This advice-column-style blog for SLPs was authored by Pam Marshalla from 2006 to 2015, the archives of which can be explored here. Use the extensive keywords list found in the right-hand column (on mobile: at the bottom of the page) to browse specific topics, or use the search feature to locate specific words or phrases throughout the entire blog.
Q: My client had a frontal lisp, but when I taught him to keep his tongue behind his teeth, he switched it to a lateral lisp. Have you seen this? What should I do?
I think it is somewhat common for a child with a frontal lisp to switch to a lateral lisp once the tongue is brought inside the mouth for the sibilants. I have seen this many times in my career, and I have had this question from many therapists throughout the years.
This is a problem of correct oral stability. You are starting with a client with a frontal lisp. Therefore, he does not have back-lateral tongue stability (it hangs forward), and he does not have jaw stability (it hangs too low).
When you have the client close his teeth (stabilize the jaw) in order to prevent the tongue from hanging too far forward, this stabilizes the jaw (it puts the jaw in the right position). But it does not stabilize the tongue correctly (it does not necessarily put the tongue in the right position). Therefore the client is free to stabilize the tongue wherever he wants. If he produces a bilateral lisp, then your client is stabilizing the middle instead of the sides of his tongue against the palate. If he produces a unilateral lisp, then your client is stabilizing the tongue only on one side.
Now you simply have to treat this like a lateral lisp. Teach the client to stabilize the tongue on both sides at its back-lateral margins, by using the Butterfly Position, and move forward from there. I describe how to teach the Butterfly Position in my book on pages 115-120.