How To Detect And Treat Pruritus

By Gary L. Dockery, DPM, FACFAS

   Pruritus is a symptom complex rather than a dermatological condition. It is a very common manifestation of skin diseases described as an itch that makes a person want to scratch. It can be frustrating and cause some patients severe discomfort. Chronic itching can lead to sleeplessness, anxiety, depression and behavioral disorders (especially in young children). Symptoms of pruritus can be a result of skin conditions such as dry skin (xerosis), atopic dermatitis, eczema and contact dermatitis. Pruritus can also present with certain internal disorders or may be due to altered processing of the itch sensation within the nervous system.

   Pruritus is not fully understood and it is considered to be a complex process involving nerves that respond to certain chemicals such as histamine that are released into the skin. The treatment for nonspecific pruritus is directed mostly at preventing dry skin while treatment for disease-specific pruritus is focused on management of the systemic condition as well as the pruritus.

   In regard to pathophysiology, varied mechanical stimuli, such as soft touch, pressure, vibration and contact with irritants such as wool fibers, can produce pruritus. Heat and electrical stimuli may also generate itch sensations. Unspecified free nerve endings in the skin receive the itch sensation. Until recently, the same pathways were thought to transmit both itch and pain. The speculation was that low-intensity stimulation of unmyelinated C-fibers caused an itch and high intensity stimulation of these fibers caused pain. This concept has now been disputed because of the differences in the features of pain and itch, namely that pain produces a withdrawal response and itch produces a desire to rub and scratch.

   Removal of the epidermis and upper dermis abolishes pruritus but not pain. Additionally, morphine relieves pain but makes itching much worse. Itch and pain can also be perceived separately at the same location simultaneously. It is now thought that pruritus and pain are different and independent sensory modalities. However, at this time, a morphologically separate end-organ for pruritus has not been positively recognized.

Understanding The Main Causes Of Pruritus

   Various authors have described several types of itch, including those related to dermatological conditions, systemic disease, direct damage to nerve fibers and psychological disorders. One may further classify pruritus by its three main causes:

   • the predisposing causes, such as genetic and allergic factors, in addition to endogenous and exogenous poisonings;
   • the chance causes due to emotional events and environmental factors such as temperature, humidity and wind; and
   • the determining causes like chemical agents, physical agents, infections and infestations.

   It is essential to differentiate the localized pruritus from the generalized pruritus. One can further separate these conditions into itch-due-to-skin diseases, in which various mediators act on the free nerve endings; itch that is associated with internal diseases; and itch of unknown origin or “idiopathic pruritus.” Note that at any given time, patients may have pruritus caused by more than one of the causes or conditions discussed.

What Causes The Itch-Scratch Cycle?

   The itch-scratch cycle is well known but not well defined. A very simple model shows the process from a primary stimulus that causes the pruritus through the transition to scratching or rubbing, which in turn causes an irritation (inflammation) to the skin and continues the cycle. Understanding the diverse causes of pruritus is essential to managing the condition.

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