A story of scarring
Sarah is an amateur singer. She tells me she was diagnosed with a vocal nodule and when she saw a surgeon, he recommended stripping it off. Confident in his abilities, she had vocal cord stripping surgery, but her singing voice never did improve. Since that time, she has just given up singing. However, she would still like to know why she remains hoarse. She asks me, “Have techniques gotten any better? Perhaps you can now fix something that my first doctor could not.”
Listening to her speaking voice, it sounds just fine. However, as she starts to produce notes through much of her middle and upper range, her voice deteriorates and breaks up. She can only make a few, very clear, somewhat random, high notes.
Using stroboscopy to examine her vocal cords in motion, one is very stiff and the other one supple, revealing the explanation for the roughness in the middle of her vocal range. At her lowest notes, the difference in stiffness causes each vocal cord to vibrate at slightly different amplitudes, but still at the same pitch, so no diplophonia. In her mid-range, the vocal cords go out of sync, each producing a different note, so the diplophonia is very audible. At her highest notes the stiff vocal cord is pulled so tight, it does not vibrate at all and all the sound comes from the flexible cord. The tone produced by her single cord vibrating is quite clear.
The stripping of one of her vocal cords appears to have removed not just the mucosa (the surface), but also the lubricating layer of the vocal cord. When she healed after the surgery, it became true that she no longer had a vocal impairment from a nodule sticking out. However, because the lubricating layer was removed by the stripping, the new mucosa lining the surface adhered directly to the vocal ligament and thyroarytenoid muscle during healing. Now, her stripped cord is much stiffer than her other vocal cord. Consequently, during many of her attempts to make a sound in her mid-range, the more supple cord is trying to oscillate at a different pitch than the stiffer side. We perceive this asymmetric stiffness as roughness.
Scarring after cancer treatment
Linus is 74 and had a vocal cord biopsy which revealed cancer and he underwent radiation therapy 12 years ago. His voice is rough. On stroboscopic exam there is a supple mucosal wave on the right. The left cord oscillates, but stiffly. He ends up with two sound sources because of the asymmetric stiffness. Interestingly, if the left vocal cord was completely stiff and did not oscillate at all, his voice would be clearer. One vocal cord would create a clear pitch and with no vibration on the stiff side, there would be no competing pitch to create roughness.
Because vocal pitch is changed by varying the tension of the thyroarytenoid muscle as well as varying the length of the vocal cords by tightening the cricothyroid muscle, two vocal cords of similar pliablity will change pitch at an identical rate. When one vocal cord has a stiffer cover than the other, a change in internal tension or cord length will alter the surface tension of each vocal cord at a different rate making them asymmetric at many pitches.
- A stiff vocal cord covering will change tension at a different rate than a pliable cover.
- Asymmetric stiffness leads to asymmetric vibrations as tension and length are adjusted.