Musculoskeletal growth in the upper arm in infants after obstetric brachial plexus lesions and its relation with residual muscle function.

Aim: Denervation after obstetric brachial plexus lesion (OBPL) is associated with reduced musculoskeletal growth in the upper arm. The aim of this study was to investigate whether reduced growth of upper arm flexor and extensor muscles is related to active elbow function and humeral length.

Method: In this study, 31 infants age less than 6 months (mean age 4.3mo; range 2.1-5.9mo; 17 males; 14 females;) with unilateral OBPL (Narakas class I, 19; II, 3; III, 2; and IV, 7) treated at the VU medical centre, in whom neurosurgical reconstruction was considered were prospectively studied using magnetic resonance imaging of both arms at a mean age of 4.3 months. Humeral length and the cross-sectional area (CSA) of elbow flexor and extensor muscles were measured in both upper arms. Paresis of elbow function was estimated when the infants were a mean age of 4.5 months using the Gilbert score.

Results: Both flexor and extensor CSAs were significantly smaller on the affected side than on the unaffected side (88% [SD 32%], p=0.020, and 88% [SD 24%], p=0.001 respectively), as was humeral length (96% [SD 7%], p=0.005) (unaffected side 100% in all cases). There was no relation between the reduction in flexor and extensor CSA and residual muscle function. In 17 out of 31 patients, hypertrophy of flexor and/or extensor muscles was observed. Humeral length was not related to muscle parameters.

Interpretation: Denervation has different effects on muscle growth and function as well as bone growth. In young infants with an OBPL, muscle size is not a predictor of muscle function. Flexion contractures of the elbow later in childhood may not be explained by a dominance of flexor muscle mass in infants.