IA-only paresis refers to weakness or paralysis of the larynx’s interarytenoid (IA) muscle—an unpaired muscle spanning between the bodies of both arytenoid cartilages—but with normal function of the other muscles in the larynx. The IA muscle helps to bring the posterior commissure together for voice production and, more specifically, to bring the bodies or “heels” of the arytenoid cartilages on each side simultaneously to the midline. The following are indicators of IA-only paresis:
- Movement: The vocal cord opens normally for breathing. From a distance, it can appear to close normally for voicing, but more intense and up-close inspection shows a persistent posterior commissure opening not only for voicing but also at the moment of cough and Valsalva maneuver. Without confirming that the heels of the arytenoids cannot come together regardless of task, the possibility of a functional posturing abnormality (such as seen for nonorganic voice disorders) cannot be ruled out. If voice change has occurred abruptly, and the above criteria pertain, IA-only paralysis can be considered; if of very gradual onset, the clinician will first want to rule out a deformity of the cricoarytenoid joints, such as can be seen with cricoid chondrosarcoma.
- Position and appearance: Position is normal during breathing, but the posterior commissure cannot be brought to full closure whether during voicing, cough, or Valsalva maneuver.
- Appearance during voicing (under strobe lighting): Vibration of the vocal cords can be normal, though, again, the persistent posterior commissure gap will be seen. The tone and bulk of the vocal cords themselves are normal.
- Voice quality: Air-wasting, and with shortened phonation time, but without the luffing and diplophonia often apparent when the thyroartyenoid (TA) muscle is also paralyzed.