Tongue thrust is an abnormal positioning of the tongue during swallowing. This condition is often present in young children, and is often not problematic. In more severe cases it can interfere with both swallowing and speech, and may cause malocclusion and other dental problems. It is sometimes called infantile swallow or reverse swallow, and it is the most common type of orofacial myofunctional disorder.
Tongue thrust is technically the positioning of the tongue between or against the teeth during swallowing, but it is also strongly correlated with the tongue remaining against or between the teeth even when at rest and during speech. Newborns and infants do this as part of their normal mode of feeding, as during suckling the tongue is properly used to help 'grab' and suck on the mother's nipple. But as the child's jaw grows and e moves onto eating solid foods, the tongue should move back in the mouth, generally being held behind and a bit below the teeth. By about five years of age children should be naturally resting their tongue within the curve of the lower alveolar ridge. That said, tongue thrust is not uncommon in older children, and will generally disappear before puberty in all but the worst cases. In most cases the effects of tongue thrust will be so mild that it does not warrant treatment.
Tongue 'thrust' makes it sound as though the child is pushing out with their tongue aggressively, but generally this is not the case. The tongue is simply resting between or against the teeth, usually anteriorly (between the incisors), but in some cases laterally (between the bicuspids or molars). The tongue is pulled in to chew and speak, and is pushed out when at rest and during the swallow.
Tongue thrust is often caused by the child failing to learn the new oral muscle pattern, possibly due to low muscle tone in the tongue or a problem with motor planning or sensory feedback. These cases are referred to as habitual tongue thrust, and are generally treated through therapy with an SLP. An orthodontist may recommend an appliance for the child to wear when not eating or speaking; these generally look like a cage to block the tongue from passing between the teeth. The IAOM is generally against these. There is some speculation that habits such as thumb sucking and drinking excessively from sippy cups may encourage children to keep these oral patterns beyond the usual developmental age.
In other cases the child has to position their tongue in this way to accommodate structural abnormalities, in which case it is referred to as obligatory tongue thrust. Obligatory tongue thrust may be caused by a number of conditions, including macroglossia, Down's syndrome, and swollen tonsils. In these cases the tongue thrust would generally not be treated until the underlying cause was corrected, if possible.
Tongue Thrust and Swallowing: Generally tongue thrust is not seen as a serious problem as far as the actual swallow goes. The child is likely to drool or spill food from the mouth, and may look somewhat odd if the tongue actually sticks out between the lips when eating, but it does not place the child at great risk for choking. Older children and adults may seek swallowing therapy to avoid social awkwardness.
Tongue Thrust and Dentition: Depending on who you ask, you will hear slightly different things about how serious the effects of tongue thrust on dentition may be. Humans are meant to have their mouths closed, with slight pressure between the upper and lower jaws. This pressure helps maintain good muscle tone in the jaw, helps encourage timely eruption of teeth, and guides good occlusion of the teeth. (FWIW, These problems, and others, are also caused by chronic mouth breathing).
Beyond this, many orthodontists claim that the pressure of tongue thrust on the teeth will cause them to be pushed out of alignment. Generally it is not a feature of tongue thrusting to have constant pressure on the teeth, and if the child presses against the teeth during swallowing it is not with great force. However, in some cases tongue thrust may be severe enough to push teeth out of alignment.
Tongue Thrust and Speech: Tongue thrust tends to interfere with the production of sibilants (the /s/, /z/, /ʒ/, and /ʃ/ (sh) sounds), and may cause a lisp in some cases. The sounds /t/, /d/, /n/, and /l/ are also occasionally affected. Children (and adults) with tongue thrust are intelligible, and often the sound distortions are nearly imperceptible. In some cases these distortions make a person's speech 'sound funny', which is generally considered good cause for therapy.