Psoriasis is a chronic inflammatory skin disease, manifesting mainly erythemato-squamous plaques , and its occurrence and development are closely related to the expression imbalance of genes related to immune dysfunction , vascular injury  and abnormal signal transduction pathway . Biological and seasonal factors are the two most common causes, among which winter and spring play an important role.
In our study, the sleep quality of patients with psoriasis was significantly affected, and their quality of life was lower than that of the control group. Skin lesions of patients with psoriasis may often be accompanied with itching and irritability, and the patients were able to be self-abased, when the skin lesions occur in the exposed parts. Furthermore, sexual activities and drive of such patients tend to be affected if psoriasis occurs in the perineum and other private parts. Some other studies have shown that psoriasis may lead to male erectile dysfunction, and may even be an independent risk factor for erectile dysfunction . Therefore, patients are prone to insomnia and decreased quality of life compared with the control group as pruritus of the skin lesions manifested more at night. Thus, patients are affected by worry, depression, anxiety, and other negative emotions. The course of psoriasis is fairly long and recurrent, and although there are several various treatment methods, curability and efficacy are elusive, thereby significantly affecting health and confidence. Because of the long-term trouble caused by the disease, the patients often showed symptoms of depression and anxiety, which affected their general quality of life. Recently, with the development of society and the continuous improvement of living standards, the biopsychosocial medical model has attracted increasing attention, and psoriasis patients’ mental health had also been given focus.
Female patients with psoriasis are more likely to suffer from depression [17, 18], which is consistent with Tian Z et al.  's on Chinese people, suggesting that there was no significant difference in depression among patients with psoriasis on different sexes. Comparing national conditions at home and abroad could be found that the number of patients with psoriasis is almost equal between men and women beyond the seas . However, in some areas, such as the south bank of the Atlantic, Pacific, and New England region, the prevalence rate of psoriasis in women was even higher than that in men. While in China, however, the opposite was true . In terms of the sample size, sex composition ratio was different at home and abroad, and the traditional concept thought that women pay more attention on their external image than men do. However, with the improvement of China's living standards, men's attention to external image is increasing.
The study found that psoriasis patients with a long course were more prone to depression than those with a short course, which was the same with the result of Lakshmy et al . Moreover, it has also been reported that psoriasis patients with short course are more prone to depression than those with long course, such as the study of Tian Z et al . Repeated attacks of the disease often turn initial hopes of a cure into disappointment. Repeated visits to the doctor often lead to anxiety and, over time, depression. Therefore, the knowledge popularization and health education of the newly diagnosed patients are particularly important in clinical practice. The body management concept of "with chronic diseases and self-discipline as medicine" should be instilled into every patient with psoriasis.
Tian Z et al.  believed that the onset of depression among psoriasis vulgaris was not related to age, but was related to the age at which psoriasis occurred. Conversely, the result of the study showed that older patients had a higher prevalence of depression. In terms of contemporary medical conditions and prospects, young patients may be more willing to believe that the medical community can overcome the "psoriasis" problem in the near future; In contrast, the older patients, based on rich social experience and lack of new knowledge, were more worried and pessimistic than the younger ones.
Studies have confirmed that dopamine, norepinephrine, and other monoamine neurotransmitters have a wide range of biological activities, which can participate in physiological responses in mental activities, mood, sleep and other central system. The biological basis for depression may decrease or lower the function of these neurotransmitters in the synaptic gap in the brain . In 1979, it was proposed that low levels of norepinephrine in the brain could lead to depression , and the hypothesis was confirmed later. However, other studies have reached the opposite conclusion, which shows that the content of NE in depressed patients was higher than that of normal people (Lake, 1982). There also was an inference that tension and anxiety can activate the hypothalamic-pituitary-adrenal axis, producing more catecholamines, which could participate in the occurrence and development of psoriasis. Our patients were older patients, chronic psoriasis, and often felt depression because they realized that the disease is relapses easily and cannot be cured. The study of Violanti JM et al. has shown that chronic anxiety can lead to depression . Moreover, the horny cell of the patients with psoriasis can secrete IL-6, TNF alpha, IL-1 alpha, glucocorticoids, among others. These inflammatory factors could affect the central nervous system, promoting the occurrence of the depression, while a large number of inflammatory cytokines could produce excessive inflammation, destroy neurogenesis, lead to neuronal dysfunction, and promote the development of depression [26, 27]. Additionally, during the whole chronic course of psoriasis, the secretion of neurotransmitters such as NE and DA was abnormal due to long-term negative emotional stress. Therefore, the patients with psoriasis included in our study were all in-patients with severe inflammatory response and high levels of inflammatory factors, which damaged structures such as neurons, resulting in decreased the content of catecholamine transmitters, and ultimately suffered depression.
Studies found that catecholamine neurotransmitters such as DA and NE could activate immune cells (lymphocytes, macrophages, and natural killer cells), had a regulatory effect on the activity of lymphocytes and macrophages by binding to the corresponding receptors on immune cells , and were important components of the nerve-endocrine-immune regulatory system, which play a significant role in the occurrence and development of psoriasis.
In the study, the level of dopamine in the blood of patients with depression was lower than that of patients without depression in the patients with severe psoriasis, and the higher the PASI score, the more likely the patients were to have depression. Further, there was no significant correlation between depression and non-depression in patients with moderate psoriasis. The role of dopamine as an intermediate transmitter in the mechanism of co-pathogenesis of psoriasis and depression, and the drugs used to alter abnormal levels of catecholamines in plasma or the central nervous system to treat both psoriasis and depression are important points to be elucidated to improve the condition of psoriasis patients complicated with mental abnormalities.
There were some evidences that the prevalence ratios of psoriasis with depression varies from 9.7% to 96% [29, 30]. In our study, psoriasis patients with depression accounted for only 20%, which was lower than most previous studies. The is because of the differences in the scales used to evaluate depression (such as PHD-9 and GAD-7 scales), and the judgment among evaluators. In the study, the alternative rating scale was selected and completed with the assistance of the physicians in charge of professional psychiatry, which would be more objective. Moreover, the study only studied psoriasis vulgaris, excluding the other three more serious types.
Some study limitations were: 1. This was a cross-sectional study, and the causal relationship between psoriasis vulgaris and depressive symptoms, the dopamine levels and other risk factors cannot be determined. 2. In order to better evaluate the skin lesions, we only investigated and evaluated psoriasis vulgaris, and the other three more severe types were not included in the study; 3. The study was a single-center study with a small sample size, especially for psoriasis patients with depression. The research should be carried out with a large sample or a multi-center study in the future.