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
to the pyramidal tract causes spastic dysarthria. Lesions of
the substantia nigra cause hypokinetic dysarthria.
Disruption of feedback loops involving the cerebellum cause
Nervous System Lesions
Finally, damage to any part of the
peripheral nervous system serving the muscles of speech
causes flaccid dysarthria.
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.
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.
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.
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
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
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.
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.
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
Aspects Of Speech
All aspects of
speech, including articulation, phonation, resonance,
prosody, rate and respiration, may be affected by
dysarthria. Dysphagia frequently accompanies
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
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.
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.
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.
will be slow and abnormal; syllables may be produced out of
Effects Of 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
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
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
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.
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.
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.