A case report: Triangular interval syndrome in an elite swimmer – A diagnostic pitfall of painless weakness

Abstract

Triangular interval syndrome (TIS) is a rare cause of radial nerve entrapment, typically described in throwing athletes or military personnel. We present a case of TIS in an elite swimmer, uniquely characterized by painless weakness and a normal initial neurological examination, challenging conventional diagnostic pathways. A 17-year-old male presented with a 2-month history of isolated performance decline, manifesting as a loss of power during the pull-through phase. He denied pain but reported mild posterior shoulder discomfort post-training. Notably, his initial neurological exam was normal, leading to diagnostic delay. Diagnosis was confirmed via MRI showing lateral triceps edema and subsequent EMG demonstrating partial radial nerve denervation with reinnervation. The athlete’s extreme training volume and identified glenohumeral internal rotation deficit (GIRD) were postulated as key biomechanical drivers. A multi-modal non-operative regimen including dry needling, neuromuscular electrical stimulation (NMES), and biomechanical correction resulted in complete resolution of symptoms and return to elite performance within two months. This case expands the etiological spectrum of TIS to include high-volume swimming. It underscores that TIS can present as an isolated motor syndrome without pain or initial electrodiagnostic abnormality. Clinicians should consider TIS in athletes with unexplained performance decline, utilizing advanced imaging and a high index of suspicion even in the face of normal preliminary investigations. Early, targeted intervention can yield excellent outcomes.