False aneurysm presenting as delayed posterior interosseus nerve palsy.

False aneurysms commonly present with a delay in symptoms of 3-6 weeks after the initial injury. These lesions may present as an enlarging nonpulsatile and painless mass causing neurologic symptoms. A review of the literature showed only 26 false aneurysms reported in the past 25 years with compression of the brachial plexus, median nerve, ulnar nerve, radial nerve, and sciatic nerves. This case is the first reported of compression of the posterior interosseus nerve by a false aneurysm and is also unusual in presenting 8 weeks after the initial injury.