I think this description is significance, because it is written clearly in MHLW guideline that selective mutism is not only a childhood problem but can have lasting effects as Hikikomori. In fact, I hear not negligible number of Japanese people who suffered (or suffer) from selective mutism become Hikikomori. One Japanese support group for children with selective mutism claims that some children with selective mutism become Hikikomori later.
I don't know what evidence the authors of the guideline have. There are few studies on long term outcome of selective mutism also in Japan. But I guess several Hikikomori cases that have the experience of selective mutism are reported among institutions for Hikikomori people, such as Mental Health and Welfare Center, Public Health Center, etc..
It is interesting to note that the guideline mentions the relationship between school refusal and selective mutism. Traditionally it has been thought that Japanese children with selective mutism in general don't suffer from school refusal, because it's conspicuous to miss school for a long time. Children with selective mutism tend to avoid being conspicuous.
Although the guideline only make passing reference to selective mutism, I want many professionals to read the sentences about the disorder. That can help to raise awareness of selective mutism.
Last but not least, please note that not all Japanese children with selective mutism become Hikikomori later.
Index of SM in Japan
