The “Texas Longhorn” Syndrome: A Case of Posterior Interosseous Nerve Palsy With Noteworthy Observations

Abstract

The posterior interosseous nerve (PIN) is quite a fascinating nerve. It wraps around the radial neck akin to the fibular nerve wrapping around the fibular neck. It can be compressed by the arcade of Frohse in a fashion similar to median nerve entrapment by the carpal tunnel. Furthermore, it divides into a number of branches, a simulacrum of the cauda equina, as it emerges from the supinator muscle. It is also associated with spaceoccupying pathologies; lipomas, and ganglion cysts being among the most common. It also has other interesting anatomical properties, such as fascicular compartmentalization, explaining PIN paralysis with radial nerve injuries. Finally, PIN lesions can lead to partial paralysis of finger extensors leading to the "Texas longhorn" hand gesture. We seize upon a case of PIN entrapment by fibrous adhesions and the arcade of Frohse, and explore the functional anatomy of the PIN.

Clinical, Electrodiagnostic, and Ultrasound Findings in 87 Patients With Finger Drop

“…PIN neuropathy was the most common cause of finger drop in our study, comprising 74% of the patients. At the radiocapitellar joint of the elbow, the radial nerve bifurcates into the PIN and the superficial radial nerve after which the PIN passes between the two heads of the supinator muscle and wraps around the radial neck [ 16 ]. As it emerges from the supinator muscle, the PIN separates into numerous divisions supplying the group of muscles of the dorsal forearm which extend to the digits at the metacarpophalangeal joints.…”

“…The PIN is most vulnerable to entrapment just beyond its origin as it passes through the fibers of the supinator muscle in the proximal forearm [ 5 ]. The PIN may be entrapped at the arcade of Frohse or compressed by space-occupying lesions (lipomas, ganglion cysts, and rheumatoid synovial overgrowth), or may experience fascicular dysfunction due to radial nerve injuries [ 5 , 16 ]. Figure 4 shows one of the patients in this series with global finger drop, in whom a cyst was detected compressing the PIN by US study.…”

“…Depending on the location of the PIN pathology, all (global) or only certain fingers may show finger drop (“Texas longhorn” hand gesture due to partial paralysis of finger extensors) [ 16 ]. Clinical distinction between partial finger drop due to cervical radiculopathy versus partial injury to the PIN (neurogenic finger drop) and those resulting from tendon injury/rupture (non-neurogenic finger drop) may be difficult without needle EMG study of the extensor muscles.…”