AANEM Connect

I recently saw a patient with numbness and tingling in the right hand that was sent to “rule out carpal tunnel.” The patient was a 36 year old female office worker that had a rather standard complaint of about a 3 week history of numbness and tingling in the third and second digit with a “feeling” of finger swelling and hand heaviness. She denied dropping objects but did state that she was beginning to have some trouble buttoning clothes with the right hand. The sensations in her right hand were beginning to occasionally wake her up at night. 
 
On physical examination her hand appeared symmetric to the contralateral hand. There was no obvious or subtle swelling, muscle asymmetry, or skin abnormalities. Manual muscle testing revealed a grade 5 to all of her hand intrinsic muscles bilaterally. Sensation was mildly reduced in the classic median nerve distribution on the volar aspect of the right hand with no abnormalities noted in the ulnar, superficial radial, or dorsal ulnar cutaneous distributions on the right. A Tinnel’s sign was present at the right wrist region, but otherwise no other provocative maneuvers caused the patient any discomfort.
 
I thought this was going to be a typical CTS assessment, but I found the motor NCV data to be unexpected and somewhat confusing. I am hoping you can shed some light on the patient’s findings.  Specifically, I performed a median sensory 14/7 cm study using an antidromic ring finger technique with ring electrodes on the third digit. Her mid-palm sensory peak latency was 1.9 ms while her 14 cm peak latency was 4.4 ms thereby yielding a trans-carpal latency of 2.5 ms. I thought, Okay, this makes sense given the symptoms: so far, the SNAP findings are suggestive of a focal median sensory neuropathy at or about the wrist region. I then performed a routine median motor study with the wrist stimulation at 8 cm resulting in a latency of 3.9 ms with a CMAP amplitude of 7 mV. No worries I thought, the median motor study is looking good at this point. But, when stimulating at the elbow, the CMAP displayed an initial positive deflection with a CMAP amplitude of 10.5 mV. This sure looked like a typical Martin-Gruber anomaly regarding the two CMAPs. I made sure the CMAP from the wrist had an abrupt negative deflection, and so I was convinced I was on the APB’s motor point. What is going on?