Metachronous ganglion cysts: illustrative case.

BACKGROUND: Ulnar neuropathy at Guyon's canal is rare, but when it occurs, it is frequently due to underlying masses, including extraneural and intraneural ganglion cysts, as well as adventitial cysts. The pathogenesis of these cysts, especially at a rare site, remains controversial. The articular theory, which is gaining popularity, posits that a joint connection via a nerve (or vascular) branch to a neighboring joint represents the pathway for their formation and propagation.

OBSERVATIONS: A recently published case of a patient with an ulnar intraneural ganglion at the wrist with a series of postoperative cystic recurrences was reevaluated, as it challenged the articular theory. No discrete articular branch had been identified on imaging or at surgery. Serial MR images were reinterpreted, which revealed that the original intraneural cyst was treated; new sequential cysts-an adventitial cyst and then a soft tissue ganglion-developed, all arising from the same wrist joint.

LESSONS: This case strengthens the principles of the articular theory for intraneural ganglion, adventitial, and soft tissue ganglion cysts. A small wrist connection from the ulnar intraneural ganglion was unrecognized and untreated. Scarring disrupted the original nerve pathway and pressure dynamics. New cysts developed through neighboring joint capsular rents along different pathways into other "compartments" (vessel or soft tissues). https://thejns.org/doi/10.3171/CASE25618.