Itching for Knowledge About Wound and Scar Pruritus

Laura K.S. Parnell, MSc

Disclosures

Wounds. 2018;30(1):17-26. 

In This Article

Abstract and Introduction

Abstract

Chronic itch continues to be a problem that plagues millions of humans and animals. Pruritus has a negative impact on patient quality of life and many patients experience sleep deprivation, anxiety, and depression, similar to patients with chronic pain. This review provides an overview of clinical pruritus research with special emphasis on itch that wound care providers may see. In addition, the need for using multifactorial questionnaires for better research in pruritus is summarized. Similarities and differences in itch characteristics, triggers, and relievers in various patient populations are discussed. A brief overview of itch receptors and pathways is provided to help the reader better understand the complexity of the resultant itch sensation. Also, some nonpharmacological and pharmacological antipruritic therapies and their mechanisms of action are included

Introduction

This overview is meant to whet the appetite for more knowledge about pruritus in skin and wound patients. There are excellent books with entire chapters dedicated to the different components involved in itch.[1,2]

For most healthy mammals, the sensation of itch is something rarely thought about. It occurs, the area is brushed or scratched, and the cycle typically ends. For protective reasons, the body alerts of a possible threat (ie, mosquito, thorn, etc.), and once the threat is removed, the sensation cycles down. Unfortunately, for millions of humans and animals, it can be difficult to turn off the itch-scratch cycle, leading to chronic pruritus.[3,4] Up to 100% of patients with atopic dermatitis, chronic idiopathic urticaria, psoriasis, burns, and possibly liver disease suffer from debilitating itch.[3–8] The number of patients affected by itch grows even larger when those with xerosis, hypertrophic scars, keloids, kidney disease, and hidradenitis suppurativa are included.[3,9–12] The negative impact itch has on depression, anxiety, and quality of life (QoL) is similar to that of chronic pain and should not be ignored.[3,7,13,14]

The elusive and complex nature of itch contributes to the lack of study and effective treatments options. A consensus paper from the International Forum on the Study of Itch[13] helpfully outlines the key factors that should be used when assessing itch: intensity, localization, frequency and duration, sensory qualities, aggravating and relieving factors, opinion on origin, treatment, affective dimensions, disability or impairments, QoL, coping, itch cognitions, and scratch response. A second consensus paper focused on clinical trial testing of antipruritics and scoring itch was released a year later to encourage the use of valid methods for better comparison of study results.[15] There are a variety of itch questionnaires, some of which are disease specific, but many stem from the McGill Pain questionnaire.[16–20]

Utilizing a multidimensional tool that captures many aspects of patient pruritus episodes provides a rich dataset of information on the complexities of itch. Simply assessing the intensity and frequency of the itch episode is not enough; however, coupling this information with each character sensation, location, and QoL issue can provide information on how itch is affecting the patient and what potential treatments might be most effective. The sensory characteristics of itch (tickling, stinging, crawling, stabbing, pinching, and burning) can present alone or in combination with other sensations.[17] Affective dimensions of itch provide an insight on itch impact on QoL and include bothersome, annoying, unbearable, and worrisome.[17] Patients with atopic dermatitis, uremia, and burn scars each have different etiologies, and it is not surprising that the itch sensory and affective dimensions of these patients also are different. In Table 1,[6,21–27] the comparison of different subject diagnoses with the 6 sensory dimensions of itch shows stinging and burning are quite elevated in burn survivors, yet burning itch in uremic patients is not common.[6,21–27] Likewise, the affective itch dimension unbearable in burn survivors is about 3 times as high as that reported in uremic patients (Table 2).[6,21,23–28] The likelihood of one antipruritic being effective in both populations is slim, and thus the clinician should consider a variety of therapeutic options. It should be noted that the severity (none, mild, moderate, severe) of the sensory and affective dimensions should be taken into account. Stinging is the most reported chronic sensation in burn survivors, but the most severe chronic sensations were crawling and burning.[26] The type of itch that causes the most distress to the patient may not always be the most frequently reported symptom.

Table 1.

Sensory dimensions of itch
Highest reported dimensions are colored red, second most reported dimension is colored orange, and third most reported dimension is colored yellow.
Articles cited6,21–27 were restricted to those that used similar itch scales. Only the presence of the characteristic is reported, not the severity. Also, some of the papers only solicited chronic itch subjects whereas others used acute and chronic itch subjects which may account for differences in reported percentages.

Table 2.

Affective dimensions of itch
Highest reported dimensions are colored red, second most reported dimension is colored orange, and third most reported dimension is colored yellow. The impact of severity on each dimension is not included because not all papers reviewed included that information.
aSubjects rated itchiness as the most bothersome of 13 complications; Likert scale average, not the percentage, was reported.28
bSubjects completed the itch questionnaire25; percentages were not listed and bothersome was the only affective dimension with a statistical difference reported.27
EB: epidermolysis bullosa pediatric

Itch can be considered acute if it lasts < 6 weeks or chronic if it exceeds 6 weeks, except in the burn community which measures acute itch as < 3 to 6 months post injury.[13,26,29,30] Itch is classified into 4 main categories: pruritoceptive, neuropathic, neurogenic, and psychogenic.[31,32] Pruritoceptive itch, also known as dermatologic itch, is due to inflammation or damage to the skin and may have sensations of crawling or tickling.[31–33] Neuropathic itch is caused by nerve damage and is typically perceived as burning, stinging, and paresthesia sensations.[31–33] Neurogenic itch (or systemic itch) does not have any nerve damage, and the perception comes from the central nervous system (CNS).[31,32] Psychogenic itch is associated with psychological abnormalities that can be further subcategorized.[14,31,32] Patients suffering from itch may have a combination of itch classes involved in the presentation. Burn survivors likely have a combination of pruritoceptive, neuropathic, and, for some, a subtype of psychogenic itch from their injuries, whereas uremic patients have neurogenic itch from unfiltered toxins. This again suggests that effective therapies for itch relief would likely require different agents.

Unlike other peripheral symptoms, itch can be initiated by the CNS simply by verbal or visual suggestion.[4] Also known as contagious itch, the sensation can be elicited in individuals without persistent itch but tends to be much more intense in patients with chronic pruritus.[4,14] Nonhuman primates also demonstrate contagious itch after watching scratching videos compared with control videos.[34] Little research into the reverse activation of periphery neurons has been studied but is theorized that it could be part of the mirror empathy system and/or classic conditioning.[4] Exploring this area of CNS activation may help elucidate future therapies, and recent neuroimaging studies of the brain suggest multiple areas are involved in the itch-scratch cycle.[35]

Intensity and itch activation can be affected by stress or other itch triggers.[14] Stress can be induced by the constant pruritus or from daily living activities, but regardless of where the stress originates, from, its presence increases the intensity and frequency of the itch.[14] As seen in Table 3,[6,21–28,36] stress, dryness, sweat, and often heat tend to trigger itching episodes or make pruritus worse regardless of the clinical diagnosis.[6,21–28,36] There are some environmental effects and activities that can trigger itching in some patient populations but calm the itch in others, such as rest and hot water. There are very few nonpharmacological effects that appear to relieve pruritus consistently, but cold water and, for some, cold temperatures and sleep seem to provide relief regardless of the clinical diagnosis related to itch. Educating patients to be aware of these triggers and relievers and to determine their personal aggravators and soothers could help avoid itch episodes from becoming so severe.

Table 3.

Itch triggers and relievers by clinical diagnosis
EB: epidermolysis bullosa pediatric; T: triggers itch episode and is colored orange; R: relieves itching and is colored green

Understanding how stress impacts itch is important, but the impact of the lack of sleep on itch is another key area of awareness.[3,14,37] Sleep allows the body to rest, heal, and recuperate while preparing it for the next day of activities. Unfortunately for the pruritic patient, evening and nighttime is when itch increases, especially in the extremities and torso.[37] Disturbed or lack of sleep leads to an increase in fatigue, stress, negative mood, and a reduced ability to cope, concentrate, learn, or work, which can in turn can lead to additional stress, impairing healing and exacerbating the itch cycle.[14,37] In Table 4,[6,21,24–28,38] the difficulty in falling asleep and staying asleep due to pruritus is a problem patients with itch frequently experience.[6,21,23–28,38] What is strikingly absent from Table 4 is information on sleeping medications from most studies. It is unclear if soporifics are not routinely offered, if the soporifics do not work, or if patients refuse to use them. What is clear from prior Table 3 is when sleep is achieved, it relieves itch for many patients.

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