Introduction

Achromobacter xylosoxidans is a non-fermenting aerobic Gram-negative bacillus that can be found in aquatic environments and aqueous fluids, such as swimming pools, well-water, dialysis solutions, chlorhexidine solutions, and on plants [1]. It was first described in 1971 by Yabuuchi and Oyama, who isolated it in ear discharges from patients with chronic otitis media [2]. Since then, it has been identified in chronic otitis media effusion along with other pathogens, as mixed flora, as well as in various human body fluids, including respiratory tract secretions and peritoneal fluid [3]. Although it is considered an opportunist pathogen with low virulence, it can cause serious infections in the immunocompromised population [3, 4]. It is associated with nosocomial infections, being bacteremia, pneumonia and chronic cystic fibrosis lung infection the most common clinical presentations [3].

Achromobacter xylosoxidans infections are challenging due to their multidrug resistance. This pathogen is innate, strain-specific resistant to beta-lactams, aminoglycosides, fluoroquinolones, aztreonam, tetracyclines and cephalosporins [1]. To date, only one case of chronic otitis externa and chronic otomastoiditis caused by A. xylosoxidans has been reported [5]. In addition, several cases of osteomyelitis due to A. xylosoxidans have been described [6,7,8,9,10,11,12,13,14], but none of them affecting the ear.

We report a unique case of osteomyelitis of the external auditory canal (EAC) caused by A. xylosoxidans in an elderly immunocompetent woman.

Case report

An 81-year-old female with no significant medical history, presented to our Otolaryngology Department with a 2-month history of mild left otalgia, otic itching and otorrhea. During this period, she had been treated by her primary care physician without any improvement. She was prescribed oral amoxicillin 500 mg every 8 h for 1 week. As part of the topical treatment, she received ciprofloxacin, beclomethasone dipropionate/clioquinol, fluocinolone and dexamethasone/polymyxin B/trimethoprim. Each treatment was prescribed for 2 weeks. Otomicroscopy revealed erythema, edema and scant otorrhea in the anteroinferior part of the EAC. The tympanic membrane (TM) was intact. An area of bone remodeling with a small cavity anterior and inferior to the bony tympanic frame was observed, with accumulation of otorrhea in it. There was no evidence of granulation tissue or bone exposure (Fig. 1).

Fig. 1
figure 1

Otomicroscopy at diagnosis. Intact tympanic membrane with an area of bone remodeling anterior and inferior to the bony tympanic frame

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No fever or facial palsy was noticed. Leukocyte count was 5930/mm3 (3.6–10.5 /mm3), C-reactive protein was 4,6 mg/L (0–5 mg/L) and erythrocyte sedimentation rate (ESR) was 25 mm/h (0–30 mm/h). A culture of the otorrhea isolated multiresistant A. xylosoxidans, only susceptible to meropenem and cotrimoxazole (Table 1). No cultures for fungal infection were needed as A. xylosoxidans was identified as the causal pathogen in the first conventional culture.

Table 1 Antibiogram of A. xylosoxidans obtained from left ear discharge
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Temporal bone computed tomography (CT) showed a thickening of the left TM with tissue accumulation at its lower insertion and excavation of EAC floor compatible with osteomyelitis (Fig. 2).

Fig. 2
figure 2

CT left temporal bone coronal plane. Excavation of EAC floor and tissue accumulation at the inferior insertion of the tympanic membrane (arrow)

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A multidisciplinary management approach was undertaken with the Infectious Disease Department. Given the compatible diagnosis of osteomyelitis, a treatment course of 6 weeks with intravenous meropenem was promptly started (2 g/8 h for the first week; 1 g/8 h for the next 5 weeks) followed by 4 weeks of oral cotrimoxazole (trimethoprim 160 mg/sulfamethoxazole 800 mg every 12 h). Topical antibiotic therapy (dexamethasone + polymyxin B + trimethoprim, 3–4 drops every 12 h) was continued throughout this time. She was hospitalized for the initial 3 weeks of treatment. Afterward, intravenous treatment was continued at home facilitated by our home hospitalization program. Symptomatology rapidly disappeared with treatment. Four months after completing the treatment, the patient remained asymptomatic. Upon otomicroscopy, the bony excavation in the lower tympanic frame persisted, stable and epithelialized, with no evidence of otorrhea or progression. A CT control also confirmed the absence of progression of the excavation of the EAC floor. Audiometry showed right normal hearing and left mild conductive hearing loss with a maximum speech discrimination of 100% in both ears, at 40 dB in the right ear and 50 dB in the left ear.

Discussion

This report presents the case of a healthy 81-year-old woman who presented with osteomyelitis of the EAC due to multidrug resistant A. xylosoxidans. To the best of our knowledge, this is the first case of osteomyelitis of the EAC caused by this pathogen. While ten cases of osteomyelitis due to A. xylosoxidans have been reported in literature [6,7,8,9,10,11,12,13,14], none have affected the ear (Table 2).

Table 2 Summary of previously reported cases of osteomyelitis caused by A. xylosoxidans
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The main differential diagnosis considered was malignant external otitis (MEO), as the clinical presentation was otalgia and chronic otorrhea. MEO usually affects elderly individuals with poorly controlled diabetes and/or immunosuppression [15]. However, our patient had no comorbidities and no evidence of immunosuppression. Another difference from MEO was the extent of the illness. MEO typically presents with extensive inflammation that progresses regionally from the EAC to the soft tissues and the bone, eventually involving the skull base [15]. In this case, mild inflammation was observed in the EAC, with bone erosion limited to a restricted area in the anteroinferior wall of the EAC.

The best imaging modality for the diagnosis and follow-up of temporal bone osteomyelitis is controversial. Traditionally, methylene diphosphonate (MDP)-technetium-99 m (99mTc) and Gallium-67 (67Ga) scans were standard for MEO diagnosis and management [16]. Recent meta-analysis and systematic reviews, such as the work by Moss et al., have challenged the reliability and efficacy of 99mTc and 67Ga scans due to poor sensitivity, specificity, and limited ability to assess disease resolution [17]. Thus, many physicians now rely upon standard CT and magnetic resonance imaging (MRI) scans due their superior anatomical resolution in diagnosing and managing osteomyelitis [18]. CT imaging provided crucial insights in identifying thickening of the left tympanic membrane, tissue accumulation, and bony erosion, consistent with osteomyelitis in our patient. Hybrid nuclear studies, specifically 18F-FDG–PET/CT, have emerged as promising tools due to their higher sensitivity, specificity, cost-effectiveness, and reduced radiation exposure. However, further extensive studies are necessary to establish its definitive role in MEO diagnosis and follow-up [19]. Based on the clinical history and CT scan findings showing compatible signs of EAC osteomyelitis, a treatment plan was promptly initiated. The patient exhibited an early positive response, obviating the need for additional imaging studies for diagnostic purposes.

Achromobacter xylosoxidans has been related to nosocomial infections in immunocompromised patients [1, 3] and it is a frequent agent in humid environments [4]. Medical history of this patient did not reveal any immunologic disorder or chronic illness that could predispose to this infection. Neither a recent contact with humid environments was detected. Aging alters the immune system and decreases its ability to fight infections [20]. The advanced age of our patient could be considered as a predisposing factor for acquiring this infection. As seen in Table 2, all other cases of osteomyelitis due to A. xylosoxidans were much younger than our patient. It remains unclear how our patient contracted this disease.

Conclusions

Chronic ear infection caused by A. xylosoxidans is uncommon and developing osteomyelitis is extremely rare. This is the first reported case of osteomyelitis of the EAC due to this organism. Otolaryngologists must be aware that atypical bacteria such as A. xylosoxidans can be responsible for chronic otitis externa. Otic cultures in chronic otorrhea play an essential role to identify the causal germ. Although A. xylosoxidans is considered an opportunistic bacteria with low virulence, its intrinsic resistance to a wide spectrum of antibiotics makes it difficult to eradicate. Early diagnosis and accurate treatment can prevent osteomyelitis progression and further related complications.