Letter to the Editor: Mediastinal Infection Caused by Heterogeneous Vancomycin-Intermediate Staphylococcus aureus after Aortic Dissection Surgery

To the Editor:

Mediastinal infection is a serious complication after cardiac surgery, with an incidence of approximately 1%–2% and a high mortality rate [1]. The main pathogenic micro-organisms that cause infection are coagulation-negative staphylococcus or Staphylococcus aureus [2]. Diagnostic criteria for mediastinal infection include positive mediastinal tissue or fluid cultures, gross anatomic or histopathologic evidence of mediastinitis, and at least one of the following symptoms: fever, chest pain, sternal instability, purulent drainage, or imaging in the mediastinum [3]. We first reported a case of mediastinal infection caused by heterogeneous vancomycin-intermediate Staphylococcus aureus (h-VISA) after acute aortic dissection surgery.

A 47-year-old male patient was admitted to the hospital because of acute aortic dissection. After admission, the patient received emergency surgery (ascending aortic replacement plus aortic arch replacement plus descending aortic stenting). On the third day after surgery, the patient presented with a fever (temperature, 38°C), and the blood tests showed that the white blood cells were elevated. After treatment with cephalosporin antibiotic agents, the above symptoms did not improve. On the fourth day after surgery, yellow viscous liquid was seen in the drainage tubes of the pericardial and mediastinal (Fig. 1A), and the fever rose to 39.4°C. Blood culture showed Staphylococcus aureus (vancomycin, minimum inhibitory concentration [MIC] = 1 mcg/mL); the patient's symptoms did not improve after vancomycin treatment. Chest computed tomography (CT) revealed that the patient's sternum was unstable (Fig. 1B), accompanied by septic shock; emergency debridement surgery was performed. During the operation, it was found that there was no infection in the skin and subcutaneous skin (Fig. 1C), but the infection in the mediastinum was obvious, and a large amount of yellow–white purulent secretions were on the surface of the heart (Fig. 1D). We performed debridement and stabilized the sternum. Staphylococcus aureus was found in purulent secretions by culture (vancomycin, MIC = 3 mcg/mL), confirmed as h-VISA. According to drug susceptibility testing, we adjusted the antibiotic to vancomycin plus moxifloxacin. After 1 week of treatment, the patient's infection symptoms resolved.

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FIG. 1.

(A) Viscous liquid seen in drainage tubes of the pericardial and mediastinal. (B) Chest computed tomography (CT) revealed that the patient's sternum was unstable. (C) There was no infection in the skin and subcutaneous skin. (D) The infection in the mediastinum was obvious, and a large amount of purulent secretions were on the surface of the heart.

According to the definition in the operation manual published by the American Clinical and Laboratory Standards Institute (CLSI) in 2011, when the MIC is 2 to 4 mcg/mL, it refers to h-VISA. Recently, h-VISA has appeared clinically, and this type of Staphylococcus aureus is less sensitive to vancomycin and vancomycin resistance of h-VISA has become a serious clinical problem [4]. It is suggested that the presence of h-VISA should be considered for Staphylococcus aureus with high vancomycin MIC value, even if it is still within the sensitive range. If h-VISA infection is not handled properly, it may become vancomycin-intermediate Staphylococcus aureus (VISA) or even vancomycin-resistant Staphylococcus aureus (VRSA), and the clinic could face the situation of having no medicine available [5].

What we want to express in this case is that when the symptoms of infection do not improve after vancomycin treatment, h-VISA should be considered, and the secretions from the infected area should be re-cultured, and appropriate antibiotic agents should be selected according to the drug susceptibility testing.