The groove in the patient’s nail, and associated splitting, was due to a digital mucous cyst seen in the photo proximal to the cuticle.
In spite of the name, these cysts do not contain mucus. Instead, they contain synovial fluid and represent an outpouching of the distal interphalangeal joint of the index finger. This same process is called a ganglion cyst when it is seen at the wrist and a Baker’s cyst when it is behind the knee. These cysts typically form from an increase in synovial fluid, often due to underlying osteoarthritis, injury, or degenerative changes of the joint capsule. If the patient is asymptomatic, no treatment is required. Half of the lesions spontaneously resolve. If the patient is symptomatic, treatment options include a steroid injection; aspirating the cyst and then using a compressive cryosurgery tip to freeze the superficial and deep aspects of the cyst to scar it down; and surgical resection of the cyst.
In this case, the patient was in pain because the abnormally shaped nail was breaking. She elected to have a steroid injection into the cyst—0.25 ml of triamcinolone 20 mg/ml. The family physician (FP) advised her that this would reduce the inflammation in the distal interphalangeal joint, hopefully reducing the size of the digital mucous cyst. The FP also indicated that she might require several injections to achieve a positive outcome and that it would take 6 months for her fingernail to return to normal.
Images and text courtesy of Daniel Stulberg, MD, FAAFP, Department of Family and Community Medicine, University of New Mexico School of Medicine, Albuquerque.