title: Distance measure error induced by displacement of the ulnar nerve when the elbow is flexed.
abstract: OBJECTIVE
  To determine the presence of ulnar nerve displacement at the elbow when it is flexed and to determine its effect on distance measurements using the conventional measurement method for nerve conduction studies (NCSs).
  
  
  METHODS
  Comparing the ultrasonography-assisted distance measurement method with the conventional measurement method.
  
  
  METHODS
  An electrodiagnostic laboratory at a university hospital.
  
  
  METHODS
  Seventy-eight elbows of 39 healthy volunteers.
  
  
  METHODS
  We used high-resolution ultrasonography in real time. Based on sonographic searching, we marked 3 points on the skin through the course of the displaced ulnar nerve when the elbow is flexed: (1) point A, 7 cm above the elbow (from the midpoint between the medial epicondyle tip and olecranon in the postcondylar groove [point M]); (2) point B, 3 cm below the elbow; and (3) point C, the point closest to the medial epicondyle tip.
  
  
  METHODS
  Distance measurements between points A, B, and C were taken. These values were compared with measurements obtained through conventional measurement methods.
  
  
  RESULTS
  Ulnar nerve displacement occurred in 24.3% (19/78) of the elbows; approximately 20.5% (16/78) were subluxation, and 3.8% (3/78) were dislocation. In the ulnar nerve displacement group, the distance between points A and C was 5.84+/-0.33 cm (range, 5.10-6.30 cm), and the distance between points B and C in the flexed position was 3.35+/-0.19 cm (range, 3.10-3.70 cm). When the conventional distance measurement was used, the ulnar nerve conduction velocity across the elbow was overestimated by approximately 5.33+/-2.29 m/s in the ulnar nerve displacement group.
  
  
  CONCLUSIONS
  This distance measurement error may be responsible for the decreased sensitivity found in NCSs that test for ulnar neuropathy at the elbow. If the NCS results are normal in patients who have clear symptoms of ulnar neuropathy, the possibility of ulnar nerve displacement at the elbow should be considered, and further investigation with ultrasonography would be beneficial.
authors.affiliation: Division of Physical Medicine & Rehabilitation, Stanford University School of Medicine, CA, USA.
authors.forename: Byung-Jo
authors.initials: BJ
authors.lastname: Kim
authors.forename: Elaine S
authors.initials: ES
authors.lastname: Date
authors.forename: Sang-Heon
authors.initials: SH
authors.lastname: Lee
authors.forename: Joon Shik
authors.initials: JS
authors.lastname: Yoon
authors.forename: Soo Yeun
authors.initials: SY
authors.lastname: Hur
authors.forename: Sei Ju
authors.initials: SJ
authors.lastname: Kim
mesh_terms: D000328:Adult; D004551:Elbow Joint / Q000000981:diagnostic imaging / Q000294:innervation*; D004568:Electrodiagnosis; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009431:Neural Conduction; D014459:Ulnar Nerve / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D020424:Ulnar Neuropathies / Q000175:diagnosis*; D014463:Ultrasonography