Scratch collapse test is used to supplement other information gathered during the history and physical examination. It can provide one more clue or confirmation of the level of nerve injury, in addition to providing insight to the hierarchy of nerve injuries within multiple levels.
• Patient seated/standing, arms at sides with elbows flexed to 90°, wrists at neutral and fingers extended.
• Examiner lightly scratches patient’s skin at area of nerve injury/compression then applies force against patient’s dorsal forearms in the direction of internal rotation. The patient resists in the direction of external rotation.
• If the patient has nerve injury/compression at that site, they will temporarily lose ability to resist the internal rotation force and ‘collapse’ their arms in that direction.
Adjunct maneuvers--add ‘freezing’ to fetter out a second downstream level of nerve compression:
• For example, in ulnar nerve compression at both the elbow (primarily) and the wrist (secondarily), you may want to operate on both at the same time so that you do not have to return for a second procedure, as relatively little risk or additional downtime is added with the relatively minor wrist-level procedure. Scratch-collapse may be positive at the elbow, but negative at the wrist. A lesser degree of compression is unmasked by ‘freezing’ (ethyl chloride spray) the elbow, with subsequent positive scratch collapse test at the wrist. See the patient example video.