Neurophysiological monitoring of S1 root function during microsurgical posterior discectomy using H-reflex and spinal nerve root potentials.

STUDY

DESIGN

Tibial H-reflex and epidural recordings of the spinal nerve root potentials (SRP) of the S1 root were performed intermittently during microsurgical posterior discectomy.

OBJECTIVE

To obtain reversible conduction dysfunction of dynamically retracted spinal nerve root S1 in an on-line manner to control the surgical manipulation and prevent iatrogenic root injury.

SUMMARY OF BACKGROUND DATA: Excessive force and duration of nerve root retraction during posterior lumbar discectomy may result in additional nerve root injury and battered root syndrome.

METHODS

The S1 root was exposed by fenestration and the epidural electrodes were placed along the root above and below the compression site in 12 patients. H wave and SRP changes were recorded intermittently during discectomy and related to different manipulative phases of surgery. The desynchronisation index of SRP was computed off-line as a ratio between a/b, where a represents the positive deflection of the signal from the zero line and b represents peak-to-peak amplitude.

RESULTS

The reflex muscle evoked potential from the soleus muscle (H wave), SRP1 (ascendant) and SRP2 (descendant volley) paralleled the surgical manipulation. The amplitude of the H wave significantly decreased and the desychronisation index of SRP1 (later evaluated in 8/12 patients) significantly increased after mobilization of the root with a dissector and in the late phase of intervertebral disc evacuation (P < 0.001), indicating conduction dysfunction related to surgical manipulation of the root.

CONCLUSION

A good surgical outcome as shown by preserved Achilles tendon and H reflexes was related to the immediate return of H wave and SRP on stopping the harmful manipulation, and preservation of the H wave and SRP at the end of surgery.SRP monitoring brings up additional information concerning initial root injury and cumulative manipulative effects. The technique of dynamic nerve root retraction in posterior lumbar discectomy can therefore be made safer.