Previous epidemiological studies indicated that people with mental illness may be at high risk of being infected with
Treponema pallidum [
10]. However, interactions between syphilis and mental illness are poorly understood. In this study, we retrospectively studied psychotic inpatients screened for syphilis to analyze the characteristics of patients in the SCZ-C and SCZ-S groups. Our results showed that the prevalence of syphilis in psychotic inpatients was 3.3% (52/1586) which is similar to previous studies (Fig.
1) [
11]. The prevalence of syphilis in this group was nearly as high as the prevalence in female sex workers (2.4–3.2%), who are regarded as a high-risk group [
12]. The high prevalence of syphilis in psychotic patients may be attributed to local factors in that Xiamen is a coastal urban city in a developing country. In addition, psychotic patients have poor self-consciousness and may not be aware of the clinical manifestations of
Treponema pallidum infection for a long time allowing the development of neurosyphilis [
13,
14].
A total of 87 schizophrenia patients (29 syphilis-positive and 58 syphilis-negative patients) were included for further analysis. We evaluated the social function and serum biochemical values in this group. Social function was assessed by NOSIE-30, a scale that has been used extensively to study schizophrenia patients [
15]. Our results indicated that schizophrenia patients with syphilis had a higher irritability score compared to schizophrenia patients without syphilis (
P < 0.05). A previous study had also shown that
Treponema pallidum infection may aggravate psychiatric symptoms [
6]. Furthermore, the psychological burden of having syphilis, which is a sexually transmitted disease, may be another explanation. As clinically expected, some of the inpatients in the SCZ-S group showed high variability in mood, which became agitated more easily. This has led to suggestions that special care and restraint might be needed in this population. In addition, among the 32 kinds of serum parameters examined, we found that CK, CK-MB, K and Cl were significantly higher in the SCZ-S group than in the SCZ-C group (
P < 0.05). It has been previously shown that serum CK levels are associated with the mood of psychotic patients [
16]. Increased CK levels were found in the majority of hospitalized patients with acute schizophrenia and patients with affective psychoses [
17]. Northoff and colleagues found that the serum CK levels in patients with tension-type schizophrenia was significantly higher than CK levels in healthy controls and those with non-nervous schizophrenia [
18]. In addition, some authors have reported higher K and Cl levels in schizophrenia patients than in healthy controls [
19‐
21]. Additionally, CK or K would be released into blood when the organs or tissues are damaged and the integrity of cell membrane is destroyed [
22]. It has long been known that a history of
Treponema pallidum infection can cause tissue damage and that the clinical manifestations result from the host immune response. A previous study reported that
Treponema pallidum multiplies at the site of inoculation and immediately disseminates throughout the body, include skin [
23], central nervous system [
24], cardiac system, liver, spleen, and kidney [
25]. As the
Treponema pallidum continue to advance without antibiotic treatment, it can cause organ damage and can result in neurosyphilis or cardiovascular syphilis. It is known that both CK and CK-MB are biomarkers for evaluating cardiac function. When the cardiovascular system was stimulated or had an inflammation reaction, CK and CK-MB levels increase, which may explain the increased CK and CK-MB levels in syphilis-positive schizophrenia patients. Then, when
Treponema pallidum invades the nervous system, patients could have some electrolyte imbalances, such as elevations in Cl level. Reagin, also called nontreponemal antigen, mainly comes from host tissue and cells destroyed by
Treponema pallidum, and the intracellular K would be released at the same time. This finding is in accordance with the previous studies [
17,
26].
Previous studies have associated biochemical changes with antipsychotic drug use. Nagamine et al. found that increased CK levels were more frequent in the olanzapine group [
16]. Bhatia and colleagues documented a moderate degree of electrolyte imbalance in schizophrenia, and serum electrolytes became normal after treatment [
21]. Therefore, we further divided these participants into four subgroups according to syphilis infection and drug taking. Elevated CK levels were found in the SCZ-S drug-free group compared to the SCZ-C drug-free group. There were no significant differences of CK-MB, K and Cl between the four subgroups (Fig.
2). We observed that the CK-MB, K and Cl levels were higher in SCZ-S drug-free patients than in SCZ-S treated patients, but there were no significant differences (
P > 0.05). This discrepancy is probably explained by the relatively small sample size in the four subgroups. As such, a more comprehensive study is needed to demonstrate the association between syphilis and antipsychotic drug use.
Of course, the shortcomings of the article must also be admitted. In the present study, social function was determined only by NOSIE. However, the NOSIE is more useful in assessing social function when it complements the Positive and Negative Syndrome Scale (PANSS) [
27]. A PANSS evaluation was excluded from this study because of missing data, making it more difficult to evaluate the clinical manifestations in the schizophrenia patients. It is still unclear whether the psychiatric symptoms in neurosyphilis patients were due to nervous system injury caused by
Treponema pallidum. This study was conducted in syphilis of any stage excluding neurosyphilis. Another limitation is that the relatively small sample size and statistical power are too low to compare biochemical levels in patient subgroups using different antipsychotic drugs.