Testing of iatrogenic lingual nerve injury using a novel psychophysical method and oral reflexes

In a case of long-term sensory loss in the tongue following third molar extraction, a novel, efficient and effective psychophysical test was used. This initial test, which includes the presentation of pairs of a real and fake stimulus, and a forced-choice response, was applied to determine whether relatively thick or thin afferent fibres of the lingual nerve were affected. The results suggested that thick fibres on the right tongue side were mainly affected. Sensory loss was confirmed and its extent assessed by a standard test determining thresholds of light touch. Furthermore, sensory function was determined by an objective test based on inhibitory reflexes in masseteric electromyographic activity following electrical stimulation of oral tissue. Reflex features that are important for within-patient diagnosis are a difference between the injured and control sides in latency of the first reflex, and also in depth-contrast in signal amplitude between both sides at the post-stimulus time of the trough of an inhibition on the uninjured side. In agreement with the findings from the psychophysical tests, the finding of the absence of an early component of both inhibitions suggested sensory loss related to dysfunction of fast-conducting, relatively thick afferent fibres of the lingual nerve. Apart from being used for medicolegal reasons, the objective reflex test may have conclusive prognostic value or may influence surgical therapeutic decisions.