Motor Speech Disorders 117also affect the rate of movement of the tongue and other articulators, contributing to distorted
and imprecise speech sound production. In addition, ataxia and hypokinesia cause slow, sluggish
movements of the tongue and other articulators. Generally, paralysis of the muscles necessary to
elevate the tip of the tongue causes the most articulatory disruptions. Muscle rigidity, including
the masked face seen in some patients with Parkinson’s disease, also impairs overall facial muscle
movements and contributes to articulatory distortions and omissions.
Dysarthrias affecting the articulatory valve can cause unintelligible speech. Intelligibility is the
ability to be understood and is usually mea sured as a percentage. For example, a patient is said to
be 25%, 50%, or 90% intelligible. Improving speech intelligibility is a primary goal in dysarthria
therapies because of the role functional communication plays in a patient’s quality of life.
Several therapies focusing on the articulatory valve can improve a patient’s speech intelligi-
bility (Tanner, 1999a, 2008). As described more fully later, slowing the rate of speech improves
intelligibility by compensating for the sluggish movements of the articulators and allowing them
to approach their ideal articulatory points of contact in dynamic speech. Speech intelligibility can
also be improved by exaggerating individual sounds. Therapies for individual phonemes include
muscle strengthening, range of motion drills, and precision and coordination exercises.
Dysarthria and Prosodic Disturbances
To vari ous degrees, all of the dysarthrias impair speech prosody, and ataxic dysarthria is fun-
damentally a prosodic disorder. Speech prosody, defined most broadly, includes pitch, loudness,
and quality of voice in addition to speech rate and f luency. Consequently, any neuromuscular
disorder affecting phoneme precision, voice quality, rate of speech, and stress intonation, either
directly or indirectly, is a prosodic disturbance. Therapies for dysarthria often require slowing the
patient’s rate of speech and syllable- by- syllable speech production. As a result, dysarthria and the
treatments for it may disrupt the patient’s prosody.
Multiple and Mixed Dysarthrias
It is common for two or more dysarthrias to occur in the same patient at the same time or for
one dysarthria to evolve into another during the course of an illness. Duffy (1995) reports that
34.7% of all dysarthrias and 31.6% of all motor speech disorders seen in the speech pathology sec-
tion at the Mayo Clinic (Rochester) from 1987 to 1990 were of the mixed variety. Spastic, flaccid,
and ataxic dysarthrias were the most common single types encountered. Hypokinetic dysarthria
was the fourth most common single variety, followed by hyperkinetic dysarthria. “A combination
of two dysarthrias represented 84% of all cases of mixed dysarthrias, 14% contained three dysar-
thria types, and 2% contained a combination of four types” (Duffy, 1995, p. 241).
Classifying motor speech disorders by the level of neurological impairment, rather than listing
and describing all the diseases and injuries to the brain and ner vous system causing neuromus-
cular disorder, is a logical, scientific, and efficient method of dysarthria diagnosis and treatment.
Although related, the therapies for each level of deficit have dif fer ent goals, objectives, and meth-
ods. However, even classifying motor speech disorders by the level of neurological impairment
has its shortcomings. The high rate of multiple and mixed dysarthrias shows that neurological
diseases and injuries often impair multiple neurological systems and that rigid localization and
compartmentalization of symptoms for neurological disorders are often fraught with error. Duffy
(1995) notes: “Unfortunately, no rule of nature obligates neurologic disease to restrict itself to the
divisions we impose upon it” (p. 234).
According to Zemlin (1998), there are 11 recognized systems in the human body (a system
comprises two or more organs combined to exhibit a functional unity): skeletal, articular (joints
and ligaments), muscular, digestive, vascular, ner vous, respiratory, urinary, reproductive, endo-
crine (glands of the body), and integumentary (skin, hair, nails). The brain and ner vous system