The double crush hypothesis was first formulated in 1973 and states that axons that have been compressed at one site become especially susceptible to damage at another site.
This theory was originally described by Upton (1973) in a study of 115 patients. A comprehensive electromyographic study of 115 patients with carpal-tunnel syndromes or lesions of the ulnar nerve at the elbow was done. In 81 cases there was electrophysiological evidence, often supported by clinical symptoms, of associated neural lesions in the neck. This association was not thought to be fortuitous, but rather the result of serial constraints of axoplasmic flow in nerve fibres.
The existence of double crush syndrome was further substantiated by Massey's (1981) study of nineteen cases of carpal tunnel syndrome co-existing with a cervical radiculopathy.