Clinical significance of asymmetrical vocal cord tension

The effects of asymmetrical tension on the vibratory pattern of the vocal cords were studied in two kinds of experiments: 1) high speed motion picture photography of artificial voice production in excised canine and human larynges, and 2) computer synthesis of voice and vocal cord vibration via a theoretical model incorporating the physiological parameters required for phonation. In both approaches the asymmetrically tensed vocal cords consistently vibrated in three distinct modes which depend partly on the rest positions of the vocal cords; Type I. For rest positions at or near closure, the two cords vibrate at the same frequency with glottal closure every period, and with tense cord preceding the lax one in phase and with the line of contact moving toward the tenser cord during the closed phase. The voice produced is not hoarse; Type II. For wider rest positions glottal closure occurs irregularly, the vibrations become complex and less periodic, and the voice becomes hoarse; Type III. The glottis never closes and the vibrations become more periodic with reduced amplitude. Supplementary stroboscopic observations suggest a precedure for diagnosing tension asymmetry and the implications for surgical treatment for disorders of vocal pitch are discussed.