Understanding the Distinction Between Essential Voice Tremor (EVT) and Dystonic Voice Tremor (DVT)
Introduction
Essential Voice Tremor (EVT)? Or is this Laryngeal Dystonia (Spasmodic Dysphonia) with an overwhelming Dystonic Voice Tremor (DVT) component? How do you distinguish between these two possibilities using vocal phenomenology alone?
A useful initial working distinction (open to some debate) is:
- EVT is fundamentally a rhythmic oscillatory disorder with no phonatory arrests (cut-outs or drop-outs) or strain. The rhythmicity is often (not always) somewhat regular, and consistent in all parts of the voice.
- DVT is noted as an accompaniment to dystonic spasms (cut-offs, drops away to whisper, strain/strangle quality). It tends towards greater variability depending on task, especially between vocal registers (chest vs. falsetto).
Consequently:
Essential Vocal Tremor | Dystonic Vocal Tremor |
| Relatively rhythmic, periodic | Often irregular |
| Continuous oscillation, including while breathing, and with vertical oscillation of larynx | Oscillation less apparent with breathing, and occasional with spasm/glitches |
| Tremor more consistent across phonatory tasks | Tremor may be less, even absent depending on task (especially falsetto). |
| Tremor often still strong in falsetto | Falsetto may reduce tremor |
| No true voice breaks, just deep “bumps” of the tremor itself | May show near-breaks or actual breaks |
| Tremor is the disorder | Tremor is an accompaniment of dystonia |
AD-SD with an Overwhelming Tremor Component.
As a mental exercise, imagine that the voice sample below is from a first visit. From this brief and incomplete voice recording, how might one decide whether her tremor is due to EVT or DVT? Listening carefully reveals two key points of potential distinction.
- While speaking, though subtle and debatable, it sounds like there are faint near-interruptions, that is, a little more than only the deep bumps of the tremor itself.
- More convincingly, in her sustained phonation, there is obvious tremor in chest voice, but far less in her falsetto. That may suggest DVT more than EVT.
Furthermore, in the chest voice productions, there seems to be a lot of variation of tremor-to-tremor amplitude and even tremor-to tremor time lapse seems to vary rather than being highly regular. This can be heard (and “intelligent ears” are usually all you need), but for teaching purposes, this is also seen in the visual (live acoustic) representation.
Compared with this challenging case, most cases of vocal tremor are more obviously EVT or DVT…but to summarize using this case (that depends on subtle cues), when eliciting speech and sustained phonation in both chest and falsetto registers, ask yourself whether you’re hearing a very regular tremor alone, or a bit of variability of the tremor speed and amplitude, with occasional spasm manifestations as well (strain, phonatory arrests (cut-offs or drop-outs to whisper).
Spoiler: For this patient, we know that her initial diagnosis years earlier was abductory spasmodic dysphonia, treated successfully with botox, and that her manifestations have evolved across many years to add tremor and some sense of adductory, more than abductory spasm. If she were being seen as a new patient, and if further elicitations continued to demonstrate #1 and 2 above, her initial working diagnosis would be “adductory spasmodic dysphonia with an overwhelming tremor component.” We say “working diagnosis,” since the manifestation of strain/spasm is so subtle, and so there would be a trial element to her initial injection.
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