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Apraxia of Speech (AOS) is a motor speech disorder (MSD) affecting an individual's ability to translate conscious speech plans into motor plans. Like other apraxias, it only affects volitional movement patterns. Dyspraxia of speech, verbal dyspraxia, and other terms, usually refer to variants of AOS, or a less severe version of the disorder.

[edit] Nature and Symptoms

Sufferers of AOS have impaired prosody, which causes their speech to be slow, highly segmented (at the syllable or word level), and is often described as 'robotic'. Because of this, they also exhibit equal syllabic stress (tec-ton-ic as opposed to tec-TON-ic), and have trouble consciously producing correct stress patterns, even though they are aware of prosodic patterns required.

Symptoms are evident only in connected speech.

[edit] Causes

AOS is often associated with Broca's Aphasia. It is actually characterised by damage to the posterior portion of Broca's area; the insula, an area underneath the inferior-anterior portion of the temporal lobe, concealed by the fissure separating the frontal and temporal lobes; and the lentiform nucleus (Dronkers, 1996). However, some studies have shown that only a percentage of patients with AOS have these lesions (Hillis, 2004).

[edit] Differential Diagnosis

AOS, in adults (where it is an acquired condition), can often be diagnosed as conduction aphasia. Key difference between AOS and other disorders are:

• Repetitive production of a single word (eg. re-fridg-er-rat-or) will tend to become progressively more intelligible in conduction aphasia sufferers, but AOS speakers will consistently produce the same errors, and repetitions will not improve
• Errors produced in a task by an AOS sufferer are consistent across type (eg. voice phoneme unvoiced, fronted consonant backed), where other illnesses produce inconsistent errors on repetition of a task
• SMR (sequential motor rate, eg. 'pataka pataka') rates are slower than AMR (alternating motor rate, eg. 'papapa' or 'kakaka') rates for AOS sufferers
• Oromotor tasks will not yield AOS symptoms, only tasks requiring production of connected speech

Not present in AOS, and ruling out its diagnosis are features such as:

• Anticipatory errors - such as a final consonant altering initial consonant eg. 'dunited states'
• Transposition errors - eg. 'park'->'carp'
• Weakness of oral structures (oromotor weakness is not the result of AOS, though it may be co-occurring if there is also a dysarthria present)

Features definitively excluding AOS are:

• Fast/normal speech rate
• Normal stress patterns, and smooth transition between words and syllables

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