Identifying clinical factors associated with Fulminant Mycoplasma pneumoniae pneumonia in children

BACKGROUND : Analyze the clinical characteristics of Fulminant pneumoniae pneumonia (FMPP), and identify the related predicting METHODS : A retrospective case-control study was performed on 345 children with Mycoplasma pneumoniae pneumonia (MPP) hospitalized in our hospital from January 2017 to June 2019. The clinical features, laboratory data and radiological findings were compared between the FMPP group, refractory Mycoplasma pneumoniae pneumonia (RMPP)group and general Mycoplasma pneumoniae pneumonia (GMPP) group. RESULTS : FMPP patients (n=69) had a higher incidence of extra-pulmonary complications and more serious radiological findings(P<0.05), besides the days of fever and the days in the hospitals were longer. FMPP patients also need more complicated treatments(P<0.05). Meanwhile, the levels of white blood cell count(WBC), C-reactive protein(CRP), lactic dehydrogenase (LDH), interleukin (IL)-6, ferritin, D-dimer, fibrinogen(FG), alanine aminotransferase(ALT) and the percentage of neutrophils in the FMPP group were significantly higher than those in the RMPP group and the GMPP group (P<0.05). In ROC curve analysis, the percentage of neutrophils, WBC, CRP, LDH, IL-6, ferritin, D-dimer and ALT were contributed to identify FMPP patients. Multivariate logistic regression analysis showed that ferritin>174.15 ng/mL, IL-6>25.475pg/ml and pleural effusion have significant predictive effects on the early diagnosis of FMPP (P<0.01). CONCLUSION : FMPP patients presented more serious clinical manifestations. Ferritin>174.15 ng/mL, IL-6>25.475pg/ml and pleural effusion were clinical factors for FMPP.


Identifying clinical factors associated with Fulminant Mycoplasma pneumoniae pneumonia in children
antibiotics for MP infections. Some patients treated regularly with macrolides for 7 days or longer and the clinical and radiological deterioration, which can be defined as refractory Mycoplasma pneumoniae pneumonia (RMPP) [4] . Some cases even complicated with hypoxemia during the disease, and the radiological imaging often manifested as diffuse consolidation of the lungs and pulmonary interstitial damage, which were called fulminant Mycoplasma pneumoniae pneumonia (FMPP) in some literatures [5] . Up to now, there is little research on FMPP, and its specific definition is not clear. But hypoxemia is a risk factor affecting the prognosis of patients. Therefore, clinicians need to recognize FMPP earlier and grasp the appropriate opportunity for reasonable therapy.
To explore the related factors predicting FMPP earlier, provide appropriate treatments and reduce complications, we retrospectively analyzed the cases of MPP hospitalized in our hospital between January 2017 and June 2019, then compared the differences of clinical features, laboratory data, and radiological findings, between the FMPP RMPP and general Mycoplasma pneumoniae pneumonia(GMPP).

Patient Selection
Clinical information: 69 patients with FMPP were admitted to the Respiratory Department of Tianjin Children's Hospital from January 2017 to June 2019. We also randomly selected 86 patients in the RMPP group and 190 patients in the GMPP group from the same period. All cases met the diagnostic criteria.
Diagnostic criteria: All patients had clinical evidence of pneumonia on admission such as a fever, cough and pneumonic infiltrations in the chest radiograph. MP infection was based on the positive results for MP polymerase chain reaction (PCR) tests of nasopharyngeal secretions (88.70%) or positive results of a serological test (11.3%). Patients underwent anti-MP IgM titrations twice at the time of admission and before discharge, and we selected whose test results was a seroconversion (negative to positive), four-fold or greater increase in IgM titers, or both high titers of 1:640 (MP-IgM antibody titer ≥ 1:160). [6] The diagnosis of RMPP was based on clinical and radiological deterioration after azithromycin treatment for 7 days or longer [4] . The diagnosis of FMPP was based on the apparent presence of MP infection with hypoxia(on room air, arterial oxygen saturation ≦92% or PaO2≦60 mmHg) [5] .  airway and lung malformations are suspected; 3. there are serious complications associated with pneumonia; 4. patients fail to respond to treatment and need to exclude other diseases such as interstitial lung disease, pulmonary tuberculosis and so on [7,8] . The percent of CT scans in FMPP, RMPP and GMPP was 100%,84.88% and 17.69% respectively.

Ethics:
The study was approved by the ethics committee of the Tianjin Children's Hospital. And the data from patients were analyzed anonymously.
Data analysis: SPSS 22.0 was used for statistical analysis. The normal distribution data was represented by mean ± SD (). One-way ANOVA was used for comparison between groups. The LSDttest was used for comparison within the group. The skewed distribution data were expressed as median (P25, P75), which comparisons were made by the Mann-Whitney U-test. And Chi-squared tests were used to compare categorical data. Receiver operating characteristic (ROC) curves were operated to evaluate candidate markers related to FMPP, and logistic regression analysis was performed to select variables associated with FMPP. The difference was considered statistically significant at P < 0.05. GMPP. There was no statistically significant difference in age and gender between the three groups.

Clinical characteristics of patients (table1)
All patients presented a cough, and 350(98.87%) patients had fever. Also, the FMPP group had a higher fever (39.1-41℃) than the other two groups(P 0.05). Moreover significant differences were observed in the incidence of rash, liver function damage, chest pain, toxic encephalopathy, embolism, dyspnea, mucous plugging between the FMPP and the other two groups(P 0.05). In this study, in total 7 patients developed embolization, which was located in the lower limb artery (2 cases), lung (4 cases) and heart (1 case).

Clinical course and treatment of patients
Regarding the clinical course, the median duration of fever was 12 (range 9-14) days in the FMPP group, 10 (range 8-12) days in the RMPP group and 9 (range 8-10) days in the GMPP group (P < 0. 01). And The median length of hospital stay was 12 (range 9 -15) days in the FMPP group, 9 (range 8-10) days in the RMPP group and 6 (range 5-7) days in the GMPP group (P < 0. 01). A total of 205 patients (57.90%) were treated with glucocorticoid after admission, and the number of FMPP group was significantly higher than that in the other two groups (100% versus 84.06%,71.05% P < 0 01).
Fiberoptic bronchoscopy was performed in 201 cases (56.77%). The number of patients using the fiberoptic bronchoscope in the FMPP group was significantly higher than that in the other two groups (84.06% vs 70.93% vs 43.16% P < 0.01).Additionally, the proportion of patients required oxygentherapy and gamma globulin in the FMPP group was higher than that in the others(P<0.01). All patients were treated with azithromycin.

Predictive values of the independent correlation factors in patients with FMPP
The ROC analysis was performed to explore predictive values of laboratory date for FMPP, and the critical value with maximum sensitivity and specificity was also determined. ROC analysis revealed that IL-6, ferritin and D-dimer were of great significance in the diagnosis of FMPP, the area of which under the curve was above 0.7.When the cut-off value for the IL-6, ferritin and D-dimer was set at  Table 4. Predictive values of the independent correlation factors in patients with FMPP.

Multiple logistic regression analysis for the related factors predicting the FMPP
To further evaluate the predictors associated with FMPP, multiple logistic regression was performed.

Discussion
Mycoplasma pneumoniae pneumonia continues to be a significant cause of childhood communityacquired pneumonia, and is usually a benign self-limited disease. On rare occasions, it manifests as a fulminant disease that leads to death [9] . Fulminant or fatal mycoplasma pneumoniae have been reported for more than 50 years [10] , death was associated with diffuse pneumonia, acute respiratory distress syndrome (ARDS), brain herniation, vascular thrombosis, and disseminated intravascular coagulation [10][11][12][13][14] . It is crucial to early diagnosis and early intervention for FMPP. However, there were still few studies on FMPP, especially in children. Therefore, we conducted a retrospective study, including 69 cases of the FMPP group, and randomly selected 86 cases of the RMPP group and 190 cases of the GMPP group as a control. All cases met the diagnostic criteria.
First of all, there was no significant difference in age and sex among the three groups, and the median age of all groups was 6 years old, which was consistent with the age of the high incidence of MPP [1] .
Secondly, the signs and symptoms in the FMPP group were more serious, and the incidence of extrapulmonary complications was higher. Dyspnea was the most common clinical manifestation of FMPP. In the study, the median time from onset to development of respiratory failure was 10 days range 9-12 , which was consistent with the study of Izumikawa et al [5] . Some literature has shown that liver function damage was the most common extrapulmonary complication of FMPP [15,16] .
In our research, 13 cases (18.84%) of FMPP complicated with liver function damage. Moreover, MP infection might contribute to hypercoagulability and cause thrombosis itself, which was serious extrapulmonary complication [17] . In our research, 13 cases (18.84%) of FMPP had embolism. These serious extrapulmonary complications also lead to longer hospitalization in patients with FMPP.
Until now, the mechanisms of FMPP have been still uncertain. Izumikawa [18] believes that it was related to the immunological hyper-reaction in the lung, which may induce lymphocyte activation, resulting in systemic impairment of cellular immunity and progression to fulminant status. In the laboratory indicators, the level of WBC, neutrophil ratio, CRP, LDH, IL-6 and ferritin were related to FMPP, which was similar to the previous case reports on FMPP [19][20][21] . Taken together, the evidence suggested a serious immune-inflammatory reaction in FMPP.
The radiological manifestations of mycoplasma pneumoniae pneumonia were various, mostly bronchial wall thickening, centrilobular nodules, ground-glass attenuation and consolidation [22] .
Besides, our study showed that the imaging findings of FMPP were not specific, mainly pulmonary inflammatory consolidation (79.71%), but FMPP was more likely to be accompanied by atelectasis, pleural effusion , and aggravated in a short period. Miyashita et al indicated that bilateral infiltrates and pleural effusion commonly present in the FMPP group compared to the other groups [16] . It further suggested the severity of the disease, which may be related to the direct invasion of MP and excessive host immune response.
As for treatment, our study showed the number of people using glucocorticoids in the FMPP group was significantly more than that in the other two groups, and only the FMPP group used gamma immunoglobulin. Interestingly, our study suggested that there was no difference in the use of azithromycin among the three groups, which was contrary to prior research [5] . However in China, the infection rate of macrolide-resistant M. pneumoniae has always been high, ranging from 69 to 100% in recent years [23] .
MP infection may cause varying degrees of respiratory mucus thrombus obstruction, even form bronchial molding, resulting in airway stenosis and occlusion [24] . We compared the incidence of mucous plugging between the three groups and found that the FMPP group was significantly higher than the other two groups. We suspect that may be related to the occurrence of hypoxemia in MPP.
Pediatric flexible fiberoptic bronchoscopy can clear respiratory secretions under direct view, relieve airway obstruction, and reduce the occurrence of complications [7] . The indications of bronchoscopy in children mainly are as follows: conventional treatment is not good, airway obstruction or atelectasis caused by inflammatory secretions or necrosis should be cleared in time and airway injury diagnosis after infection [7] . In our study, a total of 201 children (58.26%) received fiberoptic bronchoscopy intervention therapy, among which the FMPP group received more of this treatment(p<0.01).
In our case, all the children recovered and discharged from the hospital without death.
To explore the related risk factors predicting FMPP, we used the ROC curve and multivariate logistic regression analysis. ROC analysis revealed that the area under the curve of ferritin, IL-6 and D-dimer were above 0.7, which were helpful to recognize the patients in FMPP. And the optimal cutoff value for three factors was 174.15 ng/mL, 25.47pg/ml and 0.45μg/L, respectively. Besides, multiple logistic regression analysis was made to improve the predicted accuracy. We found that ferritin > 174.15 ng/mL, IL-6 >25.47pg/ml and pleural effusion were good predictors of FMPP. Ferritin not only represents iron reserves, but also an inflammatory marker [25] . When inflammation occurs, inflammatory factors act on the body to increase the production of ferritin in serum. At the same time, inflammatory factors cause degeneration and necrosis of local tissue cells, dissolution and rupture of the cell membrane, resulting in leakage of serum ferritin from damaged cells. As a result, ferritin is significantly increased in the inflammatory response. However, there is still no report about the correlation of ferritin in FMPP. Some studies [26] on RMPP reported when the ferritin level was 230 ng/mL or higher, the sensitivity and specificity for diagnosing refractory MP pneumonia were 67 and 67%, respectively. In our study, the optimal cutoff point for ferritin was 174.15 ng/mL, with a sensitivity of 82.4 % and specificity of 69.3%, and the odds ratio of logistic regression analysis was 3.430. The reason for which made it different may be the presence of mixed infection in our case. IL-6 plays an important role in the early stage of the immune response. In our study the area under the curve for IL-6 was 0.737, and the optimal cutoff point was 25.47 pg/ml, with a sensitivity of 73.5% and specificity of 68.9%, the odds ratio of logistic regression analysis was 3.005. Chen et al showed that the cutoff value of IL-6 for RMPP was 14.75pg/ml [27] . At present, it is considered that the increase of IL-6 is related to the severity and course of the disease [28] , which further suggests that there may be an excessive immune response in FMPP.
The advantage of this study is that we first explore the predictors with FMPP. Starting from the actual clinical cases, the differences between FMPP, RMPP and GMPP in large samples are compared and analyzed, and the interference of mixed factors is eliminated. It provides a strong basis for the early identification of FMPP and has a certain degree of innovation and practicality.
There are several limitations to this study. Firstly, it was a retrospective study, and there may have been some selection bias. Secondly, there may be the presence of mixed infection in some cases which cannot be detected. Thirdly, the distribution of the number of patients between the three groups is not matching, which might affect the statistic results. Therefore, In the future work, we should further carry out long-term multicenter, large sample prospective studies, and further explore the problems found in clinical work, to provide a reliable theoretical basis for early identification, early diagnosis and early intervention of FMPP.

Conclusion
Our study shows that excessive immunological inflammation may play an important role in FMPP. FER