The presence and prognosis of nerve pathology following whiplash injury: a prospective cohort study

Whiplash Associated Disorders (WAD) affect 20-50 million individuals globally each year, with up to 50% developing persistent pain. WAD grade II (WADII) is the most common type and is characterised by neck symptoms and musculoskeletal signs without apparent nerve injury on routine diagnostic testing. However, emerging evidence suggests nerve pathology may be present in some people with WADII. This longitudinal cohort study aimed to comprehensively investigate the presence, temporal patterns, and prognostic value of nerve pathology and neuropathic pain in acute WADII. A prospective longitudinal cohort study was conducted with 129 acute participants with WADII (median age 36.0 years, 58% female) and 36 healthy controls (median age 39.0 years, 61% female). Participants with WADII were recruited within four weeks of injury from local emergency departments. Data collection included bedside neurological assessments, quantitative sensory testing (QST), intraepidermal nerve fibre density, and serum neurofilament light chain (NfL) concentrations. Follow-up assessments were conducted 6- months after injury. Signs of neuropathic pain were present in 65% (84/129) of acute participants with WADII and persisted in 32% (21/66) 6-months post-injury. Bedside neurological assessment revealed somatosensory loss of function was present in 54% (70/129) of acute participants with WADII reducing to 25% (17/67) at 6-months post-injury. QST demonstrated significantly reduced cold, warm, thermal sensory limen, mechanical, and vibration detection thresholds in acute WADII compared to controls (d>0.47). Acute loss of function in at least one QST parameter was present in 67.6% (85/126) of participants with WADII. At 6-months, participants with WADII showed persistent hypoaesthesia to warm, thermal sensory limen, and mechanical detection thresholds, and decreased mechanical pain and pressure pain sensitivity compared to controls (d>0.44). These functional neurological changes were accompanied by elevated serum neurofilament light chain levels in acute participants with WADII compared to controls (d=-0.52 (95% confidence interval -0.94, -0.10). Intraepidermal nerve fibre densities at the index finger were not significantly different between groups. However, dermal MBP+/PGP+ myelinated nerve bundles at the index finger were reduced 6-months post-injury in participants with WADII compared to controls (d=0.69 (0.26, 1.11). Multivariable linear regression suggested bedside tests for hypoaesthesia at the index finger were prognostic for whiplash-related upper quadrant pain 6-months post-injury (r2=0.13, p=0.02). In conclusion, two-thirds of participants with acute WADII initially exhibited signs of neuropathic pain and nerve pathology. At the 6-month follow-up, neuropathic pain persisted in one-third of participants with WADII, while nerve pathology persisted in two-thirds. These findings challenge the traditional musculoskeletal classification of WADII and underscore the need for targeted neurological assessments and treatment.