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.
Highlights
In peripheral nerve surgery, the use of diagnostic blocks with local anesthetics, and lidocaine has been well established and widely used to determine pain generator location, and to determine if pain reduction may be obtained for the patient after a planned surgery [1] [2]
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 Common Fibular (Peroneal) Nerve (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
A positive “Phoenix Sign” exists when there is an increase in motor strength of the extensor hallucis longus (EHL) after infiltration of a small amount of lidocaine, usually 0.1 - 0.3 cc’s around the Common Fibular Nerve at the level of the fibular neck under high resolution sonographic guidance [3]
Summary
The use of diagnostic blocks with local anesthetics, and lidocaine has been well established and widely used to determine pain generator location, and to determine if pain reduction may be obtained for the patient after a planned surgery [1] [2]. We have seen patients who have had a complete drop foot for many years—well past the point on the theoretical timeline where it would be believed by any neurologist or peripheral nerve specialist that the motor endplates within the muscle fiber would still be present allowing for a return of motor function after a successful neurolysis, to have more than several grades of motor strength improvement after this diagnostic test. Because of these findings, many patients who had been told previously that there was nothing that could be done to restore normal lower extremity function have complete, or greatly improved dorsiflexion—and do not have a drop foot solely because of the findings from this test
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