The effect of levodopa on respiration and word intelligibility in people with advanced Parkinson's disease

Abstract

Objectives

Hypokinetic dysarthria is commonly encountered in Parkinson's disease (PD). Although the etiology of this dysarthria is multifactorial, disorders of respiration may strongly contribute to it. The aim of this study was to evaluate the effects of levodopa on measures of respiration and intelligibility.

Patients and methods

Vital capacity (VC), sustained vowel phonation (SVP) and phonation quotient (PQ) were determined with and without medication in 25 patients suffering from idiopathic PD. Intelligibility was evaluated by three independent speech pathologists using the word subtest protocol of the Yorkston and Beukelman “Assessment of intelligibility of dysarthric speech” (AIDS).

Results

VC was abnormal in 18/25 patients in the off-condition and 15/25 in the on-condition. SVP was normal in 22/25 patients in the off-state and in all patients in the on-state. PQ was normal in all patients in the off- and on-condition. All three respiratory parameters, as well as intelligibility, improved significantly following administration of levodopa. There were significant differences between men and women for VC and PQ, with men having the highest values for both parameters in both conditions. In none of the two conditions a correlation between respiratory parameters and intelligibility could be demonstrated.

Conclusion

This study suggests that thoracic mobility is decreased in PD, and that pharmacological treatment results in improvement, but not in normalization. Moreover, it remains unclear to what extent dyskinesias negatively influence respiratory control. Our results also demonstrate improvement of intelligibility after pharmacological treatment of PD. However, this improvement is not solely the consequence of respiratory changes.

Further research on the effects of different characteristics on intelligibility is necessary.

Introduction

Sixty to 80% of patients with Parkinson's disease (PD) develop hypokinetic dysarthria that not necessarily follows the progression of the disease [1], [2]. According to Logemann et al. [3] the first speech abnormality in Parkinsonian dysarthria is a hypophonation. Hypophonation is characterized by breathy voice quality, reduced ability to produce desired vocal intensities, reduced ability to sustain prolonged phonation and poor synchronization of exhalation and speech production [4]. Hypophonation may reflect a lack of flexibility of function and reduced control of laryngeal movements [4] but may also reflect a hyporespiratory pattern, characterized by reduced excursion of thoracic and abdominal musculature [5]. This hyporespiratory pattern may result from rigidity in these muscles [6], by irregular activation of agonist/antagonist muscles [5] or by akinesia of the intercostal muscles [7].

Studies on respiration in PD, especially vital capacity (VC) and sustained vowel phonation (SVP), have yielded relatively uniform results. PD patients show a reduction of VC and SVP, accompanied by more rapid respiratory rates with fewer variations in the amplitudes of the respiratory cycles [5].

VC is significantly decreased in PD, compared to normal controls [8]. Sathyaprabha et al. [9] showed in 35 PD patients a reduced forced vital capacity (FVC) (this is the volume of air expelled by a forced maximal expiration from a position of full inspiration) compared to normal controls. Tamaki et al. [10] investigated a correlation between thoracoabdominal movements and pulmonary function in seven patients with PD and also found a significantly decreased VC and FVC. Levodopa was shown to have a significant positive effect on thoracic excursion and hence on pulmonary functions with increased VC [7], [9]. SVP is reduced in patients with PD when compared to normal controls [2] and further decreases as the disease progresses [11], [12].

The effects of respiration on speech in PD have been examined by Solomon and Hixon [13]. They found a significant reduction in the number of syllables/expiration during reading and spontaneous speech in comparison with age-related normal controls. The authors ascribed this phenomenon to inefficient respiration.

A number of authors have reported the effect of levodopa on respiration. The results of these studies were inconsistent. Some authors reported no influence of levodopa on respiration [14], [15], [16], [17]. Rice et al. [18], on the contrary, reported irregular and fast breathing induced by levodopa in two PD patients. A possible explanation for an irregular and fast breathing is the loss of control over diaphragmatic and intercostal muscles by levodopa-induced dyskinesias. An alternative hypothesis is a deficient respiratory drive due to dopaminergic denervation of the carotid body, which is also affected in PD.

In normal persons there are remarkable gender-related differences in respiratory function. To our knowledge Stelzig et al. [19] were the only authors who reported gender-related differences in laryngeal function in respiration in PD patients. Abnormal function of vocal cord abduction and adduction were observed in 54% of the women and 39% of the men in their cohort; more patients had reduced abduction. Phase closure incompetence was found in 60.5% of the women and 49% of the men. Abnormal amplitude and mucosal waveform were seen in more male patients. Gender-related differences in PD were also found in a self-rating questionnaire by Scott et al. [20]. They found speech problems to be more frequent in male than in female patients.

The relevance of speech problems in daily communication is especially reflected in a disturbance of intelligibility. There are only a few studies about the influence of levodopa on intelligibility [6], [21], [22], [23], most of them claiming a significant improvement of intelligibility after levodopa intake.

This study investigated respiration and word intelligibility in a cohort of PD patients. More specifically it was evaluated if levodopa treatment, which has a profound impact on motor functions, affects respiration and word intelligibility.

In an untreated (off) and a treated (on) situation this study investigates if: (1) VC, SVP and the phonation quotient (PQ) are above the minimum expected values; (2) there are significant changes in respiration parameters; (3) an influence of levodopa on word intelligibility can be found; (4) gender-related differences exist in respiration and intelligibility; and (5) there is a correlation between respiration and intelligibility.