Aim Treatment decisions in obstetric brachial plexus lesions are often based on clinical paralysis of elbow flexion at 3 months of age, when electromyography (EMG) is misleading because motor unit potentials (MUPs) occur in clinically paralytic muscles. We investigated whether EMG at 1 week or 1 month identifies infants with flexion paralysis at 3 months, allowing early referral.
Method Forty-eight infants (27 females, 21 males) were prospectively studied. The presence or absence of flexion paralysis at around 1 week (median 9d; range 5–17d), 1 month (median 31d; range 24–53d), and 3 months of age (median 87d; range 77–106d) was noted for clinical (shoulder external rotation, elbow flexion, extension, and supination) and EMG parameters (denervation activity, MUPs and polyphasic MUPs in the deltoid, biceps, and triceps muscles).
Results At 1 month, the absence of biceps MUPs had a sensitivity of 95% for later flexion paralysis, and absence of deltoid MUPs had a sensitivity of 100% for flexion paralysis; the false-positive rates for the same findings were 21% and 33% respectively. EMG at 3 months was highly misleading as MUPs were seen in 19 of 20 clinically paralytic biceps muscles.
Interpretation EMG at 1 month can identify severe cases of flexion paralysis for early referral EMG of the biceps at 3 months is highly misleading; the discrepancy between the EMG and clinical testing may be due to abnormal axonal branching and aberrant central motor control.