Chapter 67 Approaching the pruritic patient
3. Is an itch a separate modality of pain or a submodality of pain?
Cutaneous nerve fibers arranged in an arborizing plexus immediately below the epidermis receive itch impulses. Unmyelinated C fibers conduct itch impulses from the skin to the ipsilateral dorsal root ganglia, ascend in the opposite anterolateral spinothalamic tract (closely associated with pain fibers), continue to the thalamus, and proceed through the internal capsule to the sensory cortex.
4. What causes an itch?
Itch is mediated by a number of local and central substances. Histamine, produced by skin mast cells, is the classical pruritus mediator. Pricking the skin with histamine produces pruritus in most individuals; however, histamine does not account for all pruritus. Other pruritus mediators include prostaglandin, serotonin, tachykinins, cytokines, and opioid receptors. Prostaglandin E1 lowers the threshold of the skin to itching provoked by histamine. Serotonin, 5-hydroxytryptamine (5-HT), may regulate itch by acting on 5-HT3 receptors. Tachykinins, such as the neuropeptide substance P, cause pruritus for reasons attributable to histamine release from mast cells. Cytokine interleukin-2 may be an important peripheral mediator of itching. Opioid receptors in the central nervous system regulate the intensity and quality of perceived itch.
Bernard JD, editor: Itch: mechanisms and management of pruritus, New York, 1994, McGraw-Hill.
Fazio SB: Pruritus, UpToDate 2000, 2005. Available at: http://www.uptodate.com.
Greaves MW, Wall PD: Pathophysiology of itching, Lancet 348:938–940, 1996.
5. Describe the difference between localized and generalized pruritus.
Pruritus occurs with a host of dermatologic conditions, or it can mark systemic disease. Some dermatologic conditions that cause pruritus are inconspicuous or nonspecific. Other dermatologic conditions are apparent on physical exam. A physician must differentiate between localized and generalized pruritus and know a variety of skin diseases that cause pruritus.
6. What is the best approach to evaluation of a patient with localized pruritus?
The most important diagnostic tools are the history and physical exam. After obtaining a thorough history, the best clinical approach is to organize the patient regionally. The physician should ask the patient if there are any lesions associated with pruritus. The patient’s physical exam should focus on identifying a primary skin lesion or disorder in the affected region. After identifying a primary lesion, the physician can categorize the skin disease based on the lesion’s morphology.
8. What is notalgia paresthetica?
Notalgia paresthetica is an acquired unilateral localized form of pruritus that develops near the inferomedial border of either scapula. The skin typically appears normal, although some patients demonstrate subtle hyperpigmentation secondary to repeated rubbing or excoriation. The cause is not understood, although there is substantial evidence to suggest that it is due to spinal nerve impingement. Occasional cases have been familial, and the term “hereditary localized pruritus” has been applied to this variant. The management typically consists of topical capsaicin cream or topical preparations containing lidocaine.
Table 67-1. Differential Diagnosis of Localized Pruritus
| LOCATION | DISEASES |
|---|---|
| Scalp | Psoriasis, seborrheic dermatitis |
| Trunk | Contact dermatitis (axillae, waistline), erythrasma (axillae), psoriasis (periumbilical), notalgia paresthetica, scabies, seborrheic dermatitis, urticaria |
| Inguinal region | Candida, contact dermatitis, erythrasma, overuse of topical steroids, pediculosis, scabies, tinea cruris |
| Anal region | Candida, contact dermatitis, gonorrhea, hemorrhoids, pinworm, psoriasis, tinea cruris |
| Hands | Contact dermatitis, scabies, eczema |
| Legs | Atopic dermatitis (popliteal fossae), dermatitis herpetiformis (knees), lichen simplex chronicus (malleoli), neurotic excoriations, nummular eczema, stasis dermatitis |
| Feet | Contact dermatitis, pitted keratolysis, tinea pedis |
9. What is the physician’s best approach when seeing a patient with generalized pruritus?
