Sunday, March 1, 2015

Ulnar neuropathy: Five new things

Campbell WW, Carroll C, Landau ME. Ulnar neuropathy at the elbow: five new things.  Neurology Clinical Practice  5:1 February , 2015 pp 35-41

1.  Authors note that the 10 cm rule was derived in 1972, in which error of velocity increased as distance decreased below 10 cm (Maynard FM and Stolow WC,  Experimental error in determination of nerve conduction velocity.  Arch Phys Med Rehabil 1972; 53: 362-372).  However, a 2003 study showed that with modern equipment, 6 cm is the new 10 cm and adequate error exists to measure velocity at 6 cm intervals (Landau ME, Diaz MI, Barner KC, Campbell WW.  Optimal distance for segmental nerve conduction studies revisited. Muscle Nerve 2003; 27: 367-369). 

Paradox is that measurements over greater distances decrease the experimental error but measurements over shorter distances increase the chance of detecting a lesion.  The type 2 error causes surgeons to use "clinical judgment" when deciding to operate.  Landau in another paper (Muscle Nerve 2003; 27: 570-574). determined the optimal distance to record velocities to minimize type I and type 2 erros is 4-6 cm.  Most ulnar lesions occur 3 cm proximal or distal to elbow.

2.  Cold elbow syndrome-- warmng the elbow can normalize some abnormalities.  (Landau ME, Barner KC, Murray ED, Campbell WW  Muscle Nerve 2005; 32: 815-817.

3.  Skinny elbow syndrome-- skinny patients are more likely to develop ulnar neuropathy.  However, they also are more likely to be overdiagnosed because rounder elbows are more likely to have a skin distance to nerve distance mismatch that overestimates NCV.  There is less margin for error and must avoid the temptation to overdiagnose.  Also must be aware of the possibility of overstimulation. 

4.  Short segment studies, called "inching" or per Kimura "centimetering," have been validated in the past ten years.  It should be considered as having additional diagnostic value in all cases of ulnar neuropathy (Visser LH, Beekman R, Franssen H.  Short segment nerve conduction studies in ulnar neuropathy at the elbow.  Muscle Nerve 2011; 43:627-635).

5.  High resolution sonography is emerging as a useful tool showing an increased cross sectinal area at the site of the compression.  However, it still is not considered definitive.

AANEM practice parameter in ulnar neuropathy at the elbow:

1.  The most important criterion is the presence of multiple internally consistent abnormalities.  Multiple abnormalities are more convincing than isolated abnormalities, which raises the possibility of "technical mishap."

    a.  Absolute motor NCV from across the elbow segment of < 50 m/s
    b.  An across the elbow segment of 10 m/s slower than the forearm segment
    c.  A decrease in the BE to AE CMAP peak amplitude of greater than 20 percent
    d.  A significant change in the CMPA potential at the AE site compared to the BE site

2.  If routine motor studies are inconclusive, the following may be used:

    a. NCS recorded from the FDI muscle
    b.  An inching or short segment study

3.  Needle examination should include the FDI, the most frequently abnormal muscle, and ulnar innervated forearm flexors.  Neither changes limited to the FDI nore sparing of the forearm flexors exclude an elbow lesion.  If ulanr innervated muscles are abnormal, the examination should be extended to include nonulnar C8/medial cord. lower trunk muscles to exclude brachial plexopathy and the cervical paraspinals to exclude radiculoapthy.

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