Clinical characteristics and results after conservative treatment or interfascicular neurolysis of 100 limbs with spontaneous anterior interosseous nerve palsy: A prospective Japanese multicenter study.

Background: Spontaneous anterior interosseous nerve (AIN) palsy is characterized by the sudden onset of upper limb pain followed by weakness of muscles mainly innervated by the AIN. Although this palsy is conventionally treated conservatively, interfascicular neurolysis to release hourglass-like fascicular constrictions has been recommended. The present study aimed to establish the clinical characteristics and treatment strategy for this condition.

Methods: One hundred limbs with spontaneous AIN palsy from 24 Japanese institutions were treated either conservatively or with interfascicular neurolysis according to the patient's intention. Patients were followed periodically from 3 months to either recovery or ≥36 months after onset. Limbs recovering to manual muscle testing grade 4 or better in both the flexor pollicis longus and flexor digitorum profundus of the index finger were rated as Good recovery, with all other results classified as Poor recovery. We performed logistic regression analysis to determine the independent factors associated with Good recovery.

Results: Good recovery was achieved in 39 of 49 limbs treated conservatively and 44 of 51 limbs by interfascicular neurolysis. All 26 limbs with conservative treatment displaying ≥1 grade of manual muscle testing improvement within 6 months after onset achieved Good recovery. All 23 limbs with Group I palsy, in which weakness was recognized only in muscles innervated by the AIN, obtained Good recovery with interfascicular neurolysis regardless of when it was performed. In the 59 limbs with no recovery at 6 months by conservative treatment, Good recovery was achieved in 30 of 36 limbs by subsequent interfascicular neurolysis and 13 of 23 limbs continuing conservative treatment. In these 59 limbs, interfascicular neurolysis and Group I palsy were significantly associated with Good recovery.

Conclusions: Conservative treatment is recommended within 6 months following symptom onset, after which time interfascicular neurolysis is advisable for cases of no improvement.