Somatosensory Misalignment: Persistent Referred Sensation After Intercostal-to-Musculocutaneous Nerve Transfer

Referred sensation (RS) is described as the sensation evoked in the skin areas distinct from the site of stimulation. The study sought to identify and map RS in participants with traumatic brachial plexus injury (TBPI) through a standardised assessment.

Conducted as an observational study, Experiment 1 involved 19 participants who underwent RS screening by stimulating 22 specific skin areas across both upper limbs, the neck and the face with a cotton swab. In Experiment 2, a comprehensive RS mapping was performed using Semmes-Weinstein monofilaments on the reinnervated forearm of three subjects who exhibited upper limb-chest RS. One participant underwent RS mapping over 6.3 years.

RSs were reported in three of four participants who underwent intercostal-to-musculocutaneous nerve (ICN-MCN) transfer and in two participants who underwent ulnar-to-musculocutaneous nerve (UN-MCN) transfer. RS within the original donor's nerve territory ('right-way') was observed following ICN-MCN transfer. Systematic mapping of 'right-way' RS after ICN-MCN transfer revealed a distinctive distribution pattern in the forearm and chest, lacking clear topographical organisation. Remarkably, longitudinal assessment of a single participant revealed a gradual, scattered expansion of RS over time.

To our knowledge, this study provides the first systematic mapping of RS in TBPI participants. Persistent 'right-way' RS following ICN-MCN transfer suggested that the significant distance between donor and recipient nerve representations in the primary somatosensory cortex (S1) may limit plastic reorganisation and restrict the disentangling of sensations between the forearm and chest. This insight underscores the complexity of nerve regeneration and opens new avenues for therapeutic approaches focused on sensory function.