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Oral-mechanism exam, cranial nerves and paediatric motor speech
The oral-mechanism exam locates the lesion from the pattern of weakness: upper motor neuron damage affects the opposite side and spares the forehead, while lower motor neuron damage affects the same side, with atrophy and fasciculations.
Why it matters in the exam
Many questions describe an exam finding — a mouth corner drooping on one side while the forehead wrinkles normally, a tongue deviating to one side with atrophy, a palate rising on one side only — and ask where the lesion is. To answer you need each cranial nerve's role in speech and swallowing: V for the jaw, VII for the lips and face, IX and X for the palate, pharynx and larynx, and XII for the tongue.
Another part concerns children: drooling caused by weakness or reduced oral sensation, cerebral palsy in which speech is often less intelligible, dyskinetic cerebral palsy with involuntary movements and poor breath coordination for speech, and cerebellar mutism after posterior fossa surgery. Non-speech oral exercises are also asked about: there is not enough evidence that they improve speech sounds.
Key points
- Upper: opposite side, forehead spared; lower: same side, whole half-face, atrophy and fasciculations.
- V jaw, VII lips and face, IX and X palate, pharynx and larynx, XII tongue.
- A palate rising on one side with hypernasality and breathy voice — high one-sided nerve X lesion.
- Drooling comes from weakness or sensation, not from a planning problem.
- Non-speech oral exercises lack sufficient evidence for improving speech sounds.
Summary table
| Finding | Interpretation | Nerve or system |
|---|---|---|
| One mouth corner droops, the forehead wrinkles normally on both sides | Upper motor neuron in the opposite hemisphere | VII, corticobulbar pathway |
| Weakness of the whole half of the face, including the forehead | Lower motor neuron on the same side (for example Bell's palsy) | VII |
| Tongue deviates to one side with atrophy and fasciculations | Lower motor neuron; the tongue deviates towards the damaged side | XII |
| Palate rises on one side only, hypernasality and breathy voice | One-sided vagus damage above the pharyngeal branch; the palate is pulled to the healthy side | X |
| Difficulty chewing and reduced facial sensation | Trigeminal nerve | V |
| Drooling in a child without excess saliva | Weakness or reduced sensation of the lips and mouth, or unstable posture | Oral motor and sensory system |
| After posterior fossa surgery the child stops speaking and is irritable, but understands | Cerebellar mutism; speech returns gradually, sometimes with ataxic dysarthria | Cerebellum |
In the full version for subscribers
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Sources
- Duffy, Motor Speech Disorders: Substrates, Differential Diagnosis, and Management, 4th edition (2020) — Neurologic bases of motor speech and the oral mechanism examination
- ASHA Practice Portal, 2024 — Dysarthria in Adults
- Lof & Watson, Language, Speech, and Hearing Services in Schools, 2008 — A nationwide survey of nonspeech oral motor exercise use
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.