Extensive motor axonal misrouting after conservative treatment of obstetric brachial plexus lesions

The aim of this cross-sectional study was to assess systematically motor function and motor misrouting in adults with conservatively treated obstetric brachial plexus lesion (OBPL).

Seventeen adults with OBPL (median age 38y; five males, 12 females) and 16 comparison participants (median age 26y; eight males, eight females) were investigated. Motor function in the group with OBPL was assessed through passive and active motion, muscle strength of the deltoid, biceps, and triceps muscles, and Mallet aggregate score and five subscores. Motor misrouting was quantified by electrically stimulating each of 10 arm muscles and recording activity from the other nine in response to this. Motor function and motor misrouting were statistically analysed using the Mann-Whitney U test and Spearman's correlation coefficient.

Motor function testing showed excellent strength but poor functional Mallet scores. Participants with OBPL had significantly more motor misrouting than comparison participants (Mann-Whitney U=31.5 [df=28], p<0.001, median difference=-4.00, 95% confidence interval [CI]=-7.00 to -1.00). Most misrouting was observed when stimulating the biceps (Mann-Whitney U=38.5 [df=31], p<0.001, median difference=-3.00, 95% CI -3.00 to -1.00), deltoid (Mann-Whitney U=68.5 [df=31], p=0.003, median difference=-1.0, 95% CI=-4.00 to 0.00 <0.001) and brachioradialis muscles (Mann-Whitney U=72.0 [df=31], p=0.002, median difference <0.001, 95% CI=-3.00 to 0.00 <0.001). There were no significant correlations between the presence of motor misrouting and impairment of motor function.

There is extensive motor misrouting in conservatively treated OBPL. The presence of this, in addition to motor functional impairment, suggests that motor misrouting should be further studied in OBPL.