Interdental Lisp vs. Lateral Lisp: What’s the Difference?

Have you noticed that your child’s /s/ or /z/ sounds different from other children’s? Maybe their “sun” sounds a little like “thun,” or their /s/ has a slushy or wet quality. These differences are often referred to as a lisp, but not all lisps are the same. Two common types are an interdental lisp and a lateral lisp. Understanding the difference can help parents know when an evaluation may be appropriate and what treatment may look like.

What Is an Interdental Lisp?

An interdental lisp, sometimes called a frontal lisp, occurs when the tongue moves too far forward and protrudes between or against the front teeth during sounds such as /s/ and /z/. Instead of producing a clear /s/, the sound may have a “th”-like quality.

For example:

● “sun” may sound like “thun”

● “sip” may sound like “thip”

● “zoo” may sound like “thoo

An interdental production is considered a distortion of the intended speech sound, rather than simply replacing one phoneme with another. The American Speech-Language-Hearing Association (ASHA) identifies distortions, including lateral /s/, as one type of speech sound error.

Interdental productions can be associated with tongue placement or oral-motor patterns. ASHA notes that a speech-language pathologist evaluating orofacial myofunctional patterns may examine tongue resting posture and the placement of the tongue during lingual-alveolar sounds, including whether productions are interdental or lateralized.

What Is a Lateral Lisp?

A lateral lisp occurs when airflow escapes over the sides of the tongue rather than being directed appropriately over the center of the tongue.

This can cause /s/, /z/, and sometimes other sibilant sounds to have a slushy, wet, or distorted quality.

For example, a child may produce:

● /s/ with a noticeably “slushy” quality

● /z/ with a similar lateralized quality

● Other sibilants, such as /ʃ/ (sh), /tʃ/ (ch), or /dʒ/ (j), with lateral airflow

Unlike an interdental lisp, a lateral lisp typically does not sound like “th.” Instead, the primary characteristic is the unusual quality created by the direction of airflow. Lateralized /s/ is specifically identified by ASHA as an example of a speech sound distortion.

What’s the Difference?

Is a Lisp Normal for Young Children?

Some speech sound errors are part of typical speech development, and children do not master every speech sound at the same age. However, the type of error, the child's age, consistency of the error, and its impact on intelligibility are all important when determining

whether an evaluation is appropriate.

ASHA recommends considering speech sound development within the context of the child's overall speech system rather than looking at one sound in isolation. A speech-language pathologist can evaluate the child's speech production, intelligibility, sound patterns, and oral-motor/orofacial factors when appropriate. It is also important to remember that not every lisp has the same developmental course. A persistent lateralized sound may warrant evaluation even when a child has otherwise typical speech development.

How Is a Lisp Evaluated?

An SLP will typically listen to how the child produces the affected sounds in different contexts.

The evaluation may include:

● Single words

● Different word positions

● Sentences

● Conversational speech

● Different sibilant sounds

● Tongue placement and movement

● Resting posture of the tongue and jaw, when relevant

● Overall speech intelligibility

● Other speech sound errors

A comprehensive assessment helps determine whether the child has an isolated articulation error, a broader speech sound disorder, or an orofacial pattern that may be contributing to the difficulty.

Can Speech Therapy Help?

Yes! Treatment for a lisp is individualized based on the child's specific error pattern.

For an interdental lisp, therapy may focus on establishing appropriate tongue placement and directing airflow correctly to produce a clear /s/ or /z/.

For a lateral lisp, treatment may focus on developing midline airflow, appropriate tongue

shaping, and accurate placement for the affected sounds.

Research and clinical literature support individualized, motor-based approaches for children with lateralized sibilants. For example, an ASHA-published case study described successful reduction of lateralized /s/ and /z/ productions following an individualized motor-based treatment approach.

A speech-language pathologist can determine what type of lisp is present, what may be

contributing to it, and whether treatment is appropriate.

How We Can Support You

If any of this sounds familiar and you're wondering whether your child has a lisp or which type it might be, we're here to help you find clarity!

We're excited to welcome Samantha, our newest speech-language pathologist at The Mouth Rehab! Samantha can complete a full speech sound evaluation to determine whether a lisp is present, what type it is, and whether treatment is recommended.

Reach out to schedule an evaluation with Samantha or another member of our team, we'd love to help your child feel confident in their speech.

Ready to Take the Next Step?

If you have concerns about your child’s feeding, speech, oral motor skills, breathing, or oral function, our team at The Mouth Rehab is here to help. Schedule a comprehensive evaluation so we can identify what’s going on and create an individualized plan for your child.

✨ Ready to book an evaluation? Click here! ✨

Written by Samantha Yi, M.S., CCC-SLP | Clinician at The Mouth Reab

Edited by Vanshi Topiwala | The Mouth Rehab Intern