Pruritus, or itch, is the predominant symptom associated with atopic dermatitis. In a US study of 304 patients with atopic dermatitis, 91% experienced itch daily (Dawn et al., 2009). The impact of persistent itch should not be underestimated as it has been shown to affect both physical and emotional/psychological quality of life (QoL) domains. Indeed, reducing itch was identified as the most important treatment goal in 36% of patients with atopic dermatitis (Schmitt et al., 2008). Furthermore, the physical damage caused by repeated scratching can exacerbate skin barrier breakdown and promote establishment of chronic disease (Blume-Peytavi & Metz, 2012). More recently, skin pain has also been identified as an important symptom associated with the burden of atopic dermatitis. A prospective, questionnaire-based study of patients with atopic dermatitis aged 13 years and older (N=305) established that 42.7% of patients had experienced skin pain in the past week, with 13.8% reporting it to be severe or very severe. Importantly, patients with severe itch and pain had significantly worse QoL scores than patients with only one or neither symptom (Vakharia et al., 2017).
With atopic dermatitis frequently representing the first step in a series of allergy-related conditions (atopic march), it is well established that patients with atopic dermatitis have an increased risk of developing asthma and allergic rhinitis. In children with severe atopic dermatitis, as many as 50% went on to develop asthma and 75% developed allergic rhinitis (Gustafsson et al., 2000). A systematic review found the prevalence of asthma in children with atopic dermatitis ranges from 14.2% to 52.7% with an odds ratio of developing asthma of 2.14 compared to children without atopic dermatitis. However, this may be higher than the risk for the general population given many of the included studies were performed in the hospital setting (van den Hulst et al., 2007). Interestingly, an Australian longitudinal study showed that childhood atopic dermatitis increased the risk of persistent allergic asthma in adulthood, but not non-allergic asthma offering potential therapeutic insights for these patients (Martin et al., 2011).
Gene mapping studies in atopic dermatitis have identified a number of loci involved in immune function that have also been implicated in other immune-mediated inflammatory diseases (Ellinghaus et al., 2013). A large retrospective cohort study identified 49,847 individuals (N = 655,815; prevalence of 7.6%) under the age of 40 years who had prevalent atopic dermatitis between 2005 and 2006. After adjusting for age and sex, patients with atopic dermatitis, compared with those without atopic dermatitis, had significantly increased risks of Crohn’s disease, ulcerative colitis and rheumatoid arthritis (Schmitt et al., 2016).
While the cause of these associations remains to be confirmed, it has been established that chronic atopic dermatitis, rheumatoid arthritis and inflammatory bowel disease all involve prominent Th1 and Th17 immune responses. It is tempting to speculate that sustained Th1/Th17 inflammation in patients with chronic atopic dermatitis leads to an increased risk of other chronic inflammatory conditions (Schmitt et al., 2016).