Chapter 61

Pruritus

Daniel W. O’Neill, Lauren E. Curtis

Definition and Epidemiology

Pruritus is a sensation that leads to a desire to scratch. It is a common symptom that can be found with many dermatologic and systemic illnesses and often leads to a high burden and impaired quality of life. A population-based cross-sectional study in 19,000 adults showed that about 8% to 9% of the general population experienced acute pruritus, and this was dominant across all age groups.¹

Pathophysiology

Pruritus is characterized by the activation of a network of distinct free nerve endings situated at the dermoepidermal junction by local mediators such as histamine and numerous other peptides and proteases as well as elevated levels of various substances associated with certain systemic diseases of renal, hepatic, endocrine, or hematologic origin.² These impulses are carried by unmyelinated C fibers to the central nervous system, where the impulses are modulated by opioid peptides. Prostaglandins in the skin lower the threshold for itching. The exact pathophysiologic mechanisms leading to itching in systemic disease are not well defined. Scratching leads to symptomatic relief by temporarily destroying the nerve endings or stimulating pain fibers. However, this often leads to the release of more mediators and the scratch-itch cycle, in which one scratch is too many and a million are not enough.

Clinical Presentation and Physical Examination

Dermatologic disorders can manifest with characteristic primary skin lesions; therefore after obtaining a basic history of the present illness the health care provider should perform a total skin examination to first identify or to exclude dermatologic disorders.² Secondary skin lesions, such as excoriations (scratches), secondary infections (e.g., impetigo), hyperkeratotic skin changes, and lichenification (thickening, which indicates chronicity), often obscure the primary lesion. If a specific diagnosis is not evident on initial examination, the history should be readdressed with particular attention to any diurnal rhythms, characteristics of associated symptoms, any variations in symptom severity, evolution of current lesion distribution, exacerbating and alleviating factors, and previous treatments. The history should also include medication use, alcohol use, past medical and psychiatric history, exposures (e.g., to people who are scratching, pets, soaps, detergents, dry air, chemicals), and complete review of systems. A complete physical examination with emphasis on evaluation for organomegaly and adenopathy is then performed. Pruritus of the scalp and face is the most common manifestation of psychogenic pruritus.³

Diagnostics

If the symptoms persist and no dermatologic cause is discovered, screening laboratory examinations include a complete blood count (CBC) with differential, serum glucose, aspartate and alanine transaminases, alkaline phosphatase, bilirubin, blood urea nitrogen (BUN), creatinine, thyroid panel, urinalysis, and chest radiograph. If indicated, a skin biopsy specimen can be sent for pathologic examination for mycosis fungoides, immunofluorescence (pemphigoid and dermatitis herpetiformis), or special staining (mastocytosis). Additional studies that may be indicated include serum ferritin level, protein electrophoresis, immunoelectrophoresis, and stool culture for ova and parasites.

On occasion, it is necessary to perform repeated evaluations in follow-up visits or to refer the patient for dermatologic or psychiatric evaluation.

Differential Diagnosis

Dermatologic disorders with pruritus as a predominant symptom are common. Some of these disorders are covered in detail in other chapters, and each has its own etiology, clinical presentation, and treatment considerations. Pruritus without diagnostic skin lesions that persists longer than 2 weeks and is undiagnosed after 2 weeks of evaluation is called pruritus of undetermined origin and may indicate a systemic disorder.⁴ It has been reported that 10% to 50% of patients with pruritus but no rash have an underlying systemic disease and up to 70% have a psychiatric one.³ Medications are also a significant cause of pruritus.

Differential Diagnosis

Pruritus

Pruritic Dermatologic Disorders

Inflammatory Disorders