Motor Speech Disorders

In motor speech disorders, the brain's control of speech is affected. Motor speech disorders include childhood dysarthria, childhood apraxia of speech, and motor speech disorder not otherwise specified.

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Developmental Dysarthria

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In childhood dysarthria, children have difficulty producing intelligible speech due to neuromuscular difficulties that impact execution of speech movements. These difficulties may be due to increased or decreased muscle tone, reduced drive to muscles, reduced force of muscle contraction, delayed initiation of movement, reduced speed of movement, restricted range of motion, or incoordination between speech systems such as respiration and phonation. These impairments can result in speech differences such as abnormal loudness, pitch, articulatory precision, resonance, voice quality, and breath support. Ultimately, a person's speech sounds distorted and less clear. About 1 in 1000 children have dysarthria (Shriberg et al., 2019).

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Childhood Apraxia of Speech

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Childhood apraxia of speech (CAS) is a neurological paediatric speech sound disorder. It occurs when a child has difficulty planning/programming speech movements. It is estimated that about 2 in 1000 children have idiopathic CAS (Shriberg et al., 2019) - CAS in which there is no known congenital or neurological disorder (e.g., Down Syndrome). The incidence is a little higher (about 4-5%) in children with neurological or congenital disorders associated with CAS (e.g., Down Syndrome, stroke, infantile spasms).

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Children with CAS may demonstrate the following types of speech difficulties (Ozanne, 2005):

• inconsistent errors - the same word or sound may be said differently each time they say it, with different errors each time

  • word examples: 'ambulance' - "ambance, amlulance, amblan", 'dada' - "deedee, dawdaw, dada"

  •  ​sound example: 'l' at the ends of words - "n, d" or omitted

• unusual 'prosody' (stress and intonation patterns)

  • add pauses between syllables

  • trouble with 'lexical stress' - stress that conveys meaning (e.g., CONtract vs conTRACT) - putting equal stress on each syllable in a word or putting stress on the wrong syllable 

• distorted vowels and consonants

• more trouble with longer words than short ones

• more trouble with volitional speech (i.e., when asked to say something specific on command) than automatic speech (i.e., saying the alphabet) and imitating sounds/words

• trouble sequencing sounds and syllables

• metathesis - switching sounds or syllables around in words (e.g., 'foliage' becomes "foilage")

• non-speech oral apraxia (NSOA) - difficulty with non-speech movements of the mouth

  • not all children with CAS have NSOA​, so although it might support a diagnosis of CAS, it does not indicate CAS

• trouble programming the parameters of speech production (i.e., timing, force, spatial parameters)​

  • oral groping (trial and error movements of the mouth)​

  • asymmetrical oral movements

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A diagnosis cannot be given for CAS until a child is at least 3 1/2 and has enough connected speech. Signs that may indicate difficulty with motor speech programming in younger or minimally verbal children may include the following (Strand, 2003):

• vowel distortions

• small phonemic inventories (only say a limited number of speech sounds)

• "go to" words/sounds (uses the same sound or word approximation for all things such as "nuh" or "dee")

• oral groping when trying to speak

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In young, minimally verbal children (e.g., <3 years), a child may present in such a way that CAS is sometimes suspected (sCAS), or it's deemed that motor speech development should be monitored. The following behaviours and features that are looked for by SLPs and are considered when making differential diagnoses between sCAS, autism spectrum disorders, language delay, or other speech sound delays/disorders:

• atypical early development (birth to 15m)

• desire and motivation to speak and interact

• play skills (should be typical, although the presence of limb apraxia can effect execution of movements needed for play)

• non-verbal communication skills

• gap between expressive and receptive language (children with only CAS will generally have good receptive skills)

• effort when speaking

• "pop-out" words (say word once and not again)

• "go-to" sound/word (a word or sound that is used to express many things, e.g., "dee" for everything; and may be strung together to form "sentences", e.g., "dee dee dee")

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There are three main causes of CAS:

1. idiopathic neurogenic speech sound disorder - CAS without any known neurological injury, abnormality, or neurobehavioural disorder

2. secondary to a neurological injury such as stroke, infection, traumatic brain injury, or tumour

3. associated with a neurobehavioural or congenital disorder such as  epilepsy or Down Syndome

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