A 28-year-old wastewater worker arrived at my office with pain in his right index finger. He said the pain had appeared gradually, and he did not recall any recent injury to the area. He complained of difficulty gripping tightly with his right hand, as well as a snapping or popping sensation of the finger, and sharp pain at the base of the digit.

He enjoyed fishing and working on his car but said the pain had worsened over three months and was now interfering with his hobbies and ability to work.

During an exam, I noticed he had a tender, swollen lump at the base of his index finger in his palm. His finger was catching or snapping when he flexed the digit, and the finger was slightly stiffer than the others on that hand.

Based on the examination, I diagnosed the patient with trigger finger. The tendons that flex the fingers into a fist originate in the forearm and attach at the fingertips, gliding back and forth to move the fingers. They are held in place against the finger bones by a series of tight, fibrous loops, arranged like the rings that hold a fishing line to a fishing pole. If the loops become too tight, or the tendon swells, the tendon gets stuck on the loop as it glides back and forth to flex the finger, causing painful snapping and stiffness. Trigger finger is an extremely common condition seen by hand surgeons.

The patient was given a steroid injection anti-inflammatory medication that brings down swelling and allows the tendon to glide smoothly again. He quickly had a complete resolution of his symptoms.

But six months later, the patient came back to my office. His symptoms had returned only a week after his injection, and now his entire finger was swollen and sausage-shaped, with mild tenderness along the entire palm surface of the finger. What was the real cause?

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There were no signs of acute infection such as fevers, chills or redness. He had not had any injury, and explained that this swelling had come on gradually over the course of months. He was having difficulty flexing the finger because of his swelling.

He was sent for an MRI to look for signs of tenosynovitis, an inflammation of the cells that line the tendon, which can result from autoimmune diseases such as rheumatoid arthritis, or from infection or mechanical irritation.

The MRI showed extensive swelling of the tendon lining, with a large buildup of inflamed tissue. He underwent surgery to remove this abnormal tissue in an effort to improve his pain and range of motion. A sample of the tissue was sent to the laboratory, and he started working immediately with a hand therapist to stretch out his finger and improve his stiffness.

The patient’s laboratory report came back positive for Mycobacterium marinum. This bacterium is closely related to tuberculosis and grows in contaminated water. Unlike tuberculosis, however, it grows best at well below body temperature. As a result, it causes a tuberculosis-like disease in cold-blooded animals, especially fish, but causes slow-growing, localized infections in humans.

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On further questioning, the patient thought that he had likely punctured the affected finger with a fish spine while fishing years ago but didn’t remember for certain. He was referred to an infectious-disease specialist and started a course of antibiotics. Because of the slow-growing nature of M marinum, his antibiotic course will last a full year, but he has already seen significant improvement in his swelling and stiffness with hand therapy.

Jacob Tulipan is a board-certified hand and wrist surgeon at Rothman Orthopaedics.