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Minnesota, provided a classification system based on the level of neuromuscular impairments
in their landmark book Motor Speech Disorders. Today, motor speech disorders are divided into
apraxia of speech and six types of dysarthrias based on their classification system. This system
has not changed significantly except for Duffy’s (1995, 2012) addition of unilateral upper motor
neuron dysarthria.
Darley and colleagues’ (1975) classification system of neurogenic communication disorders
has been widely accepted by clinicians from a variety of disciplines. Besides providing a logical
and efficient way of classifying motor speech disorders, it dramatically inf luenced aphasiology.
Although in the past some authorities had proposed detailed classification systems for neurogenic
communication disorders, clinicians typically included them in adult language disorders. The
bound aries between language and motor speech disorders were blurred, leading to unfortunate
labels such as motor aphasia and sensorimotor aphasia. Perhaps the biggest prob lem caused by
including motor speech disorders in aphasia classification systems involved clinical practice.
Although the symptoms, objectives, and methods overlap, aphasia is a language disorder distinctly
dif fer ent from the motor speech disorders and requires dif fer ent therapies.
Symbolic and Nonsymbolic Neurogenic
Communication Disorders
When addressing neurogenic communication disorders, it is con ve nient to separate them into
symbolic and nonsymbolic impairments. Symbolic disorders impair or eliminate verbal symbolic
pro cessing. A symbol is an arbitrary repre sen ta tion of real ity, and the word, whether spoken, ges-
tured, or written, is a linguistic symbol. Certain neurological diseases and disorders can impair or
destroy verbal symbolic pro cessing. Neurogenic communication disorders that impair or destroy
language are aphasia, the language of confusion resulting from traumatic brain injury, psychotic
language, and the generalized intellectual impairments seen in dementia.
Nonsymbolic neurogenic communication disorders are apraxia of speech and the dysarthrias.
Although these disorders can co- occur with symbolic disorders, in their pure form they do not
disrupt or impair symbolism. In addition, motor disorders are inf luenced by sensory impairments
that also may result from neurological deficits and diseases. Apraxia of speech is the inabil-
ity to perform voluntary movements not resulting from impaired comprehension or paralysis.
Technically, patients with dyspraxia maintain some ability to speak. Clinicians use the term
apraxia of speech to refer to both the total inability and the impaired ability to produce voluntary
speech. At the highest level of motor speech programming, the symbol (word meaning) intersects
with the articulatory program; each word contains the program for its utterance.
Motor Speech Programming and Apraxia of Speech
According to the Dyspraxia Foundation (2015), apraxia of speech (dyspraxia) is caused by
an immaturity or disruption in the way motor commands are transmitted from the brain to
the body. There are three levels of neurological impairment and resulting apraxias of speech.
At the conceptual level, ideational apraxia of speech results from the speaker’s inability to grasp
the thought driving the speech act. This type of apraxia of speech is seen in dementia and other
cognitive- linguistic disorders in which the patient has trou ble conceptualizing the idea driving an
utterance. (In ideational apraxia of speech, there is no clear discrimination between symbolic and
nonsymbolic pro cessing.) At the speech planning level, the utterance’s motor plan is formulated,
including articulatory timing, speed, and strength. This type of apraxia, sometimes called ideomo-
tor apraxia, most closely resembles the apraxia of speech described by Darley et al. (1975) discussed
later. At the planning level the motor speech (articulatory) program is created, and at the activation
level the plan is set in motion. During activation, the neural commands are sent to the muscles of
the speech mechanism and the neuromotor requirements for the utterance are met. In this type of