Speech Rate Deficits in Individuals with Parkinson's Disease: A Review of the Literature

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This article reviews the published evidence base related to speech rate deficits exhibited by dysarthric speakers with Parkinson's disease.

Following a brief review of Parkinson's disease and hypokinetic dysarthria, results of an extensive literature review are presented. Studies reviewed assessed speech rate characteristics of speakers with hypokinetic dysarthria during speaking tasks such as diadochokinesis, sentence reading, sentence imitation, reading passages, and spontaneous speech.

In general, results of these studies suggest that most Parkinson's disease patients speak at a rate that is at least somewhat different from unimpaired speakers (e.g., too slow, too fast, variable). Methodological differences notwithstanding, published findings revealed marked variability (both inter-subject and intra-subject) of the speech rate characteristics of those patients, as well as difficulty modifying their speech rates when necessary. Several factors potentially contributing to this variability are discussed.

The published evidence base provides justification for at least attempting rate control strategies in an effort to reduce overall speech rate, as well as also improve intelligibility and speech naturalness.

PARKINSON'S DISEASE AND DYSARTHRIA

Parkinson's disease (PD) is a degenerative disorder of the basal ganglia affecting motor control (Yorkston, Beukelman, Strand, & Bell, 2000). Idiopathic Parkinson's disease, the most common type, occurs in approximately 1% of the U.S. population over 50 years of age, with approximately 40,000 new cases reported each year (Yorkston, 1996). Due to motor symptoms such as tremor, rigidity, akinesia (i.e., lack of movement), and bradykinesia (i.e., slowness of movement), PD disease patients exhibit a high prevalence of speech deficits (Swigert, 1997; Yorkston et al., 2000). For example, Hartelius and Svensson (1994) surveyed 230 patients and found that 6% of the respondents reported "fast speech," 9% reported "stuttering," 27% reported "difficulty getting started," and 5% reported impaired stress or rhythm of speech. Logemann, Fisher, Boshes, and Blonsky (1978), examining speech and voice symptoms in 200 PD patients, observed rate disorders in 20% of the patients, while 45% exhibited imprecise consonant articulation. It has been estimated that 60-80% of PD patients will develop speech deficits as the disease progresses (Adams, 1994). Speech symptoms often begin with decreased loudness and progress to more severe functional limitations characterized by changes in rate, articulatory precision, and intelligibility (Yorkston et al., 2000).

In the seminal Mayo Clinic study of various groups of dysarthric speakers, Darley, Aronson, and Brown (1975) delineated the speech characteristics of 32 patients with "parkinsonism." Of these 32 speakers, 16 were judged to use excessively short phrases and 19 were judged to produce excessively short rushes of speech separated by pauses. Twenty-five participants produced inappropriate silence; this was interpreted as reflecting either difficulty initiating phonation or difficulty coordinating phonation and articulation (Darley et al., 1975). In addition, all 32 participants exhibited articulatory imprecision. The speech rates of 28 speakers were judged to be at least "somewhat deviant." Although only four participants exhibited a "festinating" pattern (i.e., acceleration during speaking, similar to the gait pattern of many PD patients), significant variability of speech rate was exhibited by 16 speakers. Finally, repetitions of word-initial phonemes were produced by 14 speakers (Darley et al., 1975). The deviant dimensions observed only in the parkinsonism group were short rushes of speech, rapid rate, and overall increases in rate from the beginning of the sample to the end. These speech features were assumed to result from reduced range of movements and fast repetitive movements, hence the term "hypokinetic" dysarthria (Darley et al., 1975).

The perceptual features of hypokinetic dysarthria seem to be consistent with the underlying pathophysiology of Parkinson's disease. For example, reduced range of motion (due to muscle rigidity) is reflected in monopitch, monoloudness, reduced stress, variable rate, short rushes of speech, and imprecise consonant articulation. Inappropriate or excessively long silences (i.e., pauses) may result …