Hypoglossal-facial anastomosis alters excitability of hypoglossal motoneurones in man.

In 6 patients who had undergone a hypoglossal-facial anastomosis, showing clinical signs of recovery, a new short-latency trigemino-hypoglossal reflex of the R1 blink reflex type was demonstrated, implying a heterotopical sprouting of trigeminal neurones towards hypoglossal motoneurones. We therefore studied the motoneuronal excitability for this R1 reflex response in order to compare the convergent properties of facial motoneurones (normal side) with those of the hypoglossal motoneurones (operated side) with the use of the classical double-shock and variable interstimulus delay (conditioning-test stimulus) technique. On normal side, conditioning stimuli (ipsi- or contralateral infraliminar supraorbital nerve) produced a clear-cut facilitation of the R1 blink reflex response within a 30-80 ms interstimulus time interval. By contrast a similar procedure remained without any effect on the R1 blink reflex response mediated via the trigeminal-hypoglossal reflex arc. These data indicate that despite the heterotopical sprouting of some axons of neurones from the principal trigeminal nucleus towards the XIIth nucleus, those hypoglossal motoneurones involved in the neoformated trigemino-hypoglossal reflex arc remain totally inexcitable by other trigeminal afferents and thus appear unable to ensure the physiological function of the normal blink reflex.