If we have a moderate sized swelling (~2mm wide and ~1mm tall – e.g. nodule) located in the mid-portion of the medial vibratory margin of the membranous vocal cord, it will tend have the following acoustic effects.
Since the comfortable speaking pitch is typically in the bottom quarter of the vocal range, the vocal cords are relatively lax during speaking. At low pitch, with either loud or soft volume, a small mass will not impair closure of the vocal cords because the cords are short and loose and the swelling can compress into the vocal cords during the closed phase of vibration. There is no air leak. The asymmetric mass difference between the two vocal cords with a small nodule is insufficient to break the phase lock and so diplophonia does not occur during speaking and the voice sounds clear.
For the same reason, maximum phonation time at comfortable speaking pitch is typical. A small swelling does not promote air leak or diplophonia in the lower vocal range.
During vocal range testing, we increase pitch initially by tightening the thyroarytenoid muscle, which stiffens the vocal cord. With further pitch elevation, the cricothyroid muscle lengthens the vocal cord, indirectly adding additional stiffness while raising pitch. The oscillatory amplitude is reduced with increasing tension. The swelling begins to stand proud of the margin of the vocal cord and it no longer compresses into the deeper layers of the vocal cord. At some pitch, the swelling begins touching the opposite vocal cord during every closing phase of the cycle. Air begins to leak from anterior and posterior to the swelling. If airflow is high enough, i.e. high-volume, the phase lock will remain. However, if volume is low and there is just enough air passing through the vocal cords to maintain entrainment, as the swelling begins to touch the opposite vocal cord with sufficient pressure, it may stop entrainment, the pure tone will cease and air will leak. When occurring at phonatory onset, this can be termed an onset delay. A delay, because the individual being examined usually quickly makes a laryngeal adjustment to break the loss of sound production, either increasing the airflow or partially opening the posterior commissure which pulls the swelling away from the opposite vocal cord and allows entrainment to resume.
Secondly, at some point the compression of the swelling against the opposite vocal cord will create an acoustic node and separate the anterior and posterior aspects of each vocal cord into two separate sound sources. Since our example swelling is in the exact central portion, the length of the anterior and posterior segments are exactly 1/2 of the original length and the vocal pitch would suddenly double, jumping about one octave, with two short segments generating an identical pitch. We could call this jump a pitch break. If the swelling were not in the exact mid-portion, the length of the anterior segment would be different than the length of the posterior segment and after the pitch break, two separate tones or diplophonia occurs.
We can rerun the scenario with a larger mass. The findings of onset delay and diplophonia will now tend to occur at a lower pitch. If the mass is large enough, such as in the case of a unilateral smoker’s polyp, the phase lock may be broken even at very low pitches and diplophonia will occur even in the low range.
Many individuals will develop compensation to avoid a rough voice, especially if the mass has enlarged slowly. The typical compensation is to open the posterior commissure, trading increased breathiness for less roughness. Breathiness from a small swelling will occur in the upper vocal range. If the swelling enlarges, then air will leak in the mid-vocal range. The farther apart the vocal cords are held, the less likely they are to be entrained at higher pitches with low subglottic pressure, since tension increases stiffness of the vocal cord. Increased tension requires higher subglottic pressure to entrain the stiffer cords.
These acoustic changes are not intrinsically dependent upon the composition of the mass or swelling. A benign nodule of the same density and mass as a carcinoma, located in the same position, will have the exact same acoustic effects. Swellings that differ in density or mass may alter the acoustic impairment somewhat. A polyp and a nodule of the same size will impair vibration at slightly different pitches because of differing densities, but the overall vocal capabilities pattern will be essentially the same. A hemorrhagic polyp may enlarge during phonatory use if it fills with blood during oscillation, and so a vocal capabilities pattern may be different in degree after extensive vocal use than after vocal rest.
Consequently, vocal capabilities pattern matching which has these characteristics (diplophonia at high, soft sound production, onset delays during swelling testing in the upper range, pitch breaks with a sudden jump upward) will direct the endoscopic examiner to look at the vocal cord’s medial vibratory margin for a mass. The smaller the mass, the more this test clues the examiner where to look. A very small vocal nodule on the inferior vibratory lip will be essentially invisible to an examiner using a fiber-optic endoscope, looking from far away (tip of scope above the epiglottis), without a stroboscope.
The same swelling will be visualized by a second examiner whose ears are tuned to vocal capabilities pattern matching, by topically anesthetizing the vocal cords if necessary, moving the endoscope very close to the vocal cords (perhaps 1 mm), utilizing a stroboscope and examining and recording the vocal cords while vibrating at high pitch and low airflow, when the small swelling is most likely to be protuberant and interrupting the vibrations of the vocal cords. Then perhaps reviewing the video frame by frame, the small, nearly hidden swelling is identified as the source of harmonic sound impairment.
Even if you assume that the first examiner hasn’t missed anything important (presupposing that a benign nodule is only important to a world-class singer - possibly a false assumption), the first examiner will be predisposed to an erroneous diagnosis, perhaps ordering inappropriate diagnostic testing, perhaps pursuing pH probe studies, esophageal manometry, prolonged prescribing of anti-reflux medication with the potential for side effects, consultations with gastroenterologists, endoscopy of the G.I. tract, fundoplication… consuming the individuals time and money, creating discomfort and wasting a healthcare system’s limited resources.
The second examiner notes the small swelling, reviews a video recording of the swelling, explains the mechanics of vocal impairment to the patient and decides on an appropriate plan of action. Plans might range from an informative discussion where the patient is willing to live with the vocal impairment, to appropriate voice therapy, to surgical excision of the lesion. Although of differing expense, all three of these treatments are appropriate and cost effective and the patient has the data and information to make a personal yet educated decision.