Published: May 8, 2026
An 81-year-old male presented to the hand surgery clinic reporting a sudden onset of weakness in his right hand’s ability to extend the middle, ring and small fingers. The index and thumb also were affected, to a lesser extent. There was no history of trauma; the symptoms began following a session of pushups four weeks prior. The patient exhibited no altered sensation in any peripheral nerve distributions distally. Muscle strength testing revealed a rating of 3/5 strength for the extensor indicis proprius (EIP) and extensor pollicis longus (EPL), while the extensor digitorum communis (EDC) showed 0/5 strength in the middle, ring and small fingers. Notably, radial deviation of the right wrist was observed with wrist extension. Additionally, tenodesis testing was normal, and the suspicion for attritional extensor tendon rupture was low.
Given the clinical presentation, posterior interosseous nerve (PIN) palsy was the primary consideration. The electromyography (EMG) results indicated a conduction block as well as axon motor loss in the radial nerve at the elbow, suggestive of right PIN neuropathy. The superficial radial nerve appeared to be spared. Notably, a cystic mass adjacent to the lateral epicondyle, in proximity to the PIN, was visualized during examination (Figure 1).
MRI revealed a hyperintense lesion at the arcade of Frohse, likely contributing to the compression of the PIN. The mass, measuring 13 millimeters, exhibited characteristics consistent with a ganglion cyst (Figure 2).
Ethan W. Blackburn, M.D.
Hand Surgeon and Orthopedic Sports Medicine Physician
Approximately three weeks following the initial clinic visit, the patient underwent exploration of the PIN and mass excision in the operating room. Intraoperatively, the PIN was found compressed by a ganglion cyst originating deep as it entered the arcade of Frohse. Clinical photos captured during the procedure depicted the cyst (Figure 3), measuring 1 centimeter by 1.5 centimeters, emerging from the radiocapitellar joint. The arcade of Frohse was released, and the ganglion cyst was subsequently decompressed, transected at its base and sent for pathology examination. Additionally, a small arthrotomy revealed an abundance of synovial fluid and fibrinous debris consistent with arthritis.
Figure 3. Clinical photos showing a ganglion cyst (white asterisk) compressing the PIN (white arrow) posteriorly as it enters the arcade of Frohse
A – Showing the relationship of the PIN (white arrow) to the ganglion cyst (white asterisk)
B – Showing retraction of the PIN and pickups grasping the decompressed ganglion cyst
The pathology report confirmed the diagnosis of a benign ganglion cyst. A repeat EMG/nerve conduction study (NCS) performed at six weeks postoperatively demonstrated reinnervation changes of the muscles innervated by the PIN. Subsequent clinic follow-ups at two weeks, eight weeks and six months postoperatively documented consistent improvement in nerve palsy symptoms. At the six-month mark, complete resolution of the PIN nerve palsy was observed, with examination revealing 5/5 wrist extension, no radial deviation and 5/5 strength of digit extension and EPL strength.
The rarity of ganglion cysts in the elbow region underscores the potential for misdiagnosis or delayed diagnosis, which can lead to cases taking months to years to be correctly identified and treated. This case further reinforces complete recovery and reinnervation is achievable following relief PIN compression through surgical excision. Clinicians should therefore consider ganglion cysts in cases of nontraumatic PIN palsy to facilitate prompt identification and treatment.