Assessment of ulnar neuropathy
The most likely site of ulnar nerve damage is in the region of the shallow ulnar groove at the elbow where the nerve is prone to stretching or injury.
Nerve conduction studies check:
• the preservation of the ulnar sensory responses from the little finger
• any evidence of slowing of the speed of nerve impulses across the site of pressure at the elbow compared to the forearm
• any drop in the size of the muscle response when a stimulus is given above, compared to below, the elbow. This would be consistent with a block in the number of nerve impulses getting through the site of entrapment.
Sometimes the site of entrapment in an ulnar neuropathy may be just below the elbow in the cubital tunnel between the two heads of Flexi carpi ulnaris. The figure below shows such a case with a dispersed low amplitude muscle potential to ADM when stimulating both above and below the elbow but above the cubital tunnel.
Figure showing reduced and dispersed size of the compound muscle action potential recorded from
Adductor Digiti Minimi (ADM) with stimulation above and below the elbow. The entrapment is within
the cubital tunnel
Less common causes of an ulnar nerve lesion include a deep ulnar nerve lesions at the level of the wrist
With a deep ulnar nerve lesion at the level of the wrist or in Guyon’s canal the following abnormalities occur:
• Prolonged distal motor latency to the 1st dorsal interosseous compared to ADM.
• Reversal of the normal distal motor latency difference between median innervated II lumbrical and ulnar innervated II interosseous.
Use of MRI or ultrasound as ancillary measures to nerve conduction studies
Electrodiagnostic studies remain the diagnostic mainstay but when electrodiagnostic findings are not localising ultrasonography or MRI may support a diagnosis of ulnar neuropathy at the elbow.
MRI of the elbow may be helpful to confirm the ulnar neuropathy at that level when electrophysiology does not localize the level. Three mm axonal MRI slices through the radial humeral joint may show increased signal within the ulnar nerve, both above and below the distal humerus, sometimes with swelling of the nerve, confirming evidence of an ulnar neuropathy (Britz G et al., 1996;Beekman et al., 2004)
False negative cases with sonography with an ulnar neuropathy at the elbow may occur with a pure demyelinating ulnar neuropathy without nerve enlargement.
Britz G et al., Ulnar nerve entrapment at the elbow: correlation of magnetic resonance imaging, clinical, electrodiagnostic and intra-operative findings. Neurosurgery March 1996, Vol 38, No. 3
Beekman R et al., Diagnostic value of high resolution sonography in ulnar neuropathy at the elbow. Neurology 2004; 62:767-773
Compiled by Stuart Mossman Neurologist FRACP MD