The patient reported noticing increasing difficulty with turning doorknobs using his right hand. On physical exam, testing of supinator strength revealed moderate, asymmetric weakness. There was also evidence of asymmetric right weak wrist dorsiflexion. Sensory function however, was noted to be intact.
The patient reported noticing increasing difficulty with turning doorknobs using his right hand. On physical exam, testing of supinator strength revealed moderate, asymmetric weakness. There was also evidence of asymmetric right weak wrist dorsiflexion. Sensory function however, was noted to be intact.
Elbow radiography was normal. Further evaluation with electromyographic studies revealed findings consistent with denervation of the posterior interosseous nerve (PIN), which is the deep branch of the radial nerve that innervates the supinator muscle, at the level of the proximal radioulnar joint.
MR imaging of the elbow was subsequently performed to evaluate the pathway of the radial nerve in the region of the arcade of Frohse for evidence of impingement of the PIN. The MR imaging study revealed a 1.3 cm heterogeneous but predominantly hyperintense mass suspicious for a schwannoma or less likely a ganglion cyst (Fig. 1a); the mass was noted to be compressing the PIN within the interosseous space, pinning this deep motor branch of the radial nerve against the adjacent radial cortex (Fig 1b). In conjunction with this mass, there was notable supinator atrophy (Fig 1c). The diagnosis of schwannoma was confirmed at surgery and following tumor resection, the patient experienced full return of his supinator strength.
PIN impingement or paralysis or palsy is an uncommon phenomenon, with suggestive clinical manifestations of paralysis or weakness of the extensors of the fingers and thumb or supinator muscle. The muscles primarily affected by PIN impingement include the supinator, the extensor carpi ulnaris, and the extensor carpi radialis brevis. This impingement causes the clinical phenomenon of the “hanging hand” which is caused by extensor dysfunction and fist weakness (from the decreased synergistic input of the extensor carpi ulnaris). This phenomenon is important to recognize as a specific entity as its presentation can mimic tennis elbow.
Reported sources of PIN compromise include ganglions (1), lipomas (2), lacerations, schwannomas (3), neurfibromas (2), bicipital bursa (4). To our knowledge, there is only one other case that reports the use of MR imaging in determining the preoperative findings of PIN impingement by a schwannoma (also referred to as a neurilemmoma). At surgery, this particular patient was found to have five schwannomas of the radial nerve at the elbow joint; three of which were seen on MR imaging along the course of the PIN (3). There was no report of atrophic muscular change on the MRI or at surgery to suggest severe impingement. In our case, the impingement of the PIN by the nearby schwannoma was clearly a chronic process that was characterized by PIN impingement significant enough to produce supinator muscle atrophy and fatty infiltration. Typically, the MR imaging findings of a schwannoma are low to intermediate signal intensity on T1W imaging and homogeneous, high signal intensity on T2W imaging. These tumors may also be differentiated from ganglion cysts by the observation of solid material within the mass and/or enhancement of tissue within the mass on post contrast images. MR imaging features that have been reported to be helpful in making the diagnosis of a nerve sheath tumor include the split fat sign, the target sign, and the string sign (5).
In summary we report a case of PIN compression by an associated nerve sheath tumor. This represents, to our knowledge, only the second report of the MR imaging features of this phenomenon. As this case has demonstrated, MR imaging may be helpful in demonstrating neoplastic masses in patients with clinically suspected peripheral nerve impingement and may also be useful in the preoperative determination of diagnosis and in planning the surgical approach.
Schwannoma causing posterior interosseous nerve impingement
Based on the provided MRI images, a round or approximately round lesion signal can be observed in the proximal area of the radial head near the elbow joint (consistent with the anatomical course of the posterior interosseous branch of the radial nerve). On T1-weighted images, the lesion appears as relatively low or intermediate signal intensity, while on T2-weighted images, it shows relatively high signal intensity, with clearly defined lesion boundaries. The lesion’s substance appears uniform or with possibly mild septation-like structure, which aligns with common MRI features of a schwannoma (schwannoma). Additionally, there is significant atrophy and fatty infiltration in the surrounding affected muscles (especially the supinator muscle), suggesting chronic compression or irritation to the nerve supply in that region. No obvious signs of bone destruction or fracture are noted, and no extensive infiltration in the surrounding soft tissues is observed.
Clinically, the patient presents with involvement of the supinator, wrist extensor muscles, etc.; MRI shows supinator muscle atrophy and a local lesion, consistent with the clinical and radiological features of compression of the deep branch of the radial nerve (PIN).
Round, well-demarcated, high signal on T2, often enhancing, possibly showing characteristic “split-fat sign” or “target sign.” Clinically, other nerve sheath tumors like neurofibroma could be considered, but based on the imaging characteristics and lesion location, schwannoma is more likely.
These typically present with fatty signals or cystic manifestations on MRI. If there is no clear enhancement of solid soft tissue within the lesion, or fat signal predominates, one may consider lipoma or ganglion cyst. However, the homogeneous solid characteristics of the lesion in this case more strongly suggest schwannoma.
Considering the patient’s age, clinical symptoms (notable weakness of the supinator and wrist extensor muscles, with relatively preserved sensory function), and MRI findings of a round solid mass causing chronic compression of the posterior interosseous nerve, the most likely diagnosis is: “Schwannoma Causing Posterior Interosseous Nerve (PIN) Compression Syndrome.” For further confirmation, surgical exploration or histopathological evaluation may be used to obtain a definitive diagnosis.
Treatment Options:
Rehabilitation and Exercise Prescription:
The above treatment and rehabilitation plan should be adjusted to individual circumstances. For patients with osteoporosis, poor cardiopulmonary function, or other comorbidities, training intensity should be modified as needed, with strict attention to safety.
This report is a reference analysis based on current case data and imaging and cannot replace an in-person consultation or professional medical advice. Specific treatment plans should be formulated and implemented by a professional medical team, in combination with the patient’s clinical presentation, laboratory tests, and pathological findings.
Schwannoma causing posterior interosseous nerve impingement
