Voice Science Talk: Paresis

           

So, some of you may be asking:  What the heck is paresis anyway?  Here’s the definition straight from my text book, Anatomy and Physiology for Speech, Language, and Hearing by Seikel, King, and Drumright:  “Paralysis refers to loss of voluntary muscle motor function, whereas paresis refers to weakness.”  In other words, paralysis is the complete loss of motor function and paresis is a partial loss of motor function, or partial disruption in the nerve signal.  So, why would loss of nerve function be a big deal in vocal function?  To begin answering that question, let’s review some basic anatomy and physiology.

The voice functions through the combined effort of laryngeal muscles and aerodynamics, which is called the myoelasticaerodynamic theory of vocal fold vibration.  In this theory, the intrinstic musculature of the larynx helps to close, or adduct, the vocal folds, through the adductor muscles at the onset, or beginning, of vocal sound, aerodynamic properties of subglottic air pressure and the Bernoulli effect (http://en.wikipedia.org/wiki/Bernoulli_effect if you want to get your physics on), keep the vocal folds in motion during voice use, and the muscular action takes place again at the end of voicing to separate the vocal folds past their usual resting position so that the air stream for inhale is more open (especially important for a singer).  (I’m not going to go too far into the aerodynamic principles here, but I might in future posts, so keep checking back in for that info.)  I will focus on the musculature since this is where the effects of paresis take place.  

The main adductors of the voice are the lateral cricoarytenoid and the transverse and oblique interaryntenoids.  The thyroarytenoid has two parts:  the internal, or vocalise portion, which is the “vocalise muscle” located under the lamina propria, and the external portion, or muscularis.  This muscle does assist with adduction, but it is primarily used to regulate vocal fold tension.  (Note to add here:  The vocalise muscle is covered in four layers of tissue:  the first is the epithelial tissue which is extremely thin and is the outer most layer, and the next three layers make up the lamina propria, which is loose connective tissue.  The complex movement of the lamina propria during vibration is what makes up much of the complex sound wave produced at the vocal fold level during vibration.  I'll be getting into this more in future posts, but wanted to introduce it here.)   The main abductor, or opener, of the voice is the posterior cricoarytenoid.  This is the muscle that keeps your vocal folds out of the air way if you’re breathing very heavy (so after a good workout, thank your cricoarytenoid for opening up the passage way for those large breaths you were taking).  The other main muscle involved in vocalizing is the cricothyroid.  This is the muscle responsible for elongating the vocal folds, which results in changing the pitch of the voice.  (*For an excellent example of this muscle in action, as well as the adductors and abductors, see this video:  http://www.youtube.com/watch?v=ajbcJiYhFKY)

Here are some visuals of these muscles to help you out:

The biggest thing to know about the voice is this:  It is a system which works on symmetry.  If the symmetry of the voice is off, due to any reason, it results in vocal dysfunction, muscular compensation, and possible damage to the vocal fold itself.  It simply is not an effective system if both sides of the voice are not working correctly.

Why is this important to understand?  Well, the larynx is innervated by two branches of the vagus nerve:  http://en.wikipedia.org/wiki/Vagus (if you're curious about it.)  One branch, the superior laryngeal branch, innervates the cricothyroid muscle and the recurrent nerve branch innervates all the other intrinsic musculature:  the adductors, abductor, and thyroarytenoid muscle itself.  Due to this, paralysis or paresis of either of these nerve sets can have different symptoms.  

If paresis is affecting the superior nerve branch, it will mainly affect the range of the voice.  Singers who experience this might experience unexpected voice breaks and/or a shortened vocal range from what is normal.  There can also be breathiness present in the voice as well due to asymmetrical vocal fold motion.  If paresis is in the recurrent nerve branch, the symptoms can vary depending on which muscle is affected, but some symptoms can include:  breathy voice quality, horse voice quality (which is breathiness + harshness), vocal fatigue and reduced volume, (or loss of vocal power).  Paresis in the recurrent nerve does not usually affect vocal range.  Although singers who recover can gain a few notes on their range, the difference is not as dramatic as with recovery from superior nerve paresis.  The symptoms that brought me in were tongue tension, breathiness, and vocal fatigue.  My asymmetry was bad enough for the left side of my tongue root to be pulled downward as the left fold went over towards the right.  If you look at the picture above that shows the root of the tongue directly anterior and superior (in front of and above) the epiglottis, you can begin to see just how much the paresis impacted my laryngeal function after nine or ten years of muscular compensation.  

Here's what to take away from this:  Nerves are the communicators that transmit the impulse from the brain down to the muscles (as well as carries signals from muscles to the brain...to let it know how everything's doing, basically.)  If that communication system is damaged, it results in a loss of muscular function, mainly causing muscle atrophy if it goes untreated for a while.  (This is why my right vocal fold was notably thinner and smaller than my left vocal fold when I saw it on the video, which also strengthens the case for my injury being rather long-standing prior to diagnosis.)  What can cause this?  Well, these nerves can be affected by a virus from a respiratory infection, which is most likely my cause, although the superior nerve branch is more commonly affected by viruses.  (That was why it was unusual for the ENT to see paresis in the recurrent nerve in voice performers like me.)  Basically, in this scenario, the virus gets down to the nerve in the tissue and irritates or obstructs part of the nerve, and the nerve then remains that way after the virus has been killed off by the immune system.  Other more serious causes can be tumor of the thyroid or neck, blunt trauma to the neck area or thyroid, intubation during surgery or emergencies, and aortic aneurysms, which could affect the left recurrent branch.  Full paralysis of these nerves is an issue if you are having thyroid or heart surgery if the nerves get severed during surgery.

My ENT sent me for a CAT scan of my neck and upper chest cavity to rule out any of these more serious reasons for recurrent nerve paresis.  He also did some basic facial muscle tests to rule out Bells’ Palsy and other neurodegenerative conditions.  (Quite frankly, given all the possible issues that could have been wrong with me, I am rather grateful it was only a simple virus that had long passed out my system that caused the trouble.  Facing voice therapy is much, much simpler than facing chemo, surgery, or an untreatable, neurodegenerative condition.)

I hope this was not too much information packed into a small entry.  If you have any questions, please post below and I’ll be happy to answer them if I know the answer.  If I don’t know the answer, I’d be happy to search and find the correct one for you as well! 

Up next:  Heading Out to Voice Therapy

(All images are public domain and can be found on wikipedia if you wish for a larger view.)