Introduction

A substantial barrier to the trial of new therapies for the treatment of peripheral nerve injuries is the absence of responsive outcome measures that correlate with functional assessments of recovery. Motor Unit Number Estimation (MUNE) is a neurophysiological test that quantifies the number of functional Motor Unit’s (MU’s) innervating a muscle. By extension, MUNE has been employed as an outcome measure in a number of pathologies associated with muscle denervation. However, application in muscle renervation is not well documented. Therefore, this study aimed to quantify the recovery of functional MU’s following a standardised surgical model of elbow flexor muscle renervation (Oberlin’s nerve transfer) using MUNE. Furthermore the relationship that of MUNE has with objective and subjective measures of functional recovery were investigated.

Methods

Patients who underwent Oberlin’s nerve transfer were identified during a retrospective review of the institute database at the Peripheral Nerve Injury Unit, Royal National Orthopaedic Hospital.  A total of 15 patients, 14 men and 1 female, with a mean age of 36.7 years (23 to 59 years) were recruited into the study after fully informed written consent. The patients were then followed up for Spike Triggered Averaging (STA) MUNE assessment at a median follow-up time of 543 days (90 to 1698 days). STA MUNE was performed in the injured and uninjured arm of each participant allowing the percentage of recovered MU’s to be quantified.

Results

The mean value of MUNE in uninjured biceps muscles was 186 (+/- 46) which is comparable to previous reports. Focusing on the nerve injured arms, MUNE demonstrated a statistically significant positive correlation (r2=0.71, p<0.01) with time following surgery. Second, MUNE demonstrated statistically significant and positive correlations with objective measures of muscle fatigue improvement (r2=0.86, p<0.01) and subjective measures of elbow flexion (Stanmore Percentage of Normal Elbow Assessment) (r2=0.94, p<0.01). A recovery of approximately 29% of MU’s was predictive of the recovery of Medical Research Council grade 3 or greater.

Conclusions

In summary, the findings of this study suggest that MUNE is a responsive outcome measure that is related to objective and subjective measures of muscular function. This has the potential to aid the clinical translation of new treatments and to act as a prognostic marker for peripheral nerve injuries.