The Neuroscience on the Web Series:
CMSD 642 Neuropathologies of Swallowing and Speech

CSU, Chico, Patrick McCaffrey, Ph.D


Chapter 15. Dysarthria vs. Apraxia: A Comparison


Etiology

Dysarthria

Dysarthric errors result from a disruption of muscular control due to lesions of either the central or peripheral nervous systems. In this way, the transmission of messages controlling the motor movements for speech is interrupted. Because it involves problems with the transfer of information from the nervous system to the muscles, dysarthria is classified as a neuromotor disorder.

Central Nervous System Lesions

Damage to the pyramidal tract causes spastic dysarthria. Lesions of the substantia nigra cause hypokinetic dysarthria. Disruption of feedback loops involving the cerebellum cause ataxic dysarthria.

Peripheral Nervous System Lesions

Finally, damage to any part of the peripheral nervous system serving the muscles of speech causes flaccid dysarthria.

Apraxia

Apraxia results from an impaired ability to generate the motor programs for speech movements rather than from the disordered transmission of controlling messages to the speech musculature. Apraxia is a planning/programming problem, not a movement problem like dysarthria.

Apraxia occurs following damage to Broca's Area, or Brodmann's area 44, which is located on the third gyrus of the left frontal lobe. Thus, apraxia is always the result of a central nervous system lesion. It is a cortical problem, not a motor impulse transmission problem like dysarthria.

Type Of Errors

Dysarthria

In dysarthria, errors are consistent and predictable.

There are no islands of clear speech; no matter what the speaking task or materials used, the patient will exhibit the same amount and types of errors.

Errors are mainly distortions and omissions. Distortions are the most common type of error in dysarthria.

Apraxia

In apraxia, errors are inconsistent and unpredictable. Different error patterns occur in spontaneous speech versus repetition. Patients' spontaneous speech contains fewer errors than does his/her speech in repetition tasks.

There are islands of clear speech; when producing over-learned material or material that has become automatic, the patient will speak clearly.

Substitutions are the most common type of error, with others normally being approximations of the targeted phoneme.

Other types of errors found in apraxic speech, listed from most to least common, include:

Repetitions
Additions
Transpositions
Prolongations
Omissions
Distortions

Errors are often perseveratory or anticipatory in nature. As in stuttering, the anticipation of errors causes dysfluent speech.

The speech of apraxics is full of groping, trial and error types of articulatory movements. This is probably related to the anticipation of errors.

Speech Sounds Affected

Dysarthria

Consonants are consistently imprecise, with the production of final and initial consonants being equally impaired. Vowels are not affected as much although, due to problems with tongue movement, they may sound too much alike.

Apraxia

For an apraxic, vowels are easier to produce than consonants. Single consonants are easier than blends. As in stuttering, final consonants are easier than those in the initial position. This may occur because initial consonants are affected by anticipatory errors. Also, perhaps once an apraxic gets speech started with the production of a vowel, production continues in a more automatic fashion. Fricative and affricates are the most difficult phonemes for apraxics to produce. (These sounds require very complex articulatory movements.)

Aspects Of Speech Affected

Dysarthria

All aspects of speech, including articulation, phonation, resonance, prosody, rate and respiration, may be affected by dysarthria. Dysphagia frequently accompanies dysarthria.

Apraxia

Apraxia is mainly a disorder of articulation. Some prosodic problems may occur as a result of the hesitations caused by the apraxic speaker's anticipation of errors. However, problems with voice, resonance, etc., are not symptomatic of this disorder.

Related Problems

Dysarthria

Changes in muscle tone may accompany dysarthria. The movements of the soft palate, lips, tongue and jaw may be impaired not only during speech, but also in the context of vegetative functions. As such, changes may affect the oral and pharyngeal stages of the swallow, dysarthria and dysphagia often co-occur.

Diadochokinesis will be slow. However, it will be normal within the limitations of the neuromuscular disorder; the patient may distort or omit phonemes but syllables will be produced in the correct order.

Apraxia

As lesions of Broca's area do not cause changes in muscle tone, apraxia may occur without such symptoms. Therefore, apraxia of speech may occur without concomitant swallowing problems. The movement of the velum, lips, tongue and jaw will only be impaired during speech. Of course, a stroke may damage Broca's area and motor tracts or other areas involved in swallowing simultaneously. In such a case, dysphagia and apraxia would be seen in the same patient.

Diadochokinesis will be slow and abnormal; syllables may be produced out of order.

Effects Of Utterance Complexity

Dysarthria

Utterance complexity does not directly affect the degree of imprecision present in a dysarthric patient's speech. Such a patient will produce single syllable and multi-syllabic words with approximately the same amount of distortion.

Apraxia

Increases in utterance complexity cause increases in the complexity of apraxic symptoms. For example, it is much easier for a patient with apraxia to produce single syllable versus multisyllable words and sentences.

Effects Of Speech Rate

Dysarthria

As the rate of a dysarthric's speech increases, the intelligibility of that person's speech will decrease proportionally. In order to improve iltelligibility, the dysarthric must learn to slow rate by articulating complex words syllable by syllable.

Apraxia

As the rate of an apraxic's speech increases, the intelligibility of that person's speech may actually increase probably to the same overlearned response as with automatic speech-days of the weeks etc..

Noting the effect of increased rate on speech intelligibility may be one way to help differentiate between dysarthria and apraxia.

Therapy

Dysarthria

For many clinicians therapy for dysarthria is strictly compensatory. If motor pathways are damaged, they cannot be repaired. The dysarthric patient must learn to use techniques that increase the intelligibility of his/her speech.

Apraxia

Although some recommend compensatory techniques most apraxics benefit more from therapy that focuses on retraining more than on compensation. Melodic Intonation Therapy (Sparks and Holland, 1976) is one technique that is frequently used with apraxic patients. It is believed that, through the use of melody and rhythm, this method stimulates the creation of more neuronal connections in the right hemisphere. This may allow the "potential Broca's area" located in the right hemisphere to begin generating motor programs that control speech production. Duffy (1995) describes the type of patient for whom MIT might help best.


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Other courses in the Neuroscience on the Web series:
CMSD 620 Neuroanatomy | CMSD 636, Neuropathologies of Language and Cognition

Copyright, 1998-2013. Patrick McCaffrey, Ph .D. .