Is the Phoenix Sign Phenomenon Due to Vasodilation? a Double-Blinded, Randomized Controlled Trial Comparing Motor Function Recovery After Diagnostic Common Fibular Nerve Block with Lidocaine and Papaverine

A double-blinded, randomised, prospective controlled trial was conducted at US Neuropathy Centers in Marietta, GA, under the approval of the Kennesaw State University Institutional Review Board. Patients with weak dorsiflexion of the ankle or a drop foot were recruited from the existing clinical practice at US Neuropathy Centers. Cohort inclusion criteria also included a stable medical and medication status, ages 18-85, speaking English as the primary language, and ambulatory. Walking aids or ankle-foot orthosis were permitted.. Based on the inclusion criteria, only 20 patients were enrolled. Patients were recruited over a period of 1 year and two months from January 2021 to March 2022 with no requirement for follow-up. The enrolment phase was concluded as the study was originally designed as a pilot investigation, with recruitment limited to a single, small medical practice. A total of 20 patients were successfully enrolled within this setting.Participants were randomised by the throw of dice to injection of 0.3 cc of Papaverine HCl 10 mg/mL or 0.3 cc of 1% Lidocaine HCl adjacent to CFN in a room separate from patient room. Injection solutions were prepared by non-blinded assistants and were visually indistinguishable. Patients and physicians are blinded to drugs received. Motor strength of the Extensor Hallucis Longus (EHL), Tibialis Anterior (TA), and Extensor Digitorum Longus (EDL) muscles were tested. Strength was rated according to the Medical Research Council Manual Muscle Testing scale 0-5. The CFN at the fibular neck was identified under high-resolution ultrasound imaging. The injection site was prepared with alcohol, and ethyl chloride was used to anaesthetise the skin. A 30-gauge 1-inch needle was inserted adjacent to, but not within, the CFN. The infiltrate was delivered at the level of the fibular neck just proximal to where the CPN coursed beneath the deep fascia of the peroneus longus. This was immediately adjacent to the first known anatomical site of compression of the CPN. 0.3cc of the drug was delivered.

The investigator then waited approximately five minutes and re-tested motor strength of the Extensor Hallucis Longus (EHL), Tibialis Anterior (TA), and Extensor Digitorum Longus (EDL) muscles manually.