Predictive Reliability of the Phoenix Sign for the Outcome of Common Fibular (Peroneal) Nerve Decompression Surgery

Abstract

A positive Phoenix sign occurs when a patient, with a suspected focal nerve entrapment of the Common Fibular (Peroneal) Nerve (CFN) at the level of the fibular neck, demonstrates an improvement in dorsifexion after an ultrasound guided infiltration of a sub-anesthetic dose of lidocaine. Less than 5 cc's of 1% or 2% lidocaine is utilized and the effect is seen within minutes after the infiltration, but usually lasts only 10 minutes. This effect may be due to the vasodilatory action of lidocaine on the microcirculation in the area of infiltration. This nerve block has significant diagnostic utility as it is highly specific in the confirmation of true focal entrapment of the CFN, has high predictive value for a patient who may undergo surgical nerve decompression if they have demonstrated a positive Phoenix Sign, and may help in the surgical decision-making process in patients who have had a drop foot for many years but still may regain some motor function after decompression. In this retrospective review, 26 patients were tested, and 25 of this cohort demonstrated a Positive Phoenix Sign (an increase in dorsiflexion strength of the Extensor Hallucis Longus muscle (EHL)). One patient had no response to the peripheral nerve block. Of the 25 patients who demonstrated a positive "Phoenix Sign" and underwent nerve decompression of the CFN, and 25 (100%) showed an increase in dorsiflexion strength of the EHL after nerve decompression surgery of the CFN. The one patient in this cohort who did not demonstrate any improvement in dorsiflexion of the EHL after the nerve block did not have any improvement after surgery.

The role of ultrasound-guided perineural injection of the tibial nerve with a sub-anesthetic dosage of lidocaine for the diagnosis of tarsal tunnel syndrome

“…Barrett et al ( 27 ) further support the hypothesis that the effect is the result of vasodilation, as they showed in several patients whose motor strength improved with plain lidocaine; however, when these same patients were infiltrated with lidocaine and 1:200,000 epinephrine (vasoconstrictor), the authors did not observe the Phoenix Sign.…”

“…In contrast, Barrett et al ( 27 ) and Nirenberg ( 28 ) obtained satisfactory and predictive results with lidocaine in surgical treatment, postulating that the patients who responded to the test with lidocaine improved with surgery.…”

“…Barrett et al ( 27 ) defined the positive “Phoenix Sign,” i.e., an increase in the motor force of the extensor hallucis longus after ultrasound-guided common peroneal nerve block with a dose of <0.5 ml and waiting 3–5 min to confirm clinical improvement.…”

“…Our objective findings confirm that lidocaine at sub-anesthetic doses has a neurophysiological effect on the nerve, leading to improved conduction velocity. This is most likely an intraneural or perineural vasodilator effect that is not related to the relaxation of the muscle ( 27 , 28 ).…”

“…These findings were not present prior to the administration of the ultrasound-guided infiltration. The authors named this phenomenon, the “Phoenix Sign,” since the common peroneal nerve that previously did not function “rose from the ashes” like the mythical Greek bird, and the patients' drop foot resolved temporarily ( 26 , 27 ).…”