Structural and functional differences between neuropathy with and without pain?

We aimed to find functional and structural differences in neuropathy between patients with and without chronic pain following nerve injury. We included 30 patients requiring hand surgery after a trauma, with 21 reporting chronic pain for more than one year after the injury, while 9 did not suffer from injury-related chronic pain. We assessed mechanical sensitivity, thermal thresholds, electrically induced pain and axon reflex erythema and cutaneous nerve fiber density in skin biopsies of the injured site and its contralateral control. Epidermal fiber density of the injured site was reduced similarly in both patient groups. Thresholds for cold and heat pain and axon reflex areas were reduced in the injured site, but did not differ between the patient groups. Only warmth thresholds were better preserved in the pain patients (35.2 vs. 38.4°C). Neuronal CGRP staining did not reveal any difference between pain and non-pain patients. Epidermal innervation density correlated best to warmth detection thresholds and deeper dermal innervation density to the area of the axon reflex erythema. No specific pattern of subjective, functional or structural parameters was detected that would separate the neuropathy patients into pain and non-pain patients. Specific staining of additional targets may help to improve our mechanistic understanding of pain development.