Quantifying the Phoenix Sign: A Double-Blind Pilot Study on Lidocaine and Papaverine-Induced Vascular and Motor Changes After Common Peroneal Nerve Block in Diabetic Neuropathy

Background: Focal entrapment of the Common Peroneal (Fibular) nerve (CPN) is the most frequent lower-extremity nerve entrapment yet it can be difficult to diagnose clinically. The Phoenix Sign—an increase in extensor hallucis longus (EHL) motor strength following lidocaine injection—may assist diagnosis. Additional observed effects include improved arterial perfusion and Doppler waveforms. Methods: In this double-blinded, randomized small pilot study, only four patients (N = 4) with diabetic peripheral neuropathy underwent bilateral peripheral nerve blocks with lidocaine or papaverine. The first leg to be tested was randomized; the contralateral leg received a different agent that was randomized initially. Pre- and post-block assessments included motor strength, Doppler velocity of dorsalis pedis and posterior tibial arteries, and near-infrared spectroscopy for microvascular perfusion. Results: All patients demonstrated increased EHL motor strength after injection with either agent. Doppler waveforms of the dorsalis pedis artery improved: lidocaine produced a 151.7% increase in blood flow velocity (p = 0.03), whereas papaverine produced a 16.8% increase (p = 0.19). Posterior tibial artery flow increased by 37.4% with lidocaine (p = 0.06) and 13.9% with papaverine (p = 0.33), but neither was statistically significant. No changes in oxygen saturation, oxyhemoglobin, deoxyhemoglobin, or total hemoglobin were observed using near-infrared spectroscopy. The consistency of motor responses across subjects supports the validity of the Phoenix Sign as a diagnostic tool. Conclusions: Peripheral nerve blocks with lidocaine or papaverine improved motor strength and macrovascular function in patients with diabetic peripheral neuropathy, though microvascular changes were not detected. These preliminary findings are consistent with the Phoenix Sign phenomenon and support further study as a potential clinical indicator. While these preliminary findings indicate support as a diagnostic tool, they are preliminary and hypothesis-generating for evaluating the Phoenix Sign as a potential clinical indicator of CPN entrapment and highlight the need for larger studies to evaluate vascular responses. Trial Registration: NCT06919289 (retrospectively registered 8 April 2025).