Let’s consider a left, partial recurrent laryngeal nerve injury - a common vocal impairment. Various injuries to the nerve supply of the larynx weaken some of the muscles and in this case the nerve injury involves the anterior branch of the recurrent laryngeal nerve. The thyroarytenoid muscle and the lateral cricoarytenoid muscle on the left side have only partial innervation.
We might hear the following acoustic effects. Beginning with comfortable speaking pitch, we notice that the patient is speaking at a higher pitch than typical. Whenever the vocal cords cannot approximate tightly from thyroarytenoid muscle lack of tension or from lack of medial rotation by the lateral cricoarytenoid muscle, the cricothyroid muscle tends to tighten, adding compensatory tension to the vocal cords. Adding tension increases the comfortable speaking pitch. If the unilateral paresis is significant enough, he may be speaking in falsetto full-time.
The voice quality is soft and the maximum phonation time is less than 10 seconds when producing sound at his comfortable speaking pitch. The softness in the voice is secondary to air leak. When the vocal cords cannot close tightly, air leaks through an incomplete closure. This may occur centrally between the membranous vocal cords and posteriorly between the vocal processes. Even when the lateral cricoarytenoid muscle remains functional, air leak occurs centrally through the paretic vocal cord, bowed from a lack of tension in the thyroarytenoid muscle as well as from atrophy and lack of mass within the vocal cord. The shortened maximum phonation time is secondary to air leak. A larger quantity of air is needed to produce vocal cord entrainment, so there is air wasting.
The lowest pitch that can be achieved is only produced softly and at a higher pitch than typical. By listening to a lower pitch, the examiner is removing the compensation provided by the cricothyroid muscle and the weak vocal cord will rest in a more lateral position as well as a more concave configuration, allowing more air leak. The highest pitch that can be produced would typically be less than normal and should be fairly clear. The weaker vocal cord still receives tension from the uninjured cricothyroid muscle but lacks intrinsic tension. The patient might be able to yell with moderate volume at a high pitch, but, at a low pitch, the yell will be either weak or there will be obvious flutter from the paretic vocal cord’s intrinsic lack of tension.
In the case of complete paralysis, the unilateral gap would be obvious during endoscopy. In this case, we have all the audible signs of weakness, but during a typical endoscopic view, no gap between the vocal cords can be perceived.
However, for the astute examiner, even a mild paresis of the anterior branch of the recurrent laryngeal nerve will also be visible. We know from vocal capabilites testing to look further. A stroboscopic examination of the vocal cords during attempted adduction at low pitch and soft volume will magnify the impairment. If necessary, even with supraglottic compensation (false cord squeeze), topical anesthesia of the larynx will allow placement of the endoscope between the false vocal cords during phonation and any gap between the membranous cords may be visualized. The endoscope may be angled beneath the arytenoids to visualize the asymmetric angles of closure between the vocal processes in a unilateral lateral cricoarytenoid muscle weakness, one vocal process remaining lateral while the normal side’s vocal process hyperextends past midline trying to reach the weakened vocal cord.
If there are abnormal findings listening to the voice, there must be corresponding visual findings on laryngoscopy. In the case of a paresis of the anterior branch of the recurrent laryngeal nerve (thyroarytenoid muscle, lateral cricoarytenoid muscle), most of the audible findings of impairment will be present at low pitch and low volume. An intact superior laryngeal nerve allows compensation from the cricothyroid muscle, which pulls the vocal cords closer together at higher pitch yielding clearer sound quality.