Although neuropharmacologic and neurosurgical approaches have had positive effects on the primary symptoms of PD, their effects on voice, speech, and swallowing have been inconsistent. Several studies have assessed the effects of levodopa and dopamine agonists on voice and speech functions in PD. Gallena et al. (47) studied the effects of levodopa on laryngeal function in six persons with early PD who were not receiving medication. They found that levodopa reduced excessive laryngeal muscle activity and vocal fold bowing and improved voice onset and offset control during speech in some patients. De Letter et al. (80) reported significant improvement in speech intelligibility with levodopa. Goberman et al. (81) examined the acoustic-phonatory characteristics of speech in nine individuals with PD and motor fluctuations before and after taking levodopa. They found that the voice F0 variability in vowels and mean F0 were higher, and intensity range was lower when on-medication, compared with off-medication. They also found that differences in speech between on- and off-medication were small, although in some individuals phonation clearly improved. Jiang et al (82) assessed the effects of levodopa on vocal function in 15 PD patients with tremor using airflow and EGG measures. The subjects were recorded as they sustained vowel phonation before and after taking medication. Speed quotient, acoustic shimmer, and extent of tremor derived from acoustic intensity contours were found to significantly decrease, and vocSPL tended to increase after medication, indicating improvement in vocal function with levodopa. Sanabria et al. (83) used acoustic measures to study the effects of levodopa treatment on vocal function in 20 PD patients before and after levodopa. When compared with pre-medication, post-medication voice F0 was significantly increased, and jitter, soft phonation index (noise parameter), and frequency tremor intensity index significantly decreased. Cahill et al. (84) studied the effects of levodopa on lip function in 16 patients with PD, using a computerized semiconductor lip pressure transducer. Lip pressures recorded during both speech and nonspeech tasks tended to improve after levodopa administration.
Although these studies indicate improvement in phonatory and articulatory functions with levodopa, numerous studies (85,86) have failed to find significant improvement in voice and speech functions with levodopa or dopamine agonists. These negative findings have raised questions regarding the role of dopamine as the sole, or major, etiologic factor in hypokinetic dysarthria and have raised the possibility that either nondopaminergic or special dopaminergic mechanisms may play an important etiologic role. Future studies should assess the therapeutic role of such non-dopaminergic mechanisms on parkinsonian speech. Interestingly, clonazepam (dosage 0.25-0.5 mg/day), a nondopaminergic agent, has been reported to significantly improve speech in 10 of 11 individuals with PD and hypokinetic dysarthria (87).
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