Coexisting Ulnar Nerve Schwannoma and Lateral Humeral Condyle Nonunion Presenting as Tardy Ulnar Nerve Palsy: A Report of a Rare Case.

Tardy ulnar nerve palsy (TUNP) is a progressive neuropathy commonly associated with cubitus valgus deformity following lateral humeral condyle nonunion. However, concurrent ulnar nerve schwannoma as a contributing factor remains undocumented. We present a rare case in which both chronic traction neuropathy and intraneural compression coexisted, with the schwannoma remaining undetected on high-resolution ultrasound (HRUS) preoperatively. A 44-year-old male professional driver presented with a six-month history of progressive ulnar neuropathy, characterized by paresthesia, intrinsic muscle weakness, and occupational impairment. Preoperative evaluation revealed severe functional limitation, with a DASH (Disabilities of the Arm, Shoulder, and Hand) score of 72 and grip strength of 40 kg compared to 70 kg contralaterally. Electrodiagnostic studies confirmed ulnar neuropathy with axonal damage. While HRUS detected nerve compression, it failed to reveal a 0.8 cm schwannoma, which was only identified intraoperatively. The patient underwent microsurgical schwannoma excision, anterior subcutaneous transposition, and microneurolysis. Postoperatively, the patient demonstrated progressive neurological recovery, achieving a DASH score of 5, restored grip strength (70 kg bilaterally), and normalized two-point discrimination (3-4 mm) within 24 months. This case highlights the dual mechanisms of ulnar nerve injury, involving both chronic traction from cubitus valgus and focal compression from an occult schwannoma. The false-negative HRUS result underscores the limitations of ultrasonography in detecting small intraneural tumors. Given the intraoperative discovery of an occult schwannoma, this case highlights the potential role of MRI in cases of unexplained or refractory ulnar neuropathy. These findings reinforce the importance of comprehensive surgical strategies in managing complex peripheral nerve pathologies.