Occult prosthetic aortic valve endocarditis caused by Propionibacterium acnes: a case report

Abstract

We present an unusual case of occult mechanical aortic valve endocarditis caused by Propionibacterium acnes (P. acnes), which occurred five years after the initial cardiac surgical procedure. The only potential source of this infection from the patient’s past medical history was a gastric band removal followed by a gastric bypass surgery, which was performed two years prior to her redo cardiac procedure. The patient’s main presenting symptom was progressive dyspnoea with worsening of gradients across the mechanical aortic valve. Of note, she had no cardinal symptoms of infection. The patient underwent redo bioprosthetic aortic valve replacement with aortic root replacement. Prosthetic valve cultures grew P. acnes, while blood cultures remained negative. She was started on intravenous antibiotics for six weeks and her postoperative course was uneventful. She remained symptom-free six months postoperatively. This case opens the discussion for the importance of administering antibiotics prophylaxis for endocarditis in patients with prosthetic heart valves undergoing gastric procedures.

Background

Propionibacterium acnes (P. acnes) is a Gram-positive anaerobic bacillus normally found in the skin flora and mucosal membranes, including the gastric mucosa.^1,2 Despite its low virulence, P. acnes can be associated with adverse symptomatology, especially in relation to prosthetic material.² Its virulence is attributed to its ability to adhere to prosthetic surfaces and formation of a surrounding biofilm layer.^3,4 Endocarditis due to P. acnes is rare and its incidence in literature varies between 0.3% and 1.8% of total cases of endocarditis and between 1.4% and 3.8% of prosthetic valve endocarditis (PVE).^4–7 Because of its low virulence, patients with P. acnes PVE usually present late during the infectious process with significant local tissue destruction, invasion of periannular structures and abscess formation.^4,8,9 Treatment of P. acnes PVE usually consists of surgical excision of the infected prosthetic material and prolonged course of antibiotics, ranging from four to six weeks up to three months.¹ P. acnes is typically sensitive to beta-lactams and clindamycin, but treatment should follow local hospital policies and sensitivity profiles.^3,4

Case history

A 57-year-old woman presented with worsening dyspnoea and increasing lethargy. She underwent an elective aortic valve replacement with a 23-mm Carbomedics Top Hat (Sulzer, Carbomedics, Austin, TX, USA) mechanical prosthetic valve in 2015 for severe symptomatic aortic stenosis. Her first postoperative transthoracic echocardiography showed a well-functioning prosthesis with slightly elevated gradients (mean gradient 14.3mmHg; peak gradient 37.7mmHg).

Her past medical history was only significant for morbid obesity; her body mass index (BMI) was 41kg/m² and body surface area 2.0m², for which she underwent a gastric band insertion in 2011. The band was subsequently removed in June 2017 followed by a gastric bypass surgery in November 2017, and her BMI at that time was 43kg/m² (body surface area 2.1m²). During these two gastric procedures, prophylactic antibiotic therapy for endocarditis was not administered nor was her cardiac surgical team consulted for an opinion.

In June 2020, she presented with progressive lethargy and dyspnoea, with no classic signs or symptoms of an underlying infectious process. Transthoracic echocardiography performed in July 2020 showed worsening gradients across the mechanical valve prosthesis (peak gradient 64mmHg, mean gradient 38mmHg and maximum velocity 409cm/second), with no significant aortic regurgitation or paravalvular leak. The patient’s coronary angiography was unremarkable and all her blood work values were within normal range including inflammatory markers.

In September 2020, she underwent an elective redo-aortic valve replacement with a bioprosthesis (patient’s choice). Intraoperative transoesophageal echocardiography (TEE) showed a well-functioning mechanical valve with no features suggestive of PVE. However, a subvalvular pannus was seen on TEE; this was obstructing the left ventricular outflow tract, while not interfering with leaflet function (Figure 1). During excision of the valve, the ease with which it detached from the annulus and the absence of resistance were atypical, suggesting an underlying incipient prosthetic dehiscence. However, the prosthesis itself seemed to be functioning well without any apparent signs of dysfunction or infection. Owing to the small annulus size, the decision was then made to replace the aortic root with a 25-mm Freestyle bioprosthesis (Freestyle; Medtronic Inc, Minneapolis, MN, USA) to minimise the likelihood of a patient–prosthetic mismatch.

Figure 1 .

Intraoperative transoesophageal image showing subvalvular ring-like stenotic pannus

Her postoperative recovery was uneventful. Blood cultures obtained at the same time as surgery remained negative after an extended period of cultivation, although her explanted prosthetic valve culture grew P. acnes. Given the exceptionally easy fashion by which her mechanical valve prosthesis was detached from the aortic annulus, it was thought that she probably had occult PVE and so she was started on a six-week course of benzylpenicillin. At her six-month follow-up visit, she was clinically stable.

Discussion

This current report presents a unique presentation of PVE due to P. acnes infection. P. acnes infection is rare,^3,4,6 occurring more in association with prosthetic valves and endocavitary pacemaker electrodes.^4,10 A previous report of P. acnes PVE suggested that early infection occurs due to contamination from handling during surgical intervention.⁴ The latency period between prosthetic valve colonisation and the clinical diagnosis of P. acnes PVE ranges from 4 months to 12 years.^4,7 This is probably due to its low virulence and the indolent early infectious process. P. acnes can remain embedded in the biofilm while the patient is devoid of bacteremia. Consequently, diagnosis is delayed, and the real incidence can become underestimated.^4,8,10 The use of perioperative antibiotic prophylaxis may also result in a smaller initial bacterial inoculum that may take longer to manifest.⁸

Patients typically present in advanced stages of the disease; clinical manifestations include congestive heart failure due to prosthetic dehiscence, which can occur in 63–85% of patients,^8,9 together with fevers and chills.^3,4 In contrast, our patient presented only with dyspnea without cardinal signs of endocarditis. Echocardiography can be helpful, as it can identify prosthetic dehiscence in 33–92% of patients with PVE as well as periannular abscesses and/or vegetations in some.^3,4,8,9 However, our patient did not exhibit signs of prosthetic dehiscence on echocardiography.

In cases of late P. acnes PVE, bacterial seeding through the bloodstream seems to be the most likely mechanism as many patients report a history of exposure to previous invasive procedures that involved the skin or mucosal membranes.⁸ Our patient did not receive prophylactic antibiotics for endocarditis during the two gastric procedures that she had undergone prior to her presentation. This is in line with current valvular management guidelines that do not recommend the use of antibiotic prophylaxis for patients receiving gastric instrumentation.^11,12 However, P. acnes has been identified in the gastric mucosa,^13,14 and invasive procedures involving the gastrointestinal mucosa have been shown to increase the risk of PVE in general;² some authors have therefore recommended the need for prophylactic antibiotics in patients with prosthetic valves undergoing invasive procedures in the gastrointestinal tract.^15–17 This controversy mandates a conversation that should take place between the gastroenterologists and cardiac surgeons to determine the best management strategy according to the patient’s clinical condition and baseline characteristics.

Finally, the treatment of P. acnes PVE involves wide surgical exploration and debridement of the infected material^18–20 and an extended course of antibiotics. Beta-lactams and clindamycin are typically used, although patients are empirically treated with vancomycin until the results of the cultures are available.^3,4 Rifampicin can also be added, due to its unique ability to penetrate into the surrounding biofilm.⁹ In patients who have a prohibitively high risk for surgery, solo antibiotic treatment has very limited curative effect¹⁸ with very high recurrence rates.^3,8 Reported hospital mortality rates are variable and range between 2% and 18.7%.^3,4,8

Conclusion

P. acnes prosthetic valve endocarditis is a serious condition with high surgical risk. Although gastric instrumentation can be the source of infection, current valvular management guidelines do not recommend antibiotic prophylaxis in patients with prosthetic heart valves. In this group of patients, a multidisciplinary team discussion should be indicated to determine the best strategy based on each patient’s clinical condition.