Intraneural Ganglion Cyst of the Ulnar Nerve at the Elbow Masquerading as a Malignant Peripheral Nerve Sheath Tumor.

BACKGROUND: Ulnar neuropathy at the elbow (UNE) is the second most common mononeuropathy of the upper extremity. One rare cause of UNE is nerve mass lesions, including intraneural ganglion cysts (IGCs). IGC imaging studies provide important information that may determine the nature of a peripheral nerve mass lesion.

CASE DESCRIPTION: We present the case of a 73-year-old woman who presented with rapid deterioration of left hand function over 2 months with weakness of fine motor control, grip strength, and dysesthesia in the ulnar nerve distribution. Preoperative imaging studies, including magnetic resonance imaging (MRI) of the elbow, postcontrast studies, diffusion-weighted imaging, and apparent diffusion coefficient measurements, suggested a highly cellular tumor. Diffusion tensor tractography also revealed imaging features suggestive of a malignant peripheral nerve sheath tumor. During the operation, a sample of the lesion was sent for frozen section. There were no features of malignancy, and the pathologist could not determine a diagnosis based on the tissue sample sent. An intraoperative decision was made not to divide the ulnar nerve above and below the lesion. The IGC was successfully managed by identifying a suitable plane of dissection and cyst resection.

CONCLUSIONS: This case demonstrates that MRI studies indicating malignant peripheral nerve sheath tumor must be considered with some caution and corroborated with supportive features on operative inspection and biopsy before radical resection is undertaken. Furthermore, for any nerve mass lesion immediately adjacent to a joint, the differential diagnosis of an IGC should be considered.