In jurisprudence, entrapment is a legal defense by which a defendant may argue that he or she should not be held criminally liable for actions which broke the law, because he/she was induced by the police to commit those acts.
In the medical world, a peripheral nerve can cause severe symptoms, due to entrapment by local anatomic structures. The nerve is an innocent bystander in these cases.
Although this is supposed to be a MRI blog, I do run into interesting cases CT and ultrasound cases as well. So, at the risk of not being topical, here is a case of an 80 year-old man who presents with extensor weakness and a mass.
A CT scan reveals a lipoma (red arrows) that is inseparable from the supinator muscle, which can no longer be seen as a discrete structure. The posterior interossesous nerve (deep branch of the radial nerve) cannot be seen, as it is markedly compressed. The superficial radial nerve (green arrow) can be seen superficial to the lipoma, with mild compression by the lipoma.
The radial nerve divides into the posterior interosseous nerve and the superficial radial nerve at the level of the distal elbow. The posterior interosseous nerve pierces the supinator muscle, and provides motor innervation to the extensor muscles in the posterior compartment of the forearm. The superficial radial nerve runs superficial to the supinator, and supplies sensation to the skin of the thumb, index, and middle fingers.
In a more distal axial section through the forearm, there is marked fatty atrophy of the extensors of the posterior compartment (red arrows). Note the normal extensor carpi radialis brevis muscle (green arrow).
The nerve to the ECRB may be coming off the superficial radial nerve (which occurs in 25% of patients), which is less compressed in this case. Alternatively, the nerve may to the ECRB may have left the posterior interosseous nerve prior to the site of lipoma compression.
In most cases, the cause for radial nerve compression is not identified on MRI, with common sites of entrapment including the tendinous arch of the supinator muscle (arcade of Frohse), the ECRB, and a leash of recurrent radial vessels.
Posterior interosseous nerve syndrome can be mistaken for a C7 radiculopathy (distinguish by looking for weakness of triceps and wrist flexors in a C7 radiculopathy) and lateral epicondylitis. Radial tunnel syndrome can also cause pain along the proximal lateral forearm; it is also due to entrapment of the radial nerve, but there are no motor symptoms.
Vic David MD