By Chris Faubel, MD —
A 50 year old male was referred to us for evaluation of his right lower extremity. Pt had a gunshot injury to the abdomen about two months ago, with a resultant right-sided retroperitoneal abscess that was subsequently drained. He presented today with significantly weakened right hip flexors (3-/5) and knee extensors (2/5), with numbness over the anterior thigh and medial leg. **note that the knee extension was more affected than the hip flexion
The usual nerve conduction testing of the peroneal and sural nerves was normal, but the saphenous nerve had no response.
EMG of the vastus medialis and intermedius showed profound (4+) positive sharp waves (PSWs) and fibrillations. The iliopsoas also showed significant PSW & fibs. All other tested muscles (sciatic nerve innervated) were normal.
As you can see in the first photo below:
- innervation to the iliacus comes from a branch off the lumbosacral plexus before the formation of the femoral nerve
- nerve supply to the psoas comes from a very proximal branch off the femoral nerve.
- what is not clearly shown is that the femoral nerve has a retroperitoneal course in the lower abdomen
The second photo shows the cutaneous supply of the saphenous nerve (L3, 4), which is just an extension off the end of the normal femoral nerve.
So to localize the lesion in this patient, we took into account that his hip flexion was fairly preserved compared to the knee extensors (quads). This can be accounted for by the fact that the innervation to the psoas portion of the iliopsoas complex was unharmed, and therefore functional enough to provide some hip flexion strength.
And we knew the lesion wasn’t in the femoral triangle (like what happens after a hematoma from a femoral artery catheterization), b/c the iliopsoas muscle was profoundly affected on EMG. Thus, a diagnosis of high femoral neuropathy was made.
Note: the lateral femoral cutaneous nerve also has a retroperitoneal course, but this patient didn’t complain of lateral thigh numbness. It would’ve been interesting to have tested it though.