Vocal Cord Paralysis
Paralysis can occur in one or both vocal folds, resulting in a breathy or weak voice quality with pitch instability. The vocal fold can be paralyzed in the midline position and sometimes cause breathing issues during heavy exercise, but most commonly the vocal fold is paralyzed in the abducted position (open). Paralysis occurs from damage to either the superior laryngeal nerve (SLN) or the recurrent laryngeal nerve (RLN). This can happen after thyroid surgeries, heart surgeries, or for no reason at all.
Superior Laryngeal Nerve Paralysis
SLN damage is the most difficult to identify because the patient usually complains of vocal fatigue, being unable to sing, and difficulty increasing pitch or volume. SLN damage can create a gap between the vocal folds after they come together (where one overlaps the other) which results in the patient being unable to achieve maximum closure. The affected vocal fold is also unable to lengthen upon pitch raising. Voice therapy can benefit anyone suffering from SLN paralysis to give the patient the maximum ability of vocal use in the presence of the paralysis, but no medical treatment is successful in treating SLN damage.
Recurrent Laryngeal Nerve Paralysis
RLN damage is easier to identify as most patients complain of diplophonia (two sounds at once), reduced volume and pitch, and breathiness. This paralysis can cause atrophy of the vocal fold(s) affected and impact the ability for the vocal folds to completely close (adduct). A combination approach of surgical treatment and behavioral voice therapy is most beneficial.
Often times there is SLN and RLN paresis which can improve spontaneously in about 6-12 months. Again, behavioral voice therapy can maximize the patient's ability to utilize the best voice possible.
Sources: Clinical Voice Pathology: Theory and Management 4th Edition Stemple, Glaze & Klaben. Benninger, MS, Crummley RL, Ford CN, Gould WJ, Hanson DG, Ossoff RH, Sataloff RT, Evaluation and treatment of the unilateral paralyzed vocal fold, Otolaryngology--head and Neck Surgery, 1994, 111(4):497-508.