By Chris Faubel, MD —
A patient presents to your clinic with a very clear presentation of ulnar neuropathy at the elbow:
- numbness in the medial hand (palmar and dorsal surfaces and medial 1.5 digits), that is worse when talking on the phone and reading a book – basically all activities when his elbows are flexed 90 degrees or more
- note: if it was only the palmar surface, then the lesion would be more likely at the wrist, b/c the dorsal ulnar cutaneous nerve supplies the dorsal medial hand and branches off before the wrist — thus not involved in Guyon’s canal lesions
- NO paresthesias in medial forearm (b/c that would be from the medial antebrachial cutaneous nerve, and therefore a lower brachial plexus lesion)
When measuring the motor conduction velocity across the elbow, we typically look for a 10m/s or more drop with a distance of 8cm [ex: 56m/s velocity for the segment from below the elbow to the wrist, but only 42m/s for the 8-cm segment across the elbow).
But this patient who had a story of bilateral symptoms that sounded a lot like ulnar neuropathy at the elbows, had a 14m/s drop across the right elbow, but only an 8m/s drop across the left. So then what…no diagnosis of ulnar neuropathy on the left? Well, the recording electrode pickup that we usually place over the abductor digiti minimi (ADM), was then switched to first dorsal interosseous (FDI).
The FDI is the most distal ulnar-innervated muscle, and is considered to be more sensitive for picking up ulnar neuropathy at the elbow. So, with that switch, the patient had a clear 15m/s drop on the left –> ulnar neuropathy at the elbow.
Why not just always use the FDI as the recording site? Well, because it’s more sensitive, it is also going to give you a higher false positive rate. So only use it when the clinical suspicion for ulnar neuropathy is high, but the testing (with pickup over the ADM) fails to show the deficiency needed to make the diagnosis.
