I don't think it is exclusively kava that is responsible for "chorea, dystonia, and spasms". I have felt this at times, and I think it is an individual neurological pattern that many of us have. The symptoms can be noticed more with a rapid onset of kava such as in the case of having a large serving early in the day or with too little food in the stomach. I think it has something to do with us not relaxing enough, or overworking our nerves, whether we realize it or not, causing them to "fall dead" temporarily during attempts to relax consciously.
Think of it like this, someone working a fast paced job might relax their arms, posture, eyes, and diaphragm a little bit in between rushes, yet not completely to the fullest potential of relaxation. The human body becomes confused because now there are two states (or more than one state) of relaxation known, and it becomes less simple which one to choose from at any given tiime.
This can be remedied with awareness, breathing, getting enough sleep, and being sure not to have kava and caffeine at the same time. I don't think kava makes this condition better or worse in the long term, however it may help us realize we have the sensations to begin with by pushing our bodies to relax so that we may do so more intentionally.
There is an equal chance the condition has nothing to do with what I said, however I feel that these kinds of spasms are natural product of our livelihoods and the ways we use our bodies, and that in any event the information I provided is useful.
If it's something you wish to treat, you can try Skullcap (scutellaria laterfloria), in the form of a warm tea. Skullcap is a nerve tonic which is especially useful at bedtime.