Since Tessa was first identified as “developmentally delayed” when she was 12 months old, she has been tested for all sorts of things in search of a diagnosis that would explain her difficulties. They often tell us that she has “some features consistent with X”, but not enough for a diagnosis. There were several times over the years that we were sure it was this or that, and then it wasn’t (hearing loss, apraxia, dyspraxia, autism, etc).
Well, it seems that now we actually have an answer.
We did a genetic test called whole exome sequencing back in December, and the results are in. Tessa has a mutation in a gene called SCN8A, which makes a voltage gated sodium ion channel. My understanding is that this particular sodium channel is mostly located on neurons in the cerebellum, and defects in the channel affect the ability of the neurons to effectively pass signals to each other. I am not a neurobiologist, but I am learning all I can about sodium channels now!
It is considered ultra rare to have a mutation in this gene, and there are only about 100-150 people with this diagnosis worldwide so far. Most of them have epilepsy, which Tessa does not, so we will have to take her to neurology to see if she might have seizures that we are unaware of. Tessa’s mutation was de novo, which means that it was not inherited from Jeff or me, but arose spontaneously. Tessa is the only one with this particular mutation so far, so the geneticist said that they are >95% sure that her mutation created her phenotype (based on the type and location of the mutation), but they can't be 100% sure. Because this type of genetic testing is new, they expect that more people will be found to have these mutations as the testing becomes more common, and we will learn more with time.