AD is a chronic inflammatory skin disease that often causes bacterial skin infections. The innate immune system of the epidermis essentially protects against the development of cutaneous infections. However, the skin of patients with AD is susceptible to infection owing to the reduction of cell-mediated immunity and the developed deficiency in the antimicrobial peptides barrier on the skin [7–9]. The skin of patients with AD is highly colonized with toxin-producing S. aureus . The skin lesion is colonized by S. aureus in up to 90% of the patients compared to that in 5–30% of healthy controls. Since S. aureus is tightly bound to the inflammation site of the skin, scratching of the skin disturbs the skin barrier and promotes its colonization .
AD is associated with a high rate of incidence of extracutaneous infections in various organs . Correlation between AD and extracutaneous infections has not been sufficiently examined until recently. According to a previous study on a large population in the United States, the ears, respiratory tract, and urinary tract are the frequently affected sites for infection in patients with AD . The bone and joint infection has also been noted; however, the concrete infected site has not been described. As far as we could find, there have been only two reports with concrete descriptions presenting with osteomyelitis of the hip and distal phalanges in children with AD [14, 15].
Although the mechanism of extracutaneous infection in patients with AD remains unknown, some past papers have documented the possibility of hematogenous dissemination of S. aureus infection.
This study suggested that AD is associated with the incidence of PS. MSSA infection was confirmed in the operative tissue samples of all five cases and the blood samples of the four cases. The same bacterium was also detected on the skin of all three patients examined. The spinal level of infection ranged from the cervical to lumbar spine. These results suggested that PS is potentially induced through hematogenous dissemination from the cutaneous infection.
We had not experienced patients with PS and AD until 2018. The number of patients with AD in Japan has demonstrated an increasing trend in recent years. According to the Ministry of Health, the number of patients with AD increased from 0.40 million in 1999 to 0.51 million in 2017 . Previous age–period–cohort (APC) analyses suggest that the period effect for AD demonstrated an increasing trend from 2002 owing to an increase in the population of the urban areas in Japan. The cohort effects for AD increased rapidly for cohorts born in the years 1950 to 1980. This tendency has been attributed to factors, such as rapid urbanization, increasingly westernized lifestyles, and improved standards of living and education [17, 18]. It is probable that the recent higher prevalence of AD will provide us with a greater number of cases of patients with PS and AD.
An accurate diagnosis of PS is relatively difficult at the early stage of the disease. Previous studies have reported that a median interval from the onset of symptoms to diagnosis is 26 days . The delayed diagnosis of PS can be attributed to its nonspecific symptoms, such as back pain and fever, which are often observed in other common diseases too. Additionally, a sign of infection is difficult to recognize by imaging studies at the early stage; therefore, the knowledge of the risk factors for PS is essential to cite PS as a differential diagnosis. Advanced age is one of the risk factors. In this study, the mean age of the patients without AD was ~70 years, whereas that of the patients with AD was around 50 years. The age of onset was significantly smaller in patients with AD. In our cases, the prevalence of AD was equivalent to that of malignancy, which is a risk factor for PS. Therefore, we considered that AD should be regarded as a risk factor for PS, especially at the middle age.
There are some limitations in this study. For instance, this is a retrospective study conducted on few cases at a single institution. Not all patients underwent a bacterial examination of the skin at the initial diagnosis, and there was no way to ensure that the same bacterium caused both cutaneous and extracutaneous infection. However, this study does provide some helpful insights for the correct diagnosis of PS associated with AD.