AANEM Connect

I recently saw an interesting and confusing right median nerve injury and I am hoping somebody may have a similar experience and/or can explain the findings. Healthy 29-year-old male fell about 10 feet from a ladder in September 2022 resulting in a radial head fracture treated nonoperatively. He developed pain in the elbow but otherwise no numbness or paresthesias. However, he developed weakness with pronation, and flexion of the DIP joint of only the index finger.  He was evaluated by orthopedics with MRI of the hand and forearm performed about 2 months post injury showing inflammation in the pronator teres and FDP as well as the radial head fracture.  There was no evidence of tendon injury to the index finger.  I saw him in December, 3 months post injury for electrodiagnostic consultation.  He stated he had no numbness and tingling, and weakness had not improved.  Physical examination notable for essentially 0/5 pronator strength (he was substituting shoulder internal rotation), no active motion of the PIP joint of the index finger.  There was obvious atrophy over the pronator teres compared with the contralateral side.  There was normal strength to the FDI, FPL, APB, wrist flexion, wrist extension, finger extension, FDS and FDP to the small, ring and middle finger.  Nerve conduction studies done bilateral showed normal and symmetric median sensory and motor.  I performed NCS to the bilateral pronator quadratus using surface electrodes and stimulating at the elbow.  The latency and amplitude were symmetric although the waveform morphology was different.  Needle EMG showed extensive positive sharp waves/fibrillation potentials and no active motor units in the pronator teres, pronator quadratus and FDP to the index finger.  Needle EMG was normal to the FCR, FDS, FPL, extensor indicis and APB.  Regarding the FDP exam, I was able to localize motor units to each finger as I advanced the needle.  After passing through the fibers to the middle finger, I encountered spontaneous activity and no motor units.

I saw him back a few days ago for repeat needle EMG. He stated other than feeling his pronation strength was somewhat better, there were no interval changes. Exam continues to show significant atrophy over the pronator teres with the FCR now showing hypertrophy. He demonstrated some active pronation but continued to have no active flexion of the DIP joint of the index finger.(I believe the FCR is now performing pronation duties). Needle EMG again showed extensive spontaneous activity with no active motor units to the pronator teres, pronator quadratus and FDP to the index finger.

Functionally, he is minimally impaired and not inclined to do anything invasive, such as exploratory surgery.

Questions/Observations:
1. Given the trauma resulting in a radial head fracture, I assume the median nerve was injured at the level of the elbow. Given the severity of denervation to the pronator muscles and FDP to the index finger, I do not have a good explanation for why other muscles were spared and why no median sensory injury occurred. Any thoughts on this?
2.  Does anybody know of anatomic variant of the anterior interosseous nerve where the pronator teres would also be innervated, and/or the AIN comes off as proximal as the elbow?
3. Regarding the FDP, I find it interesting that the only denervation is to the index finger. I assume the muscle could have more ulnar dominance with the median nerve only innervating the index finger portion of the muscle, which would explain the findings and also explain why reinnervation is not occurring from collateral sprouting of the surviving median fibers to the middle finger. Any thoughts on this?
4. Regarding the nerve conduction study to the pronator quadratus, I was surprised to have an intact conduction on the affected side, given the clinical and needle EMG findings. This is not a study I do very often. The waveform was different from the intact side, and my assumption is the electrode may have picked up signal from the FPL and/or other distal median muscles. Doing the study using a needle pickup probably would have been useful. does anybody have experience or comments on this?