Elbow Block Test: A Clinical Sign of C7 Nerve Injury in Residual Brachial Plexus Birth Injury

Concomitant limitation of shoulder abduction and external rotation is commonly seen in residual upper plexus brachial plexus birth injury. However, a subset of patients presents with preserved external rotation but diminished abduction, suggesting a disproportionate limitation. In these patients, application of an elbow extension splint improves abduction - a clinical observation termed the elbow block test (EBT). This study evaluates the clinical significance, etiological basis and pattern of brachial plexus involvement associated with EBT.

A total of 155 children (mean 5.4 years, range 2-17) were evaluated between 2019 and 2024. Active shoulder abduction was measured with and without an elbow extension splint; improvement ≥30° with the splint on defined a positive EBT. Clinical parameters, range of motion, and muscle strength were recorded. Surface EMG analysis was performed to see the muscles activation patterns associated with the EBT.

Sixty-eight patients (44%) demonstrated a positive EBT. These patients showed significantly greater passive external rotation (70° vs 22°, P < .05) and higher incidences of posterior shoulder capsular contracture (65% vs 17%), forward trumpeting (81% vs 23%), and triceps weakness (48% vs 15%) compared to negative EBT patients. EMG study revealed cocontractions between shoulder abductors and elbow flexors.

The EBT is a simple clinical tool which identifies a distinct subgroup of brachial plexus birth injury patients with C7 nerve injury, which limits shoulder abduction secondary to subscapularis weakness and altered biceps dynamics. Recognition of this clinical sign is crucial as this can influence the treatment.

Diagnostic/Prognostic III.