Unusual presentation of carpal tunnel syndrome due to Mycobacterium marinum infection of hand

Discussion

M. marinum, an atypical mycobacterium, has varied colonial characteristics and is transmitted directly from water or soil environmental reservoirs].

M. marinum is the most common pathogen infecting the hand among the atypical mycobacteria ranging from 40% to 80% of cases.1 2 Their predilection for the hand has been attributed to the relative abundance of the synovium in this region. There is also a high risk of pathogen inoculation through minor penetrating trauma.5 The hand is generally cooler than the core body temperature and is ideal for M. marinum growth, which is around 30°C.

In 1951, M. marinum was found to be a cause of human disease by Linell and Norden.⁶ In Britain, the first case described was associated with a swimming bath in Yorkshire.2 In 1962, Swift and Cohen reported an association with fish tank exposure and described the cutaneous lesion as ‘Fish tank granuloma.^{6 7} Such infections have also been described in people who keep tropical fish in aquariums at home7 or those with aquatic exposure, as fresh and saltwater may harbour it.7

Disease transmission is mainly through a puncture wound or abrasion on exposure to water contaminated by fish, shrimp, oysters or any other aquatic animal. However, there are no reports of human-to-human transmission like TB.1 2

Our patient has been a fisherman by occupation for several years and is constantly exposed to the sea. He gives a history of repeated injuries to fingers, as he is involved in handling the ropes of the boats. Disease transmission through these open wounds is suspected.

Clinical involvement in the upper extremity may involve superficial or deep structures.5 Clinically the skin lesion may present as solitary or multiple, ranging from nodular, papule-verrucous and pustular to ulcerative lesions.8 They may also present as discharging sinus, mimicking osteomyelitis.8 The lymphatic spread may result in a sporotrichoid pattern mimicking sporotrichosis fungal infection in 20%–30% of cases. However, there is no regional axillary or epitrochlear lymph node involvement, unlike sporotrichosis.6

The disease may present as bursitis, flexor or extensor tenosynovitis, septic arthritis, or osteomyelitis.9 M. marinum infection usually occurs in immunocompetent individuals without systemic symptoms, unlike TB. However, rarely in immunocompromised patients, disseminated infection with bacteraemia has been reported.10

Due to the absence of pathognomic clinical features of M. marinum infection, it is often misdiagnosed, or there is a delay in diagnosis, which can lead to severe, destructive infection. Following are the significant findings through our literature review and experience, which can pose a challenge to diagnosis.

When our patient initially presented with swelling and difficulty in movement, a differential diagnosis of TB or rheumatoid was suspected, as most clinicians do. Like other atypical variants, misdiagnosis as a rheumatological condition and treatment with immunomodulators and intralesional steroids has been described to cause devastating outcomes.11 12

On investigation, patients tend to have normal to slightly elevated inflammatory serum markers like ESR and C reactive protein.13 ESR was 30 mm/hour in our patient.

The TB skin test can only be used as a screening tool, and its utility in diagnosis is uncertain and was not done in our patient.13 In most cases, acid fast bacilli (AFB) stains tend to be negative with less than a 30% positivity rate.13 AFB stain was positive in our case, with a long non-beaded form seen more in favour of an atypical organism. This finding raised suspicion about M. marinum, and treatment was immediately started.

Culture requirements are highly specific for M. marinum, with 6 weeks of incubation, and culture yields may be as low as 42%. Culture has poor sensitivity and yields false negative results in up to 60% of cases.13 The culture was negative in our case despite the sample incubating at 30° and 37° separately.

Tissue biopsy will reveal acute inflammatory cell infiltrates, fibrinous exudate and non-caseating granulomas. These support the diagnosis but are not pathognomic, as organisms may not always be visible in histological sections.11

Polymerase chain reaction(PCR) could be diagnostic by detecting 16S ribosomal RNA, but clinical laboratories may have different testing protocols, and many centres like ours may need this advanced test available.1

The clinical presentation can be classified into benign and aggressive groups for treatment purposes.11

The benign group include asymptomatic patients with nodules and papules who respond well to conservative management, including 6–9 months of antimicrobial therapy after a diagnostic synovial biopsy.14 In contrast, patients with deep-seated infections presenting as tenosynovitis or osteomyelitis will require aggressive surgical debridement followed by postoperative antimicrobial therapy.14 The duration of antimicrobial therapy considerably varies in the literature, and it is the combination of drugs than monotherapy that is found to be more effective.15 M. marinum is invariably resistant to pyrazinamide and isoniazid and sensitive to ethambutol, rifampicin, clarithromycin and most quinolones.16

With adequate antimicrobial therapy following surgical debridement, positive results can be anticipated as early as 3 months, as seen in our patient when a combination of clarithromycin, rifampicin and doxycycline is used.

Surgery aims to remove as much disease as possible by debulking the infected and necrotic tissue, thereby augmenting antimicrobial penetration. Apart from surgical debridement, amputation or ray resection for the stiff, painful and nonfunctional disease has been described.11 Moreover, it should be a multidepartment approach, with the involvement of the infectious disease department, pathologist and microbiologist for optimum outcomes.1

In our hospital, this case of M. marinum infection of the hand was managed aptly and resulted in the patient’s full recovery.