On the Physiology of the Sensory-Collapse Test.

The sensory-collapse test (formerly the scratch-collapse test) is a physical examination finding describing a momentary inhibition of external shoulder rotation following light stimulation of an injured nerve in the ipsilateral limb. Similar to other physical examination tests designed to interrogate nerve compression, such as the Phalen or Tinel tests, its test characteristics demonstrate variation. There remains speculation about the test's existence and anatomic basis. The literature of mammalian reflex physiology was reviewed with an emphasis on the sensory pathways from the upper extremity, the extrapyramidal system, and newly discovered pathways and concepts of nociception. A clear reflex pathway is described connecting the stimulus within an injured nerve through the afferent pathways in the fasciculus cuneatus in the spinal cord directly to the lateral reticulospinal tract, resulting in the inhibition of extensor muscles in the proximal limb (eg, shoulder) and activation of the limb flexors by acting upon alpha and gamma motor neurons. The sensory-collapse test represents a reflex pathway that teleologically provides a mechanism to protect an injured nerve by withdrawal toward the trunk and away from the noxious environment.