A pilot study of elbow stiffness as a possible measure for axonal misrouting in obstetric brachial plexus lesions

Obstetric Brachial Plexus Lesion (OBPL) is a closed traction injury of the
nerves of the arm acquired during birth. Functional regeneration in OBPL is
much more complex than a peripheral nerve injury in an adult. In infants as in
adults, recovery depends on the number of outgrowing axons, but two factors
complicate matters in infants: these are 'misrouting' and the response of an
immature brain that is still developing motor programmes. If the 'tubes'
surrounding the axons are disrupted, an outgrowing axon does not necessarily
end up in the intended tube. If it does not, the sprouting axon can end up in
the wrong muscle, even one with an antagonistic function. The problem is
compounded when the sprouting axons form multiple branches that may end up in
completely different muscles. This so-called misrouting can explain
co-contractions, in which shoulder abduction and elbow flexion, or elbow
flexion and extension become irreversibly linked.
To facilitate the formation of central motor programmes co-contraction due
to misrouting of the triceps during arm flexion can temporarily be eliminated
by injecting botulinum toxin type A in the triceps. Botulinum A is believed to
have effect but the lack of prospective, randomized controlled trials shows
that botulinum A treatment for children with OBPL is still at an experimental
stage. A systematic literature review has shown that there is no consensus yet
on the exact indication criteria or outcome measures for application of
botulinum A. In order to perform such a trial, a method for the measurement of
the amount of misrouting should be defined.
We have previously assessed successfully motor misrouting in a group of
eighteen adult OBPL subjects after comparing them to sixteen controls. The
method used was based on the principle that misrouting can be assessed by
electrically stimulating any branch of a neuron which will excite all its
branches. This method was primarily aimed at understanding the branching
patterns of misrouting, not at its mechanical consequences. The aim of this
study is to evaluate elbow stiffness as a new method measuring the mechanical
constraints as a result of misrouting in OBPL.

The objective of this study is to measure the stiffness of the elbow as a
measure for axonal misrouting. This method can later aid physicians in
determining whether to use botulinum A or not in OBPL.

Pilot study which compares a group of OBPL patients with a control group. The
subjects are requested to hold the handle of a joint manipulator which applies
small movements on the handle. An EMG is recorded of their biceps and triceps
muscle in that arm which enables the measurement of the stiffness of the elbow
which can be used as a measure for axonal misrouting.

No risk is expected. The burden consists of the 1 - 1,5 hour duration of the
investigation.