Skin Deep

Posted by • April 6th, 2012

In a new Clinical Problem-Solving article, a 56-year-old woman with symptoms of carpal tunnel syndrome for 6 months presented to a referral clinic with nodular lesions on her right forearm and hand and an increase in numbness and tingling in her right hand.

The differential diagnosis of chronic nodular lesions is broad and includes infectious, inflammatory, and malignant causes. Common infectious causes include fungal and mycobacterial infections and nocardiosis. The history taking should include an assessment for potential exposures. For example, gardeners are at risk for sporotrichosis and nocardiosis, whereas tropical-fish owners who clean the aquarium may become infected with Mycobacterium marinum. Certain infections, such as leishmaniasis, myiasis, and visceral larva migrans, are associated with travel to particular geographic areas. Inflammatory causes of nodules include lupus erythematosus profundus and cutaneous polyarteritis.

Clinical Pearls

• What is the clinical presentation of M. marinum?

M. marinum, a well-described cause of cutaneous infections, can lead to a variety of clinical presentations such as nodules, ulcers, tenosynovitis, or nodular lymphangitis. The incubation period of M. marinum is typically between 2 and 4 weeks, although incubation periods of more than 30 days have been reported. The primary lesions of M. marinum are commonly described as superficial papules, nodules, plaques, ulcers, abscesses, or pustules occurring at the site of the injury. These superficial lesions are emblematic of M. marinum infections, because the pathogen typically does not invade beyond the superficial, cooler regions of the skin; it thrives at temperatures between 30 and 33 degreesC rather than at the body’s core temperature of 37 degreesC.

• What are the risk factors for M. marinum infection?

This water-borne organism is found both in naturally occurring bodies of water and in still or stagnant water, such as swimming pools and fish tanks, leading to the names “swimming pool granuloma” and “fish tank granuloma.” Infection usually occurs after direct inoculation through broken skin on exposure to fresh water or saltwater or after injuries from fish spines or oyster shells. Pointed questions regarding skin injuries associated with water, fish, or shellfish may identify exposures that could otherwise be overlooked.

Morning Report Questions

Q: How is M. marinum best diagnosed and treated?

A: The gold standard for diagnosis of M. marinum is culture. Under appropriate conditions, mycobacterial culture is positive in approximately 80% of cases of infection. M. marinum, however, grows optimally at a temperature between 30 and 33 degreesC. Unless the laboratory automatically processes skin and soft-tissue specimens at these lower temperatures for nontuberculous mycobacteria, failure to indicate the possibility of M. marinum on the initial request form frequently results in false negative results from tissue cultured at 37 degreesC. Cultures must be held for at least 6 weeks before one can conclude that the results are negative, since M. marinum, like many other mycobacteria, is a slow-growing organism. In addition, the number of organisms present in surgical specimens is usually small. Smears for acid-fast bacilli are negative in up to 95% of tissue-biopsy specimens. Clarithromycin and ethambutol are two first-line drugs associated with excellent outcomes. M. marinum is typically resistant to isoniazid.

Q: What is the value of an interferon-gamma-release assay (IGRA)?

A: IGRAs such as the QFT test and the enzyme-linked immunospot assay use enzyme-linked immunosorbance to measure antigen-specific production of interferon by circulating T cells in whole blood. Both M. tuberculosis as well as infection with atypical mycobacteria can cause tuberculin-skin-test conversion. In a meta-analysis of 59 studies, IGRAs had a sensitivity of 97.7% (95% confidence interval; 96 to 99) for latent tuberculosis infection, which is similar to the reported sensitivity of the tuberculin skin test. The specificity was 97%, which is greater than that of the tuberculin skin test in patients with a history of BCG vaccination. Neither test distinguishes between active tuberculosis and latent infection. Apart from the increased specificity, the main advantage of the IGRAs is that they require only a single patient visit. The Centers for Disease Control and Prevention has recommended that an IGRA may be used in any situation in which a tuberculin skin test is used. An interferon-gamma-release assay (IGRA) also has less cross-reactivity with atypical mycobacteria, thus a negative result in the setting of a positive PPD would help to narrow the differential diagnosis.

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