of a serious underlying systemic disease. Chronic kidney disease, liver disease, malignancy (most commonly Hodgkin lymphoma, or nervous system disorders are important underlying causes to rule out.
Psychiatric illness is diagnosis of exclusion, that is, we think of it after having ruled out organic causes. Neurotic pruritus is characterized by scratching all reachable parts of the body, with resulting lines of excoriations, most often in the arms and legs.
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